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J. Paleont., 78(4), 2004, pp. 675–689 Copyright q 2004, The Paleontological Society 0022-3360/04/0078-675$03.00

LATE CAMBRIAN AND EARLY ORDOVICIAN STEM GROUP CHITONS (MOLLUSCA: POLYPLACOPHORA) FROM UTAH AND MISSOURI MICHAEL J. VENDRASCO

AND

BRUCE RUNNEGAR

Department of Earth and Space Sciences, University of California, Los Angeles 90095-1567, ,[email protected]. ABSTRACT—Abundant silicified shell plates (valves) of some of the oldest-known chitons were recovered from the Late Cambrian Notch Peak Formation of Utah. The chiton fauna is dominated numerically by Matthevia wahwahensis new species, but also includes another mattheviid, Eukteanochiton milleri new genus and species, and a preacanthochitonid, Orthriochiton utahensis new genus and species. Robustum from the Early Ordovician Gasconade Formation of Missouri is herein reinterpreted as a septemchitonid chiton, and Hemithecella, from the Late Cambrian-Early Ordovician of the eastern United States, is once again considered a mattheviid chiton. Mattheviid valves are unique among chitons in that they are massive, elongate, and contain one or two tunnels; these characteristics have led some to exclude this family from the Polyplacophora. However, mattheviids and other chitons share many valve characters, including granules, an apical shelf, a thin anterior margin, bilateral symmetry, three valve types, and a shell layer perforated with canals. Furthermore, recently described chiton valves from the Silurian of Gotland, the Ordovician of Wisconsin, and the Cambrian-Ordovician of Missouri are gradational between the Notch Peak mattheviid valves and those of younger polyplacophorans. Also, Matthevia wahwahensis n. sp. has valves with a flat apical area, allowing for valve overlap, and so provides a link between the unusual chiton Matthevia variabilis, which had nonoverlapping, spiky valves, and Chelodes, a less disputed chiton. Known fossil assemblages reveal that, along the coastline of Laurentia during the Cambrian-Ordovician, chitons were diverse, consisting of at least four families and with much variation in valve shape.

INTRODUCTION

HE NOTCH Peak Formation of Utah contains an abundant polyplacophoran fauna. This silicified assemblage, uncovered through acid etching approximately 400 kg of calcium carbonate blocks, is among the largest collections of isolated chiton shell plates (valves) extracted from Paleozoic sediments. Cambrian chitons are rare, known only from roughly contemporaneous sediments in Missouri (Bergenhayn, 1960) and New York (Runnegar et al., 1979), making the Notch Peak assemblage one of the oldest collections of chiton fossils known. In this paper we: 1) describe these new chiton species; 2) reinterpret Robustum Stinchcomb and Darrough, 1995 and Hemithecella Ulrich and Bridge in Butts, 1941—Cambrian-Ordovician taxa from Missouri and elsewhere—as chitons; 3) provide additional evidence for the polyplacophoran affinities of mattheviids; 4) describe in detail the valve morphology of the early chitons; and 5) briefly discuss the geographic, stratigraphic, depositional, and preservational contexts of the Notch Peak chitons.

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There has been more agreement regarding the placement of Chelodes Davidson and King, 1874 within the Polyplacophora, most likely because its valves clearly overlapped each other. The thick, subconical, tunneled, non-overlapping valves of Matthevia variabilis Walcott, 1885 look different from the valves of modern polyplacophorans. However, M. wahwahensis n. sp., whose valves possess a flat to concave apical area, but with two tunnels as found in M. variabilis, provides a firm link between M. variabilis and other polyplacophorans. In addition, the mattheviids possess discrete polyplacophoran valve characters such as granules, a thin anterior margin, bilateral symmetry, an apical area, three valve types, and a shell layer perforated with canals. Furthermore, the recently described articulated chiton Echinochiton dufoei Pojeta, Eernisse, Hoare, and Henderson, 2003 from the Ordovician of Wisconsin had intermediate valves with two internal tunnels that look pyramidal in side view, similar to the morphology of mattheviids (Pojeta et al., 2003). For these reasons, we believe the mattheviids are among the earliest-known polyplacophorans. REINTERPRETATION OF CAMBRIAN–ORDOVICIAN SCLERITES FROM MISSOURI

PREVIOUS INTERPRETATIONS OF MATTHEVIIDS

There have been wide-ranging interpretations and reconstructions of mattheviids in the past (Fig. 1). Initially, Walcott (1885) considered Matthevia to be a snail, either a pteropod or a new form of equivalent rank, although he made this assignment with reservation. Knight (1941) suggested that Matthevia was more likely a representative of a new class within the Mollusca or a new phylum. Fisher (1962) agreed, placing Matthevia within the molluscan ‘‘Class Calyptoptomatida,’’ which he erected to encompass hyoliths and other organisms with subconical shells. Yochelson (1966) also agreed with Knight, but placed Matthevia in its own class (Mattheva) in the Mollusca. Yochelson reconstructed Matthevia as a two-valved mollusc. Others suggested a polyplacophoran affinity for Matthevia (Pojeta and Runnegar, 1976; Runnegar et al., 1979), though not everyone has been convinced by their arguments. For instance, Stinchcomb and Darrough (1995) and Sirenko (1997) viewed the relationship as doubtful and insufficiently substantiated. In particular, Stinchcomb and Darrough argued that fossils such as Hemithecella, a form similar to Matthevia, had a skeleton consisting of a multitude of valves (15–17). They placed these fossils and related forms (united in the Order Hemithecellitina Stinchcomb and Darrough, 1995) in an uncertain class of the Mollusca.

Stinchcomb and Darrough (1995) described many new molluscan sclerites from the Cambrian and Ordovician of Missouri. These authors were not convinced of the polyplacophoran affinities of the species they described, although they admitted that the presence of large, rounded, evenly spaced granules on the valve surface of Hemithecella and Preacanthochiton Bergenhayn, 1960 suggests such an affinity. Stinchcomb and Darrough described monoplacophoran-type muscle scars on valves of Preacanthochiton, Chelodes, and Hemithecella (Stinchcomb and Darrough, 1995, fig. 6.1, 6.2, 6.4, 6.14). However, the specimens depicted in figure 6.1, 6.2, and 6.4 seem more likely to be monoplacophoran shells than isolated valves of the genera they listed. The internal mold of Hemithecella expansa Ulrich and Bridge in Butts, 1941 shown in figure 6.14 has faint longitudinal grooves in the expanded anterior portion, but they are not clearly muscle scars. Examination of this specimen plus numerous other wellpreserved internal molds of Hemithecella have turned up no convincing signs of muscle scars, even though internal molds of monoplacophorans in the Eminence and Gasconade formations often show distinct muscle scars (Stinchcomb, 1986). Hemithecella had previously been placed in the chiton family Mattheviidae Walcott,

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FIGURE 1—Previous reconstructions of mattheviid fossils. 1, Chelodes bergmani. 2, 3, Matthevia variabilis Walcott, 1885. 4, Chelodes sp. (above) and M. variabilis (below). 5, Hemithecellid. 6, Chelodes actinis Cherns, 1998. Bergenhayn (1955), Runnegar et al. (1979), Dzik (1986), and Cherns (1999) used a polyplacophoran model, whereas Yochelson (1966) and Stinchcomb and Darrough (1995) had different interpretations of the fossils.

1886 by Runnegar et al. (1979), and we support that assignment. Hemithecella is important in understanding early chiton evolution because members of this genus possess only one tunnel rather than two, providing a link between Matthevia and Chelodes (Runnegar et al., 1979). Robustum nodum Stinchcomb and Darrough, 1995, from the Gasconade Formation, is characterized by elongate, subcylindrical valves that curved around the side of the animal. Because of these features, and the thickness in the middle of the valve, this species is herein assigned to the Septemchitonidae Bergenhayn, 1955. GEOGRAPHIC DISTRIBUTION OF MATTHEVIIDS AND OTHER CHITONS IN THE LATE CAMBRIAN AND EARLY ORDOVICIAN OF LAURENTIA

Most of the valves of Matthevia wahwahensis n. sp. were extracted from carbonates in midwestern Utah, although this species has been found elsewhere in the Great Basin (Fig. 2). In fact, Matthevia is widespread throughout and just outside of the Great Basin, having been collected from the Nopah Formation of eastern California and western Nevada, the Desert Valley Formation of eastern Nevada, the Wilberns Formation of Texas, and the Ajax Dolomite of northern Utah (Yochelson et al., 1965). The specimens from these localities photographed in Yochelson et al. (1965) and Yochelson (1966) consist of intermediate and tail valves only. Examination of these fossils, which are housed at the National Museum of Natural History (NMNH), revealed that they are strikingly similar to Matthevia wahwahensis and perhaps some specimens belong to an additional, undescribed species of Matthevia. The fossils described herein, and those from the Late Cambrian of Missouri, suggest that the western and southern shorelines of Laurentia in the Late Cambrian had a diverse assemblage of polyplacophorans. Early Ordovician chiton fossils from Oklahoma (Smith and Toomey, 1964) and Missouri (Bergenhayn, 1960; Stinchcomb and Darrough, 1995) reveal that, by this time, representatives of four stem group polyplacophoran families (Gotlandochitonidae Bergenhayn, 1955, Mattheviidae, Septemchitonidae, and Preacanthochitonidae Bergenhayn, 1960) lived along the

FIGURE 2—Maps showing localities where specimens of Matthevia wahwahensis n. sp. were collected. Collecting sites are indicated by a black square and italicized text. 2, Enlargement of the area indicated by the shaded rectangle (near Delta, labeled ‘‘2’’) in 1, showing the two sites where the majority of specimens were collected.

Laurentian coast. Furthermore, these faunas reveal that Cambrian and Ordovician chitons had valves with a diversity of shapes and growth styles. STRATIGRAPHIC CONTEXT AND LOCALITIES OF THE NOTCH PEAK FORMATION CHITONS

The Notch Peak Formation in western Utah was named by Walcott (1908) and was redefined and divided into three members (from oldest to youngest: the Hellnmaria, Red Tops, and Lava Dam) by Hintze et al. (1988). The formation consists entirely of limestone and dolomite and is characterized by high, resistant cliffs. Both conodont and trilobite biostratigraphy have revealed that these sediments were deposited during the Late Cambrian to Early Ordovician, with the period boundary in the Lava Dam Member (Hintze et al., 1988). Specimens of Matthevia wahwahensis n. sp. and Orthriochiton utahensis n. gen. and sp. were

VENDRASCO AND RUNNEGAR—CAMBRIAN AND ORDOVICIAN CHITONS FROM UTAH AND MISSOURI

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Eukteanochiton milleri were recovered from the Red Tops Member of the Notch Peak Formation at UTM coordinates 295,768 m E, 4,298,254 m N, zone 12. DEPOSITIONAL ENVIRONMENT AND PALEOECOLOGY OF THE NOTCH PEAK FORMATION

FIGURE 3—Stratigraphy of the Notch Peak Formation, where many polyplacophoran valves have been collected. 1, The stratigraphic context of the formation, indicating fossil horizons and the measured section. 2, A diagram of the measured section at Lawson Cove, Utah (LC), showing the fossil horizons (the number indicates meters above the base of the measured section) and the estimated amount of dolomite collected and acid etched from each particular bed.

collected from massive carbonate beds (0.2–1.0 m) in the upper portion of the Hellnmaria Member, and specimens of Eukteanochiton milleri n. gen. and sp. were recovered from the lower portion of the overlying Red Tops Member (Fig. 3). A majority of the specimens were collected from the Notch Peak Formation in the southern House Range and northern Wah Wah Mountains, predominantly from the localities at Lawson Cove and Steamboat Pass (Fig. 2). Locality numbers are those of the Natural History Museum of Los Angeles County, Invertebrate Paleontology Section (LACMIP). LACMIP 17160. Lawson Cove, Utah; sec. 9, T25S, R15W, Wah Wah Mountains North 1:100,000 topographic map. The specimens were collected mostly from the Gray Hills in the northwestern end of Lawson Cove, in the northern part of the Wah Wah Mountains in Millard County. Specimens of Matthevia wahwahensis and Orthriochiton utahensis were collected from the Hellnmaria Member of the Notch Peak Formation at this locality. LACMIP 17161. Steamboat Pass, Utah; sec. 19, T23S, R13W Wah Wah Mountains North 1:100,000 topographic map. Specimens were collected north of Steamboat Pass, on the east side of the canyon in the southernmost tip of the southern House Range. Specimens were collected near the painted reference section of the Notch Peak Formation (as shown in Hintze et al., 1988, fig. 8). Steamboat Pass is located 12 km west of the southwestern edge of the typically dry Sevier Lake. Specimens of M. wahwahensis were recovered from the Hellnmaria Member at UTM coordinates 295,770 m E, 4,298,222 m N, zone 12; specimens of

The sediments of the Notch Peak Formation were deposited in a shallow subtidal to peritidal environment in the middle of an extensive carbonate platform along the western margin of Laurentia (Stewart, 1991). These sediments were deposited on the subsiding miogeocline that, during the Paleozoic, formed the passive margin of western North America (Hintze, 1988). At that time, midwestern Utah was at a latitude of 5 (Scotese and McKerrow, 1990) or 10 (Suek and Knaup, 1979) degrees north. Stromatolites are common in the Notch Peak Formation and are especially abundant in the Hellnmaria Member. Hose (1961) mentioned that the stromatolites in this formation belong to the form genus Collenia Walcott, 1914, which is characterized by lateral linking between stromatolitic heads (Hallam, 1981). However, as Runnegar et al. (1979) described, the stromatolites within the Hellnmaria Member are high relief, with much of the space in between the heads filled with sediment deposited after the stromatolites formed. This stromatolite morphology is characteristic of the Cryptozoon form genus instead. The Hellnmaria Member stromatolites often show preferential growth in one direction, implying high energy current conditions, and their morphology and distribution may be suggestive of a subtidal channel paleoenvironment (Shapiro, 1994). Runnegar et al. (1979) presented several other lines of evidence suggesting a high energy, intertidal to subtidal depositional environment for the Hellnmaria Member. They documented the presence of cross-bedded dolomite sands and oncolites in addition to the domal stromatolites. Additionally, the valves of M. wahwahensis have no preferred orientation in the sediments, nor do they lie on horizontal bedding planes. Instead there is a variety of inclinations and declinations of the valves, and the dip of the long axis of the valve is often high. The random inclinations of the valves and the observation that all the valves recovered are disarticulated also support the argument for a relatively high energy depositional system. Acid etching has revealed that chiton valves make up more than 95 percent of the skeletal remains in the Hellnmaria Member, an unusual situation in the typically sparse polyplacophoran fossil record. Matthevia and the high-spired gastropod Mattherella Walcott, 1912 are the only invertebrate fossils seen in outcrops of the Hellnmaria Member. Acid-etched isolate has yielded extremely rare brachiopod valves and only a few crinoid columnal elements in addition to specimens of Mattherella and the polyplacophoran fossils. So the paleoecology of the upper part of the Hellnmaria is perplexing—stromatolites, chitons, and snails were the prime players. This observation led Runnegar et al. (1979) to conclude that the environment in which the Hellnmaria sediments were deposited was hypersaline. Matthevia from the Ajax Dolomite of the Stansibury Mountains has also been found in close association with stromatolites (Taylor et al., 1981). Matthevia variabilis was originally described from the Hoyt Limestone Member of the Theresa Dolomite, which likewise contains abundant domal stromatolites (Friedman, 1988). The consistent association between Matthevia and stromatolites was noted by Walcott (1885), and Fisher (1962) argued that Matthevia lived within the stromatolitic reefs, feeding on the algae and excretions from other reef dwellers. A similar molluscan/stromatolite association is found in the Late Cambrian–Early Ordovician of Missouri, although the Missouri fauna also includes abundant monoplacophorans and cephalopods (Stinchcomb, 1965, 1978). The Early Ordovician chitons from the Arbuckle Mountains of Oklahoma have likewise been

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FIGURE 5—Elongation of the intermediate valves of polyplacophoran species through the Paleozoic. Mattheviid species indicated by open circles.

FIGURE 4—Reconstruction of the fauna preserved in sediments of the Late Cambrian Notch Peak Formation in midwestern Utah. Large, domical stromatolites, polyplacophorans, and a gastropod are visible in the reconstruction. 1, 2, Eukteanochiton milleri n. gen. and sp.; 3–5, Matthevia wahwahensis n. sp.; 6, Mattherella; 7, 8, Orthriochiton utahensis n. gen. and sp.

recovered from beds where stromatolites and algal mat material are abundant (Smith and Toomey, 1964). The consistent association of the earliest known polyplacophorans and stromatolites suggests that chitons were living on and around the stromatolitic heads, probably feeding on the microbial mats (Fig. 4). Runnegar et al. (1979) made this suggestion for Matthevia variabilis, but it also seems to apply to the other Late Cambrian–Early Ordovician chitons of Laurentia. PRESERVATION AND DIAGENESIS OF POLYPLACOPHORAN VALVES IN THE NOTCH PEAK FORMATION

The fossilized valves of M. wahwahensis n. sp. consist of silica in most cases, secondary calcite in others, and occasionally a mixture of both minerals. The replacement of the original minerals in the valves occurred through a void-filling rather than neomorphic process. The void-fill mechanism explains: 1) the presence of large quartz crystals in many of the silicified valves; 2) the lack of microstructural detail in the calcitic specimens; 3) the sparry, euhedral calcite crystals in some of the valves; 4) a matrix that often consists of a nonplanar xenotypic mosaic of dolomite crystals with much porosity, including mouldic porosity; 5) the asymmetry in many of the specimens; 6) the variation in the length of the tunnels in silicified specimens; and 7) the mixture in some valves of chert and calcium carbonate, which can be seen in hand samples as well as thin sections. The increasing size of quartz crystals towards the center of silicified casts of M. wahwahensis indicates this is the ‘‘Pattern 1’’ silicification of Schmitt and Boyd (1981). Compaction of the sediment appears to have begun before the casts were fully formed, as shown by the sometimes asymmetrical fossils, the broken micrite envelope around many of the fossils, and the tiny faults through the specimens commonly seen in thin section. The fact that the tunnels are not fully preserved in many specimens indicates that sediment filled some tunnels but not others before the shell was dissolved. If the valves of M. wahwahensis were originally aragonitic, as is true for those of all modern polyplacophorans, the dissolution probably occurred soon after burial.

VALVE MORPHOLOGY OF THE EARLY POLYPLACOPHORANS

Elongation.The mattheviids and septemchitonids are characterized by elongate valves. Mattheviid valves are also characterized by a large apical area. These features are not typical of the valves of modern polyplacophorans. However, many other polyplacophorans from the Paleozoic have a similar elongation and large apical area. Some prominent examples include: 1) the articulated Pennsylvanian species Acutichiton allynsmithi (Hoare et al., 1983, figs. 3, 5n, o; Hoare and Mapes, 1989), whose tail valves especially show elongation and a large apical area; 2) Glyptochiton cordifer (Smith, 1971, figs. 1–15), another elongate form; and 3) Thairoplax merriami Hoare, 2000a, a Silurian chiton from northern California that has intermediate valves that are slightly longer than wide and with a large apical area (Hoare, 2000a, fig. 2j). Clearly the elongation of mattheviid valves is in line with that of other Paleozoic chitons (Fig. 5). Tunnels.One of the more striking features of mattheviids is the presence of one or two tunnels in the valves of many species. In Matthevia variabilis, these extend from the base of the valve towards the apex. In other mattheviids, the tunnels extend from the anterior portion of the valve to the apex, in between the dorsal and ventral surfaces of the valve. There are two tunnels in valves of M. variabilis, M. wahwahensis n. sp., and Eukteanochiton milleri n. gen. and sp. There is one tunnel in valves of Matthevia walcotti Runnegar, Pojeta, Taylor, and Collins, 1979, Chelodes whitehousei Runnegar, Pojeta, Taylor, and Collins, 1979, Robustum nodum, Orthriochiton utahensis n. gen. and sp., Hemithecella spp., and in problematic chitons from the Silurian of North America (Kluessendorf, 1987, pl. 1, figs. 4, 5, 7–9). In addition, internal molds of Hemithecella and Robustum show a slightly recurved midanterior margin in the same location as the dorsal tunnel in M. wahwahensis and M. variabilis; this upturned segment on the internal mold may represent a shallow depression that is a remnant of the dorsal tunnel. Some species of Chelodes likewise show remnants of a feature homologous to the dorsal tunnel of M. wahwahensis in the form of a shallow depression in the anterior portion of the valve (Cherns, 1998, pl. 1, fig. 1b; p. 4, fig. 3b). Shallow depressions also exist underneath the apical area, homologous to the ventral tunnel in M. wahwahensis, in Chelodes cf. bergmani (Cherns, 1998, pl. 1, fig. 3b, 3e), Chelodes actinis (Cherns, 1998, pl. 4, fig. 3b), Spicuchelodes pilatis (Cherns, 1998, pl. 7, figs. 2b, 3b, 4a), and Chelodes whitehousei (Runnegar et al., 1979, pl. 2, figs. 9, 32, etc.). A similar groove can be seen on the holotype of Chelodes bergmani Davidson and King, 1874, the type species of Chelodes (Cherns, 1998, pl. 1, fig. 1b). There seems to have been


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FIGURE 6—Sagittal sections of intermediate valves of mattheviids and a modern polyplacophoran, showing trends of reduction in number of tunnels and the flattening and shortening of the apical area. The apical area is indicated by the grey fill. 1, Matthevia variabilis (Cambrian); 2, Matthevia wahwahensis n. sp. (Cambrian); 3, Hemithecella expansa (Ordovician); 4, Chelodes actinis (Silurian); 5, Mopalia muscosa (Gould, 1846) (Recent).

a reduction of tunnels in chiton valves through the early Paleozoic (Fig. 6). The function of the tunnels is unclear. Runnegar et al. (1979) proposed that they had been filled with muscle tissue and documented what appeared to be an impression of tissue attachment on the flat end of a tunnel seen on an internal mold of M. variabilis (see pl. 1, fig. 27 in that publication). The situation is different in M. wahwahensis, as broken but otherwise well-preserved specimens show that both tunnels extend to near the very posterior end of the valve, tapering near the apex. This means there would not have been much surface area for muscle attachment at the end of these tunnels and suggests that they may have had another function. Cherns (1998) argued that the shallow grooves in the ventral surface of Chelodes and related species from the Silurian of Gotland most likely housed muscles that would have held the valves to each other and to the chiton’s body wall. In Matthevia, the aperture of the ventral tunnel of each intermediate valve would have been, in life, adjacent to the aperture of the dorsal tunnel on the succeeding valve. So perhaps muscles in the tunnels connected adjacent valves of Matthevia. However, the high aspect ratio of the canals as well as their apparent tapering at the end suggest an alternative function. Thin sections through valves of M. wahwahensis have not provided any data relevant to the function of the tunnels, but future discoveries of better-preserved early Paleozoic polyplacophoran valves may provide insight. Regardless of function, it is clear that as the polyplacophoran valves became progressively flattened through time, these large tunnels became reduced and were subsequently lost (Fig. 6). Traces of tunnels are seen in nonmattheviid Paleozoic polyplacophorans as well. For instance, valves of the articulated Echinochiton dufoei possess two internal tunnels (Pojeta et al., 2003). In addition, tail valves of Elachychiton juxtaterminus Hoare and Mapes, 1985 from the Pennsylvanian of Arkansas possess one tunnel and a remnant of another one dorsal to it (Hoare and Mapes, 1985, fig. 3.9). These tunnels are in the same location as those of M. wahwahensis. Similarly, Arcochiton soccus Hanger, Hoare, and Strong, 2000 from the Permian of Oregon has a shallow depression under the apical area (Hanger et al., 2000, fig. 2.27) in the same place as the ventral tunnel opening of M. wahwahensis. Granules.The early Paleozoic polyplacophorans lacked the sculptural differentiation on the surface of the valves seen in many modern polyplacophorans. Rather, they possessed valves with a simple ornament of relatively large, evenly spaced granules. The pattern of large, uniformly spaced granules is also seen in later Paleozoic chitons such as Acutichiton allynsmithi, Pterochiton tholus Hoare, Mapes, and Atwater, 1983, and Acutichiton pyrmidalus Hoare, Sturgeon, and Hoare, 1972 (Hoare et al., 1983, fig. 2). The sizes of the granules seen in the mattheviid valves

FIGURE 7—Granule diameters of fossil and modern polyplacophorans, with the diameter of the granules on the mattheviid valves included. Note that the diameters of the mattheviid granules lie within the range of granule diameters seen in other polyplacophorans.

fall within the expected range (Fig. 7) for chitons, and the granules have a similar shape and spacing to those of other polyplacophorans. Granules are seen in specimens of the following early Paleozoic chitons: Hemithecella spp. (Stinchcomb and Darrough, 1995, p. 55; Fig. 13.4); Matthevia variabilis (Runnegar et al., 1979, p. 1, fig. 29; Pojeta, 1980, fig. 14); Chelodes bergmani (Cherns, 1998, pl. 1, figs. 2d, 4); Chelodes gotlandicus (Cherns, 1998, fig. 3h); Spicuchelodes pilatis (Cherns, 1998, pl. 7, fig. 2d); and Preacanthochiton cooperi Bergenhayn, 1960 (Pojeta, 1980, fig. 14; Fig. 13.10, 13.11). Thin anterior margin.The mattheviids are characterized by a thin anterior-ventral margin of the valve at the midline. This feature is seen in most modern and fossil polyplacophorans and it allows for more snugly overlapping valves, such that the posterior apex projects only slightly above the following valve. In internal molds, it can be seen as an upward projecting anterior margin. This is the ‘‘anterior knob’’ described for Robustum nodum by Stinchcomb and Darrough (1995), it is the upturned portion at the anterior of a latex mold of a modern polyplacophoran valve, and it is seen clearly in internal molds of Hemithecella spp. (Fig. 13.1–13.3, 13.5) and Robustum spp. (Fig. 13.6–13.9). Esthete canals.Modern polyplacophorans are characterized by the perforation of the uppermost valve layer (tegmentum) with numerous canals containing sensory and/or secretory structures called esthetes (e.g., Boyle, 1974; Baxter et al., 1990; Sturrock and Baxter, 1993). Sometimes these features are seen in Paleozoic chiton valves. For instance, Hoare et al. (1983) described the configuration of the esthete canals preserved in a fractured end of a tail valve of the Pennsylvanian polyplacophoran Acutichiton pyrmidalus and Atwater (1979) described the esthete canal system in the Pennsylvanian chiton Gryphochiton simplex (Raymond, 1910). Many of the early Paleozoic polyplacophorans show evidence of esthete canals. A cotype of Septemchiton iowensis Sanders, 1964 from the Ordovician of Iowa contains numerous esthete pores on the surface of the valve (Sanders, 1964, fig. 3). The holotype of Chelodes bergmani from the Silurian of Gotland (Cherns, 1998, pl. 1, figs. 1, 2) also has pores on the abraded portion along the dorsal midline of the valve. Although Cherns (1998) argued that these pores were most likely secondary features, the size and distribution of the pores seem to suggest that they instead are esthete canal openings. These pores look similar to those visible on slightly weathered modern polyplacophoran valves as the upper shell surface becomes abraded away and the esthete canals are more exposed. In another early fossil polyplacophoran, Septemchiton? thraivensis (Reed, 1911) from the Middle Ordovician of Scotland, pores

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FIGURE 8—1–4, Schematic diagram of an intermediate valve of Matthevia wahwahensis n. sp. in lateral (1), dorsal (2), ventral (3), and anterior (4) views, showing standard measurements and terminology. 5, Drawing of a composite mold of Robustum nodum Stinchcomb and Darrough, 1995. 6, Drawing of internal mold of Hemithecella expansa. 7, Anterior view of intermediate valve of Orthriochiton utahensis n. gen. and sp.

are visible on the external surface of the valves as well as on the internal surface, according to Reed (1911), suggesting that the canals penetrated the entire valve. Thin sections made by C. D. Walcott on valves of M. variabilis reveal a pattern of lens-shaped structures. These structures were interpreted by Runnegar et al. (1979) to be solid calcareous blisters. Rather than consisting of calcareous dissepiments, however, they may represent sections of tunnels that existed, side by side, throughout the valve. If true, the pattern seen is analogous to that seen in cross sections of tree ferns where the frond stems in section appear as packed lensshaped elements. Dorsal-ventral tubes packed together may have been the primitive condition for the polyplacophoran esthete canal system. Hoare (2000b) noted that, in five species of Late Paleozoic chitons whose valves were sectioned, all esthete canals were the same size (unlike the situation in modern chiton valves). He also noted that in those species with large granules, there was one esthete canal per granule and often the walls of the esthete canals were closely packed and polygonal in cross section (seen most clearly in Hoare, 2000b, fig. 5.4, 5.6). The polygons seem likely to be homologous to the lens-shaped elements in the thin sections of valves of M. variabilis. In addition to representing dorsal-ventral esthete canals, these structures may also represent the walls of fused, hollow spicules that had coalesced to form the chiton valves. If so, the granules on the surface of chiton valves may be a remnant of the dorsal end of the fused spicules (as suggested by Pojeta, 1980). This hypothesis will be discussed in more detail in a later publication focused on broad patterns in chiton evolution. Valve growth styles.Chitons are characterized by the presence of three morphologically distinct valve types: head, intermediate, and tail. There are minor differences between the various intermediate valves of an individual (e.g., in length-to-width ratio), but these differences are not as extensive as the variation between the three main valve types. Head valves of modern polyplacophorans typically have a half- or quarter-moon-shaped outline in dorsal view and have a mixoperipheral growth style. The

intermediate valves of polyplacophorans also typically have mixoperipheral growth, whereas the tail valves usually have holoperipheral growth. In mattheviids, the head valve had holoperipheral growth whereas the intermediate and tail valves had mixoperipheral growth. As expected, there are three valve types of M. variabilis and M. wahwahensis. In both species, these consist of a small, flat valve, a subconical valve, and a narrower, taller subconical valve. Runnegar et al. (1979) argued that the small, flat plate of M. variabilis was the head valve and the tall, laterally compressed plate of this species was the tail valve. This reconstruction also makes sense for M. wahwahensis for these reasons: 1) the apical area on the tall, laterally compressed valve is convex, implying it did not overlap another valve; 2) the tail valve of the articulated Ordovician chiton Septemchiton grayiae (Rolfe, 1981, fig. 1) has a similar shape to that of the tall valve of M. wahwahensis; 3) the more vertically compressed valve of M. wahwahensis is by far the most numerous valve in the assemblage, outnumbering the tail valve by a ratio of, on average for each bed, a little over 4:1 (Vendrasco, 1999); 4) the flat to slightly concave apical area of the vertically compressed valve of M. wahwahensis suggests that it overlapped other valves; and 5) the articulated Ordovician chiton S. grayiae had a tiny head valve. TERMINOLOGY AND MEASUREMENTS

Standard measurements were taken using calipers and a protractor with the median line of the valve mounted horizontally (Fig. 8). The length, median length, width, and apical angle were measured in dorsal view; the apical length was measured in ventral view; and the height and jugal angle were measured in anterior view. The length of the embayment was determined by subtracting the total length by the median length. Only those valves with the apex, apical area, and at least one of the anteriorlateral margins preserved were measured, which meant that most

VENDRASCO AND RUNNEGAR—CAMBRIAN AND ORDOVICIAN CHITONS FROM UTAH AND MISSOURI valves in the assemblage were not measured. If only a small portion of the apex was missing, the posteriormost point was estimated by using the intersection of the lines drawn along the posterolateral margins of the valve. Similarly, the apical angle in these cases was determined by measuring the angle between these straight posterolateral margins. Because the anterior portions of the tail valves of M. wahwahensis n. sp. were fragile and hence less often preserved, only a small proportion of these valves were measured. SYSTEMATIC PALEONTOLOGY

Most specimens have been deposited at the Natural History Museum of Los Angeles County, Invertebrate Paleontology Section (LACMIP) and bear repository numbers of this institution. One specimen was deposited at the Natural History Museum of Los Angeles County, Malacology (LACM). Class POLYPLACOPHORA de Blainville, 1816 Order PALEOLORICATA Bergenhayn, 1955 Discussion.Members of this extinct order can be identified by the lack of insertion plates and sutural laminae (e.g., Van Belle, 1983), as well as typically thick and massive valves (Smith, 1960). The name Paleoloricata had been originally proposed to group those polyplacophorans with valves that lack the articulamentum sensu stricto (Bergenhayn, 1955). The articulamentum layer in modern chitons forms the sutural laminae, which project anteriorly and insert underneath each preceding valve, and the insertion plates, which project laterally and insert into the fleshy girdle. Fossil chitons that lack such projections have been united within this order. However, it has been pointed out that the presence of the articulamentum layer is often difficult to detect in the early fossils, since many Paleozoic polyplacophorans are silicified and incompletely preserved (Runnegar et al., 1979). Also, the earliest chitons had a shell that lacked the articulamentum layer (Pojeta, 1980), and so the paleoloricates are defined by a primitive character state—lack of the articulamentum. Reduction and loss of slits in the insertion plates and loss of the insertion plates themselves seem to have occurred repeatedly within the primitive modern polyplacophoran Family Lepidopleuridae (Sirenko, 1997), suggesting that uniting taxa based on the lack of these characters may produce a polyphyletic group. The Neoloricata Bergenhayn, 1955, the polyplacophoran order characterized by the possession of projections of the articulamentum layer, has been thought to be derived from the paleoloricates (e.g., Bergenhayn, 1960; Sirenko and Starobogatov, 1977). So the Paleoloricata is probably paraphyletic and this name should be used more as a descriptive term than as a taxonomic name that implies monophyly. The range of this order had been proposed as Late Cambrian through Late Cretaceous (Bergenhayn, 1955), though the specimens from the Late Cretaceous are poorly preserved and so these forms may have had projections of the articulamentum, and hence were neoloricates, but the projections were worn away. If this is true, the range of the Paleoloricata extends only to the Devonian. Suborder CHELODINA Bergenhayn, 1943 Discussion.This suborder (originally proposed as an order) was originated to unite chitons without the articulamentum valve layer (Bergenhayn, 1943). After creating the Order Paleoloricata, however, Bergenhayn in 1955 emended the description of this suborder to unite chitons with a shell composed of eight valves. The paleoloricates had been separated into two suborders: the Chelodina and the Septemchitonina (Bergenhayn, 1955). It seems the Chelodina had been retained by Bergenhayn to house all nonseptemchitonid paleoloricates. The description of a shell composed of eight valves, although supported by Van Belle (1983),

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is not useful since all polyplacophorans whose exact number of valves is known regularly have eight valves and since septemchitonids were shown to have eight valves as well (Rolfe, 1981). Smith (1960) emended the description of this suborder to chitons with valves containing a tegmentum layer that may or may not be distinctly divided into areas and valve ornament of growth lines or ridges. The first part of the description is not useful because it includes all polyplacophorans, and the second portion is the primitive state for polyplacophoran valves. Smith and Hoare (1987) had the Chelodina as the single order within the subclass Paleoloricata. Clearly the diversity of valve form exhibited by those chitons assigned to the Chelodina suggests this is not likely a monophyletic group. Family MATTHEVIIDAE Walcott, 1886 Discussion.This family was previously synonymized with the Chelodidae Bergenhayn, 1943 by Runnegar et al. (1979). The Chelodidae had originally been proposed to encompass chitons with valves lacking distinct areas set off by a sharp change in slope and/or sculpture (Bergenhayn, 1943). Smith and Hoare (1987) retained the Chelodidae in order to contain three species [Eochelodes bergenhayni Marek, 1962, Probolaeum? canadense Clarke, 1907, and Probolaeum corrugatus (Sandberger and Sandberger, 1856), the last one recently renamed Diadelochiton recavus by Hoare (2002)] not assigned to more narrowly defined paleoloricate families, while also retaining the Mattheviidae to encompass species of Calceochiton Flower, 1968, Hemithecella, Chelodes, and Matthevia. Sirenko (1997) eliminated the Mattheviidae in his taxonomic scheme, retaining the Chelodidae instead to unite Chelodes, Eochelodes Marek, 1962, and Calcheochiton. Hoare (2000b) instead suggested the recognition of the Mattheviidae in place of the Chelodidae. The mattheviids are united by the possession of elongate, thick, subconical intermediate and tail valves with a small apical angle and an apical area that extends from one-third to just over onehalf of the median length of the valve. The intermediate and tail valves have an arrowhead-shaped outline in dorsal view. The sculpture of the valves, when preserved, consists of evenly spaced granules and/or growth lines/ridges, and the valve surface is not, or is only weakly, divided into areas by a change in slope. Many forms possess one or two tunnels that extend from the anterior surface (ventral surface in M. variabilis) of the valve to near the apex. Stinchcomb and Darrough (1995) argued that Hemithecella was distinctly different from Matthevia and belonged in its own family (Hemithecellidae) in a new order (Hemithecellitina) in an uncertain molluscan class. In our opinion, however, the strong similarity between Hemithecella and Matthevia indicates the name Hemithecellidae is redundant with the Mattheviidae, and the Order Hemithecellitina is not needed. Latex molds made from intermediate valves of M. wahwahensis n. sp. (Fig. 10.29) show a nearly identical form to natural, internal molds of Hemithecella (Fig. 13.1–13.3, 13.5), except Hemithecella spp. lack the dorsal tunnel of Matthevia spp. This strong similarity in form precludes a family level distinction between Matthevia and Hemithecella. Stinchcomb and Darrough (1995) argued that the ‘‘hemithecellid’’ valves they described from Missouri occasionally were somewhat asymmetrical. However, isolated modern polyplacophoran valves are often slightly asymmetrical due to abrasion. Also, sediment compaction could have partially deformed the fossils. This is suggested by the presence of deformed internal molds of high-spired gastropods from the Eminence and Gasconade formations. All specimens of Hemithecella and Robustum examined showed, at most, slight and apparent asymmetry. In our opinion, the asymmetry is not significant enough to warrant placement of

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FIGURE 9—Matthevia wahwahensis n. sp. 1–4, Dorsal, lateral, ventral, and anterior views of the holotype, LACMIP 12821, intermediate valve, X2; 5–8, dorsal, lateral, ventral, and anterior views of an intermediate valve, paratype, LACMIP 12825, X2; 9, 10, dorsal and lateral views of an intermediate valve, paratype, LACMIP 12826, X2; 11, ventral view of an intermediate valve, paratype, LACMIP 13032, X2.5; 12, anterior view of an intermediate valve, paratype, LACMIP 13033, X2; 13, dorsal view of an intermediate valve, paratype, LACMIP 13034, X2; 14, dorsal viewof an intermediate valve, paratype, LACMIP 13035, X2; 15, lateral view of an intermediate valve, paratype, LACMIP 13036, X2; 16, anterior view of an intermediate valve, paratype, LACMIP 13037, X2; 17–20, lateral, ventral, dorsal, and anterior views of a tail valve, paratype, LACMIP


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Hemithecella and the other mattheviids outside of the Polyplacophora. Specimens from the Notch Peak Formation are often asymmetrical, but this is due to deformation of the fossils. The effects of compaction can be seen in the deformed shells of some specimens of the gastropod Mattherella as well as the broken micrite walls of Matthevia fossils seen in thin section. Calceochiton is a mattheviid known only from poorly preserved specimens from the Ordovician of New Mexico (Flower, 1968). Based on examination of material housed at the USNM and the photographs of the syntypes, it is difficult for us to distinguish this genus from other mattheviid genera. Genus MATTHEVIA Walcott, 1885 Type species.Matthevia variabilis Walcott, 1885 by monotypy and original designation. Diagnosis.Thick, massive subconical intermediate and tail valves with a barbed arrowhead-shaped outline in dorsal view, two tunnels in valve interior, elongate (length/width between 1.5 and 2.0), large embayment along the anterior margin, long apical area; intermediate valves shorter and wider than tail valves; head valves flat, holoperipheral growth style, subcentral mucro. Discussion.This genus was established as a polyplacophoran by Runnegar et al. (1979). Those authors suggested that Matthevia may have possessed seven valves only, but they emended this to eight (Runnegar and Pojeta, 1985) after Septemchiton Bergenhayn, 1955 was shown to have eight valves (Rolfe, 1981). MATTHEVIA

WAHWAHENSIS

new species

Figure 9 Matthevia variabilis RUNNEGAR, POJETA, TAYLOR, AND COLLINS, 1979, p. 1377, pl. 1, figs. 5–13, 15–18. Matthevia sp. YOCHELSON, 1966, pl. 1, figs. 6–11, 15–17, 19–22, 30– 32, 36, 37, 40, 41.

Diagnosis.Intermediate valves large (up to 4 cm long), elongate (mean length/width 1.9), subconical, barbed arrowheadshaped outline in dorsal view, apical area flat to slightly concave, two tunnels extending from anterior of valve to near the apex, deep anterior embayment; tail valve similar to intermediate valve except taller and narrower, convex apical area; head valve small (mean 1 cm long), flat, thin, oval, with subcentral mucro. Description.Intermediate valves (Fig. 9.1–9.6) subconical; elongate (mean length/width 1.9, SD 5 0.354, n 5 108); mean height, 0.714 cm (SD 5 0.217, n 5 108); flat to slightly convex jugal ridge, sharp change in slope of sides along dorsal-lateral margin, otherwise no clear valve areas; sides diverge straight from apex or flare out at anterior end of valve; deep embayment at anterior end of dorsal surface; apex pointed, small apical angle (mean 28.9 degrees, SD 5 3.15, n 5 108); apical area along ventral margin extends 44 percent of total length of valve, flat to concave, with deep rounded embayment; two tunnels penetrate valve, parallel to valve midline, extending from anterior surface to near apex, roughly oval in cross section (wider than tall), dorsal tunnel aperture sometimes circular, ventral tunnel aperture flat on ventral border (just above apical shelf); thickness of valve greatest between tunnels; slightly convex sides perpendicular to apical area; valve gently bent from anterior to posterior along ventral margin except near apex where slope increases dramatically, producing downward-trending beak; apical shelf and jugal ridge diverge from each other at a mean angle of 23.9 degrees (SD 5

FIGURE 10—Distinctiveness of the intermediate and tail valves of Matthevia wahwahensis n. sp. The tail valves of this species are more laterally compressed.

5.3, n 5 108); sculpture lacking due to coarse preservation; valve ontogeny appears roughly isometric. Tail valves (Fig. 9.17–9.24) taller, narrower than intermediate form (Fig. 10), mean height 1.3 cm (SD 5 0.3, n 5 48), mean length/width 2.6 (SD 5 0.5, n 5 48), mean apical angle 15 degrees (SD 5 4.2, n 5 48); bilaterally symmetrical; small, flat jugal ridge, flat sides extend down to ventral margin; short embayment at anterior end along the dorsal surface; apical area convex, extends, on average, 54 percent of total length of valve, with short embayment at the anterior end; two tunnels exist parallel to midline from anterior end of valve to near apex; smaller tunnel near dorsal margin of valve with constant circular diameter; larger ventral tunnel tapers from anterior margin to near apex, with oval aperture (taller than wide); greatest thickness of valves occurs between tunnels; jugal ridge and apical area diverge straight from pointed apex at a 37-degree angle (SD 5 7.6, n 5 48); prominent beak lacking in this form; anterior margin of valve diverges from apical area at 120-degree angle; sculpture lacking due to coarse preservation; valve ontogeny appears roughly isometric. Head valves (Fig. 9.25–9.27) oval, flat, thin; 1.55 times longer than wide (n 5 17); gently sloped mucro located subcentrally along midline (60 percent of length from one margin); shallow depression adjacent to mucro, producing a wavy outline in lateral view. Etymology.Named for the Wah Wah Mountains in midwestern Utah, where the majority of the valves of this species were collected. Types.Holotype, LACMIP 12821 (Fig. 9.1–9.4), and 20 paratypes, LACMIP 12822–12826, 13032–13040 (figured), and 12827 (unfigured paratype lot) from the Late Cambrian Hellnmaria Member of the Notch Peak Formation at the type locality, Lawson Cove, midwestern Utah (LACMIP Locality 17160; Fig. 2). Other material examined.The specimens described and illustrated in Runnegar et al. (1979) and Yochelson (1966) that are now placed into this new species were examined at the NMNH. Occurrence.This species is exclusive to the Late Cambrian. It has been recovered from the Notch Peak Formation at Steamboat Pass and Lawson Cove in midwestern Utah; from the Ajax

← 12822, X2; 21–24, lateral, ventral, dorsal, and anterior views of a tail valve, paratype, LACMIP 13038, X2; 25, dorsal view of a head valve, paratype, LACMIP 12824, X2; 26, 28, dorsal and lateral views of a head valve, paratype, LACMIP 13039, X2, X3, respectively; 27, lateral view of a head valve, paratype, LACMIP 13040, X3; 29, latex mold made from an intermediate valve, X1.5; 30, latex mold made from the holotype, X2.2; 31, 32, model showing reconstructed skeleton, paratypes, LACMIP 13041A-H. All specimens from LACMIP locality 17160.

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JOURNAL OF PALEONTOLOGY, V. 78, NO. 4, 2004 Occurrence.Members of this genus have only been found at the Steamboat Pass, Utah (LACMIP 17161; Fig. 2). Discussion.This genus is distinguished from Matthevia by its greatly elongated form and extensive apical area. It differs from Chelodes, Hemithecella, and Calceochiton by its elongate form and two tunnels. This genus can further be distinguished from all other mattheviids by its small size. Chelodes raaschi Kluessendorf, 1987 from the Silurian of Wisconsin has a similar form to Eukteanochiton—it has a long apical area (extending 60 percent of the length of the valve) ornamented by growth lines, and it is elongate. However, C. raaschi lacks a ventral tunnel (and, most likely, a dorsal tunnel, though this cannot be determined from the two known specimens). Also, C. raaschi has a V-shaped anterior margin of the apical area, unlike the rounded margin of Eukteanochiton milleri n. gen. and sp.

FIGURE 11—Distinctiveness of Matthevia wahwahensis n. sp. and Eukteanochiton milleri n. gen. and sp., two species found within the same stratigraphic section at Steamboat Pass, Utah. Eukteanochiton milleri is more elongate than Matthevia wahwahensis.

Dolomite in northern Utah (Runnegar et al., 1979); from the Nopah Formation of eastern California and western Nevada; and from other localities throughout the Great Basin such as the Pahranagat Range, the Delamar Mountains, and the Specter Range in Nevada, and the Wilberns Formation in Texas (Yochelson et al., 1965). Discussion.This species can be distinguished from Matthevia variabilis by the flattened aspect of the valve including a slightly concave apical area. This meant that, in life, the valves of M. wahwahensis had considerable anterior-posterior overlap. The tunnels in this species gently slope towards each other and are directed posteriorly (Fig. 6), unlike the tunnels in valves of M. variabilis. In addition, the Great Basin forms were geographically distant from M. variabilis, found along the northeast coast of the U.S., suggesting reproductive isolation. Yochelson (1966) noted that the Great Basin and east coast forms of Matthevia most likely represented different species because of the distance separating them, but he found no way to differentiate between interspecific and intraspecific variation in this genus. M. wahwahensis can be distinguished from M. walcotti by its valves with two tunnels rather than one and its slightly smaller size, and from Chelodes spp. by its valves with tunnels, a deeper anterior embayment, and a smaller apical angle. Latex and clay molds were made of specimens of M. wahwahensis (Fig. 9.29, 9.30) in order to compare this species to Hemithecella expansa, a species known mostly from internal and external molds. The tunnel in H. expansa is similar to the ventral tunnel in M. wahwahensis, but H. expansa lacks any indication of the dorsal tunnel. Even though Eukteanochiton milleri n. gen. and sp. occurs 20 m above the last occurrence of M. wahwahensis in the Notch Peak sequence at Steamboat Pass, Utah, M. wahwahensis can be distinguished from this species by its less elongate, narrower valves with a shorter apical area (Fig. 11). Genus EUKTEANOCHITON new genus Type species.Eukteanochiton milleri new species, by monotypy. Diagnosis.Valves thick, short (less than 2 cm long), elongate (mean length/width 3.3); concave apical area, extends 60 percent length of valve, ornamented by growth lines; two tunnels, extend most of length of valve; tail valves taller and narrower than intermediate plates. Etymology.Eukteanos, Greek, tall, slender.

EUKTEANOCHITON MILLERI new species Figure 12.1–12.13 Diagnosis.As for the genus. Description.Intermediate valves (Fig. 12.1–12.8, 12.13) subconical, elongate (mean length/width 3.3, SD 5 0.5, n 5 7); thick, but thin at anterior end; dorsal and ventral margins diverge at a mean angle of 10.3 degrees (SD 5 1.4, n 5 7); not very tall (mean height 0.37 cm, SD 5 0.06, n 5 7); gently convex jugal ridge; embayment at anterior margin of valve, rounded margin; ventrolateral margins of valve straight, apical angle small (mean 10.4 degrees, SD 5 4.4, n 5 7); some specimens possess a groove along dorsal midline, indicating a partially abraded jugal ridge revealing a tunnel; dorsal tunnel has small, circular, constant diameter, runs straight, parallel to midline, from anterior end to near apex; ventral tunnel close to apical area, flat on ventral side and arched dorsally, wider than long, most of thickness of valve dorsal to this tunnel; apical area extends 60 percent of length of valve, ornamented with growth lines, slightly concave, with short, rounded embayment at anterior end. Tail valves (Fig. 12.9–12.12) similar to intermediate valves, except taller and narrower; smaller ventral tunnel. Head valves unknown. Etymology.Named for James Miller, who brought the occurrence of this species to our attention. Types.Holotype, LACMIP 12837, and six paratypes, LACMIP 12838–12839, 13042–13043 (figured), and 12840 (unfigured lot) from the type locality (LACMIP 17161; Fig. 2) in the Late Cambrian Red Tops Member of the Notch Peak Formation, Saukiela junia Shelly Fossil Zone, at Steamboat Pass, midwestern Utah. Occurrence.Only at the type locality. Discussion.The greater elongation, lack of expanded, widened anterior margins, and longer apical area differentiate this species from M. wahwahensis n. sp. M. variabilis has nonoverlapping valves. The presence of the large, slightly concave apical area in Eukteanochiton milleri indicates there was much anteriorposterior valve overlap. The presence of two tunnels and the small size of the valves differentiate this species from M. walcotti, Hemithecella spp., and Chelodes spp. Chelodes raaschi has a similar elongate form and long apical area, but it lacks tunnels entirely and it has a V-shaped anterior embayment in the apical shelf, unlike the rounded margin in Eukteanochiton milleri. The groove down the midline of partially abraded specimens of this species (Fig. 12.13) looks identical to grooves seen in abraded valves of M. wahwahensis (Fig. 10.13), a species that clearly had two tunnels. Family PREACANTHOCHITONIDAE Bergenhayn, 1960 Discussion.This family was erected by Bergenhayn (1960) to encompass the four Preacanthochiton spp. he described from


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FIGURE 12—1–13, Eukteanochiton milleri n. gen. and sp., all photos X3, all specimens from LACMIP locality 17161; 1–4, dorsal, ventral, lateral, and anterior views of the holotype, LACMIP 12837, intermediate valve; 5–8, dorsal, ventral, lateral, and anterior views of an intermediate valve, paratype, LACMIP 12838; 9–11, lateral, ventral, and anterior views of a tail valve, paratype, LACMIP 12839; 12, lateral view of a tail valve, paratype, LACMIP 13042; 13, dorsal view of an intermediate valve showing groove along jugal ridge, paratype, LACMIP 13043; 14–31, Orthriochiton utahensis n. gen. and sp. All specimens from LACMIP locality 17160; 14, 15, dorsal and lateral views of an intermediate valve, paratype, LACMIP 12830, X4.5; 16–19, lateral, ventral, anterior, and posterior views of an intermediate valve, paratype, LACMIP 12834, X4; 20, posterior view of an intermediate valve, paratype, LACMIP 12835, X5; 21–23, dorsal, ventral, and lateral views of the holotype, LACMIP 12828, intermediate valve, X4; 24, 25, ventral and anterior views of an intermediate valve, paratype, LACMIP 12832, X4.5; 26, dorsal view of a possible head valve, paratype, LACMIP 12831, X4; 27, 28, dorsal and lateral views of a tail valve, paratype, LACMIP 12829, X5; 29–31, dorsal, lateral, and ventral views of a tail valve, paratype, LACMIP 12833, X5.

the Late Cambrian-Ordovician of Missouri. This family was retained by Runnegar et al. (1979), though they said of Bergenhayn’s classification (p. 1391): ‘‘It was difficult to follow the reasoning behind this taxonomy when the collections were reexamined, but one thing is certainly clear: Preacanthochiton was a tiny, apparently advanced chiton, of probable early Trempealeauan age.’’ Smith and Hoare (1987) retained this family as well, although Smith was skeptical about the assignment of Preacanthochiton depressus Bergenhayn, 1960 and Preacanthochiton cooperi, the latter being the type species of the genus, to the Polyplacophora. Hoare (2000b) recognized the Preacanthochitonidae as a valid chiton family, one that perhaps branched off from the Mattheviidae in the Late Cambrian. Genus ORTHRIOCHITON new genus Type species.Orthriochiton utahensis new species, by monotypy. Diagnosis.Small (holotype 5.0 mm long, 3.0 mm wide), delicate intermediate valves with straight side slopes diverging at a 60–70-degree angle (i.e., jugal angle); delicate tail valves with

posterior mucro and vertical posterior margin, sides of valve more rounded; possible head valves thin, flat, with subcircular outline in dorsal view. Etymology.Orthrios, Greek, dawn, named because this is one of the earliest known polyplacophoran fossils. Occurrence.These fossils have been found only at the type locality, which is located at Lawson Cove, Utah, in the upper portion of the Hellnmaria Member of the Notch Peak Formation (LACMIP 17160; Fig. 2). Discussion.The tail valve with a posterior mucro and postmucronal area directed downward, perpendicular to the dorsal ridge of the valve, differentiates this genus from Preacanthochiton spp. Also, the intermediate valve assigned to Preacanthochiton cooperi by Runnegar et al. (1979, pl. 2, fig. 64) lacks the straight sides of the valves of Orthriochiton. This genus is distinguished from Septemchiton by the straight side slopes of the intermediate valves. It is distinguished from Robustum by its much smaller size and more delicate valves. These valves were found in sediments that also contained numerous specimens of M. wahwahensis n. sp. However, the valves

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assigned to Orthriochiton are clearly not valves from juveniles of M. wahwahensis because: 1) some intermediate valves of M. wahwahensis are smaller than the average size of the individuals of O. utahensis n. gen. and sp. and they look very different (M. wahwahensis has a distinct apical area, a thick, rounded jugum, and convex sides, unlike the thin jugum and straight sides of Orthriochiton); and 2) the tail valves of juvenile individuals of M. wahwahensis have a much more pointed apex with a sloping postmucronal (apical) area. The lack of gradation between these forms in spite of the overlap in size leads us to conclude they are phylogenetically distinct. ORTHRIOCHITON UTAHENSIS new species Figure 12.14–12.31 Diagnosis.As for the genus. Description.Intermediate valves (Fig. 12.14–12.25) tentshaped, delicate, small, longer than wide (holotype 5.0 mm long, 3.0 mm wide); thin, gently sloping jugal ridge, jugal angle between 60 and 70 degrees; sides straight; apical angle between 50 and 55 degrees; ventrolateral margins straight; anteriolateral margins convex, rounded; deep embayment (extends nearly half the length of the valve); ventroposterior portion of valve along midline slightly thickened. Tail valves (Fig. 12.27–12.31) delicate, small, twice as long as wide (6.0 mm total length; 3.0 mm width); gently sloping jugal ridge, widens towards anterior margin, slopes downward slightly at the posterior end; slightly convex sides; terminal mucro with gentle slope, posterior margin flat to slightly concave, perpendicular to jugal ridge; sides diverge slightly from anterior to posterior, ventrolateral margins straight to slightly convex; short embayment (extending 15 percent of total length of valve); small depression on ventral surface of valve underneath mucro. Possible head valves (Fig. 12.26) subcircular, small (2–3 mm in diameter), flat, thin; indistinct mucro. Etymology.Derived from the state where this species was discovered (Utah). Types.One holotype (LACMIP 12828) and seven figured paratypes (LACMIP 12829–12835) taken from the type locality. Occurrence.This species has only been found at the type locality, in sediments of the upper Hellnmaria Member of the Notch Peak Formation at Lawson Cove, Utah (LACMIP 17160; Fig. 2). Discussion.This species is only known from silicified casts. Valve ornament is not preserved. The apical area is missing on the intermediate valves, probably because it has been worn away, indicating it must have been short and delicate. The apex on the intermediate valves is blunt, indicating it was probably worn down as well. The small size, thin-walled valves with a heart-shaped outline in dorsal view, and elongated tail valve with a terminal mucro suggest an affinity with other preacanthochitonids. Also, all other preacanthochitonids are known only from the Late Cambrian to Early Ordovician of Missouri (Bergenhayn, 1960). An occurrence of another member of this family in contemporaneous rocks also deposited on the west side of Laurentia (based on paleogeographic reconstructions in Scotese and McKerrow, 1990 and Dalziel, 1997) is not surprising. The tail valve with a terminal mucro and postmucronal area perpendicular to the jugal ridge differentiates this species from Preacanthochiton spp. Also, the intermediate valve of Preacanthochiton depressus (Bergenhayn, 1960, fig. 1.5) and the one assigned to Preacanthochiton cooperi by Runnegar et al. (1979, pl. 2, fig. 64) lack the straight sides of the valves of Orthriochiton utahensis. This species can be distinguished from Robustum nodum by its small size and straight sides, and from species of Septemchiton by its straight sides and less elongate valves.

Family SEPTEMCHITONIDAE Bergenhayn, 1955 Discussion.Bergenhayn (1955) originally defined this family as chitons with seven valves with complete shell coverage over the sides of the animal and valve surface divided into areas. Smith (1960) emended the diagnosis of the Suborder Septemchitonina to ‘‘Body narrow, wormlike, about 17 times longer than wide, with 7 exposed, long and narrow overlapping valves’’ (p. I50). He listed the following features of the Family Septemchitonidae: ‘‘Surface of tegmentum of all valves divided into distinct areas; valve coverage complete’’ (p. I50). Rolfe (1981) revealed that Septemchiton grayiae, the type species of the type genus of this family, has eight, not seven, valves, making Septem- a misnomer. Smith and Hoare (1987) retained this family, as did Sirenko (1997). Hoare (2000b), however, suggested omitting this family from the Polyplacophora because its members have shell plates that curve inwards along the ventral surface. However, we believe the Septemchitonidae should be retained within the Polyplacophora because septemchitonids have distinct chiton characters such as: 1) eight overlapping shell plates; 2) a head and tail valve distinct from the intermediate valves; 3) a dorsal valve surface divided into distinct ‘‘areas’’ similar to what is seen in undisputed chiton valves; and 4) a porous valve surface. Genus ROBUSTUM Stinchcomb and Darrough, 1995 ROBUSTUM NODUM Stinchcomb and Darrough, 1995 Figure 13.6–13.9, 13.13 Robustum nodum STINCHCOMB 8.11, 8.17–8.20.

AND

DARROUGH, 1995, p. 64, fig. 8.2–

Description.See Stinchcomb and Darrough (1995). Material examined.The holotype, paratype, and paratype suite of R. nodum were examined at the NMNH. Additional specimens were purchased, collected from Missouri, and donated by B. Stinchcomb. Occurrence.Upper Gasconade Formation (Early Ordovician) at Missouri localities M-1, H-5, and others (Stinchcomb and Darrough, 1995, p. 65). Discussion.Robustum nodum has elongate, subcylindrical valves with growth along the sides, an apical area, thickening in the middle, and a thin anterior margin. This species is known from numerous well-preserved internal and external molds. Latex casts made from external molds of valves of R. nodum (Stinchcomb and Darrough, 1995, fig. 8.2–8.5) show valve areas, complete overlap, and an elongate shape, indicating that these plates are intermediate valves of a septemchitonid chiton. They look very similar to the external surface of valves of S. grayiae and, in fact, internal molds of Septemchiton? thraivensis from the Ordovician of Scotland have a saddle shape similar to internal molds of R. nodum (see Reed, 1911, pl. XV, fig. 3). The cross-sectional drawing of Septemchiton aequivoca, based on several partially articulated specimens (Dzik, 1986), also shows an identical form to Robustum, as noted by Stinchcomb and Darrough (1995). The subcylindrical form of Robustum is similarly seen in Septemchiton iowensis Sanders, 1964 from the Ordovician of Iowa. The elongation of the intermediate valves (length/width slightly greater than 2) is an additional similarity between R. nodum and the other septemchitonids. Robustum nodum intermediate valves also possess a posterior tunnel and an upturned lip on the anterior margin of internal molds. Stinchcomb and Darrough (1995) argued that the tunnel is homologous to that in Hemithecella and the upturned lip represents the filling in of a remnant of the second tunnel characteristic of Matthevia. We agree. Robustum nodum appears to be a link between mattheviids and the other septemchitonids, retaining


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FIGURE 13—Polyplacophorans from the Cambrian-Ordovician of southeastern Missouri; 1, Hemithecella expansa internal mold, LACMIP 12841, X1.8; 2, H. expansa internal mold, LACMIP 12842, X1.5; 3, H. expansa composite mold, LACMIP 12843, X1.75; 4, H. expansa external mold showing granule impressions, LACMIP 12844, X2.5; 5, H. expansa internal mold, LACMIP 12845, X1.25; 6, Robustum nodum internal mold, LACMIP 13044, X2.3; 7, R. nodum composite mold, LACMIP 13045, X2; 8, R. nodum composite mold, LACMIP 13046, X1.8; 9, R. nodum composite mold, LACMIP 13047, X2.2; 10, Preacanthochiton sp., external mold showing granule impressions, LACMIP 13049, X2.8; 11, Preacanthochiton sp., external mold showing granule impressions, LACMIP 13050, X3.4; 12, latex mold of an intermediate valve of a modern chiton, Mopalia muscosa, LACM 152795, X2.5; 13, latex cast made from a composite mold of R. nodum, X2.2. Specimens in 1–9 are from the Gasconade Formation near Sullivan, Missouri (M-1 in Stinchcomb and Darrough, 1995). Specimens in 10, 11 are from the Eminence Formation, 2–4 km north of Potosi, Missouri.

one tunnel underneath the apex and the remnant of the other tunnel. This further strengthens the argument that Matthevia and Robustum are not Problematica, but were chitons. ACKNOWLEDGMENTS

We would like to thank J. Miller for guiding MV to localities of Matthevia wahwahensis n. sp. and for pointing out the occurrence of Eukteanochiton milleri n. gen. and sp. in midwestern Utah. B. Stinchcomb was instrumental in donating numerous Cambrian/Ordovician valves from Missouri and in showing MV the Missouri localities; we thank him for his hospitality, generosity, and discussions of plated mollusk phylogeny. R. J. Pojeta Jr. allowed access to the early Paleozoic chiton specimens at NMNH. R. Mapes lent us thin sections of Paleozoic chiton valves described in Hoare (2000b). D. Erwin allowed access to other fossil chitons at NMNH. D. Eernisse and D. Jacobs provided helpful comments on the relationships of the early polyplacophorans. T. Leaming assisted with laboratory work. R. Schmidtling and C. Fernandez provided artwork. R. Alkaly made thin sections. The final paper greatly benefited from reviews by R. Hoare and an anonymous reviewer. This research was supported by a field grant from the Department of Earth and Space Sciences at UCLA and a student research grant from the Geological Society of America. REFERENCES

ATWATER, D. E. 1979. Ontogeny and valve microstructure of Helminthochiton simplex (Raymond) (Polyplacophora) from the Allegheny Group (Pennsylvanian) of Ohio. Masters thesis, Bowling Green State University, Bowling Green, Ohio, 215 p. BAXTER, J. M., M. G. STURROCK, AND A. M. JONES. 1990. The structure

of the intrapigmented aesthetes and the properiostracum layer in Callochiton achatinus (Mollusca: Polyplacophora). Journal of Zoology (London), 220:447–468. BERGENHAYN, J. R. M. 1943. Preliminary notes on fossil polyplacophorans from Sweden. Geologiska Fo¨reningens i Stockholms Fo¨rhandlingar, 65:297–303. BERGENHAYN, J. R. M. 1955. Die fossilen schwedischen loricaten nebst einer vorla¨ufigen revision des systems der ganzen Klasse Loricata. ˚ rsskrift, y Fo¨ljid, Avdelningen 2,51:1–47. Lunds Universitets A BERGENHAYN, J. R. M. 1960. Cambrian and Ordovician loricates from North America. Journal of Paleontology, 34:168–178. BOYLE, P. R. 1974. The aesthetes of chitons II: fine structure in Lepidochitona cinereus (L.). Cell and Tissue Research, 153:383–398. BUTTS, C. 1941. Geology of the Appalachian Valley in Virginia. Pt. II. Fossil plates and explanations. Virginia Geological Survey Bulletin, 52: 1–271. CHERNS, L. 1998. Chelodes and closely related Polyplacophora (Mollusca) from the Silurian of Gotland, Sweden. Palaeontology, 41:545–573. CHERNS, L. 1999. Silurian chitons as indicators of rocky shores and lowstand on Gotland, Sweden. Palaios, 14:172–179. CLARKE, J. M. 1907. Some new Devonic fossils. Bulletin of the New York State Museum, 107:194–195. DALZIEL, I. W. D. 1997. Neoproterozoic-Paleozoic geography and tectonics: Review, hypothesis, environmental speculation. Geological Society of America Bulletin, 109:16–42. DAVIDSON, T., AND W. KING. 1874. On the Trimerellidae, a Paleozoic family of the palliobranchs or Brachiopoda. Quarterly Journal of the Geological Society of London, 30:124–172. DE BLAINVILLE, H. M. D. 1816. Prodrome d’une nouvelle distribution syste`matique du regne animal. Bulletin des Sciences, Societe` Philomathique de Paris, p. 51–53, 93–97. DZIK, J. 1986. Turrilepadida and other Machaeridia, p. 116–134. In A.

688


Hoffman and M. N. Nitecki (eds.), Problematic Fossil Taxa. Oxford University Press, New York. FISHER, D. W. 1962. Small conoidal shells of uncertain affinities, p. W98– W143. In R. C. Moore (ed.), Treatise of Invertebrate Paleontology, Pt. W, Miscellanea. Geological Society of America and University of Kansas Press, Lawrence. FLOWER, R. H. 1968. Some El Paso guide fossils. New Mexico Bureau of Mines and Mineral Resources Memoir, 22:1–19. FRIEDMAN, G. M. 1988. Spectacular domed microbial mats (cabbage heads) and oolitic limestone at Lester Park near Saratoga, New York. Northeastern Geology, 10:8–12. GOULD, A. A. 1846. On the shells collected by the United States Exploring Expedition, commanded by Charles Wilkes, U.S.N. Proceedings of the Boston Society of Natural History, 2:141–145. HALLAM, A. 1981. Facies Interpretation and the Stratigraphic Record. W. H. Freeman, San Francisco, California, 291 p. HANGER, R. A., R. D. HOARE, AND E. E. STRONG. 2000. Permian Polyplacophora, Rostroconchia, and Problematica from Oregon. Journal of Paleontology, 74:192–198. HINTZE, L. F. 1988. Geologic History of Utah. Brigham Young University Geology Series, Number 7, 203 p. HINTZE, L. F., M. E. TAYLOR, AND J. F. MILLER. 1988. Upper Cambrian– Lower Ordovician Notch Peak Formation in Western Utah. U.S. Geological Survey Professional Paper, 1393:1–30. HOARE, R. D. 2000a. Silurian Polyplacophora and Rostroconchia (Mollusca) from northern California. Proceedings of the California Academy of Sciences, 52:23–31. HOARE, R. D. 2000b. Considerations on Paleozoic Polyplacophora including the description of Plasiochiton curiosus n. gen. and sp. American Malacological Bulletin, 15:131–137. HOARE, R. D. 2002. New genera of Paleozoic polyplacophora. Journal of Paleontology, 76:570–573. HOARE, R. D., AND R. H. MAPES. 1985. New Mississippian and Pennsylvanian Polyplacophora (Mollusca) from North America. Journal of Paleontology, 59:875–881. HOARE, R. D., AND R. H. MAPES. 1989. Articulated specimen of Acutichiton allynsmithi (Mollusca, Polyplacophora) from Oklahoma. Journal of Paleontology, 63:251. HOARE, R. D., R. H. MAPES, AND D. ATWATER. 1983. Pennsylvanian Polyplacophora (Mollusca) from Oklahoma and Texas. Journal of Paleontology, 57:992–1000. HOARE, R. D., M. T. STURGEON, AND T. B. HOARE. 1972. Middle Pennsylvanian (Allegheny group) polyplacophora from Ohio. Journal of Paleontology, 46:675–680. HOSE, R. K. 1961. Physical characteristics of Upper Cambrian stromatolites in western Utah. U.S. Geological Survey Professional Paper, 424D:D245–D248. KLUESSENDORF, J. 1987. First report of Polyplacophora (Mollusca) from the Silurian of North America. Canadian Journal of Earth Sciences, 24: 435–441. KNIGHT, J. B. 1941. Paleozoic gastropod genotypes. Geological Society of America Special Paper, 32, 510 p. MAREK, L. 1962. New polyplacophorid family Eochelodidae n. fam. in ´ strednı´ho u´stavu geologicke´ho, the Ordovician of Bohemia. Vestnik U 38:373–375. POJETA, J. JR. 1980. Molluscan phylogeny. Tulane studies in Geology and Paleontology, 16:55–80. POJETA, J. JR. AND B. RUNNEGAR. 1976. The paleontology of rostroconch mollusks and the early history of the phylum Mollusca. U.S. Geological Survey Professional Paper, 968, 88 p. POJETA, J. JR. D. J. EERNISSE, R. D. HOARE, AND M. D. HENDERSON. 2003. Echinochiton dufoei: a new spiny Ordovician chiton. Journal of Paleontology, 77:646–654. RAYMOND, P. E. 1910. A preliminary list of the fauna of the Allegheny and Conemaugh series in western Pennsylvania. Annals of the Carnegie Museum, 7:153–154. REED, F. R. C. 1911. A new fossil from Girvan. Geological Magazine, 8:337–340. ROLFE, W. D. I. 1981. Septemchiton–a misnomer. Journal of Paleontology, 55:675–678. RUNNEGAR, B., AND J. POJETA JR. 1985. Origin and diversification of the Mollusca, p. 1–57. In E. R. Trueman and M. R. Clarke (eds.), The Mollusca. Vol. 10. Evolution. Academic Press, Orlando, Florida.

RUNNEGAR, B., J. POJETA JR., M. E. TAYLOR, AND D. COLLINS. 1979. New species of the Cambrian and Ordovician chitons Matthevia and Chelodes from Wisconsin and Queensland: evidence for the early history of polyplacophoran mollusks. Journal of Paleontology, 53:1374– 1394. SANDBERGER, G., AND F. SANDBERGER. 1856. Die Versteinerungen des Rheininschen Schichten Systems in Nassau. Kreidel and Niedner, Wiesbaden, 564 p. SANDERS, R. B. 1964. A new species of Septemchiton. Proceedings of the Oklahoma Academy of Science, 45:94–98. SCHMITT, J. G., AND D. W. BOYD. 1981. Patterns of silicification in Permian pelecypods and brachiopods from Wyoming. Journal of Sedimentary Petrology, 51:1297–1308. SCOTESE, C. R., AND W. S. MCKERROW. 1990. Revised world maps and introduction, p. 1–21. In W. S. McKerrow and C. R. Scotese (eds.), Palaeozoic Palaeogeography and Biogeography. Geological Society of London Memoir, 12. SHAPIRO, R. S. 1994. Stromatolites and thrombolites of the Upper Cambrian Notch Peak Limestone, House Range, Utah. Geological Society of America Abstracts with Programs (Cordilleran Section), 26(2):90– 91. SIRENKO, B. I. 1997. The importance of the articulamentum for the taxonomy of chitons. Ruthenica, 7:2–24. SIRENKO, B. I., AND Y. STAROBOGATOV. 1977. On the systematics of Paleozoic and Mesozoic chitons. Paleontological Journal, 3:285–293. SMITH, A. G. 1960. Amphineura, p. I41–I76. In R. C. Moore (ed.), Treatise of Invertebrate Paleontology, Pt. I, Mollusca 1. Geological Society of America and University of Kansas Press, Lawrence. SMITH, A. G. 1971. The Carboniferous genus Glyptochiton de Koninck, 1883 (Mollusca: Polyplacophora). Proceedings of the California Academy of Sciences, 37:567–574. SMITH, A. G., AND R. D. HOARE. 1987. Paleozoic Polyplacophora: a checklist and bibliography. Occasional Papers of the California Academy of Sciences, 146, 71 p. SMITH, A. G., AND D. F. TOOMEY. 1964. Chitons from the Kindblade Formation. Oklahoma Geological Survey Circular, 66, 41 p. STEWART, J. H. 1991. Latest Proterozoic and Cambrian rocks of the western United States–an overview, p. 13–38. In J. Cooper and C. Stevens (eds.), Paleozoic Paleogeography of the Western United States II (Pacific Section SEPM, Volume 67). STINCHCOMB, B. L. 1965. New Mollusca of the Potosi and Eminence Formations (Cambrian). M. A. thesis, Washington University, St. Louis, Missouri, 53 p. STINCHCOMB, B. L. 1978. The paleontology and biostratigraphy of the Lower Ordovician Gasconade Formation of Missouri. Ph.D. dissertation, University of Missouri-Rolla, 183 p. STINCHCOMB, B. L. 1986. New Monoplacophora (Mollusca) from Late Cambrian and Early Ordovician of Missouri. Journal of Paleontology, 60:606–626. STINCHCOMB, B. L., AND G. DARROUGH. 1995. Some molluscan problematica from the Upper Cambrian-Lower Ordovician of the Ozark uplift. Journal of Paleontology, 69:52–65. STURROCK, M. G., AND J. M. BAXTER. 1993. The ultrastructure of the aesthetes of Leptochiton asellus (Polyplacophora: Lepidopleurina). Journal of Zoology (London), 230:49–61. SUEK, D. H., AND W. W. KNAUP. 1979. Paleozoic carbonate buildups in the Basin and Range Province, p. 245–257. In G. W. Newman (ed.), Basin and Range Symposium and Great Basin Field Conference. Rocky Mountain Association of Geologists, Denver, Colorado. TAYLOR, M. E., J. E. REPETSKI, AND L. A. WILSON. 1981. Depositional environments and molluscan paleoecology of the Upper CambrianLower Ordovician Ajax Dolomite, Northern Stansibury Mountains, Utah, p. 132–138. In M. E. Taylor and A. R. Palmer (eds.), Cambrian Stratigraphy and Paleontology of the Great Basin and Vicinity, Western United States, Second International Symposium on the Cambrian System, Guidebook for Field Trip 1. ULRICH, E. O., AND J. BRIDGE. 1941. Hemithecella expansa, p. 19–20. In C. Butts (eds.), Geology of the Appalachian Valley in Virginia, Pt. 2. Virginia Geological Survey Bulletin, 52. VAN BELLE, R. A. 1983. The systematic classification of the chitons (Mollusca: Polyplacophora). Informations de la Societe` Belge de Malacologie, 11:1–178.

VENDRASCO AND RUNNEGAR—CAMBRIAN AND ORDOVICIAN CHITONS FROM UTAH AND MISSOURI VENDRASCO, M. J. 1999. Early evolution of the Polyplacophora (chitons). Ph.D. dissertation, University of California, Los Angeles, 232 p. WALCOTT, C. D. 1885. Note on some Paleozoic pteropods. American Journal of Science, 30:17–21. WALCOTT, C. D. 1886. Second contribution to the studies of the Cambrian faunas of North America. U.S. Geological Survey Bulletin, 30: 1–369. WALCOTT, C. D. 1908. Nomenclature of some Cambrian Cordilleran formations. Smithsonian Miscellaneous Collections, 53:1–12. WALCOTT, C. D. 1912. New York Potsdam-Hoyt fauna. Smithsonian Miscellaneous Collections, 57:251–304.

689

WALCOTT, C. D. 1914. Cambrian geology and paleontology. III., No. 2. Pre-Cambrian Algonkian algal flora. Smithsonian Miscellaneous Collections, 64:77–156. YOCHELSON, E. L. 1966. Mattheva, a proposed new class of mollusks. U.S. Geological Survey Professional Paper, 523-B:B1–B11. YOCHELSON, E. L., J. F. MCALLISTER, AND A. RESO. 1965. Stratigraphic distribution of the Late Cambrian mollusk Matthevia Walcott 1885. U.S. Geological Survey Professional Paper, 525-B:B73–B78. ACCEPTED 19 OCTOBER 2003