Lucas, S.G. and Sullivan, R.M., eds., 2018, Fossil Record 6. New Mexico Museum of Natural History and Science Bulletin 79.
A TRACE FOSSIL ASSEMBLAGE FROM THE THAYNES GROUP (TRIASSIC), IDAHO
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MARTIN LOCKLEY1 , SPENCER G. LUCAS2, JAMES F. JENKS3, ALLAN J LERNER2 , TRICK RUNIONS 1 and JUDE BURTON1 1 Dinosaur Trackers Research Group, University of Colorado at Denver, PO Box, 173364, Denver, CO 80217; -email:
[email protected]; 2New Mexico Museum of Natural History, 1801 Mountain Road N. W., Albuquerque, NM 87104; 31134 West Johnson Ridge Lane, West Jordan, Utah 84084
Abstract—An important Triassic fossil locality in the Triassic Thaynes Group, near Paris, Idaho, has yielded a low diversity ichnoassemblage of invertebrate and vertebrate traces. The trace fossils occur in a thinly bedded, greenish/brownish, calcareous siltstone and thinly bedded, laminated fine sandstones, which facilitates excellent preservation of fine detail. Vertebrate activity is represented by the fish swim trace Undichna, and invertebrate activity is represented by Kouphichnium, Taenidium, cf. Lockeia, cf. Gordia and cf. Helminthopsis. Relatively abundant plant material (cycad and bennettitalean) is also found at this locality. The ichnoassemblage reported here is the first well documented ichnoassemblage from the Thaynes Group in Idaho, and it is likely that these strata have a high potential to yield other ichnoassemblages. The ichnoassemblage described here is a low diversity representative of the Cruziana ichnofacies dominated by horizontal burrowers (and no vertical structures) and trails of epibenthic organisms. The traces and sediments indicate rapid deposition in subtidal waters, but the low ichnodiversity likely indicates stressful conditions, probably caused by the high depositional energy. This ichnoassemblage fits well with ideas that the marine benthos had definitely recovered from the Permo-Triassic extinction by Spathian time.
INTRODUCTION The ichnofauna of the Lower Triassic Thaynes Group is not particularly well known. Between 2009 and 2018, the present authors collected various trace and body fossils from outcrops west of the small town of Paris, about 6 miles (10 km) northwest of St. Charles, situated on the northwestern shore of Bear Lake, Idaho (Fig. 1). The trace and body fossils obtained from the study area formed the basis of a previous report (Lerner et al., 2017) and inform the present study. Diverse specimens have been collected for several institutions, including the University of Colorado Museum of Natural History (UCM), the New Mexico Museum of Natural History and Science (NMMNH) and the Moab Giants Museum (MGL) The first report on the fauna from this site dealt specifically with Vaderlimulus tricki (Lerner et al., 2017), a new austrolimulid (horseshoe crab) genus and species, and only briefly mentioned the occurrence of horseshoe crab and fish traces, Kouphichnium and Undichna, respectively. Here, we present a more detailed description of the ichnofauna, report an ammonite from the locality, and discuss the paleoecological and paleoenvironmental implications of the ichnoassemblage. STRATIGRAPHIC CONTEXT According to the geological map produced by Mansfield and Girty (1927), the assemblage described here comes from strata of the Lower Triassic Thaynes Group (Fig. 1). Kummel (1954) subdivided Triassic rocks of the Thaynes Formation on the eastern side of Bear Lake into eight lithologic units, of which the lowest five (listed below) are recognizable in the Paris Canyon area on the western side of Bear Lake. In ascending stratigraphic order: lower limestone (includes the classic middle Smithian Meekoceras beds [Hyatt and Smith, 1905; Smith, 1932]), lower shale (includes the late Smithian Anasibirites beds at the base where developed [Kummel, 1953, 1954] and, higher in the unit, the early Spathian Paris Biota [Brayard et al., 2017]), middle limestone (includes the early Spathian Tirolites Zone of Smith [1932]), middle shale (includes the classic early Spathian Columbites beds [Hyatt and Smith, 1905; Smith, 1932; Kummel, 1969; Guex et al., 2010; Jenks et al., 2013]), upper calcareous
siltstone (includes flattened early Spathian ammonoids and trace fossils), and an unnamed ~460 m thick sequence unique to the Paris Canyon/Sleight Canyon/Hammond Creek area, consisting of gray shaly limestone, olive-gray shales and gray, nodular, argillaceous limestone (includes early Spathian Procolumbites beds and middle Spathian Prohungarites and Silberlingeria beds [Guex et al., 2010]). The trace fossil locality is in the NMMNH locality database as locality 12236, and in the UCM data base as locality 676; precise map coordinates are available to qualified researchers (see below). This locality is west of Paris, Idaho (Fig. 1), in the upper calcareous siltstone unit of the Thaynes Formation, which we hereafter refer to as the calcareous siltstone unit of the Thaynes Group. A measured stratigraphic section at locality 12236 (Fig. 1) indicates that the trace fossils, and the holotype specimen of the xiphosuran Vaderlimulus tricki (Fig. 2A) published by Lerner et al. (2017) as well as an associated ammonoid fossil (Fig. 2B), were collected from a 2.5-m-thick interval of thinly laminated calcareous siltstone with very thin intercalations of silty shale. This fossiliferous interval has its base about 27 m above a definite lithologic change from relatively thickbedded dolomicrite and intercalated shale to thinly laminated calcareous siltstone and sandstone. The stratigraphic section of the calcareous siltstone interval that we measured (Fig. 1) is dominated by very thinly laminated siltstone that in its lower part is gradational with the underlying shale interval. Kummel (1954, pl. 39, section 2) measured a complete stratigraphic section of the calcareous siltstone unit at Hot Springs Ridge east of Bear Lake that is about 412 m thick. West of Bear Lake, in the Paris/ Sleight canyons area, the calcareous siltstone unit may be much thinner, about 229 m thick (see Kummel, 1954, pl. 39, section 1), but cover and local structure make a complete thickness indeterminate with present data. Nevertheless, it is certain that locality 12236 is stratigraphically low in the calcareous siltstone unit. Hoffman et al. (2015, figs. 6-7) assigned the strata in Sleight Canyon that include locality 12236 to the Dinwoody Formation. However, these strata are lithologically quite distinct from a complete stratigraphic section of the Dinwoody Formation on the northern flank of Paris Canyon (Kummel, 1954, pl. 35, fig.
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FIGURE 1. Stratigraphic section of a part of the Thaynes Group showing lower portion of “calcareous siltstone” unit with photo (top right) of lithology at trace fossil horizon. Inset (lower right) shows location of site. Compare with Lerner et al. (2017, fig. 1). 1; also see Wignall and Hallam, 1992, fig. 21), only 2.3 km to the SW. The base of this Dinwoody Formation section (contact with the underlying Phosphoria Formation) is at UTM zone 12, 463061E, 4674265N (datum NAD 83), and the top of the Dinwoody Formation (contact with the base of the overlying Thaynes Group) is at 463394, 4674449. Common lithotypes in this Dinwoody section are 0.3-0.5-m-thick beds of lime mudstone, muddy wackestone and (less common) bivalve packstone, unlike the strata we measured in Sleight Canyon and that Hoffman et al. (2015) assigned to the Dinwoody Formation. Furthermore, farther east on the northern flank of Paris Canyon, the upper calcareous siltstone unit is well exposed at 464075, 4674953, where it dips to the northeast, as do beds we assign to the calcareous siltstone unit in Sleight Canyon. The Paris Canyon and Sleight Canyon outcrops of the calcareous siltstone unit are lithologically identical, being dominated by thinly laminated calcareous siltstone (Fig. 1). There is some normal faulting in the Paris/Sleight canyons area, with the faults generally striking nearly east-west and down to the north. So, in order for the Dinwoody Formation to be exposed in Sleight Canyon at locality 12236, the structure would have to be reversed and have brought a block of Dinwoody Formation up over a throw of about 400
m or more (local approximate thickness of the Thaynes Group section between the top of the Dinwoody and the base of the calcareous siltstone unit: Kummel, 1954, pl. 39). Finally, an ammonoid found at locality 12236 (Fig. 2B) appears to be of Spathian age. Given its completely compacted preservation and lack of a preserved suture line, identification is problematic, even at the generic level. However, its involute coiling combined with a relatively high whorl and narrowly rounded venter suggest affinity with the Early Spathian genus Bajarunia Dagys, 1983. Indeed, the species Bajarunia pilata (Hyatt and Smith, 1905) is a common component of the underlying Columbites beds (see Guex et al., 2010 and Jenks et al., 2013). Thus, we here identify it as a probable Bajarunia sp. Lerner et al (2017, p. 290) erroneously placed locality 12236 “on the northern side of Paris Canyon….in the lower shale unit of the Thaynes Group…strata of early Spathian age.” This mistake was primarily based on incorrectly locating the site in Paris Canyon, which would put it much lower in the local stratigraphic section of the Thaynes Group. More recent work by us has thus established the correct location, stratigraphic position and age of locality 12236 as on the northern flank of Sleight Canyon, in the calcareous siltstone unit of the Thaynes Group and of early Spathian age. MATERIAL AND METHODS Both body and trace fossil specimens have been recovered from the calcareous sandstone unit of the Thaynes Group, at different times, mostly by the present authors. As noted in the previous section, the most notable find is a nearly complete austrolimulid body fossil designated as the type of Vaderlimulus tricki (Lerner et al., 2017). This specimen (UCM 140.25, Fig, 2A) is part of the University of Colorado Museum of Natural History (UCM) collections, which also includes the horseshoe crab-produced trails Kouphichnium (UCM 140.27-29 and UCM 140. 31-32) described here (Figs. 3-7). Several specimens have trace fossils on both the upper and lower surfaces of slabs, including UCM 140.32, which reveals the fish trace Undichna on the underside (Figs. 8-9). The locality is referred to as L-00676 in University of Colorado Museum of Natural History catalog and locality 12236 in the New Mexico Museum of Natural History and Science (NMMNH) catalog. Replicas of UCM 140.25-29 and UCM 140.31-32 were donated to the New Mexico Museum of Nature and Science (NMMNH), and additional replicas have been reposited with the Moab Giants Museum (MGL), together with additional original material. Subsequent visits to the site produced an ammonite (MGL 114: Fig. 2B) and a large number of additional trace fossils, including several examples of the fish trace Undichna, and various invertebrate traces. Selected specimens have been reposited in the Moab Giants (MGL) collections in the series MGL 100-MGL 126, with replicas of some of these specimens donated to UCM and NMMNHS to supplement the existing collections. DESCRIPTION OF MATERIAL Invertebrate Traces I: Kouphichnium Kouphichnium (Nopsca, 1923) is one of the best known arthropod trails, due in large part to the classic work of Caster (1938), who convincingly attributed the trails to horseshoe crab producers: see Häntzschel (1975) for summary. Following Caster (1938) and Häntzschel (1975), we recognize that Kouphichnium is very variable, and often, but by no means always characterized by a symmetrical arrangement of imprints on either side of a midline impression created by the telson. The configuration on either side of the midline generally consists of bifid or V-shaped imprints made by the anterior four pairs of walking legs, arranged as two chevron-like series leading laterally to larger digitate, often tetradactyl, pusher traces representing walking
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FIGURE 2. Selected fossils from the calcareous siltstone unit of the Thaynes Group in southeastern Idaho. A, Vaderlimulus tricki (UCM 140 25), holotype; and B, ammonite that probably belongs to the genus Bajarunia (MGL 114). leg 5 and marking the outer margin of the trackway. Assigned specimens of Kouphichnium from the Thaynes Group in the study area include UCM 140.27-29, 140.31-32A and MGL 126, which collectively encompass a total of eight unequivocal trackways. Specimen UCM 140.27 is the widest (width ~12 cm), with a well-defined telson impression (Fig. 3A). Specimen UCM 140.28 was illustrated by Lerner et al. (2017, fig. 2A), and shows a trackway ~10 cm wide with a wellpreserved medial telson trace (Fig. 3B here). Specimen UCM 140.29 (Fig. 4) contains two narrow trackways, each ~2.0 cm wide. Likewise, specimens UCM 140.31 and 140.32 (Fig. 5) also contain narrow trackways comparable to those of specimen UCM 140.29. Specimen MGL 126 (Fig. 6) contains two trackways, a smaller one ~2.0 cm wide and a larger trackway ~4.0 cm wide. In the section that follows we describe each of these specimens in detail from a systematic viewpoint. Invertebrate Traces II: Kouphichnium Taxonomy Ichnogenus Kouphichnium Nopsca, 1923 Specimens: MGL 126, UCM 140.27, UCM 140.28, UCM 140.29, UCM 140.31 and UCM 140.32B (Figs. 3-6) Description: The specimens show trackways that are composed of two generally symmetrical track rows of variably shaped (round ellipsoidal to nail-like) imprints, some in chevronlike arrangement, and with most showing pusher impressions (walking leg 5) and continuous or intermediate medial (telson) impressions. Most trackways are relatively narrow, with external widths of 2 to 4 cm, although two specimens (Fig. 3) are relatively wide, with external widths of ~10 cm (UCM 140.28) and ~12 cm (UCM 140.27). All trackways extend from the edge to edge of the slabs on which they occur, with the longest trackway length being 23 cm (UCM 140.31). Trackway courses are generally straight or have a gentle curve (UCM 140.31). UCM 140.29 contains two trackways of similar external widths that are aligned side by side, with generally straight courses. MGL 126 shows two simple trackways primarily composed of pusher imprints and without medial telson impressions. The
two trackways have differing external widths (2 cm, 4 cm) (Fig. 6A). UCM 140.27 contains two track rows of somewhat poorly defined walking leg and pusher imprints. At the anterior medial portion of the trace there are about three pairs of symmetrically opposite, 15-20 mm long imprints that are arranged nearly perpendicular to the midline. The specimens are preserved in either concave epirelief (UCM 140.27, 140.32) or convex hyporelief (UCM 140.28, UCM 140.29, UCM 140.31). Remarks: As noted above, Kouphichnium is one of the best known arthropod trails due in large part to the classic work of Caster (1938), who first attributed the trails to horseshoe crab producers. A long overdue revision of Kouphichnium and its ichnospecies has recently been provided by Shu et al. (2018), which we follow here. As noted by these authors, Kouphichnium was widely distributed and particularly prolific from the latest Permian to the Early Triassic. Specimens UCM 140.27, UCM 140.28, UCM 140.29, UCM 140. 31 and UCM 140.32 are assigned to K. lithographicum Oppel, 1862, due to the characteristic arrangement and variable nature of the imprints, as well as the presence of pusher impressions and medial telson impressions within the trackways. Specimen MGL 126 is assigned to K. didactylum Willard, 1935 due to its two trackways being simple in nature and demonstrating an absence of medial impressions. Shu et al. (2018) considered that ichnospecific differences of K. lithographicum and K. didactylum resulted from differing behaviors by the producers. Although a distinction between K. lithographicum and K. didactylum has merit, particularly for descriptive purposes, it is worth noting that the ichnospecific variation might also be the result of undertrack registration (taphonomy) rather than differing behaviors (ethology), in which case K. didactylum would be a taphotaxon. UCM 140.27, the widest specimen in the sample at 12 cm, is unusual in that it intergrades with another horseshoe crab trace, Arborichnus Romano and Meléndez, 1985, as seen by the several pairs of symmetrically opposite imprints near the anterior midline of the trace (Fig. 3A). The type locality for Arborichnus, a rarely reported ichnotaxon, is from the Carboniferous of Spain. Lerner and Lucas (2005) provided
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FIGURE 3 Photographs and line drawings of UCM 140.27 and 140.28, respectively. See text for details. Scale bars are 10 cm.
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FIGURE 4. Specimen UCM 140.29 shows two trackways. Line drawing of right hand trackway is shown for clarity. Left and right scale bars are 10 cm and 5 cm, respectively.
the only North American report of this ichnotaxon, which came from abundant material found at the Union Chapel Mine (Lower Pennsylvanian, Pottsville Formation, Alabama). The intergradation between Kouphichnium and Arborichnus seen on UCM 140.27 might have been produced when the behavior of the horseshoe crab producer shifted from walking (Kouphichnium) to resting (Arborichnus). The Arborichnus-like imprints likely represent proximal elements of the walking legs. Unfortunately, UCM 140.27 is on a small, fragmentary slab, so that any trace of continuing forward motion of the producer is lost at the edge. In addition to the numbered specimen described above, at least one specimen, recently found by one of us (TR), shows large, well-preserved pusher marks up to 3 cm long (Fig. 6BC). A sequence of 3 pusher traces indicates spacing of ~6.0 cm between sequential traces. The size indicates a large tracemaker, but, as only side of the trackway appears to be preserved, the width of the trail cannot be determined. The study site is also notable for having produced Vaderlimulus tricki, the first horseshoe crab specimen to have been reported from the Triassic of North America (Lerner et al., 2017). Vaderlimulus, assigned to the extinct family Austrolimulidae, consists of a single, well-preserved individual that displays long, widely splayed genal spines (Fig. 2A). Vaderlimulus has an external prosomal width of 7 cm. The external width of its predicted trackway can be extrapolated by using a formula (I = EW x 1.5) from Gaillard (2011), in which I is equal to the producer’s prosomal width and EW is equal to the external width of the trackway. Vaderlimulus would accordingly have produced a trackway with an external width of about 4.6 cm, which is within the low range (2-12 cm) of the sample described here. However, we cannot prove Vaderlimulus made the trackways, or even whether it was an autochthonous rather than an allochthonous element within the local biota. It is possible that the referred Kouphichnium trackways, particularly the wider specimens, were produced by limulid horseshoe crabs
FIGURE 5. Kouphichnium specimens UCM 140.31 (A) and 140.32 (B). Note that the reverse side of UCM 140.32 reveals the fish trace Undichna illustrated in Figure 8.
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FIGURE 6. Kouphichnium trackways from the Lower Triassic of Idaho. A, MGL 126 with small and large traces highlighted for clarity, B-C, photograph and line drawing of unnumbered specimen, showing large pusher marks. Scale bars = 5 cm.
rather than austrolimulid horseshoe crabs, which were generally small animals (Lerner et al., 2017). Austrolimulids have been considered to have mostly occupied freshwater environments (Lamsdell, 2016). It is therefore possible that Vaderlimulus was transported as a carcass from a more freshwater setting together with the abundant terrestrial plant material found at the site. Vaderlimulus might also have been tolerant of a range of salinities, which would have allowed it to have inhabited a variety of transitional coastal settings. A more definite determination of the life habitat of Vaderlimulus is further hindered by its having been collected as a float slab. In this respect, Vaderlimulus might have originated from a more landward, or even perhaps freshwater, lens at some level of the hillside other than the layers that produced the trace fossils. Further controlled collecting of this important locality therefore appears warranted. The Kouphichnium trackways record horseshoe crab locomotion but little else in the way of their behaviors. Extant horseshoe crabs such as Limulus polyphemus are prodigious burrowers and diggers, which they do for various reasons including feeding and concealment. There is in this respect, with the exception of the intergraded Arborichnus-like specimen already discussed, an absence of burrowing, digging or resting traces (e.g., Selenichnites, Protolimulus) that might otherwise be expected from the study site. However, if the local Kouphichnium trails, which represent walking behavior, were produced by Vaderlimulus individuals living in this paleoenvironment, it is possible that it wasn’t a burrowing form. The elongated and widely splayed genal spines of Vaderlimulus might have hindered its ability to effectively burrow, although this is conjectural. As intimated above Kouphichnium is a widely known ichnogenus, with reported ichnospecies, and a distribution in space and time that broadly corresponds with that of limulid body fossil occurrences. The co-occurrence of Vaderlimulus tricki with Kouphichnium lithographicum described here is reminiscent of other sites where both trace and body fossils have been reported, notably in the classic “dead in its tracks” scenario recorded for Mesolimulus in the Solnhofen Limestone (Lomax and Racay, 2012). Caster (1938) also illustrated several examples of full body impressions associated with Upper Devonian limulid trails. While compelling evidence for limulid tracemakers, these occurrences are also technically trace fossils, not body fossils. Invertebrate Traces III: Others In addition to the distinctive Kouphichnium traces, the study site also yields a low diversity ichnoassemblage of traces mostly made by arthropods and bivalves. Microbially-induced sedimentary structures are rare, but an example is NMMNH P-80340 (Fig. 7A), which preserves a bumpy texture of low, flat-topped crests (5-10 mm in diameter) separated by narrow depressions that do not bifurcate. It is broadly similar to “Rugalichnus” (cf. Porada et al., 2008; Stimson et al., 2017), but we do not attempt a more definitive identification. Shallow, unbranched horizontal burrows with a backfill of crescentic menisci have straight burrow courses and diameters of 2-5 mm (Fig. 7B). They are readily assigned to Taenidium (cf. Keighley and Pickerill, 1994) and were likely made by arthropods (Minter et al., 2007). There are also unbranched, shallow horizontal burrows that display winding segments (Fig.7 C). These burrows are smooth and unlined with diameters of 2-3 mm. Because of the irregular winding, we assign these burrows to cf. Gordia, even though on the specimen illustrated they cannot be seen to overcross (cf. Buatois et al., 1998). Gordia is a facies-crosser that has been attributed to various invertebrate producers (e. g., Gaigalis and Uchmann, 2004).
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A very common component of the ichnofauna consists of oval to almond-shaped impressions that are rounded at one end and tapered at the other (Fig. 7D-E). They have smooth surfaces, are 5-12 mm long and up to 6 mm wide. Assignment to cf. Lockeia is based on the diagnostic almond shape (e. g., Lucas and Lerner, 2006), though it is possible that at least some of these marks were made by plant debris (seeds). These are generally considered to be bivalve resting traces, though other interpretations have been offered (Mángano et al., 2002). A relatively common trace consists of unbranched, shallow horizontal burrows that alternate between straight segments and irregular, broad, low meanders (Fig. 7F). These burrows have diameters of 1-2 mm. Their morphology supports assignment to cf. Helminthopsis, generally considered to have been produced by arthropods (e. g., Han and Pickerill, 1995; Lerner et al., 2007). Vertebrate Traces Undichna The sinuous, sinusoidal or wave-like trace Undichna, is highly distinctive and attributed to the contact made by fish fins while swimming in situations where they made contact with subaqueous substrates. This general interpretation is noncontroversial among ichnologists. Minter and Braddy (2006) reviewed 14 named ichnospecies of Undichna, which range in age from Carboniferous to Pleistocene, but considered only nine as valid. These traces have variable, and in some cases, complex morphologies. Undichna was originally described by Anderson (1976) based on Permian traces from South Africa. She named three ichnospecies U. bina, U. simplicitas and U. solentia “on the basis of increasing complexity” (op. cit. p. 397). She recognized “unpaired” as well as paired sinusoidal “waves” and inferred that the unpaired wave with the greatest amplitude was made by a single centrally located fin such as the caudal fin, whereas the paired traces, with lesser amplitudes, would have been made by paired fins (pectoral or pelvic). Minter and Braddy (2006) reviewed Undichna, illustrating the nine ichnospecies they regarded as valid, showing a range of morphologies, of increasing complexity, from a simple sinuous trail in the case of Unichna unisulca named by Gibert et al. (1999), to U. septemsulcata, with seven waves named by Wisshak et al. (2004), from the Devonian. Minter and Braddy (2006) demonstrated that there are multiple examples of Undichna with between two and six waves: i.e., between the 1 and 7 (“uni” and “sept”) extremes of wave complexity. At least three diagnostic examples of the sinusoidal fish swim trace Undichna occur in the Thaynes sample described here. These are designated as MGL-102. The latter specimen represents the undersurface of UCM 140.32A, which has a Kouphichnium trackway on the upper surface. This surface was illustrated by Lerner et al. (2017 fig. 2B) as an uncataloged specimen, but not described in detail. In the sections that follow we describe each of the specimens in detail. It is noteworthy that all three specimens are relatively simple, consisting of either one or two waves. Trace MGL 100 is the longest, best preserved and most regular example of Undichna, dominated by a single unpaired, primary wave that can be followed continuously for 36 cm across the slab. The trace is ~2.0 mm wide with between 2.0 and 2.5 wave lengths registered with a consistent amplitude of about 4.0 cm and wavelength of about 13.0 cm. A second, narrower and less-conspicuous secondary wave occurs as a sinuous trace about 0.5 mm wide. These secondary wave traces are more conspicuous (more deeply incised) on one side than another: i.e., clearly seen in the inter-crest sulci on the “upper” side view shown in Figure 8, and faint on the “lower” side. The primary and secondary wave lengths are similar, but the amplitude of the secondary wave is less (~ 2 cm). Viewed from the perspective shown in Figure 8, the crests of the two waves are slightly offset,
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FIGURE 7. Selected invertebrate ichnofossils from NMMNH locality 12236 in the Lower Triassic Thaynes Group. A, Microbiallyinduced sedimentary structure, NMMNH P-80340 in convex hyporelief. B, Taenidium (T) and cf. Helminthopsis (H), P-80339 in concave epirelief. C, cf. Gordia, P-80338 in concave epirelief. D-E, cf. Lockeia, P-80335, part and counterpart, in convex hyporelief (D) and concave epirelief (E). F, cf. Helminthopsis, P-80332 in concave epirelief.
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FIGURE 8. A, photograph and B, line drawing of specimen MGL 100, showing Undichna trace with wide, well-developed primary wave and narrower secondary wave. Note the primary wave has a greater amplitude and cuts across the secondary wave, with lower amplitude. Both waves are similar in length. See text for details. and it is apparent that the wider, primary wave trace is continuous and cuts across the narrower, secondary wave trace. A scenario in which the smaller, less undulating trace was registered by the pectoral fin, before it was cross cut by the caudal fin, appears to fit the available evidence provided by this specimen. This interpretation would also be consistent with the inferences of Anderson (1976) that such a simple, primary wave trace was probably made by a single caudal fin. Trace MGL 102 is similar in morphology to MGL 100, although only 20 cm long. It is also preserved as a natural cast on the underside of the slab illustrated by Lerner et al. (2017, fig 2B) and there described as an “uncatalogued specimen at the University of Colorado Museum of Natural History (UCM).” This specimen is now cataloged as UCM 140.32, with the upper side (UCM 140.32A) showing a Kouphichnium trace, preserved as a natural impression (concave epirelief) illustrated here (Fig 5B), and the lower side (UCM 140.32B) showing Undichna (Fig. 9), preserved as a natural cast (convex hyporelief). This specimen is also preserved as replica MGL 102. The trace reveals a single, sinuous wave extending for about 1.5 cycles. The amplitude is about 3.5 cm, and the wavelength about 14.0 cm. There are no secondary traces associated with the sinusoidal wave. Trace MGL 101 is less well-preserved than MGL 100 and UCM 140.32A (=MGL 102), with the more sinuous part
(wave) about 15 cm long, a wavelength of about 8.5 cm and an amplitude of 2.0 cm (Fig 10). This trace appears compound, with a second trace of uncertain relationship roughly following the midline of the sinuous wave trace. The surface also shows a relatively large (1.7 cm wide) pusher imprint, which compare with those illustrated in Figure 6B,C. DISCUSSION The ichnoassemblage reported here is the first well documented ichnoassemblage from the Thaynes Group in Idaho. The unit that hosts this ichnoassemblage, the calcareous siltstone unit of the Thaynes Group, is extensively exposed in the Bear Lake region. Therefore, it likely contains high potential to yield other ichnoassemblages. From an ichnofacies point of view, the ichnoassemblage described here is a low diversity representative of the Cruziana ichnofacies. Thus, it is dominated by horizontal burrowers (and no vertical structures) and trails of epibenthic organisms, characteristic features of the Cruziana ichnofacies (e. g., MacEachern et al., 2007). The traces and sediments at the study site indicate rapid deposition in subtidal waters. However, the low ichnodiversity likely indicates stressful conditions, probably caused by the high depositional energy. In terms of recent ideas about recovery from the PermoTriassic mass extinction (e.g., Fraiser and Bottjer 2009; Zhang
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FIGURE 9 (facing page). A, photograph and B, line drawing of specimen MGL 102, showing Undichna trace registered as single sinuous wave. Note that this specimen, also cataloged as UCM 140.32B, is the underside of a slab with Kouphichnium preserved on the upper surface (UCM 140.32A), and previously illustrated by Lerner et al. (2017, fig. 2B). See text for details.
FIGURE 10. A, photograph of MGL 101 showing Undichna trace and relatively large tetradactyl Kouphichnium pusher imprint in white circle. Inset B is line drawing of same specimen at reduced scale. et al., 2018), this ichnoassemblage seems to be a normal, low diversity Cruziana ichnofacies assemblage, not unlike those found in younger rocks. Therefore, this ichnoassemblage fits well with ideas that the marine benthos had definitely recovered from the Permo-Triassic extinction by Spathian time. ACKNOWLEDGMENTS Permission to collect the specimens described here was obtained from the landowners, the City of Paris (Idaho), through the Bear Lake County Assessor’s office. We particularly thank Lynn Lewis, Bear Lake County Assessor, for his kind cooperation. We thank Matt Stimson, New Brunswick Museum, and Lida Xing, China University of Geosciences, Beijing, for their helpful reviews of this MS. REFERENCES Anderson, A M., 1976, Fish trails from the early Permian of South Africa: Palaeontology, v. 19, p. 397-409. Buatois, L. A., Mángano, M. G., Maples, C. G. and Lanier, W. P., 1998, Ichnology of an Upper Carboniferous fluvio-estuarine paleovalley: The Tonganoxie Sandstone, Buildex Quarry, eastern Kansas: Journal of Paleontology, v. 72, p. 152-180. Brayard, A., Krumenacker, L.J., Botting, J.P., Jenks, J.F., Bylund, K.G., Fara, E., Vennin, E., Olivier, N., Goudemand, N., Saucéde, T., Charbonnier, S., Romano, C., Doguzhaeva, L., Thuy, B., Hautmann, M., Stephen, D.A., Thomazo, C. and Escarguel, G., 2017, Unexpected Early Triassic marine ecosystem and the rise of the Modern evolutionary fauna: Science Advances, v. 3, 11 p. Caster, K. E., 1938, A restudy of the tracks of Paramphibius: Journal of Paleontology, v. 12, p. 3-60. Dagys, A.S., 1983, Morphology, systematics and evolution of the genus Nordophiceras (Ammonoidea); in Dagys, A.S. and Dubatolov, V.N., eds., Morphology and systematics of Phanerozoic invertebrates: Moscow, Nauka, p. 37-51 (in Russian). Fraiser, M. L. and Bottjer, D. J., 2009, Opportunistic behavior of
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