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4, Brachiopods; 3, Acrotretid brachiopod, CPC 37163; 4, Acrothelid brachiopod, CPC 37166. ... As yet, no formal stages have been recognized for the Cambrian.
J. Paleont., 78(3), 2004, pp. 574–590 Copyright q 2004, The Paleontological Society 0022-3360/04/0078-574$03.00

HALKIERIIDS IN MIDDLE CAMBRIAN PHOSPHATIC LIMESTONES FROM AUSTRALIA SUSANNAH M. PORTER* Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford St., Cambridge, Massachusetts 02138

ABSTRACT—Halkieriids are part of a distinctive Early Cambrian fauna, the ‘‘Tommotian fauna’’ sensu Sepkoski (1992), that is preserved mostly as phosphatic and secondarily phosphatized skeletal elements. The distinctiveness of the Tommotian fauna is ascribed, in part, to its preferential elimination during the end-Early Cambrian mass extinction event (the ‘‘Botomian extinction’’). Newly discovered halkieriids in phosphatic limestones of the Middle Cambrian (Ptychagnostus gibbus Zone) Monastery Creek Formation, Georgina Basin, Australia, now indicate that this group not only survived the end-Early Cambrian extinction, but was at least locally abundant thereafter. Most of the Georgina halkieriid sclerites can be accommodated within a single species, Australohalkieria superstes new genus and species, described and partly reconstructed here. Remaining sclerites probably represent two additional but rare halkieriid species. Additional newly discovered sclerites may have affinities with the sachitids, another problematic ‘‘Tommotian’’ taxon related to the halkieriids. Rare wiwaxiid sclerites extend the taphonomic and geographic distribution of this clade. The Monastery Creek Formation provides a valuable window on Middle Cambrian life, both because it provides information that is distinct from but complementary to other, similarly aged windows (e.g., the Burgess Shale) and because it represents a taphonomic window similar to those that preserve Early Cambrian small shelly problematica. A decline during the Cambrian in conditions necessary for the early diagenetic phosphatization of shallow-shelf and platform limestones may have effectively closed this taphonomic window, potentially biasing apparent patterns of diversity change through the period.

INTRODUCTION

Cambrian fossil record is characterized by unique skeletal assemblages that include the spongelike archaeocyathans and a number of mostly problematic small skeletal elements, known collectively as small shelly fossils (SSFs; Matthews and Missarzhevsky, 1975). These assemblages are distinctive enough from Middle and Upper Cambrian assemblages that Sepkoski (1992) recognized the ‘‘Tommotian’’ fauna as a discrete evolutionary fauna separate from his Cambrian fauna sensu stricto. In part, this distinctiveness reflects an extinction event at the end of the Early Cambrian [the ‘‘Botomian extinction’’ (Palmer, 1982; Signor, 1992; Zhuravlev and Wood, 1996)] that eliminated many ‘‘Tommotian’’ taxa s. Sepkoski (1992) (Zhuravlev and Wood, 1996). The extinction thus marks the evolutionary turnover from an early fauna populated mostly by skeletal problematica to the more familiar Cambrian fauna s.s. To understand better the nature and extent of this turnover, it is important to control for preservational mode and environment, as taxa under consideration may be taphonomically or facies restricted. Because many elements of the ‘‘Tommotian’’ fauna appear to be both facies restricted and preserved by early diagenetic phosphatization (e.g., Brasier and Hewitt, 1979; Brasier, 1990; Landing, 1992; Mount and Signor, 1992; Dzik, 1994; Khomentovskii and Karlova, 1994), it is useful to compare them with taphonomically and environmentally comparable assemblages of Middle Cambrian age. The Monastery Creek Formation of the Middle Cambrian Georgina Basin, Australia, offers one such opportunity. It contains an SSF assemblage that, like many of its well-studied Early Cambrian counterparts, preserves secondarily phosphatized skeletal elements associated with condensed horizons in phosphatic limestones (Southgate and Shergold, 1991; Gravestock and Shergold, 2001). For the most part, the Monastery Creek assemblage confirms the pattern highlighted by Sepkoski (1992): many of its dominant constituents, such as echinoderms, inarticulate brachiopods,

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HE EARLY

* Current address: Department of Geological Sciences, University of California, Santa Barbara, California 93106-9630, ,[email protected].

nontrilobite arthropods, and trilobites, are typical of the Cambrian fauna s.s. Also present, however, at relatively high abundances, are halkieriids. Halkieriids were sluglike metazoans armored with a coat of mineralized sclerites and two prominent shells at either end (Conway Morris and Peel, 1990, 1995). Their phylogenetic affinities with modern groups are problematic, but they appear to fall somewhere within the Lophotrochozoa, possibly representing stem-group molluscs (e.g., Bengtson, 1992; Runnegar, 2000), stem-group annelids (e.g., Jell, 1981; Conway Morris and Peel, 1995), stem-group brachiopods (Williams and Holmer, 2002; Holmer et al., 2002; Cohen et al., 2003; Conway Morris and Peel, 1995), or a stem group of a combination of lophotrochozoan phyla (Conway Morris and Peel, 1995). [Conway Morris and Peel (1995) argued for specific stem-group connections between the Sirius Passet halkieriid and brachiopods; between Thambetolepis and annelids (cf. Jell, 1981); and between a Siberian halkieriid species (Bengtson and Conway Morris, 1984) and the annelid1brachiopod clade.] Halkieriids are a common and diverse component of the ‘‘Tommotian’’ fauna, where they are represented by abundant sclerites (e.g., Qian and Bengtson, 1989; Bengtson et al., 1990; Conway Morris and Chapman, 1997; Conway Morris et al., 1998) and rare articulated specimens (Conway Morris and Peel, 1990, 1995). However, with the exception of possible halkieriid shells reported from the Burgess Shale (Conway Morris, 1995), halkieriids have not been identified in post-Botomian successions. Their presence in the Monastery Creek Formation, thus, provides the first compelling evidence that halkieriids survived the end-Early Cambrian extinction. GEOLOGY

The Georgina Basin, a shallow intracratonic basin covering ;325,000 km2 in western Queensland and the Northern Territory, Australia (Fig. 1.1), formed during a Late Proterozoic phase of crustal extension (Lindsay et al., 1987; Southgate and Shergold, 1991). By Middle Cambrian time, marine conditions prevailed in the basin and phosphogenesis was widespread (Southgate and Shergold, 1991). Today, diverse assemblages of phosphatized fossils can be found in Middle Cambrian rocks throughout the basin (e.g., Fleming, 1973; Runnegar and Jell, 1976; Jones and Mackenzie, 1980; Henderson and Mackinnon, 1981; Shergold and

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FIGURE 1—Geologic map, modified from Rogers and Crase [1980; as illustrated in Soudry and Southgate (1989)] and Sandstrom (1986), showing the location of the Rogers Ridge locality within the Georgina Basin, Australia. 1, The Georgina Basin. 2, The Burke River Outlier and the Duchess Embayment. 3, The Rogers Ridge locality (indicated by hollow star) within the Duchess Embayment.

Southgate, 1986; Southgate et al., 1988; Mu¨ller and Hinz, 1992; Hinz-Schallreuter, 1993; Mu¨ller and Hinz-Schallreuter, 1993; Kruse, 1998; Mehl, 1998; personal observations). A particularly diverse and well-preserved assemblage—and the focus of this study—occurs in the Monastery Creek Formation [formerly referred to as the Beetle Creek Formation (Russell and Trueman, 1971)] exposed in the Burke River Structural Belt ;140 km southeast of Mt. Isa, near the town of Duchess in western Queensland (Fig. 1.2). Although fossils have been reported from three localities in the Duchess region (Mt. Murray, Rogers Ridge, and Phosphate Hill), sample collection for this study was limited for logistical reasons to Rogers Ridge (Fig. 1.3). In the Burke River Outlier, the Monastery Creek Formation disconformably overlies the Thorntonia Limestone. It is overlain, with a conformable to laterally disconformable contact, by the organic-rich Inca Shale, which contains fossiliferous carbonate nodules in the Rogers Ridge area (Mu¨ller and Hinz-Schallreuter, 1993). The Monastery Creek Formation is divided into two members: the lower Siltstone Member, a 50–60-m-thick succession containing white fissile siltstone, chert, and minor pelletal phosphorite; and the upper Monastery Creek Phosphorite Member, a 10–15-m-thick unit comprising grainstone phosphorite, chert,

phosphatic and siliceous siltstone, shale, sandstone, and phosphatic limestone (de Keyser and Cook, 1972; Soudry and Southgate, 1989). Phosphatized fossils occur throughout the Monastery Creek Phosphorite Member; the fossils discussed in this study come from two beds located at ;6 and ;7.5 m above the contact with the Siltstone Member, and ;8.7 and ;7.2 m, respectively, below a marker bed containing hollow chert nodules (noted in Mu¨ller and Hinz, 1992). The unit from which the fossils derive consists of tabular, fine-grained limestones ;10–15 cm thick, devoid of hummocky cross-stratification or other cross-lamination, suggesting deposition below fair-weather wave base. Some beds are silicified and most contain abundant ferruginized and phosphatic hardgrounds. Fossils occur in high concentrations in these rocks, with as many as ;20 fossils per mm2 evident in thin section. Also present in varying abundance are nonskeletal phosphatic clasts. Middle Cambrian phosphatic sediments from the Duchess Region were deposited in a restricted embayment that was bounded by land on its northern, western, and southern sides, and whose eastern connection with the Burke River Outlier, an appendage of the Georgina Basin (see Fig. 1.1, 1.2), was restricted by shallow banks (Russell and Trueman, 1971).

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In the sequence stratigraphic framework formulated by Southgate and Shergold [1991; see also Gravestock and Shergold (2001)] for Middle Cambrian sediments of the Georgina Basin, the Monastery Creek Formation forms a progradational package (referred to as parasequence set 2) within the transgressive systems tract of their sequence 2. Phosphatic and glauconitic hardgrounds deposited on the transgressive surface of parasequence set 2 indicate sediment starvation; subsequent to this transgression, phosphatic carbonates, including the fossiliferous units from Rogers Ridge, were deposited in the structural lows, or subbasins, of the embayment (Russell and Trueman, 1971; Southgate and Shergold, 1991). Age.The Monastery Creek Formation contains Ptychagnostus gibbus, diagnostic of the Late Templetonian/Floran Stage of ¨ pik et al., 1957; O ¨ pik, 1979). the Middle Cambrian of Australia (O As yet, no formal stages have been recognized for the Cambrian System, but attempts at constructing internationally recognized divisions of Cambrian time have been made. The latest of these (Geyer and Shergold, 2000), based largely on trilobite biostratigraphy, correlates the Late Templetonian/Floran of Australia with intervals that all fall within the Middle Cambrian, wherever it is recognized (including the Delamaran/Early Marjuman of Laurentia, the Amgan of Siberia, the Acadian and St. David’s series of Avalonia, and the Taijiangian of South China; Geyer et al., 2000). THE ASSEMBLAGE

The Monastery Creek Phosphorite Member contains a diverse assemblage of fossils, representatives of which are shown in Figure 2. Bradoriid head shields (or carapaces), including those of Zepaera and Monasterium (Fleming, 1973), are the most common fossil fragments (Fig. 2.1, 2.2). Occurring at lower abundances are shells of juvenile acrotretid (Fig. 2.3) and acrothelid brachiopods (Fig. 2.4), probable echinoderm fragments (Fig. 2.5), and hyolith shells similar to those of Hyptiotheca and Actinotheca spp. from the Early Cambrian of Australia (Fig. 2.6; Bengtson et al., 1990). Halkieriid sclerites (e.g., Fig. 2.7) are the fifth most abundant fossils, constituting about five percent of all identifiable specimens (of course, each halkieriid animal produced many sclerites). Present at lower abundance levels are trilobite fragments [including larval hypostomes; Fig. 2.8, 2.9; cf. Speyer and Chatterton (1989)], chancelloriid sclerites (Fig. 2.10), hollow spheres 50–100 mm in diameter that possibly represent egg capsules (Fig. 2.11; Roy and Fa˚hræus, 1989; Walossek et al., 1993), cap-shaped fossils of possible molluscan affinity (Fig. 2.12; cf. Qian and Bengtson, 1989), and paleoscolecid worm fragments (Fig. 2.13). Many fragments (37 percent) are unidentifiable. All of these taxa have been preserved by phosphate, but their mode of preservation varies, possibly reflecting differences in original mineralogy. MATERIALS AND METHODS

Fossils were extracted from the carbonate matrix by maceration in 10–15 percent acetic acid, picked using a dissecting microscope, mounted on stubs, coated in gold and/or palladium, and

viewed with a LEO 982 scanning electron microscope at 5 or 10 kV. All figured specimens have been deposited in the Commonwealth Palaeontological Collections (CPC), Geoscience Australia, Canberra, under accession numbers CPC 37156–CPC 37228. SYSTEMATIC PALEONTOLOGY

Taxonomic treatment of disarticulated fossils is difficult and fraught with uncertainty (Bengtson, 1985a). This is particularly the case with halkieriid sclerites, which are known to be highly variable within a single scleritome. Nevertheless, early halkieriid taxonomies (e.g., Bengtson and Conway Morris, 1984; Qian and Bengtson, 1989; Bengtson et al., 1990) assumed that certain sclerite characteristics (e.g., the presence and nature of lateral zones and wall ornamentation) were uniform within a single species, an assumption supported by subsequently discovered articulated specimens (Conway Morris and Peel, 1995). Guided by both these taxonomies and descriptions of articulated halkieriids (Conway Morris and Peel, 1995), I use overall sclerite morphology, and in particular the nature of the lateral zones, as a species-level character. The majority of sclerites can be assigned to a single species, Australohalkieria superstes n. gen. and sp. In the following sections, I will describe the sclerites of this species in detail, reconstruct their original composition, morphology, wall structure, and position in the scleritome, and compare this species with Early Cambrian halkieriids. The remaining Georgina sclerites are distinctive enough to discourage their inclusion in A. superstes, but they are not abundant enough to provide detail sufficient for taxonomic description. These sclerites will be discussed briefly in the section, ‘‘Other sclerites.’’ Terminology used in sclerite description is drawn from Bengtson and Conway Morris (1984) and illustrated in Figure 3. Briefly stated, one end of the sclerite is called the base, and the remainder, the blade (Fig. 3.1). The basal region can be identified by the presence of a flattened facet, typically with an indentation housing a foramen (Fig. 3.2). The concave side of the sclerite is defined as its lower surface, the convex side as upper (Fig. 3.3). The plane of the base is defined as that which is perpendicular to the basal facet and aligned along its longest dimension (Fig. 3.4); similarly, the plane of the blade corresponds to the dimensions of greatest flattening (Fig. 3.4). In most cases, the sclerites are asymmetric: the blade may curve within the plane of the blade. The side to which the blade curves is referred to as the inferior side because, when imbricated, this side lies below the adjacent sclerite (Bengtson and Conway Morris, 1984). Similarly, the superior side lies above the adjacent sclerite (Fig. 3.1). Family HALKIERIIDAE Poulsen, 1967 Discussion.Poulsen (1967) erected the family Halkieriidae, but he did not provide a diagnosis for it, or for the genus Halkieria. In his restudy of the type material, Bengtson (1985b) published a diagnosis for the Halkieriidae, later used by Qian and Bengtson (1989). Bengtson et al. (1990) modified this diagnosis, →

FIGURE 2—Representative fossils from the Monastery Creek Phosphorite Member together with distribution of their relative abundances. Note that abundances are based on number of identifiable fossil fragments (Total N 5 1,131), which in the case of the echinoderms, halkieriids, trilobites, chancelloriids, and paleoscolecids, do not represent the number of individuals present. The following refers to both the picture and the graph: 1, 2, Bradoriids; 1, Monasterium sp., Commonwealth Palaeontological Collection Accession Number (CPC) 37165; 2, Zepaera sp., CPC 37156. 3, 4, Brachiopods; 3, Acrotretid brachiopod, CPC 37163; 4, Acrothelid brachiopod, CPC 37166. 5, Probable echinoderm fragments, CPC 37167. 6, Hyoliths, CPC 37160. 7, Halkieriid sclerites, CPC 37169. 8, 9, Hypostomes of larval trilobites; 8, CPC 37217; 9, CPC 37226. 10, Chancelloriid sclerites, CPC 37157. 11, Possible eggs, CPC 37158. 12, Cap-shaped fossils of possible molluscan affinities, CPC 37164. 13, Paleoscolecids, CPC 37225. Scale bar on bottom right represents 400 mm for 1, 2, 6; 600 mm for 3; 200 mm for 4, 5, 7, 8, 10, 13; 240 mm for 9; 100 mm for 11; and 160 mm for 12.

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FIGURE 3—Terminology used to describe halkieriid sclerites. Sclerite drawn for example is of the cultrate type. See text for explanation.

noting the presence of distinctive sclerite types and lateral sclerite zones that may comprise longitudinal canals (canals that flank the central canal) or may be divided into lateral canals or camerae (numerous canals extending outward from the central canal; these can be thought of as transversely divided longitudinal canals). The diagnosis presented in Conway Morris and Peel (1995, p. 310) modified those of both Bengtson et al. (1990) and Qian and Bengtson (1989), stating that halkieriid sclerites had ‘‘lateral canals of varying length.’’ It is unclear whether this diagnosis embraces sclerites with undivided, longitudinal canals (including the sclerites discussed here). The strong similarity between these sclerites and those with lateral canals, however, suggests that they do belong in the same family. Genus AUSTRALOHALKIERIA new genus Type species.A. superstes n. sp. Other species.Halkieria parva Conway Morris in Bengtson et al., 1990. Diagnosis.Halkieriids with lateral zones comprising undivided longitudinal canals. Etymology.Latin australis, southern, and halkieria, a reference to its geographic locality and its similarity to the genus Halkieria. Discussion.Early diagnoses for the genus Halkieria either explicitly (Bengtson et al., 1990) or implicitly (Bengtson, 1985b; Qian and Bengtson, 1989) included those halkieriids with longitudinal canals (Bengtson et al., 1990). Conway Morris and Peel (1995), however, restricted the diagnosis of Halkieria such that only those halkieriids with ‘‘short lateral canals’’ are included (p. 310). Under this restricted definition, those sclerites with longitudinal canals [including the Georgina sclerites and the Australian species originally described as Halkieria parva (Bengtson et al., 1990)] must be placed in a new genus. AUSTRALOHALKIERIA SUPERSTES new species Figures 2.7, 4, 6, 8, 9 Diagnosis.Species of Australohalkieria n. gen. with sclerites that possess a flattened central canal. Flattening begins at midpoint of blade, where longitudinal canals originate, and is only visible on the upper surface of the sclerite.

Description.Longitudinal canals are cylindrical in shape and extend beyond the central canal, resulting in a distal sclerite tip with two points separated by a rounded valley. Includes four variable sclerite types, distinguishable from each other by base-blade transition and shape of basal facet. Basal facet commonly with indentation, perforated by a small (5–10 mm diameter) foramen connected to a short tube. Wall composed of fibrous plates imbricated in the distal direction, resulting in appearance of parallel transverse lineations on both upper and lower sclerite surfaces. Sclerites range from ;250 to ;1,000 mm in length. Etymology.From the Latin superstes, surviving, with reference to the fact that this species is part of a lineage that survived the end-Early Cambrian extinction event. Types.Holotype: CPC 37189, Figure 4.4. Paratypes: CPC 37159, 37161, 37169–37171, 37173–37184, 37190–37200, 37211–37216, 37219, 37222, 37224. From samples PK98–41 and PK98–42; Figures 2.5, 4.1–4.3, 4.5, 4.6, 8, 9. Other material examined.Two hundred and forty-five sclerites. Sclerites are represented by internal molds (e.g., Figs. 4.3, 4.6, 8.1–8.4, 8.9, 8.11, 9.4, 9.12, 9.16, 9.17), with, in some cases, part or all of the phosphatized wall (e.g., Figs. 4.1, 4.2, 4.4, 4.5, 8.5–8.8, 8.10, 8.12–8.21, 9.1–9.3, 9.5, 9.6, 9.8, 9.9, 9.11, 9.13– 9.15, 9.18, 9.19). Outer surfaces may also be encrusted with diagenetic apatite (e.g., Figs. 10.12–10.14). Occurrence.Middle Cambrian (Late Templetonian/Floran) Monastery Creek Phosphorite Member, Rogers Ridge, Burke River Outlier (samples PK98–41,242,244,245,247,248). AUSTRALOHALKIERIA PARVA (Conway Morris, 1990) new combination Halkieria parva CONWAY MORRIS figs. 41–48.

IN

BENGTSON

ET AL.,

1990, p. 73–77,

Diagnosis.Species of Australohalkieria n. gen. with sclerites that possess an unflattened central canal; longitudinal canals arise at or near base of blade. Material examined.Bengtson et al. (1990) examined material from acid maceration residues. For this publication, figures 41– 48 in Bengtson et al. (1990) were examined.

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FIGURE 4—Examples of A. superstes blade structure. 1–3, views of the lower surface of siculate sclerites with varying degree of wall preservation; 1, CPC 37211; 2, CPC 37212; 3, CPC 37170. 4–6, as in 1–3, but for the upper surface of cultrate sclerites; 4, CPC 37189; 5, CPC 37190; 6, CPC 37171. Scale bar represents 150 mm for 1, 2, 4, 5; 110 mm for 3; and 90 mm for 6.

Occurrence.Australohalkieria parva n. comb. occurs in Early Cambrian rocks in the Flinders Ranges (Ajax Limestone: Mt. Scott Ranges locality) and Yorke Peninsula (Parara Limestone: Curramulka, Horse Gully, and Kulpara localities); South Australia (Bengston et al., 1990). Discussion.See Bengtson et al. (1990) for a detailed description of this species. DESCRIPTION AND RECONSTRUCTION OF AUSTRALOHALKIERIA SUPERSTES SCLERITES

Original composition.Halkieriid sclerites were originally calcareous in composition (e.g., Bengtson and Missarzhevsky, 1981; Bengtson and Conway Morris, 1984; Bengtson et al., 1990; Conway Morris and Peel, 1995). An aragonitic mineralogy is indicated by the presence of ,1-mm-diameter needlelike fibers suggestive of fibrous aragonite in the wall of Australohalkieria superstes sclerites (Fig. 6.2, 6.6, 6.10; cf. Runnegar, 1985) and by the preservation of A. superstes sclerites, similar to that of other taxa in the assemblage known to be originally aragonitic (e.g., hyoliths and chancelloriids; James and Klappa, 1983; Bengtson et al., 1990; Mehl, 1996; Kouchinsky, 2000a, 2000b), and distinct from that of calcitic taxa. In addition to an aragonitic layer, there is evidence for an outer organic coating (see the section, Wall structure, below). Morphology.Proximally, the sclerites of Australohalkieria superstes n. gen. and sp. have a single internal cavity. At the midpoint of the blade, the cavity divides into three roughly equal parts: a central canal flanked by two longitudinal canals. At the point of division, the upper surface of the central canal slopes sharply downward and comes in close contact with the lower surface such that the canal becomes extremely flattened; indeed,

in internal molds, the prong that represents the central canal is commonly broken off [Figs. 4.3, 4.6, 8.1, 8.2, 8.9–8.11, 9.4, 9.16 (although see Fig. 9.7)]. No pores that connect longitudinal canals to the central canal have been observed (cf. A. parva; Bengtson et al., 1990), although the extremely close proximity (;1–2 mm separation) of canals suggests a very thin intervening wall. The three-part blade structure is limited to the internal part of the sclerite. On the outer surface of the sclerite, the wall is continuous; only the flattened aspect of the central canal, visible on the upper surface of the sclerite, and the two-pointed tip hint at this internal structure (e.g., Figs. 4.4, 4.5, 8.6, 8.8, 8.14, 9.19). On the lower surface, the wall lacks any indication of the inner compartmentalization (e.g., Figs. 4.1, 8.5, 8.7, 8.12, 8.13, 8.15, 9.3, 9.5, 9.18, 10.3). The basal region is variable in structure (indeed, this is the basis for distinguishing sclerite types; see next section), but can be identified by the presence of an indentation (e.g., Fig. 8.12, 8.13, 8.18, 8.19) and/or a foramen (e.g., Figs. 8.2–8.4, 8.10, 8.11, 8.16, 8.20, 8.21, 9.10, 9.16, 9.17). The foramen is most obvious in internal molds (although see Fig. 8.20) where it takes the form of a sharply delineated circular depression, 5–10 mm in diameter and at least as deep. This suggests that the sclerite wall curved into the cavity, forming a short canal or tube (cf. fig. 7.1 in Conway Morris and Chapman, 1997). The presence on one specimen of wall fragments that curve inward into the foramen (Fig. 8.21) and the presence of phosphatic infill (of the tube) in the center of the foramen (Fig. 8.3), surrounded by a narrow valley where the original tube wall was located, confirm this interpretation. A reconstruction of A. superstes sclerite morphology is illustrated in Figure 5.

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FIGURE 5—A reconstruction of A. superstes blade structure, including cross sections at two different locations (indicated by the dashed lines). See text for explanation.

The most distinctive aspect of A. superstes n. gen. and sp. is the flattened central canal that forms a depression on the upper distal surface of the sclerites. Ascribing function to this structure has obvious difficulties, but one possible explanation is that the depressions were involved in sensory activity (cf. Jell, 1981). As discussed below, what is interpreted to be the mineralized part of the sclerite does not extend across the depressed region; only the outer layer of the wall, interpreted to be originally organic (see below), extends across it. Thus, the depression represents an area where there might have been easy exchange of gases, ions, or simple biomolecules, possibly through a permeable organic layer, between the sclerite’s internal tissue and the external environment. If the dorsal surface of A. superstes n. gen. and sp. was covered by a tightly integrated scleritome, similar to Early Cambrian articulated specimens (Conway Morris and Peel, 1995), sensory receptors on the upper (outer) surface of sclerite tips might have been useful. Many extant animals have similar structures that permit sensory reception on surfaces covered by a mineralized exoskeleton. For example, some arthropods have hollow setae with slits or pores in the cuticle that allow contact between chemoreceptors and the external environment; and polyplacophorans, possible relatives of halkieriids (Bengtson, 1992), have a system of minute vertical canals in their shell plates into which photoreceptors, called aesthetes, extend (e.g., Baxter et al., 1987, 1990). Aesthetes are probably homologous with the sensory papillae of aplacophorans, which, interestingly, are associated with the formation of spicules in this group (Haas and Kriesten, 1975; Fischer et al., 1980; as cited in Bengtson, 1992). Wall structure.The wall of A. superstes sclerites consists of two layers. The inner layer is composed of longitudinally oriented fibers [Fig. 6.1–6.3; also noted in other halkieriids (Bengtson,

1992) and in chancelloriids (Mehl, 1996; Kouchinsky, 2000a)] bundled together into transverse, distally inclined, imbricated ‘‘rows’’ (e.g., Fig. 6.1, 6.3, 6.6). The imbricated rows are present everywhere on the sclerite except the flattened area on the upper surface, where the central canal is located (e.g., Fig. 9.19). In most areas, the rows directly overlap one another (e.g., Fig. 6.1, 6.3), but on the upper distal surface, spaces occur between the rows (e.g., Fig. 6.6–6.10). In some specimens, these ‘‘spaces’’ are preserved as secondary phosphatic infill, commonly with grooves that reflect molded fibers (e.g., Fig. 6.8, 6.9). The function of the spaces is not known, but it may have been similar to that of the sclerite’s internal cavity—thought to have been tissue- or fluidfilled during life and have aided in sclerite mineralization (Bengtson and Missarzhevsky, 1981). Their restriction to the upper distal surface of the sclerite suggests that, like the depressed area of the sclerite discussed above, the spaces may have been involved in sensory reception. The lowermost part of the inner layer appears to have been continuous (e.g., Figs. 6.5, 7), although ;1-mm bumps on the surface of some internal molds (Figs. 6.4, 8.9) suggest that the inner surface of this layer was pitted. Covering both the rows and the spaces between them is the second outer layer of the sclerite wall. In well-preserved specimens, this layer forms an undulating coat that covers the entire sclerite (Fig. 6.11). Possibly, this layer is a diagenetic encrustation, but several features argue against this. First, internal molds of the central canal are regular and smooth on their upper, flattened surface, indicating the presence of a primary boundary layer, on which the mold templated. The lower, fibrous layer of the wall is not present in this region; only the outer layer is. Collectively, these observations suggest that the thin outer layer is a primary feature of the sclerite wall. Second, the outer layer does not fill in the spaces between the rows, as would be expected if it were an encrustation. Instead, it extends straight from one ridge to the next, bridging the intervening spaces (Fig. 6.8). In sclerites where the spaces are represented by secondary phosphatic infill (e.g., Fig. 6.9, 6.10, 6.12), there is a sharp line of contact between the outer layer and the infill (see, in particular, Fig. 6.12), suggesting the outer layer acted as a template for the phosphatic infill. Third, unlike diagenetic encrustation observed on some fossils in the Rogers Ridge assemblage (e.g., Fig. 10.14) and elsewhere (e.g., Xiao and Knoll, 1999), apatite crystals forming the layer are not elongate or oriented perpendicular to the coated surface but instead are spheroidal and randomly oriented, a fabric typical of phosphatized organic material (Fig. 6.13; Allison, 1988; Martill, 1988; Wilby, 1993; Wilby and Briggs, 1997; Xiao and Knoll, 1999; Zhao and Bengtson, 1999). Collectively, these features suggest that the outer layer represents a primary organic layer that coated the sclerite. An outer organic layer has also been observed in sclerites of chancelloriids (J.-Y. Chen, personal commun., cited in Mehl, 1996, although see Bengtson and Hou, 2001), metazoans which may be closely related to the halkieriids (Bengtson and Missarzhevsky, 1981; Bengtson and Hou, 2001; although see, e.g., Butterfield and Nicholas, 1996). Its function is unknown, but it may have been involved in sclerite mineralization or protection, similar to the molluscan periostracum (Taylor and Kennedy, 1969; Tevesz and Carter, 1980; Bottjer, 1981). Indeed, if halkieriids are →

FIGURE 6—Details of A. superstes sclerite wall structure. 1, detail of sclerite shown in Figure 8.7; 2, detail of sclerite shown in Figure 9.1; 3, detail of sclerite shown in Figure 9.9; 4, detail of sclerite shown in Figure 9.7; 5, detail of sclerite shown in Figure 8.16; 6, CPC 37172; 7, CPC 37221; 8, detail of sclerite shown in Figure 9.2; 9, detail of sclerite shown in Figure 9.2; 10, detail of sclerite shown in Figure 2.5; 11, detail of sclerite shown in Figure 9.5; 12, arrows point to line of contact between the phosphatic infill and the outer layer; CPC 37220; 13, cross section of outer layer; CPC 37223. Scale bar represents 20 mm for 1, 5, 10; 4 mm for 6, 7; 15 mm for 2, 8; 30 mm for 3; 50 mm for 4, 11; 10 mm for 9; 2 mm for 12; and 3 mm for 13.

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FIGURE 7—Model of A. superstes sclerite wall structure. See text for explanation.

stem-group molluscs (Bengtson, 1992; Runnegar, 2000), the organic layer may be homologous with the periostracum. A tentative reconstruction of the sclerite wall is illustrated in Figure 7. RECONSTRUCTION OF THE AUSTRALOHALKIERIA SUPERSTES SCLERITOME

Specimens of articulated halkieriids (Halkieria evangelista Conway Morris and Peel, 1995) from the Early Cambrian Sirius Passet fauna, Greenland, provide a useful guide for reconstructing the scleritome of A. superstes n. gen. and sp. The scleritome of H. evangelista is bilaterally symmetric, and on either side it is composed of three zones, each consisting of a distinctive sclerite type. ‘‘Palmate’’ sclerites are situated in the dorsal zone of the animal’s body. These are smaller than other sclerite types, bear prominent ribs on their upper surfaces, and are tightly adpressed to the animal’s body (Conway Morris and Peel, 1995). ‘‘Cultrate’’ sclerites are situated on the lateral surfaces of the animal’s body. They also bear prominent ribs and are tightly adpressed to the body, but are larger than the palmate sclerites. ‘‘Siculate’’ sclerites are situated on the ventrolateral margin on either side of the animal’s body. They are approximately the same size as cultrate sclerites, but lack prominent ribs. Rather than being tightly adpressed to the animal’s body, siculate sclerites appear to have extended outward at a 45–90-degree angle. In addition to the scleritome, the articulated specimens bear two bilaterally symmetric shells, one at the anterior and the other at the posterior end of the body. Using these sclerite characteristics, I have divided the A. superstes sclerites into palmate, cultrate, and siculate types. Each sclerite type includes roughly equal numbers of right- and lefthanded forms (e.g., see symmetry pairs in Fig. 8.10–8.13), as would be expected if the scleritome of A. superstes n. gen. and sp. were bilaterally symmetric. Palmate sclerites (Fig. 8.1–8.7).Twelve specimens, ;250– 650 mm in length. In these sclerites, the basal facet is long and

narrow, mirroring the flattened cross section of the blade. On one side of the sclerite the base may extend beyond the width of the blade, forming a projection, or ‘‘auricle’’ (e.g., Fig. 8.1–8.6, Bengtson and Conway Morris, 1984). A shallow indentation on the basal facet is long and narrow, like the facet itself (faintly visible in Fig. 8.2–8.4, 8.5, 8.6). The foramen is located within this indentation, offset to one side toward the auricle (Fig. 8.2, 8.4). The base is offset from the blade by a ;90-degree bend, such that the plane of the base is perpendicular to the plane of the blade. Other than at the blade-base transition, the blade is not strongly curved. On the upper surface of the blade, a central rib may run from the basal transition to the flattened central canal. These sclerites are assigned to the palmate type because they exhibit the same characteristics as palmate sclerites of H. evangelista. They are the smallest of the sclerite types and have prominent ribs on their upper surface. The sharp angle between the blade and the basal facet suggests that, like the palmate sclerites of H. evangelista, they were adpressed to the body surface. Finally, these sclerites share a number of features with Early Cambrian sclerites from disarticulated assemblages identified as palmate, including a strongly flattened base and an auricle (Bengtson and Conway Morris, 1984; Qian and Bengtson, 1989; Bengtson et al., 1990). Cultrate sclerites (Figs. 4.4–4.6, 8.8–8.21).Eighty-eight specimens, ;300–1,000 mm in length. In these sclerites the basal facet is circular to polygonal in shape, with an even rim, ;10 mm wide, around the facet border (Fig. 8.12, 8.13, 8.18, 8.19). The foramen is located in the center of the facet. Like the palmate sclerites, the plane of the base is oriented perpendicular to that of the blade, but it may be offset from the blade by a broad constriction (e.g., Fig. 8.10, 8.11, 8.15–8.17). Most sclerites, when viewed from either the upper or lower surface, have a proximal end that is angular in shape, with the sides of the sclerite bending sharply toward each other to meet in a point (Figs. 4.4–4.6, 8.8– 8.16). The sclerite edge dips more steeply away from this point on the inferior side of the sclerite; the sclerite base is oriented such that the longest dimension of the facet is parallel to this edge (Fig. 8.10–8.13, 8.16, 8.18). The upper surface commonly has a central rib, extending from the proximal point to the inferior side of the central canal (Figs. 4.4–4.6, 8.8, 8.9, 8.14). These sclerites are considered cultrate because, like cultrate sclerites of H. evangelista, they are larger than the palmate sclerites, exhibit prominent ribs, and have an angle between the basal facet and blade that suggests adpression against the animal’s body. In addition, these sclerites are similar to cultrate sclerites identified in other disarticulated assemblages (Bengtson and Conway Morris, 1984; Bengtson et al., 1990; Conway Morris and Peel, 1995), particularly in the appearance of their proximal tips (see especially fig. 51 in Bengtson and Conway Morris, 1984), the shape of their basal facets, and their base-blade transition (e.g., figs. 44c and 45g in Bengtson et al., 1990). Siculate sclerites (Figs. 2.5, 4.1–4.3, 9.1–9.19).Sclerites consistent with a siculate assignment can be divided into two groups, although rare transitional forms exist. Group 1 (Figs. 2.5, 4.1–4.3, 9.1–9.10).—Eighty-four specimens, ;400–1,000 mm in length. The basal facet is circular to elliptical and indentation is faint or lacking. The foramen is offset →

FIGURE 8—Palmate sclerites (1–7) and cultrate sclerites (8–21) of A. superstes n. gen. and sp. 1, CPC 37218; 2–4, CPC 37173; 5, CPC 37222; 6, CPC 37191; 7, CPC 37174; 8, CPC 37175; 9, CPC 37192; 10, CPC 37176; 11, CPC 37177; 12, CPC 37193; 13, CPC 37194; 14, CPC 37195; 15, CPC 37178; 16, CPC 37224; 17, CPC 37179; 18, detail of 12; 19, detail of 15; 20, 21, detail of 10. Scale bar represents 180 mm for 1, 7; 100 mm for 2; 5 mm for 3; 70 mm for 4; 220 mm for 5, 6, 10; 250 mm for 8, 15; 350 mm for 9; 150 mm for 11, 18; 330 mm for 12; 200 mm for 13, 14, 16, 17; 50 mm for 19; 80 mm for 20; and 40 mm for 21.

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FIGURE 10—Other sclerites. Type I (1–16); variant A (1–14), variant B (15, 16). Type II (17–23). 1, CPC 37185; 2, CPC 37186; 3, 4, CPC 37201; 5, 6, CPC 37187; 7, CPC 37202; 8, CPC 37203; 9, 10, CPC 37204; 11–14, CPC 37205; box in 13 indicates area shown in 14; 15, CPC 37206; 16, CPC 37188; 17, CPC 37168; 18, CPC 37162; 19, 20, CPC 37207; 21, CPC 37208; 22, 23, CPC 37227. Scale bar represents 140 mm in 1; 120 mm for 2, 3; 35 mm for 5; 50 mm for 4, 11, 20, 23; 100 mm for 6, 12, 13, 21, 22; 150 mm for 7, 8; 125 mm for 9; 25 mm for 10; and 75 mm for 17–19.

← FIGURE 9—Siculate (1–10) and possible siculate (11–19) sclerites of A. superstes n. gen. and sp. 1, CPC 37161; 2, CPC 37180; 3, CPC 37181; 4, CPC 37159; 5, CPC 37219; 6, CPC 37213; 7, CPC 37214; 8, CPC 37182; 9, CPC 37183; 10, detail of specimen in Figure 4.3; 11, CPC 37196; 12, CPC 37197; 13, CPC 37198; 14, CPC 37184; 15, CPC 37199; 16, 17, CPC 37200; 18, CPC 37215; 19, CPC 37216. Scale bar represents 100 mm for 1, 3; 150 mm for 2, 5, 6; 200 mm for 4, 7; 125 mm for 8, 11, 12, 18; 350 mm for 9; and 25 mm for 10; 160 mm for 14, 16, 19; 65 mm for 17; 140 mm for 15; and 90 mm for 13.

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toward the upper surface of the sclerite (Fig. 9.10). The most distinctive aspect of these specimens, however, is the sigmoidal curve that distinguishes the base from the blade: distally the blade curves sharply away from the upper side of the sclerite and then curves back such that the basal facet is perpendicular to the plane of the blade. In some cases, the base is also offset by a broad constriction (e.g., Fig. 4.1, 4.2). The degree of sigmoidal curvature varies among sclerites from barely perceptible (Fig. 9.6–9.8) to strongly pronounced (Figs. 2.5, 4.1–4.3, 9.1–9.5, 9.9). Like palmate and cultrate sclerites, these are asymmetric: the curvature is commonly more pronounced on one side than on the other (Fig. 9.6–9.8) and the plane of the blade may be twisted slightly. In all of these specimens, the upper surface lacks a rib. Group 2 (Fig. 9.11–9.19).—Twenty-eight specimens, ;400– 500 mm in length. In these specimens the shape of the basal facet reflects the roughly elliptical cross section of the blade. A shallow, broad indentation on the facet is similar in shape to the facet, and the foramen is slightly offset toward the inferior side of the sclerite (Fig. 9.16, 9.17). The sclerite bends sharply between the base and the blade such that the plane of the base is offset from that of the blade by ;45–90 degrees. The longitudinal axis of the base (perpendicular to the plane of the basal facet) is also bent ;45 degrees relative to the longitudinal axis of the blade, toward the inferior side of the sclerite. In some specimens, a central rib may be present on the upper surface of the sclerite, extending from the edge of the basal facet to the base of the central canal (Fig. 9.12, 9.13). Like the siculate sclerites of H. evangelista, these sclerites are comparable in size to the cultrate sclerites and ribs are rare or absent. In addition, the position of the basal facet in these specimens suggests that they projected outward from the animal’s body at a ;45–90-degree angle, like siculate sclerites on H. evangelista. Finally, the sigmoidal curvature typical of Group 1 is observed in sclerites from disarticulated assemblages that are also identified as siculate (Bengtson et al., 1990, fig. 47b, c). Shells.A distinctive feature of articulated Early Cambrian halkieriids is the presence of two symmetric shells, one at either end of the body (Conway Morris and Peel, 1990, 1995). Since this discovery, putative halkieriid shells have been identified in a number of disarticulated assemblages (e.g., Bengtson, 1992; Conway Morris, 1995; Conway Morris and Chapman, 1997; Conway Morris et al., 1998). No shells that might be ascribed to halkieriids have been recognized within the Monastery Creek assemblage. This may reflect their relative rarity [H. evangelista possesses one shell for every ;1,000 sclerites (Conway Morris and Peel, 1995)], or it may indicate that the scleritome of A. superstes n. gen. and sp. did not possess shells. Whether such an absence would reflect a loss of shells in the Australohalkieria lineage or a plesiomorphic state within the halkieriids is not known. COMPARISON WITH OTHER HALKIERIIDS

All halkieriids possess calcareous, hollow sclerites arranged in a tightly imbricated, differentiated scleritome. In addition, their sclerite walls share a similar ultrastructure, consisting of distally inclined, fibrous elements—a pattern also observed in chancelloriids (Bengtson et al., 1990; Mehl, 1996; Bengtson, 1999; Kouchinsky, 2000a; Bengtson and Hou, 2001), siphogonuchitids (Qian and Bengtson, 1989), and sachitids (Bengtson et al., 1990). There is substantial variation within the halkieriids, however. Sclerites may have lateral zones consisting of longitudinal canals [e.g., Australohalkieria parva (Bengtson et al., 1990)] or multiple lateral canals [e.g., Halkieria mira (Conway Morris and Chapman, 1997); Thambetolepis delicata (Jell, 1981; Bengtson et al., 1990)], or lateral zones may be absent altogether (e.g., Fig. 10.17–10.22). Sclerite walls may be composed of distally inclined rows [A. parva (Bengtson et al., 1990)], plates [e.g., H. sp. (Bengtson and

Conway Morris, 1984); H. sthenobasis (Qian and Bengtson, 1989)], or tubercles, or a combination of these [e.g., A. parva, T. delicata (Bengtson et al., 1990); H. mira]. Sclerites may also vary in size (;250 mm to ;3,000 mm in length) and in relative numbers of palmate, cultrate, and siculate types. Of the variable and widespread Early Cambrian halkieriid species, A. superstes n. gen. and sp. is most similar to A. parva, also from Australia (Bengtson et al., 1990). Unlike other halkieriids, both species have undivided longitudinal canals and a wall structure consisting of distally inclined, more or less continuous rows. These similarities suggest that A. parva was closely related to A. superstes n. gen. and sp., a hypothesis formalized by the inclusion of A. parva in the new genus Australohalkieria. A. parva and A. superstes n. gen. and sp. may have belonged to a single clade of Australian halkieriids that survived Botomian extinction. Why this clade would have survived while other halkieriid clades apparently did not is not known. It is interesting to note, however, that the only archaeocyathans known to have survived the Botomian extinction also occur in Gondwana (Debrenne et al., 1984; Wood et al., 1992). Despite similarities to A. parva, A. superstes n. gen. and sp. is nevertheless quite distinct from all Early Cambrian halkieriids. Most notably, it differs in its possession of a distally flattened central canal and two-pronged tip (see ‘‘Description and reconstruction of Australohalkieria superstes sclerites’’), but it also differs in scleritome composition and sclerite size. Scleritomes of Early Cambrian halkieriids possess many more palmate and cultrate than siculate sclerites [e.g., Bengtson and Conway Morris, 1984; Qian and Bengtson, 1989; Bengtson et al., 1990; Conway Morris and Peel, 1995; in some cases, definitive siculate sclerites could not be identified (Conway Morris and Chapman, 1997)]. In contrast, siculate sclerites of A. superstes n. gen. and sp. are more abundant than either cultrate or palmate sclerites; palmate sclerites are rare. This could reflect either postmortem sorting of sclerite types (palmate sclerites are on average smaller than the other types) or, more likely, a scleritome whose ventrolateral zone is expanded in area relative to lateral and dorsal zones. A. superstes sclerites are significantly smaller than Early Cambrian halkieriid sclerites (250–1,000 mm vs. 600–3,000 mm; the presence of larger fossils in the Georgina assemblage and the fact that most Early Cambrian halkieriids are also phosphatized suggests that this difference is not due to taphonomic selectivity). This suggests that the individual(s) represented in the Georgina assemblage were juvenile, their scleritome was composed of many more sclerites than those of Early Cambrian halkieriids, or the species was relatively small. OTHER SCLERITES

Two other morphologically distinct blade types occur in the Georgina assemblage, each with either one or two sclerite morphologies. While it cannot be ruled out that these sclerites belong to A. superstes n. gen. and sp., their inclusion would imply a scleritome that was much more complex (with respect to structural variability as well as the number of sclerite types) than those of Early Cambrian halkieriids. More likely, the structural types correspond to species that were rare in the assemblage and, therefore, are only represented by sclerite types that were most abundant in the scleritome. Lack of information on the range of sclerite variability cautions against erecting species names for these specimens, and, therefore, only a form classification is presented (cf. Bengtson, 1985a). Type I (Fig. 10.1–10.16).The internal structure of these sclerites is difficult to discern due to the absence of specimens preserved only as internal molds; in all specimens of this type the wall is preserved, almost always in its entirety. The few specimens that expose part of the internal mold show that, like the

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FIGURE 11—Putative sachitid sclerites (1–6) and wiwaxiid sclerites (7–11). 1, 2, CPC 37209; 3–6, CPC 37210; in 4, upper box indicates area shown in 5, lower box indicates area shown in 6; 7, 9, 10, wiwaxiid specimen 1; note complex tubercles in 9, indicated by arrows; note fine parallel lines in 10. Unfortunately, specimen was destroyed during manipulation (sample PK98–41; SEM stub PK98–41-HP2); 8, 11, wiwaxiid specimen 2; CPC 37228. Scale bar represents 400 mm for 1; 60 mm for 2; 40 mm for 3; 450 mm for 4; 10 mm for 5, 9; and 20 mm for 6; 100 mm for 7; 50 mm for 8; 4 mm for 10; and 30 mm for 11.

blade of A. superstes n. gen. and sp., the distal section is divided into three canals (Fig. 10.11). This is also suggested by the distal tips of the sclerites that, like those of A. superstes n. gen. and sp., feature two points (representing each side canal) separated by a valley (Fig. 10.9, 10.10, 10.13). Unlike A. superstes n. gen. and sp., however, the middle canal is not flattened. The upper surface of sclerites of this type instead may exhibit a broad central rib or swelling (Fig. 10.7, 10.12, 10.13), probably representing the central canal. The wall structure appears comparable to that of A. superstes sclerites (note e.g., the imbricated fibrous rows on the sclerite in Fig. 10.2). There are two variants with this kind of blade, which differ in base shape: Variant A (Fig. 10.1–10.11).—Fourteen specimens. In these specimens, the basal facet is not distinct from the surface of the blade. Instead, the proximal part of the blade curves smoothly around a 180-degree angle and tapers to a subrounded point, maintaining the same cross-sectional thickness throughout. The foramen is triangular in shape, about 15 mm at its widest, and is situated at this proximal point, on the upper surface of the blade (Fig. 10.3–10.6, 10.9, 10.12). Variant B (Fig. 10.15, 10.16).—Only two specimens of this type have been identified, but they are distinctive enough to warrant separation from the specimens mentioned above. Although the sclerite blade is flattened as in variant A, the base is cylindrical with a rounded basal facet. In addition, at the base-blade transition, the sclerites only curve around a 90-degree angle (rather than 180 degrees), such that the presumed basal facet is oriented parallel to the plane of the blade. No information is known about the shape, size, or position of the foramen. The sharp angle between the base and the blade of both of these variants suggests that they probably occupied the dorsal or lateral zones, where the palmate and cultrate sclerites occur. It is not clear whether these sclerites occupied additional distinctive A.

superstes sclerite zones or whether they belong to an additional, rare Australohalkieria species. Type II (Fig. 10.17–10.23).Seven specimens. In these sclerites there are no distinct lateral zones. Instead, the flattened canal remains undivided throughout, tapering distally to a single point. A central rib may be separated from thickened margins by two broad depressions (Fig. 10.22), or may slope evenly to thin edges (Fig. 10.18). Proximally, the blade curves slightly (Fig. 10.17– 10.19) or strongly (Fig. 10.21, 10.22) and ends in a flattened base (Fig. 10.19–10.21), comparable to palmate sclerites in Early Cambrian halkieriids. On some sclerites, the base may flare on one (Fig. 10.21) or both (Fig. 10.19) sides so that it is wider than the blade. A small (;5 mm diameter) foramen lies in the center of the basal facet, within a linear indentation that extends the width of the facet (Fig. 10.19, 10.20). The wall is generally not well preserved in these sclerites, with the exception of one specimen that possesses small (7 mm) holes (Fig. 10.22, 10.23) in a phosphatic matrix (a structure similar to that of putative sachitid sclerites, described below). Assuming that the presence and nature of the lateral zones is a genus-level character of halkieriids, these specimens, which lack lateral zones, represent an additional Middle Cambrian halkieriid genus, distinct from Australohalkieria n. gen. OTHER NEW TAXA

?Sachitids (Fig. 11.1–11.6).Two distinctive, spiniform specimens have also been recovered from the assemblage. These sclerites are roughly circular in cross section, long and narrow in dimension, and they lack a distinctive basal region. The distal region of the sclerites exhibits roughly triangular holes, ;7 mm wide, regularly arranged in a phosphatic matrix (Fig. 11.3–11.5). These holes appear to represent tubercles that did not become phosphatized and were dissolved during maceration. Partially

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phosphatized tubercles can be seen in Figure 11.3. (The surrounding phosphatic matrix may represent a diagenetic encrustation that molded the tubercles, or, as in the halkieriid sclerites, it may represent an outer organic layer.) On the proximal part of the sclerite, the wall structure is similar to that of A. superstes sclerites, suggesting that the tubercles coalesced into imbricated plates (Fig. 11.6). While it is plausible that these specimens represent rare spiniform sclerites belonging to A. superstes n. gen. and sp. or another halkieriid (cf. Bengtson et al., 1990), they are most similar, both in morphology and wall structure, to sclerites of the coeloscleritophoran family Sachitidae (cf. Hippopharangites Bengtson in Bengtson et al., 1990), previously known only from the Early Cambrian. Wiwaxiids.Of special note are two wiwaxiid sclerites, each possibly representing a distinct species [Fig. 11.7–11.11; Wiwaxia sp. from the Monastery Creek Formation is also reported but not figured in Southgate and Shergold (1991)]. These originally organic sclerites (Conway Morris, 1985) are preserved as secondarily phosphatized casts, comparable to the phosphatized paleoscolecidan cuticles and possible eggs also found in the assemblage (Fig. 2). Their presence in the assemblage extends not only the taphonomic distribution of this group (known previously only from organic specimens in siliciclastic sediments) but also, together with articulated specimens from the Early Cambrian Emu Bay Shale, Australia (C. Nedin, personal commun., 2000), their geographic range. [Wiwaxiids were known previously only from North America and China (Conway Morris, 1985; Butterfield, 1994; Zhao et al., 1994).] One specimen exhibits the same fine structure observed in sclerites from the Burgess Shale (Butterfield, 1990), including longitudinal ribs, rows of tubercles, and submicrometer lineations (Fig. 11.7, 11.9, 11.10). The other (Fig. 11.8, 11.11) is similar to a specimen from the Mount Cap Formation, northwestern Canada, figured in Butterfield (1994, fig. 2b). CONCLUSIONS

The phosphatic limestones of the Monastery Creek Formation, Georgina Basin, provide a window on Middle Cambrian life that is valuable for at least three reasons. First, like other phosphatized assemblages (e.g., Mu¨ller, 1985; Xiao et al., 1998), the Monastery Creek Formation preserves a diverse array of taxa, many of which are not preserved under conventional circumstances. Many are preserved in very fine detail, providing insight into the structure and function of their skeletal parts (cf. Mu¨ller and Hinz, 1992; Hinz-Schallreuter, 1993; Mu¨ller and Hinz-Schallreuter, 1993; Walossek et al., 1993). Second, the Monastery Creek Formation complements other preservational windows on life during this time. For example, articulated halkieriids in fine-grained siliciclastics from Greenland (Conway Morris and Peel, 1990, 1995) provide unique information about scleritome organization that can be used as a guide to reconstruct halkieriid species from disarticulated assemblages. They provide little information, however, about sclerite morphology and wall structure, information that can be helpful in elucidating phylogenetic relationships. For this, we must rely on phosphatic preservation. Combining information from different preservational windows, then, provides an integrated view not available from any single window. In addition to providing complementary views of single organisms, different windows may give us different perspectives on community structure. The Burgess Shale is approximately the same age as the Monastery Creek Formation, but it does not provide definitive evidence for halkieriids (Conway Morris, 1995). It also lacks the diversity of molluscs that is recorded in phosphatic rocks of the same age from Australia (Runnegar and Jell,

1976; personal observations). This may be due to a restriction in the geographic range of these taxa, but it also likely reflects environmental differences between the Burgess Shale and the Monastery Creek Formation. Clearly, to get a more complete view of Middle Cambrian communities, we must sample more than one window. A single assemblage, no matter how well preserved, allows a glimpse of only one environment. Finally, because the Monastery Creek Formation preserves taxa in the same way and in the same environment as most Early Cambrian small-shelly-fossil-rich successions, it allows us to view evolving Cambrian life through a single taphonomic window. Because unique taphonomic windows can preserve taxa that otherwise may not be represented in the fossil record, the opening and closing of these windows, i.e., temporal variation in the prevalence of specific taphonomic conditions, can bias the fossil record (e.g., Cherns and Wright, 2000). One condition necessary for phosphatization of small shelly taxa, namely the availability of phosphate, may have decreased through Cambrian time (Cook and McElhinny, 1979). If so, this may have contributed to the pattern of ‘‘Tommotian’’ faunal decline (Sepkoski, 1992). By comparing faunas of differing age through the same window we are able to control for these variations. The discovery of halkieriids in the Monastery Creek Formation suggests that further exploration of Middle and Upper Cambrian phosphatic limestones may yield more ‘‘Tommotian’’ taxa, indicating that, rather than short-lived experiments that gave way to successful crown-group taxa, these problematic organisms may have been long-lived and important components of Cambrian communities. Certainly, this appears to be true of the Halkieriidae. ACKNOWLEDGMENTS

Thanks to S. Bengtson who first pointed out the similarities to other halkieriid sclerites. Thanks also to P. Southgate, J. Laurie, M. Walter, and the Australian Geological Survey Organization for help with field logistics, and to A. H. Knoll for help in the field. Discussions with A. H. Knoll, P. Southgate, J. Shergold, S. Bengtson, and J. Payne were extremely helpful. Critical reviews by S. Conway Morris and G. Budd substantially improved the paper. Y. Lu assisted with SEM. T. Moran and M. Goh were useful resources for Latin names. Financial assistance from an NSF Graduate Student Fellowship and a National Research Council Postdoctoral Fellowship (NASA Astrobiology) to S. M. Porter and a NASA exobiology grant (# NAG5-3645) to A. H. Knoll is gratefully acknowledged. REFERENCES

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