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J. Paleont., 78(3), 2004, pp. 574±590 Copyright ᭧ 2004, The Paleontological Society 0022-3360/04/0078-574$03.00

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

ABSTRACTÐHalkieriids 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 event (the ``Botomian extinction''). Newly discovered 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 can be accommodated within a single , Australohalkieria superstes new and species, described and partly reconstructed here. Remaining sclerites probably represent two additional but rare halkieriid species. Additional newly discovered sclerites may have af®nities with the sachitids, another problematic ``Tommotian'' related to the halkieriids. Rare wiwaxiid sclerites extend the taphonomic and geographic distribution of this . 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 ) 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 phosphati- zation of shallow-shelf and platform limestones may have effectively closed this taphonomic window, potentially biasing apparent patterns of diversity change through the period.

INTRODUCTION nontrilobite , and , are typical of the Cam- brian fauna s.s. Also present, however, at relatively high abun- HE EARLY Cambrian record is characterized by unique skeletal assemblages that include the spongelike archaeocy- dances, are halkieriids. Halkieriids were sluglike metazoans ar- T mored with a coat of mineralized sclerites and two prominent athans and a number of mostly problematic small skeletal ele- shells at either end (Conway Morris and Peel, 1990, 1995). Their ments, known collectively as small shelly (SSFs; Matthews phylogenetic af®nities with modern groups are problematic, but and Missarzhevsky, 1975). These assemblages are distinctive they appear to fall somewhere within the , pos- enough from Middle and Upper Cambrian assemblages that Sep- sibly representing stem-group molluscs (e.g., Bengtson, 1992; koski (1992) recognized the ``Tommotian'' fauna as a discrete Runnegar, 2000), stem-group (e.g., Jell, 1981; Conway evolutionary fauna separate from his Cambrian fauna sensu stric- Morris and Peel, 1995), stem-group (Williams and to. In part, this distinctiveness re¯ects an at the Holmer, 2002; Holmer et al., 2002; Cohen et al., 2003; Conway end of the Early Cambrian [the ``Botomian extinction'' (Palmer, Morris and Peel, 1995), or a stem group of a combination of 1982; Signor, 1992; Zhuravlev and Wood, 1996)] that eliminated lophotrochozoan phyla (Conway Morris and Peel, 1995). [Con- many ``Tommotian'' taxa s. Sepkoski (1992) (Zhuravlev and way Morris and Peel (1995) argued for speci®c stem-group con- Wood, 1996). The extinction thus marks the evolutionary turnover nections between the halkieriid and brachiopods; from an early fauna populated mostly by skeletal problematica to between Thambetolepis and annelids (cf. Jell, 1981); and be- the more familiar Cambrian fauna s.s. tween a Siberian halkieriid species (Bengtson and Conway Mor- To understand better the and extent of this turnover, it ris, 1984) and the annelidϩbrachiopod clade.] Halkieriids are a is important to control for preservational mode and environment, common and diverse component of the ``Tommotian'' fauna, as taxa under consideration may be taphonomically or facies re- where they are represented by abundant sclerites (e.g., Qian and stricted. Because many elements of the ``Tommotian'' fauna ap- Bengtson, 1989; Bengtson et al., 1990; Conway Morris and pear to be both facies restricted and preserved by early diagenetic Chapman, 1997; Conway Morris et al., 1998) and rare articu- phosphatization (e.g., Brasier and Hewitt, 1979; Brasier, 1990; lated specimens (Conway Morris and Peel, 1990, 1995). How- Landing, 1992; Mount and Signor, 1992; Dzik, 1994; Khomen- ever, with the exception of possible halkieriid shells reported tovskii and Karlova, 1994), it is useful to compare them with from the (Conway Morris, 1995), halkieriids have taphonomically and environmentally comparable assemblages of not been identi®ed in post-Botomian successions. Their presence in the Monastery Creek Formation, thus, provides the ®rst com- Middle Cambrian age. The Monastery Creek Formation of the pelling evidence that halkieriids survived the end-Early Cam- Middle Cambrian Georgina Basin, Australia, offers one such op- brian extinction. portunity. It contains an SSF assemblage that, like many of its well-studied Early Cambrian counterparts, preserves secondarily GEOLOGY phosphatized skeletal elements associated with condensed hori- zons in phosphatic limestones (Southgate and Shergold, 1991; The Georgina Basin, a shallow intracratonic basin covering 2 Gravestock and Shergold, 2001). ϳ325,000 km in western Queensland and the Northern Territory, For the most part, the Monastery Creek assemblage con®rms Australia (Fig. 1.1), formed during a Late Proterozoic phase of the pattern highlighted by Sepkoski (1992): many of its domi- crustal extension (Lindsay et al., 1987; Southgate and Shergold, 1991). By Middle Cambrian time, marine conditions prevailed in nant constituents, such as , inarticulate brachiopods, the basin and phosphogenesis was widespread (Southgate and Shergold, 1991). Today, diverse assemblages of phosphatized fos- sils can be found in Middle Cambrian rocks throughout the basin * Current address: Department of Geological Sciences, University of Cal- (e.g., Fleming, 1973; Runnegar and Jell, 1976; Jones and Mac- ifornia, Santa Barbara, California 93106-9630, Ͻ[email protected]Ͼ kenzie, 1980; Henderson and Mackinnon, 1981; Shergold and 574 PORTERÐMIDDLE CAMBRIAN HALKIERIIDS FROM AUSTRALIA 575

FIGURE 1ÐGeologic map, modi®ed 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; MuÈller and Hinz, 1992; phosphatic and siliceous siltstone, shale, sandstone, and phos- Hinz-Schallreuter, 1993; MuÈller and Hinz-Schallreuter, 1993; phatic (de Keyser and Cook, 1972; Soudry and South- Kruse, 1998; Mehl, 1998; personal observations). A particularly gate, 1989). Phosphatized fossils occur throughout the Monastery diverse and well-preserved assemblageÐand the focus of this Creek Phosphorite Member; the fossils discussed in this study studyÐoccurs in the Monastery Creek Formation [formerly re- come from two beds located at ϳ6 and ϳ7.5 m above the contact ferred to as the Beetle Creek Formation (Russell and Trueman, with the Siltstone Member, and ϳ8.7 and ϳ7.2 m, respectively, 1971)] exposed in the Burke River Structural Belt ϳ140 km below a marker bed containing hollow chert nodules (noted in southeast of Mt. Isa, near the town of Duchess in western Queens- MuÈller and Hinz, 1992). The unit from which the fossils derive land (Fig. 1.2). Although fossils have been reported from three consists of tabular, ®ne-grained limestones ϳ10±15 cm thick, de- localities in the Duchess region (Mt. Murray, Rogers Ridge, and void of hummocky cross-strati®cation or other cross-lamination, Hill), sample collection for this study was limited for suggesting deposition below fair-weather wave base. Some beds logistical reasons to Rogers Ridge (Fig. 1.3). are silici®ed and most contain abundant ferruginized and phos- In the Burke River Outlier, the Monastery Creek Formation phatic hardgrounds. Fossils occur in high concentrations in these disconformably overlies the Thorntonia Limestone. It is overlain, rocks, with as many as ϳ20 fossils per mm2 evident in thin sec- with a conformable to laterally disconformable contact, by the tion. Also present in varying abundance are nonskeletal phos- organic-rich Inca Shale, which contains fossiliferous carbonate phatic clasts. nodules in the Rogers Ridge area (MuÈller and Hinz-Schallreuter, Middle Cambrian phosphatic sediments from the Duchess Re- 1993). The Monastery Creek Formation is divided into two mem- gion were deposited in a restricted embayment that was bounded bers: the lower Siltstone Member, a 50±60-m-thick succession by land on its northern, western, and southern sides, and whose containing white ®ssile siltstone, chert, and minor pelletal phos- eastern connection with the Burke River Outlier, an appendage of phorite; and the upper Monastery Creek Phosphorite Member, a the Georgina Basin (see Fig. 1.1, 1.2), was restricted by shallow 10±15-m-thick unit comprising grainstone phosphorite, chert, banks (Russell and Trueman, 1971). 576 JOURNAL OF , V. 78, NO. 3, 2004

In the sequence stratigraphic framework formulated by South- viewed with a LEO 982 scanning electron microscope at 5 or 10 gate and Shergold [1991; see also Gravestock and Shergold kV. (2001)] for Middle Cambrian sediments of the Georgina Basin, All ®gured specimens have been deposited in the Common- the Monastery Creek Formation forms a progradational package wealth Palaeontological Collections (CPC), Geoscience Australia, (referred to as parasequence set 2) within the transgressive sys- Canberra, under accession numbers CPC 37156±CPC 37228. tems tract of their sequence 2. Phosphatic and glauconitic hard- grounds deposited on the transgressive surface of parasequence SYSTEMATIC PALEONTOLOGY set 2 indicate sediment starvation; subsequent to this transgres- Taxonomic treatment of disarticulated fossils is dif®cult and sion, phosphatic carbonates, including the fossiliferous units from fraught with uncertainty (Bengtson, 1985a). This is particularly Rogers Ridge, were deposited in the structural lows, or subbasins, the case with halkieriid sclerites, which are known to be highly of the embayment (Russell and Trueman, 1971; Southgate and variable within a single scleritome. Nevertheless, early halkieriid Shergold, 1991). taxonomies (e.g., Bengtson and Conway Morris, 1984; Qian and Age.The Monastery Creek Formation contains Ptychagnos- Bengtson, 1989; Bengtson et al., 1990) assumed that certain scler- tus gibbus, diagnostic of the Late Templetonian/Floran Stage of ite characteristics (e.g., the presence and nature of lateral zones the Middle Cambrian of Australia (OÈ pik et al., 1957; OÈ pik, 1979). and wall ornamentation) were uniform within a single species, an As yet, no formal stages have been recognized for the Cambrian assumption supported by subsequently discovered articulated System, but attempts at constructing internationally recognized specimens (Conway Morris and Peel, 1995). Guided by both these divisions of Cambrian time have been made. The latest of these taxonomies and descriptions of articulated halkieriids (Conway (Geyer and Shergold, 2000), based largely on biostratig- Morris and Peel, 1995), I use overall morphology, and in raphy, correlates the Late Templetonian/Floran of Australia with particular the nature of the lateral zones, as a species-level char- intervals that all fall within the Middle Cambrian, wherever it is acter. recognized (including the Delamaran/Early Marjuman of Lauren- The majority of sclerites can be assigned to a single species, tia, the Amgan of , the Acadian and St. David's series of Australohalkieria superstes n. gen. and sp. In the following sec- Avalonia, and the Taijiangian of South ; Geyer et al., 2000). tions, I will describe the sclerites of this species in detail, recon- struct their original composition, morphology, wall structure, and THE ASSEMBLAGE position in the scleritome, and compare this species with Early The Monastery Creek Phosphorite Member contains a diverse Cambrian halkieriids. The remaining Georgina sclerites are dis- assemblage of fossils, representatives of which are shown in Fig- tinctive enough to discourage their inclusion in A. superstes, but ure 2. Bradoriid head shields (or carapaces), including those of they are not abundant enough to provide detail suf®cient for tax- Zepaera and Monasterium (Fleming, 1973), are the most common onomic description. These sclerites will be discussed brie¯y in fossil fragments (Fig. 2.1, 2.2). Occurring at lower abundances the section, ``Other sclerites.'' are shells of juvenile acrotretid (Fig. 2.3) and acrothelid brachio- Terminology used in sclerite description is drawn from Bengt- pods (Fig. 2.4), probable fragments (Fig. 2.5), and son and Conway Morris (1984) and illustrated in Figure 3. Brie¯y hyolith shells similar to those of Hyptiotheca and Actinotheca spp. stated, one end of the sclerite is called the base, and the remainder, from the Early Cambrian of Australia (Fig. 2.6; Bengtson et al., the blade (Fig. 3.1). The basal region can be identi®ed by the 1990). Halkieriid sclerites (e.g., Fig. 2.7) are the ®fth most abun- presence of a ¯attened facet, typically with an indentation housing dant fossils, constituting about ®ve percent of all identi®able spec- a foramen (Fig. 3.2). The concave side of the sclerite is de®ned imens (of course, each halkieriid produced many sclerites). as its lower surface, the convex side as upper (Fig. 3.3). The plane Present at lower abundance levels are trilobite fragments [includ- of the base is de®ned as that which is perpendicular to the basal ing larval hypostomes; Fig. 2.8, 2.9; cf. Speyer and Chatterton facet and aligned along its longest dimension (Fig. 3.4); similarly, (1989)], chancelloriid sclerites (Fig. 2.10), hollow spheres 50±100 the plane of the blade corresponds to the dimensions of greatest ␮m in diameter that possibly represent egg capsules (Fig. 2.11; ¯attening (Fig. 3.4). In most cases, the sclerites are asymmetric: Roy and FaÊhrñus, 1989; Walossek et al., 1993), cap-shaped fos- the blade may curve within the plane of the blade. The side to sils of possible molluscan af®nity (Fig. 2.12; cf. Qian and Bengt- which the blade curves is referred to as the inferior side because, son, 1989), and paleoscolecid worm fragments (Fig. 2.13). Many when imbricated, this side lies below the adjacent sclerite (Bengt- fragments (37 percent) are unidenti®able. son and Conway Morris, 1984). Similarly, the superior side lies All of these taxa have been preserved by phosphate, but their above the adjacent sclerite (Fig. 3.1). mode of preservation varies, possibly re¯ecting differences in original mineralogy. Family HALKIERIIDAE Poulsen, 1967 Discussion.Poulsen (1967) erected the family Halkieriidae, MATERIALS AND METHODS but he did not provide a diagnosis for it, or for the genus Hal- Fossils were extracted from the carbonate matrix by maceration kieria. In his restudy of the type material, Bengtson (1985b) pub- in 10±15 percent acetic acid, picked using a dissecting micro- lished a diagnosis for the Halkieriidae, later used by Qian and scope, mounted on stubs, coated in gold and/or palladium, and Bengtson (1989). Bengtson et al. (1990) modi®ed this diagnosis,

FIGURE 2ÐRepresentative fossils from the Monastery Creek Phosphorite Member together with distribution of their relative abundances. Note that abundances are based on number of identi®able fossil fragments (Total N ϭ 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 , 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 af®nities, CPC 37164. 13, Paleoscolecids, CPC 37225. Scale bar on bottom right represents 400 ␮m for 1, 2, 6; 600 ␮m for 3; 200 ␮m for 4, 5, 7, 8, 10, 13; 240 ␮m for 9; 100 ␮m for 11; and 160 ␮m for 12. PORTERÐMIDDLE CAMBRIAN HALKIERIIDS FROM AUSTRALIA 577 578 JOURNAL OF PALEONTOLOGY, V. 78, NO. 3, 2004

FIGURE 3ÐTerminology 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 Description.Longitudinal canals are cylindrical in shape and zones that may comprise longitudinal canals (canals that ¯ank the extend beyond the central canal, resulting in a distal sclerite tip central canal) or may be divided into lateral canals or camerae with two points separated by a rounded valley. Includes four var- (numerous canals extending outward from the central canal; these iable sclerite types, distinguishable from each other by base-blade can be thought of as transversely divided longitudinal canals). The transition and shape of basal facet. Basal facet commonly with diagnosis presented in Conway Morris and Peel (1995, p. 310) indentation, perforated by a small (5±10 ␮m diameter) foramen modi®ed those of both Bengtson et al. (1990) and Qian and connected to a short tube. Wall composed of ®brous plates im- Bengtson (1989), stating that halkieriid sclerites had ``lateral ca- bricated in the distal direction, resulting in appearance of parallel nals of varying length.'' It is unclear whether this diagnosis em- transverse lineations on both upper and lower sclerite surfaces. braces sclerites with undivided, longitudinal canals (including the Sclerites range from ϳ250 to ϳ1,000 ␮m in length. sclerites discussed here). The strong similarity between these Etymology.From the Latin superstes, surviving, with refer- sclerites and those with lateral canals, however, suggests that they ence to the fact that this species is part of a lineage that survived do belong in the same family. the end-Early Cambrian extinction event. Types.Holotype: CPC 37189, Figure 4.4. Paratypes: CPC Genus AUSTRALOHALKIERIA new genus 37159, 37161, 37169±37171, 37173±37184, 37190±37200, Type species.A. superstes n. sp.  37211±37216, 37219, 37222, 37224. From samples PK98±41 and Other species. Halkieria parva Conway Morris in Bengtson PK98±42; Figures 2.5, 4.1±4.3, 4.5, 4.6, 8, 9. et al., 1990.   Other material examined. Two hundred and forty-®ve scler- Diagnosis. Halkieriids with lateral zones comprising undivid- ites. Sclerites are represented by internal molds (e.g., Figs. 4.3, ed longitudinal canals.  4.6, 8.1±8.4, 8.9, 8.11, 9.4, 9.12, 9.16, 9.17), with, in some cases, Etymology. Latin australis, southern, and halkieria, a refer- part or all of the phosphatized wall (e.g., Figs. 4.1, 4.2, 4.4, 4.5, ence to its geographic locality and its similarity to the genus Hal- 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± kieria. 9.15, 9.18, 9.19). Outer surfaces may also be encrusted with dia- Discussion.Early diagnoses for the genus Halkieria either ex- genetic apatite (e.g., Figs. 10.12±10.14). plicitly (Bengtson et al., 1990) or implicitly (Bengtson, 1985b; Occurrence.Middle Cambrian (Late Templetonian/Floran) Qian and Bengtson, 1989) included those halkieriids with longi- tudinal canals (Bengtson et al., 1990). Conway Morris and Peel Monastery Creek Phosphorite Member, Rogers Ridge, Burke Riv- (1995), however, restricted the diagnosis of Halkieria such that er Outlier (samples PK98±41,Ϫ42,Ϫ44,Ϫ45,Ϫ47,Ϫ48). only those halkieriids with ``short lateral canals'' are included (p. AUSTRALOHALKIERIA PARVA (Conway Morris, 1990) new 310). Under this restricted de®nition, those sclerites with longi- combination tudinal canals [including the Georgina sclerites and the Australian species originally described as Halkieria parva (Bengtson et al., Halkieria parva CONWAY MORRIS IN BENGTSON ET AL., 1990, p. 73±77, 1990)] must be placed in a new genus. ®gs. 41±48. AUSTRALOHALKIERIA SUPERSTES new species Diagnosis.Species of Australohalkieria n. gen. with sclerites Figures 2.7, 4, 6, 8, 9 that possess an un¯attened central canal; longitudinal canals arise Diagnosis.Species of Australohalkieria n. gen. with sclerites at or near base of blade. that possess a ¯attened central canal. Flattening begins at mid- Material examined.Bengtson et al. (1990) examined material point of blade, where longitudinal canals originate, and is only from acid maceration residues. For this publication, ®gures 41± visible on the upper surface of the sclerite. 48 in Bengtson et al. (1990) were examined. PORTERÐMIDDLE CAMBRIAN HALKIERIIDS FROM AUSTRALIA 579

FIGURE 4ÐExamples 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 ␮m for 1, 2, 4, 5; 110 ␮m for 3; and 90 ␮m for 6.

Occurrence.Australohalkieria parva n. comb. occurs in Early in internal molds, the prong that represents the central canal is Cambrian rocks in the Flinders Ranges (Ajax Limestone: Mt. commonly broken off [Figs. 4.3, 4.6, 8.1, 8.2, 8.9±8.11, 9.4, 9.16 Scott Ranges locality) and Yorke Peninsula (Parara Limestone: (although see Fig. 9.7)]. No pores that connect longitudinal canals Curramulka, Horse Gully, and Kulpara localities); South Australia to the central canal have been observed (cf. A. parva; Bengtson (Bengston et al., 1990). et al., 1990), although the extremely close proximity (ϳ1±2 ␮m Discussion.See Bengtson et al. (1990) for a detailed descrip- separation) of canals suggests a very thin intervening wall. The tion of this species. three-part blade structure is limited to the internal part of the sclerite. On the outer surface of the sclerite, the wall is continu- DESCRIPTION AND RECONSTRUCTION OF AUSTRALOHALKIERIA ous; only the ¯attened aspect of the central canal, visible on the SUPERSTES SCLERITES upper surface of the sclerite, and the two-pointed tip hint at this  Original composition. Halkieriid sclerites were originally cal- internal structure (e.g., Figs. 4.4, 4.5, 8.6, 8.8, 8.14, 9.19). On the careous in composition (e.g., Bengtson and Missarzhevsky, 1981; lower surface, the wall lacks any indication of the inner com- Bengtson and Conway Morris, 1984; Bengtson et al., 1990; Con- partmentalization (e.g., Figs. 4.1, 8.5, 8.7, 8.12, 8.13, 8.15, 9.3, way Morris and Peel, 1995). An aragonitic mineralogy is indi- 9.5, 9.18, 10.3). cated by the presence of Ͻ1-␮m-diameter needlelike ®bers sug- The basal region is variable in structure (indeed, this is the basis gestive of ®brous in the wall of Australohalkieria for distinguishing sclerite types; see next section), but can be superstes sclerites (Fig. 6.2, 6.6, 6.10; cf. Runnegar, 1985) and identi®ed by the presence of an indentation (e.g., Fig. 8.12, 8.13, by the preservation of A. superstes sclerites, similar to that of other taxa in the assemblage known to be originally aragonitic 8.18, 8.19) and/or a foramen (e.g., Figs. 8.2±8.4, 8.10, 8.11, 8.16, (e.g., hyoliths and chancelloriids; James and Klappa, 1983; Bengt- 8.20, 8.21, 9.10, 9.16, 9.17). The foramen is most obvious in son et al., 1990; Mehl, 1996; Kouchinsky, 2000a, 2000b), and internal molds (although see Fig. 8.20) where it takes the form distinct from that of calcitic taxa. In addition to an aragonitic of a sharply delineated circular depression, 5±10 ␮m in diameter layer, there is evidence for an outer organic coating (see the sec- and at least as deep. This suggests that the sclerite wall curved tion, Wall structure, below). into the cavity, forming a short canal or tube (cf. ®g. 7.1 in Con- Morphology.Proximally, the sclerites of Australohalkieria way Morris and Chapman, 1997). The presence on one specimen superstes n. gen. and sp. have a single internal cavity. At the of wall fragments that curve inward into the foramen (Fig. 8.21) midpoint of the blade, the cavity divides into three roughly equal and the presence of phosphatic in®ll (of the tube) in the center of parts: a central canal ¯anked by two longitudinal canals. At the the foramen (Fig. 8.3), surrounded by a narrow valley where the point of division, the upper surface of the central canal slopes original tube wall was located, con®rm this interpretation. A re- sharply downward and comes in close contact with the lower construction of A. superstes sclerite morphology is illustrated in surface such that the canal becomes extremely ¯attened; indeed, Figure 5. 580 JOURNAL OF PALEONTOLOGY, V. 78, NO. 3, 2004

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 ¯attened 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 in®ll, commonly with grooves that re¯ect molded ®bers (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 cavityÐthought to have been tissue- or ¯uid- ®lled during life and have aided in sclerite mineralization (Bengt- son 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 ϳ1-␮m 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 FIGURE 5ÐA reconstruction of A. superstes blade structure, including second outer layer of the sclerite wall. In well-preserved speci- cross sections at two different locations (indicated by the dashed lines). mens, this layer forms an undulating coat that covers the entire See text for explanation. sclerite (Fig. 6.11). Possibly, this layer is a diagenetic encrusta- tion, but several features argue against this. First, internal molds of the central canal are regular and smooth on their upper, ¯at- The most distinctive aspect of A. superstes n. gen. and sp. is tened surface, indicating the presence of a primary boundary lay- the ¯attened central canal that forms a depression on the upper er, on which the mold templated. The lower, ®brous layer of the distal surface of the sclerites. Ascribing function to this structure wall is not present in this region; only the outer layer is. Collec- has obvious dif®culties, but one possible explanation is that the tively, these observations suggest that the thin outer layer is a depressions were involved in sensory activity (cf. Jell, 1981). As primary feature of the sclerite wall. Second, the outer layer does discussed below, what is interpreted to be the mineralized part of not ®ll in the spaces between the rows, as would be expected if the sclerite does not extend across the depressed region; only the outer layer of the wall, interpreted to be originally organic (see it were an encrustation. Instead, it extends straight from one ridge below), extends across it. Thus, the depression represents an area to the next, bridging the intervening spaces (Fig. 6.8). In sclerites where there might have been easy exchange of gases, ions, or where the spaces are represented by secondary phosphatic in®ll simple biomolecules, possibly through a permeable organic layer, (e.g., Fig. 6.9, 6.10, 6.12), there is a sharp line of contact between between the sclerite's internal tissue and the external environment. the outer layer and the in®ll (see, in particular, Fig. 6.12), sug- If the dorsal surface of A. superstes n. gen. and sp. was covered gesting the outer layer acted as a template for the phosphatic in®ll. by a tightly integrated scleritome, similar to Early Cambrian ar- Third, unlike diagenetic encrustation observed on some fossils in ticulated specimens (Conway Morris and Peel, 1995), sensory re- the Rogers Ridge assemblage (e.g., Fig. 10.14) and elsewhere ceptors on the upper (outer) surface of sclerite tips might have (e.g., Xiao and Knoll, 1999), apatite crystals forming the layer been useful. Many extant have similar structures that per- are not elongate or oriented perpendicular to the coated surface mit sensory reception on surfaces covered by a mineralized exo- but instead are spheroidal and randomly oriented, a fabric typical . For example, some arthropods have hollow setae with of phosphatized organic material (Fig. 6.13; Allison, 1988; Mar- slits or pores in the cuticle that allow contact between chemore- till, 1988; Wilby, 1993; Wilby and Briggs, 1997; Xiao and Knoll, ceptors and the external environment; and polyplacophorans, pos- 1999; Zhao and Bengtson, 1999). Collectively, these features sug- sible relatives of halkieriids (Bengtson, 1992), have a system of gest that the outer layer represents a primary organic layer that minute vertical canals in their shell plates into which photorecep- coated the sclerite. An outer organic layer has also been observed tors, called aesthetes, extend (e.g., Baxter et al., 1987, 1990). Aes- in sclerites of chancelloriids (J.-Y. Chen, personal commun., cited thetes are probably homologous with the sensory papillae of apla- in Mehl, 1996, although see Bengtson and Hou, 2001), metazoans cophorans, which, interestingly, are associated with the formation which may be closely related to the halkieriids (Bengtson and of spicules in this group (Haas and Kriesten, 1975; Fischer et al., Missarzhevsky, 1981; Bengtson and Hou, 2001; although see, 1980; as cited in Bengtson, 1992). e.g., Butter®eld and Nicholas, 1996). Its function is unknown, but Wall structure.The wall of A. superstes sclerites consists of it may have been involved in sclerite mineralization or protection, two layers. The inner layer is composed of longitudinally oriented similar to the molluscan (Taylor and Kennedy, 1969; ®bers [Fig. 6.1±6.3; also noted in other halkieriids (Bengtson, Tevesz and Carter, 1980; Bottjer, 1981). Indeed, if halkieriids are

FIGURE 6ÐDetails 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 in®ll and the outer layer; CPC 37220; 13, cross section of outer layer; CPC 37223. Scale bar represents 20 ␮m for 1, 5, 10;4␮m for 6, 7;15␮m for 2, 8;30␮m for 3;50␮m for 4, 11;10␮m for 9;2␮m for 12; and 3 ␮m for 13. PORTERÐMIDDLE CAMBRIAN HALKIERIIDS FROM AUSTRALIA 581 582 JOURNAL OF PALEONTOLOGY, V. 78, NO. 3, 2004

narrow, mirroring the ¯attened 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 ϳ90-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 ¯attened central canal. These sclerites are assigned to the palmate type because they exhibit the same characteristics as palmate sclerites of H. evan- gelista. They are the smallest of the sclerite types and have prom- inent 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. Fi- nally, these sclerites share a number of features with Early Cam- FIGURE 7ÐModel of A. superstes sclerite wall structure. See text for brian sclerites from disarticulated assemblages identi®ed as pal- explanation. mate, including a strongly ¯attened base and an auricle (Bengtson and Conway Morris, 1984; Qian and Bengtson, 1989; Bengtson et al., 1990). stem-group molluscs (Bengtson, 1992; Runnegar, 2000), the or- Cultrate sclerites (Figs. 4.4±4.6, 8.8±8.21).Eighty-eight ganic layer may be homologous with the periostracum. A tenta- specimens, ϳ300±1,000 ␮m in length. In these sclerites the basal tive reconstruction of the sclerite wall is illustrated in Figure 7. facet is circular to polygonal in shape, with an even rim, ϳ10 RECONSTRUCTION OF THE AUSTRALOHALKIERIA SUPERSTES ␮m wide, around the facet border (Fig. 8.12, 8.13, 8.18, 8.19). SCLERITOME 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 Specimens of articulated halkieriids (Halkieria evangelista the blade, but it may be offset from the blade by a broad con- Conway Morris and Peel, 1995) from the Early Cambrian Sirius striction (e.g., Fig. 8.10, 8.11, 8.15±8.17). Most sclerites, when Passet fauna, , provide a useful guide for reconstructing viewed from either the upper or lower surface, have a proximal the scleritome of A. superstes n. gen. and sp. The scleritome of end that is angular in shape, with the sides of the sclerite bending H. evangelista is bilaterally symmetric, and on either side it is composed of three zones, each consisting of a distinctive sclerite sharply toward each other to meet in a point (Figs. 4.4±4.6, 8.8± type. ``Palmate'' sclerites are situated in the dorsal zone of the 8.16). The sclerite edge dips more steeply away from this point animal's body. These are smaller than other sclerite types, bear on the inferior side of the sclerite; the sclerite base is oriented prominent ribs on their upper surfaces, and are tightly adpressed such that the longest dimension of the facet is parallel to this edge to the animal's body (Conway Morris and Peel, 1995). ``Cultrate'' (Fig. 8.10±8.13, 8.16, 8.18). The upper surface commonly has a sclerites are situated on the lateral surfaces of the animal's body. central rib, extending from the proximal point to the inferior side They also bear prominent ribs and are tightly adpressed to the of the central canal (Figs. 4.4±4.6, 8.8, 8.9, 8.14). body, but are larger than the palmate sclerites. ``Siculate'' sclerites These sclerites are considered cultrate because, like cultrate are situated on the ventrolateral margin on either side of the an- sclerites of H. evangelista, they are larger than the palmate scler- imal's body. They are approximately the same size as cultrate ites, exhibit prominent ribs, and have an angle between the basal sclerites, but lack prominent ribs. Rather than being tightly ad- facet and blade that suggests adpression against the animal's body. pressed to the animal's body, siculate sclerites appear to have In addition, these sclerites are similar to cultrate sclerites identi- extended outward at a 45±90-degree angle. In addition to the ®ed in other disarticulated assemblages (Bengtson and Conway scleritome, the articulated specimens bear two bilaterally sym- Morris, 1984; Bengtson et al., 1990; Conway Morris and Peel, metric shells, one at the anterior and the other at the posterior end 1995), particularly in the appearance of their proximal tips (see of the body. especially ®g. 51 in Bengtson and Conway Morris, 1984), the Using these sclerite characteristics, I have divided the A. su- shape of their basal facets, and their base-blade transition (e.g., perstes sclerites into palmate, cultrate, and siculate types. Each ®gs. 44c and 45g in Bengtson et al., 1990). sclerite type includes roughly equal numbers of right- and left- Siculate sclerites (Figs. 2.5, 4.1±4.3, 9.1±9.19).Sclerites con- handed forms (e.g., see symmetry pairs in Fig. 8.10±8.13), as sistent with a siculate assignment can be divided into two groups, would be expected if the scleritome of A. superstes n. gen. and although rare transitional forms exist. sp. were bilaterally symmetric. Group 1 (Figs. 2.5, 4.1±4.3, 9.1±9.10).ÐEighty-four speci- Palmate sclerites (Fig. 8.1±8.7).Twelve specimens, ϳ250± mens, ϳ400±1,000 ␮m in length. The basal facet is circular to 650 ␮m in length. In these sclerites, the basal facet is long and elliptical and indentation is faint or lacking. The foramen is offset

FIGURE 8ÐPalmate sclerites (1±7) and cultrate sclerites (8±21)ofA. 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 ␮m for 1, 7; 100 ␮m for 2;5␮m for 3;70␮m for 4; 220 ␮m for 5, 6, 10; 250 ␮m for 8, 15; 350 ␮m for 9; 150 ␮m for 11, 18; 330 ␮m for 12; 200 ␮m for 13, 14, 16, 17;50␮m for 19;80␮m for 20; and 40 ␮m for 21. PORTERÐMIDDLE CAMBRIAN HALKIERIIDS FROM AUSTRALIA 583 584 JOURNAL OF PALEONTOLOGY, V. 78, NO. 3, 2004 PORTERÐMIDDLE CAMBRIAN HALKIERIIDS FROM AUSTRALIA 585

FIGURE 10ÐOther 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 ␮min1; 120 ␮m for 2, 3;35␮m for 5;50␮m for 4, 11, 20, 23; 100 ␮m for 6, 12, 13, 21, 22; 150 ␮m for 7, 8; 125 ␮m for 9;25␮m for 10; and 75 ␮m for 17±19.

FIGURE 9ÐSiculate (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 ␮m for 1, 3; 150 ␮m for 2, 5, 6; 200 ␮m for 4, 7; 125 ␮m for 8, 11, 12, 18; 350 ␮m for 9; and 25 ␮m for 10; 160 ␮m for 14, 16, 19;65␮m for 17; 140 ␮m for 15; and 90 ␮m for 13. 586 JOURNAL OF PALEONTOLOGY, V. 78, NO. 3, 2004 toward the upper surface of the sclerite (Fig. 9.10). The most Conway Morris, 1984); H. sthenobasis (Qian and Bengtson, distinctive aspect of these specimens, however, is the sigmoidal 1989)], or tubercles, or a combination of these [e.g., A. parva, T. curve that distinguishes the base from the blade: distally the blade delicata (Bengtson et al., 1990); H. mira]. Sclerites may also vary curves sharply away from the upper side of the sclerite and then in size (ϳ250 mm to ϳ3,000 mm in length) and in relative num- curves back such that the basal facet is perpendicular to the plane bers of palmate, cultrate, and siculate types. of the blade. In some cases, the base is also offset by a broad Of the variable and widespread Early Cambrian halkieriid spe- constriction (e.g., Fig. 4.1, 4.2). The degree of sigmoidal curva- cies, A. superstes n. gen. and sp. is most similar to A. parva, also ture varies among sclerites from barely perceptible (Fig. 9.6±9.8) from Australia (Bengtson et al., 1990). Unlike other halkieriids, to strongly pronounced (Figs. 2.5, 4.1±4.3, 9.1±9.5, 9.9). Like both species have undivided longitudinal canals and a wall struc- palmate and cultrate sclerites, these are asymmetric: the curvature ture consisting of distally inclined, more or less continuous rows. is commonly more pronounced on one side than on the other (Fig. These similarities suggest that A. parva was closely related to A. 9.6±9.8) and the plane of the blade may be twisted slightly. In all superstes n. gen. and sp., a hypothesis formalized by the inclusion of these specimens, the upper surface lacks a rib. of A. parva in the new genus Australohalkieria. A. parva and A. Group 2 (Fig. 9.11±9.19).ÐTwenty-eight specimens, ϳ400± superstes n. gen. and sp. may have belonged to a single clade of 500 ␮m in length. In these specimens the shape of the basal facet Australian halkieriids that survived Botomian extinction. Why re¯ects the roughly elliptical cross section of the blade. A shallow, this clade would have survived while other halkieriid ap- broad indentation on the facet is similar in shape to the facet, and parently did not is not known. It is interesting to note, however, the foramen is slightly offset toward the inferior side of the scler- that the only archaeocyathans known to have survived the Bo- ite (Fig. 9.16, 9.17). The sclerite bends sharply between the base tomian extinction also occur in (Debrenne et al., 1984; and the blade such that the plane of the base is offset from that Wood et al., 1992). of the blade by ϳ45±90 degrees. The longitudinal axis of the Despite similarities to A. parva, A. superstes n. gen. and sp. is base (perpendicular to the plane of the basal facet) is also bent nevertheless quite distinct from all Early Cambrian halkieriids. ϳ45 degrees relative to the longitudinal axis of the blade, toward Most notably, it differs in its possession of a distally ¯attened the inferior side of the sclerite. In some specimens, a central rib central canal and two-pronged tip (see ``Description and recon- may be present on the upper surface of the sclerite, extending struction of Australohalkieria superstes sclerites''), but it also dif- from the edge of the basal facet to the base of the central canal fers in scleritome composition and sclerite size. Scleritomes of (Fig. 9.12, 9.13). Early Cambrian halkieriids possess many more palmate and cul- Like the siculate sclerites of H. evangelista, these sclerites are trate than siculate sclerites [e.g., Bengtson and Conway Morris, comparable in size to the cultrate sclerites and ribs are rare or 1984; Qian and Bengtson, 1989; Bengtson et al., 1990; Conway absent. In addition, the position of the basal facet in these spec- Morris and Peel, 1995; in some cases, de®nitive siculate sclerites imens suggests that they projected outward from the animal's could not be identi®ed (Conway Morris and Chapman, 1997)]. In body at a ϳ45±90-degree angle, like siculate sclerites on H. evan- contrast, siculate sclerites of A. superstes n. gen. and sp. are more gelista. Finally, the sigmoidal curvature typical of Group 1 is abundant than either cultrate or palmate sclerites; palmate sclerites observed in sclerites from disarticulated assemblages that are also are rare. This could re¯ect either postmortem sorting of sclerite identi®ed as siculate (Bengtson et al., 1990, ®g. 47b, c). types (palmate sclerites are on average smaller than the other Shells.A distinctive feature of articulated Early Cambrian types) or, more likely, a scleritome whose ventrolateral zone is halkieriids is the presence of two symmetric shells, one at either expanded in area relative to lateral and dorsal zones. A. superstes end of the body (Conway Morris and Peel, 1990, 1995). Since sclerites are signi®cantly smaller than Early Cambrian halkieriid this discovery, putative halkieriid shells have been identi®ed in a sclerites (250±1,000 ␮m vs. 600±3,000 ␮m; the presence of larg- number of disarticulated assemblages (e.g., Bengtson, 1992; Con- er fossils in the Georgina assemblage and the fact that most Early way Morris, 1995; Conway Morris and Chapman, 1997; Conway Cambrian halkieriids are also phosphatized suggests that this dif- Morris et al., 1998). No shells that might be ascribed to halkieriids ference is not due to taphonomic selectivity). This suggests that have been recognized within the Monastery Creek assemblage. the individual(s) represented in the Georgina assemblage were This may re¯ect their relative rarity [H. evangelista possesses one juvenile, their scleritome was composed of many more sclerites shell for every ϳ1,000 sclerites (Conway Morris and Peel, 1995)], than those of Early Cambrian halkieriids, or the species was rel- or it may indicate that the scleritome of A. superstes n. gen. and atively small. sp. did not possess shells. Whether such an absence would re¯ect a loss of shells in the Australohalkieria lineage or a plesiomorphic OTHER SCLERITES state within the halkieriids is not known. Two other morphologically distinct blade types occur in the Georgina assemblage, each with either one or two sclerite mor- COMPARISON WITH OTHER HALKIERIIDS phologies. While it cannot be ruled out that these sclerites belong All halkieriids possess calcareous, hollow sclerites arranged in to A. superstes n. gen. and sp., their inclusion would imply a a tightly imbricated, differentiated scleritome. In addition, their scleritome that was much more complex (with respect to struc- sclerite walls share a similar ultrastructure, consisting of distally tural variability as well as the number of sclerite types) than those inclined, ®brous elementsÐa pattern also observed in chancel- of Early Cambrian halkieriids. More likely, the structural types loriids (Bengtson et al., 1990; Mehl, 1996; Bengtson, 1999; correspond to species that were rare in the assemblage and, there- Kouchinsky, 2000a; Bengtson and Hou, 2001), siphogonuchitids fore, are only represented by sclerite types that were most abun- (Qian and Bengtson, 1989), and sachitids (Bengtson et al., 1990). dant in the scleritome. Lack of information on the range of sclerite There is substantial variation within the halkieriids, however. variability cautions against erecting species names for these spec- Sclerites may have lateral zones consisting of longitudinal canals imens, and, therefore, only a form classi®cation is presented (cf. [e.g., Australohalkieria parva (Bengtson et al., 1990)] or multiple Bengtson, 1985a). lateral canals [e.g., Halkieria mira (Conway Morris and Chapman, Type I (Fig. 10.1±10.16).The internal structure of these scler- 1997); Thambetolepis delicata (Jell, 1981; Bengtson et al., 1990)], ites is dif®cult to discern due to the absence of specimens pre- or lateral zones may be absent altogether (e.g., Fig. 10.17±10.22). served only as internal molds; in all specimens of this type the Sclerite walls may be composed of distally inclined rows [A. par- wall is preserved, almost always in its entirety. The few speci- va (Bengtson et al., 1990)], plates [e.g., H. sp. (Bengtson and mens that expose part of the internal mold show that, like the PORTERÐMIDDLE CAMBRIAN HALKIERIIDS FROM AUSTRALIA 587

FIGURE 11ÐPutative 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 ®ne 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 ␮m for 1;60␮m for 2;40␮m for 3; 450 ␮m for 4;10␮m for 5, 9; and 20 ␮m for 6; 100 ␮m for 7;50 ␮m for 8;4␮m for 10; and 30 ␮m for 11. blade of A. superstes n. gen. and sp., the distal section is divided superstes sclerite zones or whether they belong to an additional, into three canals (Fig. 10.11). This is also suggested by the distal rare Australohalkieria species. tips of the sclerites that, like those of A. superstes n. gen. and sp., Type II (Fig. 10.17±10.23).Seven specimens. In these scler- feature two points (representing each side canal) separated by a ites there are no distinct lateral zones. Instead, the ¯attened canal valley (Fig. 10.9, 10.10, 10.13). Unlike A. superstes n. gen. and remains undivided throughout, tapering distally to a single point. sp., however, the middle canal is not ¯attened. The upper surface A central rib may be separated from thickened margins by two of sclerites of this type instead may exhibit a broad central rib or broad depressions (Fig. 10.22), or may slope evenly to thin edges swelling (Fig. 10.7, 10.12, 10.13), probably representing the cen- (Fig. 10.18). Proximally, the blade curves slightly (Fig. 10.17± tral canal. The wall structure appears comparable to that of A. 10.19) or strongly (Fig. 10.21, 10.22) and ends in a ¯attened base superstes sclerites (note e.g., the imbricated ®brous rows on the (Fig. 10.19±10.21), comparable to palmate sclerites in Early Cam- sclerite in Fig. 10.2). brian halkieriids. On some sclerites, the base may ¯are on one There are two variants with this kind of blade, which differ in (Fig. 10.21) or both (Fig. 10.19) sides so that it is wider than the base shape: blade. A small (ϳ5 ␮m diameter) foramen lies in the center of Variant A (Fig. 10.1±10.11).ÐFourteen specimens. In these the basal facet, within a linear indentation that extends the width specimens, the basal facet is not distinct from the surface of the of the facet (Fig. 10.19, 10.20). The wall is generally not well blade. Instead, the proximal part of the blade curves smoothly preserved in these sclerites, with the exception of one specimen around a 180-degree angle and tapers to a subrounded point, that possesses small (7 ␮m) holes (Fig. 10.22, 10.23) in a phos- maintaining the same cross-sectional thickness throughout. The phatic matrix (a structure similar to that of putative sachitid scler- foramen is triangular in shape, about 15 ␮m at its widest, and is ites, described below). situated at this proximal point, on the upper surface of the blade Assuming that the presence and nature of the lateral zones is (Fig. 10.3±10.6, 10.9, 10.12). a genus-level character of halkieriids, these specimens, which lack Variant B (Fig. 10.15, 10.16).ÐOnly two specimens of this lateral zones, represent an additional Middle Cambrian halkieriid type have been identi®ed, but they are distinctive enough to war- genus, distinct from Australohalkieria n. gen. rant separation from the specimens mentioned above. Although the sclerite blade is ¯attened as in variant A, the base is cylindrical OTHER NEW TAXA with a rounded basal facet. In addition, at the base-blade transi- tion, the sclerites only curve around a 90-degree angle (rather ?Sachitids (Fig. 11.1±11.6).Two distinctive, spiniform spec- than 180 degrees), such that the presumed basal facet is oriented imens have also been recovered from the assemblage. These scler- parallel to the plane of the blade. No information is known about ites are roughly circular in cross section, long and narrow in di- the shape, size, or position of the foramen. mension, and they lack a distinctive basal region. The distal re- The sharp angle between the base and the blade of both of gion of the sclerites exhibits roughly triangular holes, ϳ7 ␮m these variants suggests that they probably occupied the dorsal or wide, regularly arranged in a phosphatic matrix (Fig. 11.3±11.5). lateral zones, where the palmate and cultrate sclerites occur. It is These holes appear to represent tubercles that did not become not clear whether these sclerites occupied additional distinctive A. phosphatized and were dissolved during maceration. Partially 588 JOURNAL OF PALEONTOLOGY, V. 78, NO. 3, 2004 phosphatized tubercles can be seen in Figure 11.3. (The surround- 1976; personal observations). This may be due to a restriction in ing phosphatic matrix may represent a diagenetic encrustation that the geographic range of these taxa, but it also likely re¯ects en- molded the tubercles, or, as in the halkieriid sclerites, it may rep- vironmental differences between the Burgess Shale and the Mon- resent an outer organic layer.) On the proximal part of the sclerite, astery Creek Formation. Clearly, to get a more complete view of the wall structure is similar to that of A. superstes sclerites, sug- Middle Cambrian communities, we must sample more than one gesting that the tubercles coalesced into imbricated plates (Fig. window. A single assemblage, no matter how well preserved, al- 11.6). lows a glimpse of only one environment. While it is plausible that these specimens represent rare spini- Finally, because the Monastery Creek Formation preserves taxa form sclerites belonging to A. superstes n. gen. and sp. or another in the same way and in the same environment as most Early halkieriid (cf. Bengtson et al., 1990), they are most similar, both Cambrian small-shelly-fossil-rich successions, it allows us to view in morphology and wall structure, to sclerites of the coeloscleri- evolving Cambrian life through a single taphonomic window. Be- tophoran family Sachitidae (cf. Hippopharangites Bengtson in cause unique taphonomic windows can preserve taxa that other- Bengtson et al., 1990), previously known only from the Early wise may not be represented in the fossil record, the opening and Cambrian. closing of these windows, i.e., temporal variation in the preva- Wiwaxiids.Of special note are two wiwaxiid sclerites, each lence of speci®c taphonomic conditions, can bias the fossil record possibly representing a distinct species [Fig. 11.7±11.11; (e.g., Cherns and Wright, 2000). One condition necessary for sp. from the Monastery Creek Formation is also reported but not phosphatization of small shelly taxa, namely the availability of ®gured in Southgate and Shergold (1991)]. These originally or- phosphate, may have decreased through Cambrian time (Cook and ganic sclerites (Conway Morris, 1985) are preserved as second- McElhinny, 1979). If so, this may have contributed to the pattern arily phosphatized casts, comparable to the phosphatized paleos- of ``Tommotian'' faunal decline (Sepkoski, 1992). By comparing colecidan cuticles and possible eggs also found in the assemblage faunas of differing age through the same window we are able to (Fig. 2). Their presence in the assemblage extends not only the control for these variations. The discovery of halkieriids in the taphonomic distribution of this group (known previously only Monastery Creek Formation suggests that further exploration of from organic specimens in siliciclastic sediments) but also, to- Middle and Upper Cambrian phosphatic limestones may yield gether with articulated specimens from the Early Cambrian Emu more ``Tommotian'' taxa, indicating that, rather than short-lived Bay Shale, Australia (C. Nedin, personal commun., 2000), their experiments that gave way to successful crown-group taxa, these geographic range. [Wiwaxiids were known previously only from problematic organisms may have been long-lived and important North America and China (Conway Morris, 1985; Butter®eld, components of Cambrian communities. Certainly, this appears to 1994; Zhao et al., 1994).] One specimen exhibits the same ®ne be true of the Halkieriidae. structure observed in sclerites from the Burgess Shale (Butter- ®eld, 1990), including longitudinal ribs, rows of tubercles, and ACKNOWLEDGMENTS submicrometer lineations (Fig. 11.7, 11.9, 11.10). The other (Fig. Thanks to S. Bengtson who ®rst pointed out the similarities to 11.8, 11.11) is similar to a specimen from the Mount Cap For- other halkieriid sclerites. Thanks also to P. Southgate, J. Laurie, mation, northwestern , ®gured in Butter®eld (1994, ®g. M. Walter, and the Australian Geological Survey Organization for 2b). help with ®eld logistics, and to A. H. Knoll for help in the ®eld. Discussions with A. H. Knoll, P. Southgate, J. Shergold, S. Bengt- CONCLUSIONS son, and J. Payne were extremely helpful. Critical reviews by S. The phosphatic limestones of the Monastery Creek Formation, Conway Morris and G. Budd substantially improved the paper. Y. Georgina Basin, provide a window on Middle Cambrian life that Lu assisted with SEM. T. Moran and M. Goh were useful re- is valuable for at least three reasons. First, like other phosphatized sources for Latin names. Financial assistance from an NSF Grad- assemblages (e.g., MuÈller, 1985; Xiao et al., 1998), the Monastery uate Student Fellowship and a National Research Council Post- Creek Formation preserves a diverse array of taxa, many of which doctoral Fellowship (NASA Astrobiology) to S. M. Porter and a are not preserved under conventional circumstances. Many are NASA exobiology grant (# NAG5-3645) to A. H. Knoll is grate- preserved in very ®ne detail, providing insight into the structure fully acknowledged. and function of their skeletal parts (cf. 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