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Paleoecology and taphonomy of the Early Cambrian Maotianshan Shale biota chancelloriid Allonnia junyuani: Adaptation to nonactualistic Cambrian substrates
Palaeogeography, Palaeoclimatology, Palaeoecology 277 (2009) 149–157
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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Paleoecology and taphonomy of the Early Cambrian Maotianshan Shale biota chancelloriid Allonnia junyuani: Adaptation to nonactualistic Cambrian substrates Tristan J. Kloss a,⁎, Stephen Q. Dornbos a, Jun-Yuan Chen b a b
Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53201-0413, USA Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, China
a r t i c l e
i n f o
Article history: Received 19 September 2008 Received in revised form 20 February 2009 Accepted 1 March 2009 Keywords: Chancelloriid Ichnofabric Bioturbation Nonactualistic substrates
a b s t r a c t The Cambrian radiation marks a period of extensive ecologic innovation within metazoan communities. Among these innovations were increasing levels of vertical bioturbation and a subsequent transition from Proterozoic-style firm unlithified substrates with very low levels of bioturbation to Phanerozoic-style soft substrates with a high water content and well-developed mixed layer. This transition, termed the “agronomic revolution” had a significant impact on benthic organisms already adapted to firm unlithified substrates, leading to evolutionary and ecological changes in these organisms known as the Cambrian Substrate Revolution (CSR). Chancelloriids are morphologically unusual sessile suspension feeders restricted to the Early and Middle Cambrian, and as such are a likely candidate to have been affected by the CSR. Examination of exceptionally preserved specimens of the species Allonnia junyuani from the Maotianshan Shale of Yunnan Province, China, in addition to a study of the rocks in which they are preserved, indicate that A. junyuani was likely a shallow sediment sticker that used a tapered blunt basal end for insertion into firm unlithified substrates. Bioturbation levels in the Maotianshan Shale are very low (ichnofabric index average = ii1) suggesting that Proterozoic-style firm unlithified substrates dominated this environment, consistent with the morphological adaptations of A. junyuani. These results suggest that A. junyuani was adapted to nonactualistic environments, and this may have partially contributed to its unusual morphology and extinction by the Late Cambrian. Complete, well-preserved specimens from the study group were very rare (3.7% of total), whereas partly or completely disarticulated specimens dominated (77.5% of total), suggesting that, despite the Burgess Shale-type preservation apparent in the Maotianshan biota, A. junyuani typically underwent significant pre-burial decay. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The beginning of the Cambrian, ca. 542 million years ago, marked a time of major ecologic change within marine communities. In the wake of the rapid diversification of metazoans are the first reported occurrences of Cambrian predation, extensive biomineralization among animals, and intensive bioturbation of the seafloor (Bengtson, 1994; Bottjer et al., 2000). Both predation (e.g. Nedin, 1999; Vannier and Chen, 2002; Vannier and Chen, 2005) and biomineralization (e.g. Bengtson, 1994; Brennan et al., 2004) on both a broad scale and at the species level have received much attention. Seilacher and Pflüger (1994) examined the broad-scale effects of bioturbation upon benthic metazoans during the Cambrian period, while more recent studies have focused on the response from specific taxa to intensive bioturbation (McIlroy and Logan, 1999; Bottjer et al., 2000; Dornbos and Bottjer, 2000; Droser et al., 2002; Dornbos, 2006).
⁎ Corresponding author. E-mail address: [email protected] (T.J. Kloss). 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.03.002
Increasing vertical bioturbation levels through the Early Cambrian led to a fundamental change in shallow subtidal seafloor substrates. During the Neoproterozoic normal marine shallow subtidal seafloor settings were dominated by microbial mats, leading to substrates with low water content and a sharp water–sediment interface (Seilacher and Pflüger, 1994). With notable exceptions during the aftermaths of several mass extinctions (e.g. Pruss et al., 2004; Sheehan and Harris, 2004), the widespread distribution of microbially mediated substrates in shallow subtidal settings is largely unheard of in later Paleozoic and younger sediments; their presence in the Early Cambrian therefore represents a nonactualistic marine environment (Bottjer, 1997). Increasing bioturbation, in concert with other possible factors (i.e. grazing, changing water chemistry) led to the destruction of this microbially mediated substrate and the establishment of a new dominant substrate with higher water content and a well-developed mixed layer. The transition from firm, unlithified “Proterozoic-style” substrates to soft, soupy “Phanerozoicstyle” substrates has been termed the “agronomic revolution” (Seilacher and Pflüger, 1994). The ecologic effect changing substrates had on organisms already adapted to Proterozoic-style seafloors is known as the “Cambrian Substrate Revolution” (CSR) (Bottjer et al., 2000). Earlier
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detailed studies on organisms impacted by the CSR have focused largely on echinoderms (Dornbos and Bottjer, 2000; Parsley and Prokop, 2004; Dornbos, 2006). The chancelloriid species Allonnia junyuani was probably adapted to nonactualistic substrates and hence likely affected by the CSR. That hypothesis will be tested through the use of direct analysis of bioturbation levels within the Maotianshan Shale and morphological and taphonomic analysis of specimens of A. junyuani. 2. Previous research 2.1. Maotianshan Shale biota Samples of Allonnia junyuani used for this study are part of the Lower Cambrian Maotianshan Shale of southwestern China. Like the slightly younger Middle Cambrian age Burgess Shale of British Columbia, the Maotianshan Shale from the Early Cambrian preserves soft-bodied organisms in exceptional detail (Hou et al., 2004). Many organisms show minimal evidence for decay and transport prior to burial, suggesting that normal agents for biodegradation, i.e., scavenging and bacterial decay, were often inhibited (Hou et al., 2004). The details of Burgess Shale-type preservation in the Maotianshan Shale biota are still under debate. Some organisms show evidence for minimal or no transport following death, such as worms found still in their burrows and sponges oriented upright perpendicular to bedding, indicating in situ burial (Babcock et al., 2001; Hou et al., 2004). 2.2. Chancelloridae Chancelloriids have been reconstructed (Janussen et al., 2002; Randell et al., 2005) as sac-shaped sessile benthic animals covered in rows of spiny sclerites. The sclerites appear to protrude from beneath an integuement (Janussen et al., 2002), are arranged in a series of rows that converge at the apical end of the body (Randell et al., 2005), and do not interact with one another. Individual sclerites are composed of a series of fused rays, with the number and orientation of rays differing between genera and species; it is even possible for multiple sclerite arrangements to occur in a single scleritome (Janussen et al., 2002; Randell et al., 2005). The chancelloriid integument has a granular appearance in hand specimen, and under magnification is composed of numerous closely packed “pits” that Janussen et al. (2002) describe as cellular. Several partially flattened specimens of the species Allonnia junyuani from the Maotianshan Shale biota have been observed to have an internal cavity (Janussen et al., 2002). This, combined with the apical convergence of sclerites, suggest that chancelloriids were filter feeding organisms with an inferred apical opening. Due to sclerite arrangement, and the observation that they do not appear to serve as structural support for the chancelloriid body, it is believed sclerites served largely a defensive role (Janussen et al., 2002; Randell et al., 2005). The common preservation of wellarticulated scleritomes in mudstones relative to other rock units led Janussen et al. (2002) to conclude that chancelloriids were sessile mud dwellers attaching to substrates via a basal root bulb. While individual sclerites are a common constituent of the Cambrian small shelly fauna (Janussen et al., 2002), well-articulated scleritomes are rare and appear to be restricted to Konservat Lagertstätten such as the Burgess Shale, Wheeler Shale, and the Maotianshan Shale biotas (Rigby, 1978; Conway Morris and Peel, 1990; Janussen et al., 2002). The earliest taxonomic descriptions of the Chancelloridae, based upon the morphology of their sclerites, depicted them as sponges (Walcott, 1920). Bengtson and Missarzhevsky (1981), realizing the hollow nature of chancelloriid sclerites, revised their taxonomic affinities and placed them in the broad Order Coeloscleritophora. This assignation is problematic, as the coeloscleritophorans are likely an assemblage of many unrelated forms. The slug-like coeloscleritophorans – the wiwaxiids and halkieriids– are interpreted as a monophyletic group within the lophotrochozoans (Conway Morris
and Caron, 2007), whereas chancelloriids possibly represent a basal metazoan group (Janussen et al., 2002). Chancelloriids have also been compared to ascidians (Mehl, 1996) and cnidarians (Randell et al., 2005); these, too, base the associations at least in part on similarities between individual chancelloriid sclerites and the homologous structures of their counterparts. Complete chancelloriid scleritomes begin to reveal the limitations of drawing comparisons to other taxa based primarily upon sclerite morphology. While chancelloriid sclerites bear some resemblance to sponge spicules, they do not act as a structural framework for the organism as spicules do; rather, they are primarily a protective measure (Janussen et al., 2002). They also lack the siphons that are critical to the lifestyle of ascidians (Randell et al., 2005). Randell et al. (2005) partially take into account chancelloriid morphology in comparing them to octocorals; but chancelloriids lack the tentacles, mesenteries, and actinopharynx that typify cnidarians. For an excellent summary of previous research regarding chancelloriid taxonomy, we refer the reader to Janussen et al., 2002. 2.2.1. Allonnia junyuani Allonnia junyuani is described in two different, but distinct, forms: the first appearing as rather small individuals 5–7 cm across with large, closely spaced sclerites or more or less uniform orientation (Chen et al., 1996; ‘Form A’ of Janussen et al., 2002); and the second appearing much larger, up to 20 cm long, with smaller randomly oriented sclerites (Chen et al., 1996; ‘Form B’ of Janussen et al., 2002). Both forms have 3 + 0 sclerites, although typically the basal ray is broken off in preserved specimens, leaving a characteristic “wishbone” shape to the rays. It is unclear as to whether the disparities in body size between the two forms represent two distinct species of Allonnia, or whether they are rather subspecies of A. junyuani (Janussen et al., 2002). Based upon similarities in descriptions, Form A was likely previously described as Allonnia phrixothrix by Bengtson and Hou (2001). Thus, it is possible that only Form B is A. junyuani, whereas Form A is simply a synonym for A. phrixothrix. The majority of well-preserved specimens studied here are of the Form A (phrixothrix) type. 2.3. Cambrian Substrate Revolution (CSR) As previously mentioned, the continual loss of Proterozoic-style, microbially mediated substrates through the Cambrian forced a response from metazoans that had adopted biomat-related lifestyles (Seilacher, 1999). This Cambrian Substrate Revolution (CSR) led to drastic changes in attachment strategies in immobile suspension feeders, and has been most effectively demonstrated in three early echinoderm groups: the edrioasteroids, the eocrinoids, and the helicoplacoids (Bottjer et al., 2000; Dornbos and Bottjer, 2000; Parsley and Prokop, 2004; Domke and Dornbos, 2005; Dornbos, 2006). Early to Middle Cambrian edrioasteroids display both sediment resting and direct sediment attachment strategies (Domke and Dornbos, 2005), but Late Cambrian forms are almost exclusive in their use of hard substrate attachment (Bottjer et al., 2000; Dornbos, 2006). Likewise, Early and Middle Cambrian eocrinoids, with their primitive stem morphology, were sediment resters, sediment stickers, or hard substrate attachers; Late Cambrian eocrinoids were only hard substrate attachers and developed “true” crinoid-like stems (Bottjer et al., 2000). Helicoplacoids are an example of non-response to the CSR: perhaps because they were unable to adapt from their sediment sticking lifestyle, these echinoderms were extinct by the Late Cambrian (Dornbos and Bottjer, 2000). The effects of the CSR are evident not only in animal morphology but environmental distribution, as well. Trace fossils created by grazing molluscs indicate that they were present in shallow subtidal settings during the Neoproterozoic and Early Cambrian (Seilacher, 1999; Dornbos et al., 2004). Modern distributions of grazing molluscs, however, show their restriction to rocky nearshore and deep sea settings where microbial mats are still abundant (Bottjer et al., 2000). This switch from shallow subtidal marine shelf to deep marine or
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rocky nearshore is consistent with a response to changes in substrate conditions from Proterozoic-style firm unlithified substrates to modern Phanerozoic-style soft substrates in shallow subtidal settings (Bottjer et al., 2000). Broad changes in community ecology in response to the CSR have also been documented among benthic metazoans in the Burgess and Maotianshan Shales (Dornbos et al., 2005). Benthic metazoans from the Maotianshan Shale almost exclusively exhibit adaptations for Proterozoic-style substrates (sediment resters, sediment attachers), suggesting that the CSR was still in a very early stage of development (Dornbos et al., 2005). Although preserved in a more offshore depositional environment, the later Middle Cambrian Burgess Shale shows marked decreases in metazoans adapted for Proterozoic-style substrates and an increase in hard substrate attachers, indicating that the CSR had escalated. The Burgess Shale is thus more representative of a transitional period between substrate conditions (Dornbos et al., 2005). Overall, the declining trend in benthic metazoans adapted to firm nonactualistic substrates is a predicted consequence of the CSR (Bottjer et al., 2000). In their paper, Dornbos et al. (2005) interpreted chancelloriid genera Allonnia and Chancelloria as sediment stickers adapted to Proterozoic-style substrates. This represents an initial attempt to incorporate chancelloriid ecology into the larger framework of the CSR. While chancelloriids are commonly reported in Early and Middle Cambrian deposits (e.g. Sdzuy, 1969; Rigby, 1978; Butterfield and Nicholas, 1996; Mehl, 1998; Janussen et al., 2002; Randell et al., 2005) there are no known published occurrences of chancelloriids in the Late Cambrian or younger strata. While other explanations are possible, this temporal distribution is consistent with a response to the CSR. 3. Geologic setting The Chengjiang biota is preserved within the Maotianshan Shale, the middle member of the Lower Cambrian Yu'anshan Formation of southwestern China (Fig. 1) (Chen and Zhou, 1997). The Yu'anshan Formation is the oldest Cambrian stratigraphic unit in the region containing trilobites (Dornbos et al., 2005). It is a 150 km thick sequence of shallowing upward siliciclastics that likely represents deposition in a shallow, tidally influenced marine shelf setting (Chen and Zhou, 1997; Babcock et al., 2001). It is composed of three members: the lower Black Shale Member (20 m thick), the middle fossiliferous Maotianshan Shale member (60 m), and the upper Siltstone Member (60 m) (Dornbos et al., 2005). Underlying the Yu'anshan Formation is the Shiyantuo Formation, a ~ 50 m thick shallowing upward siliciclastic sequence that likely represents a slightly deeper depositional setting than the Yu'anshan Formation (Dornbos et al., 2005). The Maotianshan Shale is known from a number of localities in the Yunnan Province of southwest China, including several near the town of Chengjiang, approximately 50 km southeast of the capital of Kunming (Hagadorn, 2002). The specimens of Allonnia junyuani examined in this study were originally collected from a 2–3 m interval of the Maotianshan Shale near the village of Shankou, Anning, Yunnan Province. 4. Methods In order to better understand how the chancelloriid Allonnia junyuani interacted with its substrate, to characterize the substrate conditions of its local environment, and to examine the potential ramifications the CSR may have had for this genus, 626 previously collected specimens of A. junyuani and 2.3 m of Maotianshan Shale strata from core samples were examined. The specimens of A. junyuani used during this study are currently housed at the Early Life Research Center of the Nanjing Institute of Geology and Paleontology, Chengjiang, China. The Maotianshan Shale core samples
Fig. 1. General Lower Cambrian stratigraphy of Yunnan Province, China. Crosshair indicates location of Mt. Maotianshan (from Dornbos and Chen, 2008).
are deposited in the University of Wisconsin-Milwaukee Department of Geosciences. Bioturbation levels in the Maotianshan strata were assessed on an mm-scale through 2.3 of total strata using the ichnofabric index (ii) method of Droser and Bottjer (1986). It is a semiqualitative method of visually determining the relative amount of bioturbation in a core or vertical outcrop, with several categories, or ichnofabric indices (ii), describing extent of bioturbation: ii1—no bioturbation observed, primary sedimentary structures preserved; ii2—up to 10% of original sedimentary structures disturbed (bioturbated); ii3—10–40% original sedimentary structures disturbed; ii4—40–60% original sedimentary structures disturbed; ii5—completely bioturbated, original sedimentary structures destroyed (Droser and Bottjer, 1986). Ichnofabric indices of 1 or 2 are expected of Proterozoic-style firm substrates, whereas values of 5 are expected of Phanerozoic-style substrates with a well-developed mixed layer. In addition to bioturbation levels, strata were examined for evidence of features suggestive of microbial mats (Scheiber, 1999). Sedimentary structures in the Maotianshan samples are visible with the naked eye, although the surfaces of the samples were wetted to enhance overall visibility. Specimens of Allonnia junyuani were utilized for study of their taphonomy and paleoecology. Specimens were categorized into one of
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Fig. 2. Representative specimens of each taphonomic grade. A. Exceptionally well preserved, well articulated; B. well preserved, articulated; C. poorly preserved, partially disarticulated; D. disarticulated. Scale bar = 1 cm.
four taphonomic grades, based upon preservational quality (Fig. 2): Grade A—very well preserved; sclerites preserved in life orientation, soft parts show no degradation; Grade B—well preserved; sclerites preserved in life orientation, soft parts show some degree of degradation; Grade C—partly disarticulated; sclerites show some disarticulation, soft parts are poorly preserved or absent; Grade D— completely disarticulated; sclerites completely disarticulated, no soft part preservation. Specimens of taphonomic Grade D were also subdivided into two additional categories based upon geometry of sclerites: unidirectional—sclerites arranged with distal end of rays oriented in a single direction; radial—sclerites arranged with distal end of rays oriented approximately 360° around a central point. The percentage of specimens in each taphonomic grade was calculated, and these were used to describe the preservation of A. junyuani. This taphonomic information ultimately assisted in the proper paleoecological interpretation of A. junyuani.
Specimens were also observed with respect to morphological characteristics that indicate adaptations to one substrate type or another. Several authors (e.g. Seilacher, 1999; Dornbos et al., 2005) have identified at least three strategies for adaptation to Proterozoicstyle substrates in the Cambrian: 1) sediment attachers lived permanently attached to unlithified seafloor sediment; 2) shallow sediment stickers embedded a basal end into the substrate; and 3) sediment resters lived resting on the substrate and showed no adaptations for soupy substrates. Organisms living on soft Phanerozoic-style substrates utilized a separate suite of attachment strategies (Thayer, 1975, 1983; Dornbos et al., 2005): 1) attachment to hard substrates for stabilization; 2) presence of root-like holdfasts or ballast structures; 3) broad distribution of body mass (‘Snowshoe Strategy’ of Thayer (1975)); and 4) long skeletal extensions for insertion into sediment (‘Iceberg Strategy’ of Thayer (1975)). Both suites of substrate adaptations are distinct, but only one is easily
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Fig. 3. Ichnofabric index data from the studied 2.3 m thick Maotianshan Shale core sample. N = 2287 mm.
recognized in the morphology of complete, well-articulated Allonnia junyuani specimens. Due to the method of collecting Maotianshan fossil samples, correlating Allonnia junyuani specimens to a specific level within the strata that were examined in cores is not possible. However, both the fossils and the cores were collected from the same member, indicating that the 2.3 m of Maotianshan strata is broadly representative of the substrates on which the chancelloriid specimens were living. 5. Core analysis of Maotianshan Shale strata Analysis of 2.3 m of core strata indicates a dominance of nonbioturbated thin beds and laminae within the Maotianshan Shale, with an average ii of 1 (Fig. 3). Not only are non-bioturbated laminae dominant, but the range of observed bioturbation levels is relatively restricted: 96.9% of strata are unbioturbated (ii1), 2.7% of strata are minimally bioturbated (ii2), 0.4% show moderate bioturbation (ii3), and none of the analyzed strata is heavily or completely bioturbated (ii4, ii5). It should be noted that some mud laminae did exhibit microbioturbation; however, as the ichnofabric index only accounts for macroscopic bioturbation and the laminae were not destroyed by the microbioturbation, such observations are not reflected in this study. The extremely low bioturbation levels represented in these rocks are indicative of relatively firm, unlithified substrates of the Proterozoic-style and indicate a lack of mixed layer development. While Proterozoic-style substrates are commonly associated with microbial mat cover, the strata examined in this study present no direct evidence for microbial mats in the Maotianshan Shale. However, Dornbos et al. (2005) noted the presence of muddy intraclasts interspersed among thin beds within the Maotianshan, indicating that seafloors were relatively cohesive in order to produce such rip-up clasts. Additional trace fossil evidence reported by Droser et al. (2002) suggests that Early Cambrian siliciclastic substrates were relatively firm despite the lack of evidence for microbial mat cover. Trace fossil preservation is typically hampered in shallow subtidal substrates due to the high water content of bioturbated sediments (Jensen et al., 2005); the excellent preservation of many Early Cambrian surface trace fossils in siliciclastic substrates indicates that bioturbation and water content of the substrate were relatively low (Droser et al., 2002; Jensen et al., 2005).
6. Taphonomy of chancelloriids Percentages of each taphonomic grade represented in the study group are as follows: Grade A—3.7%; Grade B—18.8%; Grade C—12.6%; Grade D—64.9% (Fig. 4). Articulated specimens are rare, whereas the majority (97.5%) of specimens are partially or completely disarticulated. The high degree of disarticulation among Allonnia junyuani specimens indicates significant decay either prior to or following burial. Considering the exceptional degree of preservation among the Maotianshan biota as a whole, post-burial decay seems unlikely; therefore, the majority of disarticulation observed in A. junyuani is attributed to pre-burial decay of specimens. This decay was not likely associated with transport, however, as even Grade D specimens are still identifiable as individual scleritomes and not random assemblages of sclerites. Dornbos et al. (2005) identified thin beds of the Maotianshan Shale exhibiting grading and small-scale cross bedding, suggesting an environment of rapid deposition. Rapid deposition would likely limit redistribution of disarticulated sclerites and promote exceptional preservation of chancelloriid sclerites. The overwhelming abundance of Grades C and D specimens suggest that A. junyuani readily disarticulated upon death.
Fig. 4. Pie chart of taphonomic results. A—3.7%; B—18.8%; C—12.6%; D—64.9%. N = 626.
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Fig. 5. Primary modes of chancelloriid collapse during final stages of decay. A. Chancelloriid body collapses sideways, possibly under influence of a weak current, resulting in preservation of a unidirectional scleritome. B. Chancelloriid body collapses top-down, resulting in preservation of a radial scleritome. Scale bar = 1 cm.
Disarticulated specimens from the study group also yield important paleoecological information regarding the substrate adaptations of Allonnia junyuani. Grade D specimens can be further subdivided into two categories based upon the geometry of scleritomes and orientation of individual sclerite rays within the scleritome (Fig. 5). Unidirectional scleritomes are specimens with an elongated scleritome that mimics the general morphology of well-articulated specimens; in addition, the distal end of sclerite rays all point predominantly in a
single direction. Nearly all (98.0%) of the 406 disarticulated specimens show this unidirectional style of preservation. Radial scleritomes compose a much smaller (2%) taphonomic component: scleritomes which are circular in shape, with the distal end of sclerite rays oriented ~360° around a central point within the scleritome. Unidirectional and radial scleritomes are suggestive of two different modes of chancelloriid body collapse during the final stages of decay (Fig. 5). Under typical conditions, as the soft parts of Allonnia junyuani
Fig. 6. A. Complete specimen of Allonnia junyuani with tapered, rounded basal end. B. Same specimen as A, with basal end highlighted for clarity. C. Specimen of helicoplacoid echinoderm, from the White-Inyo Mountains, USA, for comparison. U.S. one cent coin for scale (from Dornbos and Bottjer, 2000). Scale bars = 0.5 cm.
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decay, the body of the organism collapses unto one side; this type of collapse preserves the general morphology of the living chancelloriid (elongate shape) and orients sclerites in one direction. The dominance of unidirectional collapse may result, in part, by weak bottom currents acting on the body; chancelloriids, as likely filter feeders, would have required such a current to capture suspended trophic resources. In some rare cases, the chancelloriids collapsed down upon themselves, resulting in a near uniform radial orientation in sclerite direction. The preservation of radial scleritomes is indicative of the living orientation of A. junyuani: oriented perpendicular to bedding, living erect upon the seafloor. This orientation is similar to organisms that utilized shallow sediment sticking to survive on the substrate of the Maotianshan Shale (Dornbos and Bottjer, 2000; Domke and Dornbos, 2005). Hence, the presence of radial scleritomes suggests that A. junyuani was likely a shallow sediment sticker, an adaptive strategy for survival on the firm, Proterozoic-style substrates of the Maotianshan. 7. Adaptive morphology of Allonnia junyuani Of the 23 Grade A specimens, 8 have their lower end preserved, making them complete enough to permit observations regarding
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substrate adaptations. These specimens of Allonnia junyuani exhibit a tapered, rounded basal end (Figs. 6A,B and7A), with no obvious structures for attachment to hard substrates or stabilization on Phanerozoic-style soft substrates. These basal blunt ends are most consistent with the interpretation of A. junyuani as a shallow sediment sticker. This interpretation is further strengthened by the morphological similarity between the basal end of A. junyuani and another shallow sediment sticker from the Early Cambrian, the helicoplacoid echinoderms. As a whole, A. junyuani is roughly analogous to the bauplan and paleoecology of helicoplacoids: a sac-like body with a lower, tapered basal end, living upright on the seafloor as a suspension feeder (Fig. 6C) (Dornbos and Bottjer, 2000). The interpretation of a suspension feeding lifestyle for helicoplacoids in based in part upon the termination of the ambulacra along the lower third of the body (Dornbos and Bottjer, 2000); it is interesting to note that, while not directly observed in this study, the reduction of chancelloriid sclerites toward the basal end of the body has been commonly reported elsewhere (Janussen et al., 2002; Randell et al., 2005). Perhaps this is further indication of their shallow sediment sticking life position, as the basal end embedded in the sediment would have no use for defensive sclerites.
Fig. 7. A. Specimen of Allonnia junyuani with tapering basal end (outlined). B. Specimen of Chancelloria eros, Middle Cambrian Wheeler Shale. Basal end noted by arrow. C. Specimen of Chancelloria eros, Middle Cambrian Wheeler Shale, with tapered basal end (outline). Scale bars = 1.0 cm.
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The similarities in broad body shape between helicoplacoids and chancelloriids are likely partially due to convergent evolution to living upon a firm Proterozoic-style substrate. It seems reasonable to assume that based upon morphology both helicoplacoids and chancelloriids should have formed similar responses to the CSR. Among the earliest groups of sessile suspension feeding echinoderms, only helicoplacoids were extinct by the Middle Cambrian; in contrast, edrioasteroids and eocrinoids evolved the ability to attach to hard substrates and survived into the Ordovician (Bottjer et al., 2000). Helicoplacoids lacked sufficient adaptations to Phanerozoic-style substrates and were possibly unable to adapt to changes in substrate conditions (Dornbos and Bottjer, 2000). Similar to helicoplacoids, chancelloriids are a common component of Early through Middle Cambrian shelly faunas (Sdzuy, 1969; Rigby, 1978; Butterfield and Nicholas, 1996; Mehl, 1998; Clausen and Alvaro, 2006), but sclerite occurrences are unknown in Late Cambrian or younger strata. The apparent extinction of chancelloriids by the Late Cambrian mirrors the earlier extinction of helicoplacoids, and at least suggests that chancelloriid extinction may well be related to their inability to adapt to changing substrate conditions. When compared to the Early Cambrian Allonnia junyuani, complete scleritomes of Chancelloria eros from the Wheeler Shale do not exhibit any major changes to the general tapered rounded design of the basal end (Fig. 7B, C; Janussen et al., 2002; Fig. 3.2), suggesting that as late as the Middle Cambrian chancelloriids had not yet evolved adaptations to Phanerozoic-style substrates. Additional work is necessary to determine if, and to what extent, changing substrate conditions were involved in the extinction of chancelloriids. The paleoecology of both the chancelloriids and the helicoplacoids might serve as useful analogs for many Cambrian Fauna sessile suspension feeders that are interpreted to have used Proterozoic-style substrate adaptation strategies (e.g. Dornbos et al., 2005). If shallow sediment stickers as a whole were particularly susceptible to changing substrates, then the decline of much of the Cambrian Fauna may be linked in part to substrate transitions during the CSR. 8. Conclusions Core analysis of the Maotianshan Shale supports previous work (Dornbos et al., 2005) indicating that bioturbation levels are extremely low (average ii = 1.0), and minimal to moderate levels of bioturbation are exceedingly rare (3.1% total). The Maotianshan Shale depositional environment was therefore dominated by firm unlithified Proterozoic-style substrates with limited mixed layer development. Graded bedding (Dornbos et al., 2005) also suggest periods of rapid deposition of the shale. The taphonomic study indicates that Allonnia junyuani readily disarticulated upon death. Well-articulated specimens were likely preserved during periods of rapid deposition. Disarticulated specimens are further subdivided into unidirectional and radial geometries that define the nature of collapse of the chancelloriid body during the final stages of decay. Unidirectional geometries exhibit elongated scleritomes and sclerite oriented in a single direction, and indicate a side-on collapse of the body, possibly under influence of a weak current. Radial geometries exhibit a circular scleritome with sclerites uniformly distributed in orientation, indicating a top-down collapse of the body. The presence of radial sclerites suggests that chancelloriids were living upright on the seafloor, a lifestyle consistent with a shallow sediment sticker living on firm Proterozoic-style substrates. Complete scleritomes of Allonnia junyuani reveal a tapered, rounded basal end that fits the lifestyle of a shallow sediment sticker. The general morphology of A. junyuani is comparable to that of helicoplacoid echinoderms, another shallow sediment sticker found in Early Cambrian fossil groups. The conspicuous similarities between A. junyuani and helicoplacoids suggest that substrate conditions have, at least in part, led to the convergent evolution of broad body shape in both groups. The extinction of chancelloriids by the Late Cambrian
may also have resulted from their inability to evolve adaptations for stabilization in Phanerozoic-style substrates, possibly mirroring the earlier extinction of helicoplacoids. Allonnia junyuani was a sessile suspension feeding organism adapted to firm Proterozoic-style substrates that were dominant in environments represented by the Maotianshan Shale. As such, the CSR undoubtedly impacted chancelloriids, but their response to this event is still largely unknown. Their morphological similarities to other sessile shallow sediment stickers, like the helicoplacoids, and their extinction by the Late Cambrian, may indicate that they were physically unable to adapt to changing substrates. Based upon precursory examinations of specimens of Chancellloria eros from the Wheeler Shale, this scenario seems plausible; however, additional work is required to determine the extent of the role the CSR may have played in the ultimate demise of chancelloriids and the history of the larger Cambrian Fauna.
Acknowledgements This work was supported through grants to SQD from the UWM Research Growth Initiative and the UWM Graduate Research Committee Award, and to JYC by the National Science Foundation of China (Grant 40432006; 40772001). The authors thank three anonymous reviewers for their helpful comments.
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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Paleoecology and taphonomy of the Early Cambrian Maotianshan Shale biota chancelloriid Allonnia junyuani: Adaptation to nonactualistic Cambrian substrates Tristan J. Kloss a,⁎, Stephen Q. Dornbos a, Jun-Yuan Chen b a b
Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53201-0413, USA Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, China
a r t i c l e
i n f o
Article history: Received 19 September 2008 Received in revised form 20 February 2009 Accepted 1 March 2009 Keywords: Chancelloriid Ichnofabric Bioturbation Nonactualistic substrates
a b s t r a c t The Cambrian radiation marks a period of extensive ecologic innovation within metazoan communities. Among these innovations were increasing levels of vertical bioturbation and a subsequent transition from Proterozoic-style firm unlithified substrates with very low levels of bioturbation to Phanerozoic-style soft substrates with a high water content and well-developed mixed layer. This transition, termed the “agronomic revolution” had a significant impact on benthic organisms already adapted to firm unlithified substrates, leading to evolutionary and ecological changes in these organisms known as the Cambrian Substrate Revolution (CSR). Chancelloriids are morphologically unusual sessile suspension feeders restricted to the Early and Middle Cambrian, and as such are a likely candidate to have been affected by the CSR. Examination of exceptionally preserved specimens of the species Allonnia junyuani from the Maotianshan Shale of Yunnan Province, China, in addition to a study of the rocks in which they are preserved, indicate that A. junyuani was likely a shallow sediment sticker that used a tapered blunt basal end for insertion into firm unlithified substrates. Bioturbation levels in the Maotianshan Shale are very low (ichnofabric index average = ii1) suggesting that Proterozoic-style firm unlithified substrates dominated this environment, consistent with the morphological adaptations of A. junyuani. These results suggest that A. junyuani was adapted to nonactualistic environments, and this may have partially contributed to its unusual morphology and extinction by the Late Cambrian. Complete, well-preserved specimens from the study group were very rare (3.7% of total), whereas partly or completely disarticulated specimens dominated (77.5% of total), suggesting that, despite the Burgess Shale-type preservation apparent in the Maotianshan biota, A. junyuani typically underwent significant pre-burial decay. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The beginning of the Cambrian, ca. 542 million years ago, marked a time of major ecologic change within marine communities. In the wake of the rapid diversification of metazoans are the first reported occurrences of Cambrian predation, extensive biomineralization among animals, and intensive bioturbation of the seafloor (Bengtson, 1994; Bottjer et al., 2000). Both predation (e.g. Nedin, 1999; Vannier and Chen, 2002; Vannier and Chen, 2005) and biomineralization (e.g. Bengtson, 1994; Brennan et al., 2004) on both a broad scale and at the species level have received much attention. Seilacher and Pflüger (1994) examined the broad-scale effects of bioturbation upon benthic metazoans during the Cambrian period, while more recent studies have focused on the response from specific taxa to intensive bioturbation (McIlroy and Logan, 1999; Bottjer et al., 2000; Dornbos and Bottjer, 2000; Droser et al., 2002; Dornbos, 2006).
⁎ Corresponding author. E-mail address: [email protected] (T.J. Kloss). 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.03.002
Increasing vertical bioturbation levels through the Early Cambrian led to a fundamental change in shallow subtidal seafloor substrates. During the Neoproterozoic normal marine shallow subtidal seafloor settings were dominated by microbial mats, leading to substrates with low water content and a sharp water–sediment interface (Seilacher and Pflüger, 1994). With notable exceptions during the aftermaths of several mass extinctions (e.g. Pruss et al., 2004; Sheehan and Harris, 2004), the widespread distribution of microbially mediated substrates in shallow subtidal settings is largely unheard of in later Paleozoic and younger sediments; their presence in the Early Cambrian therefore represents a nonactualistic marine environment (Bottjer, 1997). Increasing bioturbation, in concert with other possible factors (i.e. grazing, changing water chemistry) led to the destruction of this microbially mediated substrate and the establishment of a new dominant substrate with higher water content and a well-developed mixed layer. The transition from firm, unlithified “Proterozoic-style” substrates to soft, soupy “Phanerozoicstyle” substrates has been termed the “agronomic revolution” (Seilacher and Pflüger, 1994). The ecologic effect changing substrates had on organisms already adapted to Proterozoic-style seafloors is known as the “Cambrian Substrate Revolution” (CSR) (Bottjer et al., 2000). Earlier
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detailed studies on organisms impacted by the CSR have focused largely on echinoderms (Dornbos and Bottjer, 2000; Parsley and Prokop, 2004; Dornbos, 2006). The chancelloriid species Allonnia junyuani was probably adapted to nonactualistic substrates and hence likely affected by the CSR. That hypothesis will be tested through the use of direct analysis of bioturbation levels within the Maotianshan Shale and morphological and taphonomic analysis of specimens of A. junyuani. 2. Previous research 2.1. Maotianshan Shale biota Samples of Allonnia junyuani used for this study are part of the Lower Cambrian Maotianshan Shale of southwestern China. Like the slightly younger Middle Cambrian age Burgess Shale of British Columbia, the Maotianshan Shale from the Early Cambrian preserves soft-bodied organisms in exceptional detail (Hou et al., 2004). Many organisms show minimal evidence for decay and transport prior to burial, suggesting that normal agents for biodegradation, i.e., scavenging and bacterial decay, were often inhibited (Hou et al., 2004). The details of Burgess Shale-type preservation in the Maotianshan Shale biota are still under debate. Some organisms show evidence for minimal or no transport following death, such as worms found still in their burrows and sponges oriented upright perpendicular to bedding, indicating in situ burial (Babcock et al., 2001; Hou et al., 2004). 2.2. Chancelloridae Chancelloriids have been reconstructed (Janussen et al., 2002; Randell et al., 2005) as sac-shaped sessile benthic animals covered in rows of spiny sclerites. The sclerites appear to protrude from beneath an integuement (Janussen et al., 2002), are arranged in a series of rows that converge at the apical end of the body (Randell et al., 2005), and do not interact with one another. Individual sclerites are composed of a series of fused rays, with the number and orientation of rays differing between genera and species; it is even possible for multiple sclerite arrangements to occur in a single scleritome (Janussen et al., 2002; Randell et al., 2005). The chancelloriid integument has a granular appearance in hand specimen, and under magnification is composed of numerous closely packed “pits” that Janussen et al. (2002) describe as cellular. Several partially flattened specimens of the species Allonnia junyuani from the Maotianshan Shale biota have been observed to have an internal cavity (Janussen et al., 2002). This, combined with the apical convergence of sclerites, suggest that chancelloriids were filter feeding organisms with an inferred apical opening. Due to sclerite arrangement, and the observation that they do not appear to serve as structural support for the chancelloriid body, it is believed sclerites served largely a defensive role (Janussen et al., 2002; Randell et al., 2005). The common preservation of wellarticulated scleritomes in mudstones relative to other rock units led Janussen et al. (2002) to conclude that chancelloriids were sessile mud dwellers attaching to substrates via a basal root bulb. While individual sclerites are a common constituent of the Cambrian small shelly fauna (Janussen et al., 2002), well-articulated scleritomes are rare and appear to be restricted to Konservat Lagertstätten such as the Burgess Shale, Wheeler Shale, and the Maotianshan Shale biotas (Rigby, 1978; Conway Morris and Peel, 1990; Janussen et al., 2002). The earliest taxonomic descriptions of the Chancelloridae, based upon the morphology of their sclerites, depicted them as sponges (Walcott, 1920). Bengtson and Missarzhevsky (1981), realizing the hollow nature of chancelloriid sclerites, revised their taxonomic affinities and placed them in the broad Order Coeloscleritophora. This assignation is problematic, as the coeloscleritophorans are likely an assemblage of many unrelated forms. The slug-like coeloscleritophorans – the wiwaxiids and halkieriids– are interpreted as a monophyletic group within the lophotrochozoans (Conway Morris
and Caron, 2007), whereas chancelloriids possibly represent a basal metazoan group (Janussen et al., 2002). Chancelloriids have also been compared to ascidians (Mehl, 1996) and cnidarians (Randell et al., 2005); these, too, base the associations at least in part on similarities between individual chancelloriid sclerites and the homologous structures of their counterparts. Complete chancelloriid scleritomes begin to reveal the limitations of drawing comparisons to other taxa based primarily upon sclerite morphology. While chancelloriid sclerites bear some resemblance to sponge spicules, they do not act as a structural framework for the organism as spicules do; rather, they are primarily a protective measure (Janussen et al., 2002). They also lack the siphons that are critical to the lifestyle of ascidians (Randell et al., 2005). Randell et al. (2005) partially take into account chancelloriid morphology in comparing them to octocorals; but chancelloriids lack the tentacles, mesenteries, and actinopharynx that typify cnidarians. For an excellent summary of previous research regarding chancelloriid taxonomy, we refer the reader to Janussen et al., 2002. 2.2.1. Allonnia junyuani Allonnia junyuani is described in two different, but distinct, forms: the first appearing as rather small individuals 5–7 cm across with large, closely spaced sclerites or more or less uniform orientation (Chen et al., 1996; ‘Form A’ of Janussen et al., 2002); and the second appearing much larger, up to 20 cm long, with smaller randomly oriented sclerites (Chen et al., 1996; ‘Form B’ of Janussen et al., 2002). Both forms have 3 + 0 sclerites, although typically the basal ray is broken off in preserved specimens, leaving a characteristic “wishbone” shape to the rays. It is unclear as to whether the disparities in body size between the two forms represent two distinct species of Allonnia, or whether they are rather subspecies of A. junyuani (Janussen et al., 2002). Based upon similarities in descriptions, Form A was likely previously described as Allonnia phrixothrix by Bengtson and Hou (2001). Thus, it is possible that only Form B is A. junyuani, whereas Form A is simply a synonym for A. phrixothrix. The majority of well-preserved specimens studied here are of the Form A (phrixothrix) type. 2.3. Cambrian Substrate Revolution (CSR) As previously mentioned, the continual loss of Proterozoic-style, microbially mediated substrates through the Cambrian forced a response from metazoans that had adopted biomat-related lifestyles (Seilacher, 1999). This Cambrian Substrate Revolution (CSR) led to drastic changes in attachment strategies in immobile suspension feeders, and has been most effectively demonstrated in three early echinoderm groups: the edrioasteroids, the eocrinoids, and the helicoplacoids (Bottjer et al., 2000; Dornbos and Bottjer, 2000; Parsley and Prokop, 2004; Domke and Dornbos, 2005; Dornbos, 2006). Early to Middle Cambrian edrioasteroids display both sediment resting and direct sediment attachment strategies (Domke and Dornbos, 2005), but Late Cambrian forms are almost exclusive in their use of hard substrate attachment (Bottjer et al., 2000; Dornbos, 2006). Likewise, Early and Middle Cambrian eocrinoids, with their primitive stem morphology, were sediment resters, sediment stickers, or hard substrate attachers; Late Cambrian eocrinoids were only hard substrate attachers and developed “true” crinoid-like stems (Bottjer et al., 2000). Helicoplacoids are an example of non-response to the CSR: perhaps because they were unable to adapt from their sediment sticking lifestyle, these echinoderms were extinct by the Late Cambrian (Dornbos and Bottjer, 2000). The effects of the CSR are evident not only in animal morphology but environmental distribution, as well. Trace fossils created by grazing molluscs indicate that they were present in shallow subtidal settings during the Neoproterozoic and Early Cambrian (Seilacher, 1999; Dornbos et al., 2004). Modern distributions of grazing molluscs, however, show their restriction to rocky nearshore and deep sea settings where microbial mats are still abundant (Bottjer et al., 2000). This switch from shallow subtidal marine shelf to deep marine or
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rocky nearshore is consistent with a response to changes in substrate conditions from Proterozoic-style firm unlithified substrates to modern Phanerozoic-style soft substrates in shallow subtidal settings (Bottjer et al., 2000). Broad changes in community ecology in response to the CSR have also been documented among benthic metazoans in the Burgess and Maotianshan Shales (Dornbos et al., 2005). Benthic metazoans from the Maotianshan Shale almost exclusively exhibit adaptations for Proterozoic-style substrates (sediment resters, sediment attachers), suggesting that the CSR was still in a very early stage of development (Dornbos et al., 2005). Although preserved in a more offshore depositional environment, the later Middle Cambrian Burgess Shale shows marked decreases in metazoans adapted for Proterozoic-style substrates and an increase in hard substrate attachers, indicating that the CSR had escalated. The Burgess Shale is thus more representative of a transitional period between substrate conditions (Dornbos et al., 2005). Overall, the declining trend in benthic metazoans adapted to firm nonactualistic substrates is a predicted consequence of the CSR (Bottjer et al., 2000). In their paper, Dornbos et al. (2005) interpreted chancelloriid genera Allonnia and Chancelloria as sediment stickers adapted to Proterozoic-style substrates. This represents an initial attempt to incorporate chancelloriid ecology into the larger framework of the CSR. While chancelloriids are commonly reported in Early and Middle Cambrian deposits (e.g. Sdzuy, 1969; Rigby, 1978; Butterfield and Nicholas, 1996; Mehl, 1998; Janussen et al., 2002; Randell et al., 2005) there are no known published occurrences of chancelloriids in the Late Cambrian or younger strata. While other explanations are possible, this temporal distribution is consistent with a response to the CSR. 3. Geologic setting The Chengjiang biota is preserved within the Maotianshan Shale, the middle member of the Lower Cambrian Yu'anshan Formation of southwestern China (Fig. 1) (Chen and Zhou, 1997). The Yu'anshan Formation is the oldest Cambrian stratigraphic unit in the region containing trilobites (Dornbos et al., 2005). It is a 150 km thick sequence of shallowing upward siliciclastics that likely represents deposition in a shallow, tidally influenced marine shelf setting (Chen and Zhou, 1997; Babcock et al., 2001). It is composed of three members: the lower Black Shale Member (20 m thick), the middle fossiliferous Maotianshan Shale member (60 m), and the upper Siltstone Member (60 m) (Dornbos et al., 2005). Underlying the Yu'anshan Formation is the Shiyantuo Formation, a ~ 50 m thick shallowing upward siliciclastic sequence that likely represents a slightly deeper depositional setting than the Yu'anshan Formation (Dornbos et al., 2005). The Maotianshan Shale is known from a number of localities in the Yunnan Province of southwest China, including several near the town of Chengjiang, approximately 50 km southeast of the capital of Kunming (Hagadorn, 2002). The specimens of Allonnia junyuani examined in this study were originally collected from a 2–3 m interval of the Maotianshan Shale near the village of Shankou, Anning, Yunnan Province. 4. Methods In order to better understand how the chancelloriid Allonnia junyuani interacted with its substrate, to characterize the substrate conditions of its local environment, and to examine the potential ramifications the CSR may have had for this genus, 626 previously collected specimens of A. junyuani and 2.3 m of Maotianshan Shale strata from core samples were examined. The specimens of A. junyuani used during this study are currently housed at the Early Life Research Center of the Nanjing Institute of Geology and Paleontology, Chengjiang, China. The Maotianshan Shale core samples
Fig. 1. General Lower Cambrian stratigraphy of Yunnan Province, China. Crosshair indicates location of Mt. Maotianshan (from Dornbos and Chen, 2008).
are deposited in the University of Wisconsin-Milwaukee Department of Geosciences. Bioturbation levels in the Maotianshan strata were assessed on an mm-scale through 2.3 of total strata using the ichnofabric index (ii) method of Droser and Bottjer (1986). It is a semiqualitative method of visually determining the relative amount of bioturbation in a core or vertical outcrop, with several categories, or ichnofabric indices (ii), describing extent of bioturbation: ii1—no bioturbation observed, primary sedimentary structures preserved; ii2—up to 10% of original sedimentary structures disturbed (bioturbated); ii3—10–40% original sedimentary structures disturbed; ii4—40–60% original sedimentary structures disturbed; ii5—completely bioturbated, original sedimentary structures destroyed (Droser and Bottjer, 1986). Ichnofabric indices of 1 or 2 are expected of Proterozoic-style firm substrates, whereas values of 5 are expected of Phanerozoic-style substrates with a well-developed mixed layer. In addition to bioturbation levels, strata were examined for evidence of features suggestive of microbial mats (Scheiber, 1999). Sedimentary structures in the Maotianshan samples are visible with the naked eye, although the surfaces of the samples were wetted to enhance overall visibility. Specimens of Allonnia junyuani were utilized for study of their taphonomy and paleoecology. Specimens were categorized into one of
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Fig. 2. Representative specimens of each taphonomic grade. A. Exceptionally well preserved, well articulated; B. well preserved, articulated; C. poorly preserved, partially disarticulated; D. disarticulated. Scale bar = 1 cm.
four taphonomic grades, based upon preservational quality (Fig. 2): Grade A—very well preserved; sclerites preserved in life orientation, soft parts show no degradation; Grade B—well preserved; sclerites preserved in life orientation, soft parts show some degree of degradation; Grade C—partly disarticulated; sclerites show some disarticulation, soft parts are poorly preserved or absent; Grade D— completely disarticulated; sclerites completely disarticulated, no soft part preservation. Specimens of taphonomic Grade D were also subdivided into two additional categories based upon geometry of sclerites: unidirectional—sclerites arranged with distal end of rays oriented in a single direction; radial—sclerites arranged with distal end of rays oriented approximately 360° around a central point. The percentage of specimens in each taphonomic grade was calculated, and these were used to describe the preservation of A. junyuani. This taphonomic information ultimately assisted in the proper paleoecological interpretation of A. junyuani.
Specimens were also observed with respect to morphological characteristics that indicate adaptations to one substrate type or another. Several authors (e.g. Seilacher, 1999; Dornbos et al., 2005) have identified at least three strategies for adaptation to Proterozoicstyle substrates in the Cambrian: 1) sediment attachers lived permanently attached to unlithified seafloor sediment; 2) shallow sediment stickers embedded a basal end into the substrate; and 3) sediment resters lived resting on the substrate and showed no adaptations for soupy substrates. Organisms living on soft Phanerozoic-style substrates utilized a separate suite of attachment strategies (Thayer, 1975, 1983; Dornbos et al., 2005): 1) attachment to hard substrates for stabilization; 2) presence of root-like holdfasts or ballast structures; 3) broad distribution of body mass (‘Snowshoe Strategy’ of Thayer (1975)); and 4) long skeletal extensions for insertion into sediment (‘Iceberg Strategy’ of Thayer (1975)). Both suites of substrate adaptations are distinct, but only one is easily
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Fig. 3. Ichnofabric index data from the studied 2.3 m thick Maotianshan Shale core sample. N = 2287 mm.
recognized in the morphology of complete, well-articulated Allonnia junyuani specimens. Due to the method of collecting Maotianshan fossil samples, correlating Allonnia junyuani specimens to a specific level within the strata that were examined in cores is not possible. However, both the fossils and the cores were collected from the same member, indicating that the 2.3 m of Maotianshan strata is broadly representative of the substrates on which the chancelloriid specimens were living. 5. Core analysis of Maotianshan Shale strata Analysis of 2.3 m of core strata indicates a dominance of nonbioturbated thin beds and laminae within the Maotianshan Shale, with an average ii of 1 (Fig. 3). Not only are non-bioturbated laminae dominant, but the range of observed bioturbation levels is relatively restricted: 96.9% of strata are unbioturbated (ii1), 2.7% of strata are minimally bioturbated (ii2), 0.4% show moderate bioturbation (ii3), and none of the analyzed strata is heavily or completely bioturbated (ii4, ii5). It should be noted that some mud laminae did exhibit microbioturbation; however, as the ichnofabric index only accounts for macroscopic bioturbation and the laminae were not destroyed by the microbioturbation, such observations are not reflected in this study. The extremely low bioturbation levels represented in these rocks are indicative of relatively firm, unlithified substrates of the Proterozoic-style and indicate a lack of mixed layer development. While Proterozoic-style substrates are commonly associated with microbial mat cover, the strata examined in this study present no direct evidence for microbial mats in the Maotianshan Shale. However, Dornbos et al. (2005) noted the presence of muddy intraclasts interspersed among thin beds within the Maotianshan, indicating that seafloors were relatively cohesive in order to produce such rip-up clasts. Additional trace fossil evidence reported by Droser et al. (2002) suggests that Early Cambrian siliciclastic substrates were relatively firm despite the lack of evidence for microbial mat cover. Trace fossil preservation is typically hampered in shallow subtidal substrates due to the high water content of bioturbated sediments (Jensen et al., 2005); the excellent preservation of many Early Cambrian surface trace fossils in siliciclastic substrates indicates that bioturbation and water content of the substrate were relatively low (Droser et al., 2002; Jensen et al., 2005).
6. Taphonomy of chancelloriids Percentages of each taphonomic grade represented in the study group are as follows: Grade A—3.7%; Grade B—18.8%; Grade C—12.6%; Grade D—64.9% (Fig. 4). Articulated specimens are rare, whereas the majority (97.5%) of specimens are partially or completely disarticulated. The high degree of disarticulation among Allonnia junyuani specimens indicates significant decay either prior to or following burial. Considering the exceptional degree of preservation among the Maotianshan biota as a whole, post-burial decay seems unlikely; therefore, the majority of disarticulation observed in A. junyuani is attributed to pre-burial decay of specimens. This decay was not likely associated with transport, however, as even Grade D specimens are still identifiable as individual scleritomes and not random assemblages of sclerites. Dornbos et al. (2005) identified thin beds of the Maotianshan Shale exhibiting grading and small-scale cross bedding, suggesting an environment of rapid deposition. Rapid deposition would likely limit redistribution of disarticulated sclerites and promote exceptional preservation of chancelloriid sclerites. The overwhelming abundance of Grades C and D specimens suggest that A. junyuani readily disarticulated upon death.
Fig. 4. Pie chart of taphonomic results. A—3.7%; B—18.8%; C—12.6%; D—64.9%. N = 626.
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Fig. 5. Primary modes of chancelloriid collapse during final stages of decay. A. Chancelloriid body collapses sideways, possibly under influence of a weak current, resulting in preservation of a unidirectional scleritome. B. Chancelloriid body collapses top-down, resulting in preservation of a radial scleritome. Scale bar = 1 cm.
Disarticulated specimens from the study group also yield important paleoecological information regarding the substrate adaptations of Allonnia junyuani. Grade D specimens can be further subdivided into two categories based upon the geometry of scleritomes and orientation of individual sclerite rays within the scleritome (Fig. 5). Unidirectional scleritomes are specimens with an elongated scleritome that mimics the general morphology of well-articulated specimens; in addition, the distal end of sclerite rays all point predominantly in a
single direction. Nearly all (98.0%) of the 406 disarticulated specimens show this unidirectional style of preservation. Radial scleritomes compose a much smaller (2%) taphonomic component: scleritomes which are circular in shape, with the distal end of sclerite rays oriented ~360° around a central point within the scleritome. Unidirectional and radial scleritomes are suggestive of two different modes of chancelloriid body collapse during the final stages of decay (Fig. 5). Under typical conditions, as the soft parts of Allonnia junyuani
Fig. 6. A. Complete specimen of Allonnia junyuani with tapered, rounded basal end. B. Same specimen as A, with basal end highlighted for clarity. C. Specimen of helicoplacoid echinoderm, from the White-Inyo Mountains, USA, for comparison. U.S. one cent coin for scale (from Dornbos and Bottjer, 2000). Scale bars = 0.5 cm.
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decay, the body of the organism collapses unto one side; this type of collapse preserves the general morphology of the living chancelloriid (elongate shape) and orients sclerites in one direction. The dominance of unidirectional collapse may result, in part, by weak bottom currents acting on the body; chancelloriids, as likely filter feeders, would have required such a current to capture suspended trophic resources. In some rare cases, the chancelloriids collapsed down upon themselves, resulting in a near uniform radial orientation in sclerite direction. The preservation of radial scleritomes is indicative of the living orientation of A. junyuani: oriented perpendicular to bedding, living erect upon the seafloor. This orientation is similar to organisms that utilized shallow sediment sticking to survive on the substrate of the Maotianshan Shale (Dornbos and Bottjer, 2000; Domke and Dornbos, 2005). Hence, the presence of radial scleritomes suggests that A. junyuani was likely a shallow sediment sticker, an adaptive strategy for survival on the firm, Proterozoic-style substrates of the Maotianshan. 7. Adaptive morphology of Allonnia junyuani Of the 23 Grade A specimens, 8 have their lower end preserved, making them complete enough to permit observations regarding
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substrate adaptations. These specimens of Allonnia junyuani exhibit a tapered, rounded basal end (Figs. 6A,B and7A), with no obvious structures for attachment to hard substrates or stabilization on Phanerozoic-style soft substrates. These basal blunt ends are most consistent with the interpretation of A. junyuani as a shallow sediment sticker. This interpretation is further strengthened by the morphological similarity between the basal end of A. junyuani and another shallow sediment sticker from the Early Cambrian, the helicoplacoid echinoderms. As a whole, A. junyuani is roughly analogous to the bauplan and paleoecology of helicoplacoids: a sac-like body with a lower, tapered basal end, living upright on the seafloor as a suspension feeder (Fig. 6C) (Dornbos and Bottjer, 2000). The interpretation of a suspension feeding lifestyle for helicoplacoids in based in part upon the termination of the ambulacra along the lower third of the body (Dornbos and Bottjer, 2000); it is interesting to note that, while not directly observed in this study, the reduction of chancelloriid sclerites toward the basal end of the body has been commonly reported elsewhere (Janussen et al., 2002; Randell et al., 2005). Perhaps this is further indication of their shallow sediment sticking life position, as the basal end embedded in the sediment would have no use for defensive sclerites.
Fig. 7. A. Specimen of Allonnia junyuani with tapering basal end (outlined). B. Specimen of Chancelloria eros, Middle Cambrian Wheeler Shale. Basal end noted by arrow. C. Specimen of Chancelloria eros, Middle Cambrian Wheeler Shale, with tapered basal end (outline). Scale bars = 1.0 cm.
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The similarities in broad body shape between helicoplacoids and chancelloriids are likely partially due to convergent evolution to living upon a firm Proterozoic-style substrate. It seems reasonable to assume that based upon morphology both helicoplacoids and chancelloriids should have formed similar responses to the CSR. Among the earliest groups of sessile suspension feeding echinoderms, only helicoplacoids were extinct by the Middle Cambrian; in contrast, edrioasteroids and eocrinoids evolved the ability to attach to hard substrates and survived into the Ordovician (Bottjer et al., 2000). Helicoplacoids lacked sufficient adaptations to Phanerozoic-style substrates and were possibly unable to adapt to changes in substrate conditions (Dornbos and Bottjer, 2000). Similar to helicoplacoids, chancelloriids are a common component of Early through Middle Cambrian shelly faunas (Sdzuy, 1969; Rigby, 1978; Butterfield and Nicholas, 1996; Mehl, 1998; Clausen and Alvaro, 2006), but sclerite occurrences are unknown in Late Cambrian or younger strata. The apparent extinction of chancelloriids by the Late Cambrian mirrors the earlier extinction of helicoplacoids, and at least suggests that chancelloriid extinction may well be related to their inability to adapt to changing substrate conditions. When compared to the Early Cambrian Allonnia junyuani, complete scleritomes of Chancelloria eros from the Wheeler Shale do not exhibit any major changes to the general tapered rounded design of the basal end (Fig. 7B, C; Janussen et al., 2002; Fig. 3.2), suggesting that as late as the Middle Cambrian chancelloriids had not yet evolved adaptations to Phanerozoic-style substrates. Additional work is necessary to determine if, and to what extent, changing substrate conditions were involved in the extinction of chancelloriids. The paleoecology of both the chancelloriids and the helicoplacoids might serve as useful analogs for many Cambrian Fauna sessile suspension feeders that are interpreted to have used Proterozoic-style substrate adaptation strategies (e.g. Dornbos et al., 2005). If shallow sediment stickers as a whole were particularly susceptible to changing substrates, then the decline of much of the Cambrian Fauna may be linked in part to substrate transitions during the CSR. 8. Conclusions Core analysis of the Maotianshan Shale supports previous work (Dornbos et al., 2005) indicating that bioturbation levels are extremely low (average ii = 1.0), and minimal to moderate levels of bioturbation are exceedingly rare (3.1% total). The Maotianshan Shale depositional environment was therefore dominated by firm unlithified Proterozoic-style substrates with limited mixed layer development. Graded bedding (Dornbos et al., 2005) also suggest periods of rapid deposition of the shale. The taphonomic study indicates that Allonnia junyuani readily disarticulated upon death. Well-articulated specimens were likely preserved during periods of rapid deposition. Disarticulated specimens are further subdivided into unidirectional and radial geometries that define the nature of collapse of the chancelloriid body during the final stages of decay. Unidirectional geometries exhibit elongated scleritomes and sclerite oriented in a single direction, and indicate a side-on collapse of the body, possibly under influence of a weak current. Radial geometries exhibit a circular scleritome with sclerites uniformly distributed in orientation, indicating a top-down collapse of the body. The presence of radial sclerites suggests that chancelloriids were living upright on the seafloor, a lifestyle consistent with a shallow sediment sticker living on firm Proterozoic-style substrates. Complete scleritomes of Allonnia junyuani reveal a tapered, rounded basal end that fits the lifestyle of a shallow sediment sticker. The general morphology of A. junyuani is comparable to that of helicoplacoid echinoderms, another shallow sediment sticker found in Early Cambrian fossil groups. The conspicuous similarities between A. junyuani and helicoplacoids suggest that substrate conditions have, at least in part, led to the convergent evolution of broad body shape in both groups. The extinction of chancelloriids by the Late Cambrian
may also have resulted from their inability to evolve adaptations for stabilization in Phanerozoic-style substrates, possibly mirroring the earlier extinction of helicoplacoids. Allonnia junyuani was a sessile suspension feeding organism adapted to firm Proterozoic-style substrates that were dominant in environments represented by the Maotianshan Shale. As such, the CSR undoubtedly impacted chancelloriids, but their response to this event is still largely unknown. Their morphological similarities to other sessile shallow sediment stickers, like the helicoplacoids, and their extinction by the Late Cambrian, may indicate that they were physically unable to adapt to changing substrates. Based upon precursory examinations of specimens of Chancellloria eros from the Wheeler Shale, this scenario seems plausible; however, additional work is required to determine the extent of the role the CSR may have played in the ultimate demise of chancelloriids and the history of the larger Cambrian Fauna.
Acknowledgements This work was supported through grants to SQD from the UWM Research Growth Initiative and the UWM Graduate Research Committee Award, and to JYC by the National Science Foundation of China (Grant 40432006; 40772001). The authors thank three anonymous reviewers for their helpful comments.
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