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Disparity, Decimation and the Cambrian "Explosion": Comparison of Eady Cambrian and Present Faunal Communities with Emphasis on Velvet Worms (Onychophora)

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Disparity, Decimation and the Cambrian Rev. Bio!. Trop., 48(2/3): 333-35 1, 2000 www.ucr.ac.cr www.ots.ac.cr www.ots.duke.edu Disparity, decimation and the Cambrian "explosion": comparison of eady Cambrian and Present faunal communities with emphasis on velvet worms (Onychophora) Julián Monge-Nájera1 and Xianguang HouZ 1 Biología Tropical, Universidad de Costa Rica, San José, Costa Rica. Fax (506) 2075550; [email protected] 2 Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing 210008, China; [email protected] Abstract: The controversy about a Cambrian "explosion" of morphological disparity (followed by decima­ tion), eladogenesis and fossilization is of central importanee for the history of life. This paper revisits the con­ troversy (with emphasis in onychophorans, which inelude emblematic organisms such as Hallucigenia), pre­ sents new data about the Chengjiang (Cambrian of China) faunal community and compares it and Ihe Burgess Shale (Cambrian of Canada) with an ecologically similar bul modern tropical marine site where onychopho­ rans are absent, and with a modern neotropical terrestrial onychophoran community. Biovolume was estimated from material collected in Costa Rica and morphometric measurements were made on enlarged images of fos­ sils. CaJTlbriantropical mudflats were characterized by the adaptive radiation of two contrasting groups: the va­ gile arthropods and the sessile poriferans. Arthropods were later replaced as the dominant benthic laxon by polychaetes. Vagility and the exoskeleton may explain the success of arthropods from the Cambrian to the mo­ dernmarine and terrestrial communities, both in population and biovolume. Food ecological displacement was apparen t in the B. Shale, but not in Chengjiang or the terrestrial community. When only hard parts were pre­ served, marine and terrestrial fossil deposits of tropical origin are even less representative than deposits produ­ ced by temperate taxa, Chengjiang being an exception. Nutrient limitations might explain why deposit feeding is less important in terrestrial onychophoran communities, where carnivory, scavenging and omnivory (asso­ ciated with high motility and life over the substrate) became moreimportant. Fossil morphometry supports the interpretation of "lobopod animal s" as onychophorans, whose abundance in Chengjiang was equal lo their abundance in modern communities. The extinction of marine onychophorans may reflect domination of the in­ faunal habitat by polychaetes. We conclude tha! (1) a mature ecological community structure was generalized during the Cambrian, and even biodiversity and equitability indices were surprisingly elose lo modern values; (2) the morphological diversity and geographic distribution of onychophorans indicate a significantpre-Cam­ brian evolutionary history which does no! support the "explosion" hypothesis; (3) disparity among phyla was not as important as the explosion-decimation model predicIs, bu! in the case of onychophorans, disparíty wÍl­ hin the phylum was greater than it is today and ¡ts reduction may have been associated with migration into the sediment when large predators evolved. Key words: Disparity, decimation, "explosion", community ecology, feeding, habitat, fossil, Metazoa, evolu­ lion, Chengjiang, Burgess Shale, Cambrian, Recent, Costa Rica. The evolution of the earliest metazoans me nts have centered on how representative the has been the subject of debate in recent years, fossil record is and the usefulness of techni­ particularly regarding the speed of change and ques such as cladism and specially molecular the diversity of general body plims (Hou and analysís (Bergstrom 1986) whose varied re­ Bergstrom 1995, Fortey et al. 1996). Argu- sults have shown it to be premature (Hou and 334 REVISTA DE BIOLOGÍA TROPICAL Bergstrom 1995, Fortey et al. 1996, Abouheif comm.) and a more meaningful comparison of et al. 1998, Valentine et al. 1999). the Cambrian tropical community requires in­ Early century attempts to place sorne dif­ formation about modero tropical biotas becau­ fícult taxa into extant phyla were abandoned se the best known Cambrian faunas were tropi­ for the proposal of new phyla in the 1970's, cal (Hou et al. 1991, Monge-Nájera 1995). but recent work with better fossil material re­ In the last decade, the Chinese Cheng­ sulted in another change of opinion, and now jiang fauna mentioned only briefly in a 1985 much material has been reassigned to known review of Cambrian soft-bodied faunas or "la­ phyla (Hou and Bergstrom 1994, Hou et al. gersUitten" (Conway Morrís 1985) has beco­ 1995, Fortey el al. 1996, Sprinkle and Co­ me a mayor source of significant díscoveries, llins 1998). among them, six of the nine species of fossil Limítations of the fossil record are less onychophorans reliably described to date important when softparts have been preserved (Hou et al. 1991, Chen et al. 1995, Hou and (Hou 1999). "Lobopod" worms are among the Bergstrom 1997). most interesting animal groups found in Cam­ It ¡neludes marine fauna of approxima­ brian soft-bodied communities. For almost a tely upper Atdabanian dating, about 10 mi­ century authors have disagreed about their ta­ llíon years older than B. Shale, thus provi­ xonomíc position, although recent studies have ding a unique view of a fauna that lived supported the hypothesis that they were early shortly after the so caBed "Cambrian explo­ ancestors of onychophorans (Hou and Bergs­ sion", a characteristic of the greatest interest trom 1995, Monge-Nájera 1995). even if such an "explosion" never took place Modern onychophorans are part of terres­ (Fortey et al. 1996). trial ecosystems in tropical and south tempera­ The Chengjiang fauna occurs in the te regions (Monge-Nájera 1994 a, b), but the Yu' anshan Member of the Lower Cambrian Cambrian species were marine and basically Qiongzhusi Formation in Yunnan Province, tropical (Hou & Bergstrom 1995, Monge-Ná­ but ít was first discovered in Chengjiang jera 1995, 1996). County, about 50 km from Kunming, the capi­ The only detailed quantitative synecologi­ tal of Yunnan Province (southeast China). The cal study of a fossil community with onychop­ good preservation of Chengjiang specimens horans described Burgess Shale (B. Shale) of significantly improved the reconstruction of southwest Canada (Briggs and Whittington several taxa (e.g. Hou et al. 1991, Hou and 1985, Conway Morrís 1986). However, at that Bergstrom 1994, Hou et al. 1995, Rigby and time no appropriate tropical data were availa­ Hou 1995). Despite the geographic and chro­ ble for comparison. Conway Morris (1986) ba­ nological distance, both sites have sorne gene­ sically compared B. Shale with temperate ra in common and were dominated by arthro­ communities and coneluded that they were si­ pods; differing mainly in the rarity of deuteros­ milar, particularly in the proportion of preda­ tomes (a chordate and a crinoid have been tors, in the presence of a few species that do­ found ú�cently, H.x., pers. observo 1999) and minate numerically, and when compared with the lower proportion of trilobites in Cheng­ modern deep sea communities, in the occu­ jiang (Hou el al. 1991). rrenceof certain types of ancient taxa (Conway This fauna ineluded a variety of inverte­ Morrís 1986). brates preserved in extraordinary detail (Jin et It ís now known that there are differences al. 1993, Hou and Bergstrom 1994, Hou et al. between tropical and temperáte mudflat com­ 1995, Rigby and Hou 1995). Sorne of these munities (for example, tropical communities have been identified as onychophorans, also tend to have smaller populations of each taxon known as "lobopods", "lobopodians" or "on­ and also individuals with smaller bodies: Alon­ copodophores" (Hou el al. 1995, Monge-Ná­ gi 1989, Vargas 1996, J.A. Vargas 1999 pers, jera 1995, Chen el al. 1995). However, the INTERNATIONAL JOURNAL OF TROPICAL BIOLOGY AND CONSERVATION 335 identification of these animals, with which (1987, 1988a). The fossil communities deve­ most authors from temperate countries are not loped at a greater depth than the P. Morales familiar, is often difficult (e.g. Rhebergen and community, but still within the photic zone, Donovan 1994). so the overall patterns (precisely the type of This paper presents new data concerning patterns studied here) are not expected to dif­ the Chengjiang community and compares it fer greatly, the main probable difference and B. Shale with a similar but modern tropi­ being the greater physiological stress in the cal marine site where onychophorans are ab­ intertidal P. Morales habitat (J.A. Vargas sent, and with a modern neotropical terrestrial 1999 pers. comm.). Standard ecological indi­ onychophoran community. Terrestrial com­ ces, biodiversity (H) and equitability (J) we­ munities are expected to be different from re calculated from number of specimens per marine communities, and their inclusion here taxon reported by Vargas (1987, 1988a), requires justification: the Coronado terrestrial Conway Morris e 1986) and Hou et al. community was included because this paper e 1991). Habitat, fossilization potential and stresses our common group of interest: ony­ body length information was provided by chophorans, which were marine in the Cam­ Vargas (1995 pers. comm., Table 1). Biovo­ brian and are terrestrial in the Presento We lume was estimated ftom the Vargas collec­ wanted to examine how the assemblages in tions and from ten samples collected by the which they have evolved differed from the senior author in November 1992 with a cy­ beginning of the phylum to the present.
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