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4.3 Understanding the assembly of animal body plans in the context of the Cambrian explosion (Gehling, Jacobs, Kouchinsky, Porter, Runnegar, Webster)
The Cambrian Period witnessed one of the most significant events in the history of life: an exponential increase in animal diversity and complexity, commonly referred to as the "Cambrian explosion". Despite recent advances in our understanding of animal relationships (Peterson and Eernisse 2001) and Cambrian geochronology (Bowring and Erwin 1998), important questions remain, perhaps the foremost of which is, what triggered the Cambrian explosion, and what makes it so unusual? Addressing these questions requires that we understand not only the steps by which early animal body plans evolved, but also the extrinsic and intrinsic controls on that evolution. We will study four topics that will elucidate these issues: (1) The stratigraphic record of the first multicellular (Ediacaran) animals (Gehling, Runnegar) (2) The nature of the developmental program controlling skeleton formation (Jacobs, Kouchinsky) (3) The evolution of developmental constraints in early Cambrian trilobites (Webster) (4) The sequence of body plan evolution in the lophotrochozoan phyla (Porter, Runnegar). These studies will combine insights from evolutionary developmental biology and data from the fossil record to obtain a clearer understanding of the Cambrian explosion. 4.3.1 Ediacaran biodiversity: Prelude to the Cambrian explosion Ediacaran organisms have been biologically enigmatic since they were first discovered (and thought to be pseudofossils) in Newfoundland during the 19th century (Gehling et al. 2000). However, recent discoveries of numerous new fossils at remarkably rich sites in the Flinders Ranges of South Australia, in southern Namibia, on the shores of the White Sea, and in Newfoundland have demonstrated that the Ediacaran biotas really do record the initial diversification of multicellular animal life. What is particularly striking, is that the older assemblages (ca. 565 Myr old) lack evidence in the form of body fossils and trace fossils for animals of bilaterian grade (mobile, bilaterally symmetrical, cerebral). In contrast, there is ample evidence for bilaterian grade animals in the younger assemblages worldwide. Ediacaran fossils are no longer limited to single stratigraphic horizons that are isolated by unfossiliferous strata from the succeeding Cambrian explosion. In Newfoundland, for example, Ediacaran fossils have been found beneath virtually every volcanic ash layer (~65 in all) over some 2.5 km of stratigraphic thickness (Narbonne and Gehling 2003). Furthermore, the oldest of these fossiliferous levels is not far above the Gaskiers Tillite, arguably the youngest (580 Myr; Bowring et al. 2002) of the Neoproterozoic snowball Earth glacial events. The time has come to put all of this globally-distributed paleontologic and biostratigraphic information together in order to: (1) Test the idea that the last one or two Snowball Earth glaciations were implicated in the appearance and diversification of the Ediacaran biota. (2) Document the progressive evolution of biocomplexity through the terminal Proterozoic period (soon to be formally named the Ediacaran). (3) Explore further the connections with the Cambrian explosion. Gehling will conduct studies of the most important sites outside Australia. 4.3.2 The evolution of mineral skeletons The evolution of skeletons is thought to have fed back through predator-prey evolution to increase specificity of interactions that contributed to the rapid diversification of Cambrian bilaterian animals. Preliminary work (Jacobs et al. 2000) suggests that aspects of the growth of shells and other mineralized skeletal elements are under common developmental control in a wide range of living animals. Although skeletal ultrastructures and materials in different animal phyla are distinct, common features of organic matrices found with skeletons suggest that the process may have a single common ancestry. Homology of bilaterian skeleton formation, if proven, would suggest that the onset of skeletogenesis played a critical role in the Cambrian radiation. To better understand the relationship between the evolution of bilaterian skeletogenesis and the Cambrian explosion, we will: (1) explore the relationships of developmental genes that appear to be associated with skeletogenesis in unstudied animals; (2) compile all available ultrastructural information pertaining to setae, spicules, and similar skeletal precursor organs in order to develop a model of the evolutionary origin of the simplest skeletal elements; and (3) use this model to choose candidate genes for further study. The engrailed gene, best known for its role in bounding the exoskeletal units of arthropod segments, appears to bound skeletal elements in other invertebrate phyla including shells of molluscs, setae of annelid polychaetes and the arm plates of brittle star echinoderms (Jacobs et al. 2000, Seavers et al. 2002). Thus, the engrailed gene is one of several clues linking the process of skeletogenesis across a range of bilaterian animals. To assess the breadth of this phenomenon we propose to measure the expression of engrailed in related phyla that have mineralized hard parts, including brachiopods, echiurids, sipunculids, and tube dwelling annelids. The engrailed gene has already been sequenced from echiurids and sipunculans.
4.3.3 Role of constraints on animal development through morphometric studies of Cambrian trilobites The magnitude and rate of evolutionary innovation associated with the Cambrian explosion was not sustained in post-Cambrian times. This implies either ecological constraint, where filled niches limited opportunity, or developmental constraint, where evolution of development limited the ability of organisms to evolve new morphology regardless of ecological opportunity. Cambrian trilobites are the best group to test competing ecological and developmental explanations for the post-Cambrian decrease in evolutionary innovation. Detailed reconstruction of paleoenvironments across significant Cambrian extinction events permit control for ecological factors and assessment of responses to changes of diversity that reduce the degree to which niches are filled. This, in combination with detailed examination of the developmental series through time, permits a formal comparison of the ecological and developmental limits on morphological diversification. The research will use detailed morphometry (Webster et al., 2001; Webster, 2003) to investigate new collections of high-quality, silicified (uncompacted) trilobites made at high stratigraphic resolution both across and between extinction events. These studies will lead to a greater understanding of the unique patterns of metazoan evolution associated with the Cambrian explosion.
4.3.4 Using stem group taxa to order characters important in body plan evolution Modern mobile animals, technically known as bilaterians, are characterized by complex, functionally and developmentally integrated anatomies. Insight into how such complexity arose can be gained through study of the Cambrian fossil record, which contains a number of "oddball" animals that have unfamiliar morphologies or lack features found in modern groups. Recently it has been recognized that these problematical animals are not members of extinct phyla (Gould 1989). Instead, they are often stem-group representatives of modern groups. When they are placed within a phylogenetic framework, stem-group taxa can help reveal the evolutionary steps through which modern body plans arose (Budd 2002). Halkieriids are among the most widespread, diverse, and abundant early Cambrian "oddball" animals. These scaly organisms are either stem-group members of the lophotrochozoans or, more likely, of one of its contituent phyla such as the molluscs, brachiopods, or annelids (Annelida, Brachiopoda, Mollusca; Jell 1981; Bengtson 1992; Conway Morris and Peel 1995; Runnegar 2000; Holmer et al. 2002). Porter and Runnegar will study the morphology and ontogeny of halkieriids and their relatives in unprecedented detail. First, ultrastructural studies will be conducted to reconstruct the original composition, morphology, and structure of halkieriid sclerites (Porter 2002; in press); variation within the group necessitates comprehensive representation of halkieriid taxa. Second, 3D models of halkieriid scleritomes, constructed from laser scanned images of well-preserved sclerites (Lyons et al. 2002) and guided by data from articulated specimens (Conway Morris and Peel 1995), will be used to test competing hypotheses of halkieriid ontogeny and skeletal growth. These data, combined with those obtained through similar investigations of fossil and modern lophotrochozoan groups, will lead to a detailed cladistic analysis of selected lophotrochozoans and, hopefully, provide some insight into how complex body plans arose. Halkieriid specimens from Australia and Siberia are in existing collections. Scanning electron microscopy and laser scanning of specimens, and fieldwork in China will allow us to obtain new collections from Early Cambrian horizons known for exceptional preservation of halkieriids and other problematical fossils.
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