A Sturtian A
Figure 10.7 Stratigraphie distribution of the Ediacara biota. Solid triangles, glaciations; C, calcified metazoans; T, position of the Twitya disks. (Based on Narbonne 2005.)
shallow-water siliciclastic sediments, consisting of clasts of silicic-rich rocks, or volcanic ash, more rarely carbonates or even turbi-dites. The sediments were deposited during specific events, such as a storm, and are usually termed event beds. Deep-water biotas are also known such as those from Mistaken Point in Newfoundland. The style of preservation plays an important role in understanding these organisms (Narbonne 2005). The widespread development of algal mats, prior to the Cambrian substrate revolution (see p. 330), suggests that these too aided preservation, sometimes providing "death masks", of these non-skeletal organisms.
Although morphologically diverse, the Edi-acaran organisms have many features in common. All were soft-bodied, with high surface to volume ratios and marked radial or bilateral symmetries. These thin, ribbon-shaped animals may have operated by direct diffusion processes where oxygen entered through the skin surface, so gills and other more complex internal organs were perhaps not required. Most Ediacaran organisms have been studied from environments within the photic zone; many collected from deeper-water deposits are probably washed in. Provincialism among these Upper Proterozoic biotas was weak with many taxa having a nearly worldwide distribution. It is possible that the flesh of the Ediacaran organisms lit tered areas of the Late Precambrian seafloor; predators and scavengers had yet to evolve in sufficient numbers to remove it.
Traditionally the Ediacaran taxa, a collection of disks, fronds and segmented bodies, have been assigned to a variety of Phanerozoic invertebrate groups on the basis of apparent morphological similarities. In many cases considerable speculation is necessary and many assumptions are required to classify these impressions. Most of the species have been assigned to coelenterate groups, although some taxa have been identified as, for example, arthropods or annelids. Michael Fedonkin (1990), however, suggested a form classification based on the morphology and structure of these fossils. Key areas of his classification are summarized in Box 10.3 and typical examples illustrated in Figure 10.8. The bilateral forms were probably derived from an initial radial body plan. The concept and classification of the Ediacara biota is in a state of flux and Fedonkin's classification is one of a number of attempts to rationalize the group, assuming the majority are in fact animals. Some have argued, nevertheless, that the Ediacarans are organisms unrelated to modern metazoans (Box 10.4), or are even Fungi.
Box 10.3 The Ediacaran animals: a form classification
RADIATA (RADIAL ANIMALS)
Three main classes are defined. Most colonial organisms in the fauna, for example Charnia, Char-niodiscus and Rangea, are assigned to coelenterates and were part of the sessile benthos. The affinities of these animals have been debated in detail, but their close similarity to the sea-pens suggests an assignment to the pennatulaceans.
• These animals have a concentric body plan with a large disk-shaped stomach and the class includes mostly sessile forms such as Cyclomedusa and Ediacaria. About 15 species of jellyfishlike animals have been described and in some, for example, Eoporpita tentacles are preserved
• Medusa-like animals with more complex internal structures, for example Hielmalora Class TRILOBOZOA
• Characterized by a unique three-rayed pattern of symmetry. Tribrachidium and Albumares are typical members of the group
BILATERIA (BILATERAL ANIMALS)
This division contains both smooth and segmented forms. Smooth forms
• These morphotypes are rare. They include Vladimissa and Platypholinia, which may be turbel-larians, a type of platyhelminthes worm
• Much of the Ediacara fauna is dominated by segmented taxa inviting comparisons with the annelids and arthropods. Dickinsonia, for example, may represent an early divergence from the radial forms whereas Spriggina, although superficially similar to some annelids and arthropods, possesses a unique morphology
There is little doubt that the Ediacara biotas dominated the latest Precambrian marine ecosystem, occupying a range of ecological niches and pursuing varied life strategies probably within the photic zone (Fig. 10.10). There is no evidence to suggest that any of the Edia-caran organisms were either infaunal or pelagic, thus in contrast to the subsequent Cambrian Period, life was restricted to the seabed. It is also possible that these flattened organisms hosted photosymbiotic algae, maintaining an autotrophic existence in the tranquil "garden of Ediacara" as envisaged by
Mark McMenamin (1986), although this model has its opponents. McMenamin considered that the ecosystem was dominated by medusoid pelagic animals, and that attached, sessile benthos and infaunal animals were sparse; the medusoids have been reinterpreted as bacterial colonies or even holdfasts. Food chains were thus probably short and the trophic structure was apparently dominated by suspension and deposit feeders.
Although provincialism was weak among the Ediacara biotas, three clusters have been rec-
Cyclomedusa (a) Radiata
Figure 10.8 Some typical Ediacara fossils: (a) the Radiata, which have been associated with the cnidarians, and (b) the Bilateria, which may be related to the annelids and arthropods. Ediacaria (x0.3), Charnia (x0.3), Rangea (x0.3), Cyclomedusa (x0.3), Medusinites (x0.3), Dickinsonia (x0.6), Spriggina (x1.25), Tribrachidium (x0.9) and Praecambridium (x0.6). (Redrawn from various sources by Anne Hastrup Ross.)
ognized based on multivariate biogeographic analysis (see p. 45) by Ben Waggoner (2003): (i) the Avalon assemblage is from deep-water, volcaniclastic settings in eastern Newfoundland; (ii) the White Sea assemblage represents the classic Vendian section in the White Sea, Russia; and (iii) the Nama assemblage is a shallow-water association from Namibia, West Africa. Unfortunately the distribution of these assemblages does not match any paleo-geographic models for the period and the clusters may rather represent a mixture of environmental and temporal factors (Grazh-dankin 2004).
The Ediacara biota, as a whole, became extinct about 550 Ma. Nevertheless, in terms of longevity, the ecosystem was very successful and a few seem to have survived into the Cambrian. The rise of predators and scavengers together with an increase in atmospheric oxygen may have at last prevented the routine preservation of soft parts and soft-bodied organisms. More importantly, the Ediacara body plan offered little defense against active predation. There is abundant evidence for Cambrian predators: damaged prey, actual predatory organisms and the appearance of defense structures, such as trilobite spines and multielement skeletons. All suggest the existence of a predatory life strategy that was probably established prior to the beginning of the Cambrian Period. The Proterozoic-Cambrian transition clearly marked one of the largest faunal turnovers in the geological record, with a significant move from soft-bodied, possibly photoautotrophic, animals to heterotrophs relying on a variety of nutrient-gathering strategies. It is, however, still uncertain whether a true extinction, or the slamming shut of a taphonomic window, accounted for the disappearance of the Edia-cara biota from the fossil record.
Although the Ediacara biotas were overwhelmingly dominated by soft-bodied organisms,
The apparently unique morphology and mode of preservation of the Ediacara biota has led to much debate about the identity and origins of the assemblage. Adolf Seilacher (1989) argued that these organisms were quite different from anything alive today in terms of their constructional and functional morphology (Fig. 10.9). Apart from a distinctive mode of preservation, the organisms all share a body form like a quilted air mattress: they are rigid, hollow, balloon-like structures with sometimes additional struts and supports together with a significant flexibility. Seilacher termed the Ediacaran organisms vendobionts, meaning organisms from the Vendian, and he speculated about their unique biology. Reproduction may have been by spores or gametes. The skin must have been flexible, although it could crease and fracture, and it must have acted as an interface for diffusion processes. This stimulating and original view of the Ediacarans, however, remains controversial. Several members of the Vendobionta have been interpreted as regular metazoans, suggesting a less original explanation for the Ediacara group.
Leo Buss and Adolf Seilacher (1994) suggested a compromise. Their phylum Vendobionta includes cnidarian-like organisms lacking cnidae, the stinging apparatus typical of the cnidarians. Vendobionts thus comprise a monophyletic sister group to the Eumetazoa (ctenophorans + bilaterians). This interpretation requires the true cnidarians to acquire cnidae as an apomorphy for the phylum.
The vendobiont interpretation has opened the doors for a number of other interpretations and the understanding of Ediacaran paleobiology is as open as ever: some authors have suggested the Edia-carans are giant protists, lichens, prokaryotic colonies or fungus-like organisms. However most agree that the Ediacara assemblage includes some crown- and stem-group sponges and cnidarians, a conclusion proposed by Sprigg in the late 1940s. This is supported by biomarker and molecular clock
minute conical shells were also present in some Ediacaran successions, including localities in Brazil, China, Oman and Spain. Cloudina was possibly a cnidarian-type organism with a unique shell structure having new layers forming within older layers. Moreover it was probably related to a suite of similar shells such as Sinotubulites, Nevadatubulus and Wyattia that also occurred close to the Pre-cambrian-Cambrian boundary. In addition to complex multicellularity, modularity, locomotion and predation, biomineralization was already far advanced in the Late Proterozoic, providing a link with what was to follow in the Nemakit-Daldynian assemblages of the earliest Cambrian. Some of the shells of Cloudina are bored, suggesting the presence of predators (Fig. 10.11), although it is not certain the animals were still living when bored.
Small shelly fauna_
A distinctive assemblage of small shelly fossils has now been documented in considerable detail from the Precambrian-Cambrian transition; the assemblage is most extravagantly developed in the lower part of the Cambrian defined on the Siberian platform, traditionally called the Tommotian, which gives its name
to the fauna. A great deal is now known about the stratigraphic distribution and paleobio-geography of these organisms through current
Figure 10.12 Elements of the Tommotian-type or small shelly fauna. Magnification approximately x20 for all, except Fomitchella which is about x40. (Based on various sources.)
Hertzina interest in the definition of the base of the Cambrian System. Nevertheless, the biological affinities of many members of the Tom-motian fauna have yet to be established. The assemblage, although dominated by minute species, together with small sclerites of larger species, represents the first major appearance of hard skeletal material in the fossil record, some 10 myr before the first trilobites evolved (see p. 363).
This type of fauna is not restricted to the Tommotian Stage; small shelly fossils are also common in the overlying Adtabanian Stage (see below) and similar assemblages of mainly phosphatic minute shells have been reported from younger condensed sequences in the Paleozoic. The shell substance of the carbonate skeletons within the fauna seems to have been controlled by the ambient seawater chemistry; Nemakit-Daldynian assemblages were mainly aragonite, whereas younger shells were mainly calcitic (Porter 2007). Tommotian-type faunas probably finally disappeared with the escalation of predation during the Mesozoic.
Some scientists such as Stephen Jay Gould suggested the less time-specific term, small shelly fossils to describe these assemblages. The fauna is now known to include a variety of groups united by their minute size and sudden appearance near the base of Cam brian. The small shelly fauna probably dominated the earliest Cambrian ecosystems when many metazoan phyla developed their own distinctive characteristics, initially at a very small scale. Nevertheless, some of this small size may be a preservational artifact, since phosphatization only works at a millimeter scale.
Many of the Tommotian skeletons (Fig. 10.12) were retrieved from residues after the acid etching of limestones; thus there is a bias towards acid-resistant skeletal material in any census of the group as a whole. Moreover, there is currently discussion concerning whether the acid-resistant skeletons of the Tommotian-type animals were primary constructions or secondary replacement fabrics. Or perhaps these shells survived in the sediments because of particular chemical conditions in the oceans at the time that allowed phosphatic fossils to survive (Porter 2004). The Tommotian animals had skeletons composed of a variety of materials. For example, Cloudina and the anabaritids were tube-builders that secreted carbonate material, whereas Mobergella and Lapworthella consisted of sclerites comprising organisms that secreted phosphatic material; Sabellidites is an organic-
walled tube possibly of an unsegmented worm.
Many of the Tommotian animals are form taxa (that is, named simply by their shapes) because the biological relationships of most cannot be established and often there are few clues regarding the function and significance of each skeletal part. Most are short-lived and have no obvious modern analogs. Two groups are common - the hyolithelminthids have phosphatic tubes, open at both ends, whereas the tommotiids are usually phosphatic, cone-shaped shells that seem to belong in bilaterally symmetric sets.
Discoveries of near-complete examples of Microdictyon-like animals from the Lower Cambrian of China have helped clarify the status and function of some elements of the Tommotian fauna. These worms have round to oval plates arranged in pairs along the length of the body, which may have provided a base for muscle attachment associated with locomotion. As noted previously, many of the small shelly fossils are probably the sclerites of larger multiplated worm and worm-like animals (Box 10.5).
The Meishucunian Stage of South China has yielded some of the most diverse Tommotian-type assemblages in strata of Atdabanian age (see Appendix 1). Qian Yi and Stefan Bengt-son (1989) have described nearly 40 genera that belong to three largely discrete, successive assemblages through the stage. First, the Anabarites-Protohertzina-Arthrochites assemblage is dominated by tube-dwelling organisms such as Anabarites; the Siphongu-chites-Paragloborilus assemblage contains mobile mollusk-like and multiplated organisms together with some tube-dwellers and possible predators; whereas the Lapworth-ella-Tannuolina-Sinosachites association has mainly widespread multiplated animals.
Many of these fossils are known from Lower Cambrian horizons elsewhere in the world, highlighting the global distribution of many elements of the fauna. However, the three "community" types are rather mysterious, and probably represent different ecosystems, but it is hard to speculate further.
Although it is still unclear whether many of the Tommotian skeletons are single shells or single sclerites and the autecology of most groups is unknown, the assemblage was certainly the first example in evolution of a skel-etalized benthos. Very few of the Tommotian skeletal parts exceed 1 cm; nevertheless many shells were the armored parts of larger wormlike animals. And both mobile and fixed forms occurred together with archaeocyathans and non-articulate brachiopods. The microben-thos of the Tommotian was succeeded by a more typical Cambrian fauna, dominated by trilobites, non-articulate brachiopods, mono-placophoran mollusks and primitive echino-derms together with the archaeocyathans during the Atdabanian Stage (Fig. 10.14).
The Cambrian explosion suddenly generated many entirely new and spectacular body plans (Box 10.6) and coincides with the appearance of the Bilateria over a relatively short period of time (Conway Morris 1998, 2006). This rapid diversification of life formed the basis for Stephen Jay Gould's bestseller, Wonderful Life (1989), which took its title from the Frank Capra 1946 film It's a Wonderful Life. The rapid appearance of such a wide range of apparently different animals has suggested two possible explanations. The "standard" view is that the diversification of bilaterians happened just as fast as the fossils suggest, and that some reasons must be sought to explain why many different animal groups apparently acquired mineralized skeletons at the same time. An alternative view arose after initial molecular studies had suggested that animals diverged some 800 myr before the beginning of the Cambrian (e.g. Wray et al. 1996). If these molecular views were correct, then the absence of fossils of modern animal phyla through the Proterozoic would have to be explained by an interval of cryptic evolution of probable micro- and meioscopic organisms, living between grains of sand, operating beneath the limits of detection prior to the explosion (Cooper & Fortey 1998). Greater refinement of Cambrian stratigraphy, the taxonomy and phylogeny of key Cambrian taxa and their relative appearance in the fossil
Box 10.5 Coelosclerites, mineralization and early animal evolution
The coeloscleritophorans are an odd group of animals based on the unique structure of their sclerites that appeared first in the Tommotian (Fig. 10.13). The sclerites are made of thin mineralized walls surrounding a cavity with a small basal opening. Once formed, the sclerites did not grow and were secreted by the mineralization of organic material occupying the cavity. The sclerites have longitudinal fibers and overlapping platelets within the mineralized wall. These animals may be extremely important in understanding the origin of biomineralization and the fuse for the Cambrian explosion, as argued by Stefan Bengtson (2005). Coelosclerites may be structures that are not known in any living animal but that were shared by both the bilaterians and non-bilaterians and probably characterized both ecdysozoans and spiralians. Coelosclerites may then have been lost, possibly by progenesis (see p. 145) from the larval to juvenile stages. If these features were developed in larger bilaterians then it is possible that within the Ediacara fauna giant forms - tens of centimeters in length - lurked, adorned by spiny and scaly sclerites. This is a controversial but nonetheless stimulating view that adds even more variety to our interpretations of early metazoan evolution.
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