Rdb

in oxygen levels in the deep ocean (Canfield et al. 2007).

Body fossil evidence

Body fossils of basal metazoans in the Edia-caran Period are few and far between. The morphology of an early metazoan fossil must be clearly described and convincingly illustrated, different organs and tissues identified, and comparisons drawn with other extant and fossil organisms. Many Upper Precam-brian successions have been subjected to intense metamorphism and tectonism (see p. 48) and are now located in some of the Earth's mountain belts. The chances of finding adequately preserved fossils are slight. Nevertheless, the earliest undoubted metazoans occur within the widespread Ediacara biota (see p. 242) dated at approximately 600-550 Ma. Moreover the fact that a relatively advanced metazoan, the mollusk Kimberella, possibly equipped with a foot and radula (see p. 330), occurs within the Ediacara biota from southern Australia and Russia could suggest a history of metazoan evolution prior to the Ediacaran. But although a strong case can be made for a significant Proterozoic record for the cnidarians and sponges and perhaps some other metazoans, the Cambrian explosion still marks the arrival, center stage, of the bilateri-ans (Budd 2008).

Trace fossil evidence

Trace fossils are the behavior of organisms recorded in the sediment (see p. 510). By their very nature they occur in place and thus cannot be transported or reworked by currents. Nevertheless these too must be convincingly demonstrated as biogenic and the age of their enclosing sediments accurately determined. If and when metazoans developed locomotory organs, such as the molluskan foot, and digestive systems, we might expect to find burrows and trails together with fecal pellets. Records of trace fossils from rocks older than 1 Ga in India (Seilacher et al. 1998) and over 1.2 Ga in the Stirling biota of Australia (Rasmussen et al. 2002) generated considerable excitement (Fig. 10.2). Both suggested metazoan life older than 1 Ga but both are now considered questionable (Jensen 2003). The oldest undoubted locomotory

Figure 10.2 Putative trace fossils from the Precambrian of Australia, showing Myxomitodes, a presumed trail of a mucus-producing multicellular organism about 1.8-2 billion years old from Stirling Range, Western Australia. (Photo is approximately 65 mm wide.) (Courtesy of Stefan Bengtson.)

trace fossils are from about 550 Ma (Droser et al. 2002) from northwest Russia, whereas fecal strings have been reported from rocks some 600 Ma (Brasier & Mcllroy 1998) suggesting the existence of an ancient digestive system. In fact no convincing trace fossils are known from successions older than the Mari-noan glaciation (635 Ma), the second main icehouse event associated with snowball Earth (see p. 112).

Embryo fossil evidence

Fossil Neoproterozoic embryos are now known from a number of localities, although claims that they represent sulfur-oxidizing bacteria or that they are not embryos at all have their advocates. Some of the best studied examples are from the Doushantuo Formation, South China. The part of the formation yielding the embryos was first dated at approximately 580 Ma, predating much of the Edia-caran but postdating the Marinoan glaciation. Revised dates seem to suggest that the faunas are younger and that they overlap with the older Ediacaran assemblages. Cell division and cleavage patterns are obvious although it is difficult to assign the material to distinct metazoan groups in the absence of juvenile and adult forms. There are, however, a lack of epithelia even in clusters of over 1000 cells suggesting that the embryos examined are those, at best, of stem-group metazoans (Hagadorn et al. 2006); they could equally well be fungi or rangeomorphs (enigmatic frond-like fossils). Nonetheless the Doushan-tuo embryos, although unplaced taxonomi-cally, provide our earliest body fossil evidence for probable metazoan life, albeit very basal, and a fascinating insight into embryologic processes in deep time (Donoghue 2007) (Box 10.1).

Molecular evidence

Not only have the morphologies of organisms evolved with time, but so too have their molecules. This forms the basis of the concept of the molecular clock (see p. 133). The molecular clock has opened up tremendous possibilities to date, independently of direct fossil evidence, the times of divergence of say the mammals from the reptiles or the brachio-pods from the mollusks. Nevertheless, attempts to date the divergences of the various groups of metazoans have proved controver sial. For example, the last common ancestor of the bilaterians, the metazoan clade excluding the sponges and cnidarians, has been variously placed at anywhere between 900 and 570 Ma. Why is there such a spread of ages in a seemingly exact science? The rates of molecular evolution in various groups are unfortunately not constant. The vertebrates appear to have reduced their rates of molecular change through time. So, using the slow vertebrate rates of molecular evolution to calibrate the date of origin of Bilateria gives dates that are too ancient (900 Ma). On the other hand, using mean bilaterian rates of molecular evolution gives a date (570 Ma) that is more in keeping with evidence from the fossil record (e.g. Budd & Jensen 2000) and thus makes the Cambrian explosion much more of an explosion of animals rather than fossils (Peterson et al. 2004). Nevertheless the most recent molecular clock data (Peterson et al. 2008) suggest a major phase of metazoan radiation within the Ediacaran, prior to that in the Cambrian. This radiation probably set the agenda for metazoan macroevolution for the rest of geological time.

Box 10.1 Synchrotron-radiation X-ray tomographic microscopy

Fossil embryos from the Upper Neoproterozoic and Cambrian are providing some important clues about the origin and early evolution of the metazoans. They are, however, tiny and notoriously hard to study. Nevertheless Phil Donoghue and his colleagues (2006) are beginning to accumulate a large amount of new information on the composition, structure and cell division within these minute organisms together with their modes of preservation. Synchrotron-radiation X-ray tomographic microscopy (SRXTM) has provided a whole new way of scanning embryos without actually destroying them (Fig. 10.3). The embryos, most of them 1 mm across or smaller, are held steady in a high-energy beam of photons, and multiple "slices" are produced, spaced a few microns apart. Using imaging software, these slices can be combined to create a detailed three-dimensional model of the internal structure of the fossil. Embryos assigned to the bilaterian worm, Markuelia, together with Pseudooides, variously show the process of cell cleavage and development of possible blastomeres, clusters of cells produced by cell division after fertilization, rather than yolk pyramids, which are more typical of the arthropods. This high-tech methodology has already demonstrated a real prospect for identifying the animals themselves and charting their early stages of development, some 600 Ma. It also can reject the claims that such fossils were the planula larvae of cnidarians, minute bilaterians or the early stages of gastrulation (see p. 240) of hydrozoans or bilaterians. It has, however, been recently suggested that many of these embryonic structures were created by bacteria (see p. 190). But not all.

Read more about this topic at http://www.blackwellpublishing.com/paleobiology/.

Figure 10.3 Animal embryos from the Doushantou Formation, China. (a) Surface of embryo based on tomographic scans together with (b) an orthoslice revealing subcellular structures analogous to modern lipids and (c) an orthoslice at the boundary between two cells. (c, f) Two-cell embryo of the sea urchin Heliocidaris showing lipid vesicles for comparison. (e) Orthoslice rendering of a possible embryo revealing internal structures. (g-i) Models of tetrahedrally arranged cells. Relative scale bar (see top left): 170 |im (a-d, f), 270 |im (e), 150 |im (g-i). (Courtesy of Philip Donoghue.)

Figure 10.3 Animal embryos from the Doushantou Formation, China. (a) Surface of embryo based on tomographic scans together with (b) an orthoslice revealing subcellular structures analogous to modern lipids and (c) an orthoslice at the boundary between two cells. (c, f) Two-cell embryo of the sea urchin Heliocidaris showing lipid vesicles for comparison. (e) Orthoslice rendering of a possible embryo revealing internal structures. (g-i) Models of tetrahedrally arranged cells. Relative scale bar (see top left): 170 |im (a-d, f), 270 |im (e), 150 |im (g-i). (Courtesy of Philip Donoghue.)

Biomarker evidence

Biomarkers, essentially the biochemical fingerprints of life, have become increasingly important in astrobiology, where they have been sought in the quest for extraterrestrial life. But they are also of considerable importance in the investigation of Precambrian life (see p. 188), where other lines of evidence are lacking. Thus amino acids, hopanes, some types of hydrocarbons, evidence of isotopic fractionation in carbon (12C) and biofilms are strong indicators of life forms. More exciting is the fact that specific biomarkers may be related to particular groups of organisms. Significantly, biomarkers associated with meta-zoan demosponges (see p. 262) have now been reported from rocks older than the Edia-caran, confirming the presence of basal meta-zoans at this time. But since the sponges are paraphyletic, biomarkers from the homoscle-romorph sponges (see p. 262) would also have to be present to prove the presence of the eumetazoans.

Neoproterozoic

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