There are lots of good reasons for being interested in Neoproterozoic geobiology and palaeobiology. It was during this half-billion-year run-up to the Phanerozoic that the Earth experienced some its greatest convulsions, including supercontinental breakup (and partial reassembly), recurrent bouts of glaciation, unparalleled fluctuations in the carbon cycle, increasing levels of atmospheric oxygen and, of course, the rise and radiation of animals. The emphasis on biology – whether geo or palaeo – is a heartening sign of the times, a recognition that palaeontologists, ecologists and evolutionary biologists have just as much to contribute to the understanding of planetary function as the commandeering geophysicists and geochemists. Biological innovation is of course what defines the end of the Neoproterozoic era, and it this fin de siècle that ties together the diverse contributions of this multi-authored, multidisciplinary volume.
Body fossils offer our most direct, if imperfect, account of ancient life. In Chapter 1 Susannah Porter reviews the early fossil record of (non-metazoan) heterotrophic eukaryotes, noting that heterotrophy is both the primitive condition of this domain and, in terms of kingdom-level diversity, by far the most common means of making a living. So it's curious that their Proterozoic record is so sparse – little more than some testate amoebae and possible fungi. Porter considers the possibility that the pattern is real, perhaps the consequence of a Meso–Neoproterozoic nitrogen crisis, but then moves on to more likely explanations such as preservational bias and the facile assumption that most Proterozoic microfossils are the remains of photoautotrophs.
Most Proterozoic microfossils are taxonomically and physiologically unresolved and consequently assigned to the Acritarcha, a waste-basket group of form-taxa that is perennially misrepresented as unicellular phytoplankton. In Chapter 2, Huntley and colleagues bypass much of this systematic trainwreck by carrying out a taxon-free morphometric analysis of the early acritarch record. Significantly, they find that acritarch disparity – the different ways in which these microfossils were constructed – remains conspicuously flat through most of the Proterozoic, only rising with the onset of the Ediacaran, and again in the early Cambrian. Using a similar approach, Xiao & Dong (Chapter 3) identify a broadly similar pattern among Proterozoic ‘macroalgae’. In both these studies the authors are impressed with how the patterns correspond to the Ediacaran–Cambrian radiations of animals, etc., but are unable to shake themselves clear of the current biogeochemical bandwagon that explains all evolutionary innovation in terms of ‘permissive environment’.
Whether or not the appearance of eumetazoans provides sufficient explanation for the many associated phenomena, there is no question that the Ediacaran rang in a fundamentally new style of macroecology and macroevolution (Butterfield, Reference Butterfield2007). In Chapter 4, Bottjer & Clapham present an overview of Ediacaran palaeobiology emphasizing the importance of fossil Lagerstätten in resolving this weirdly transitional world where microbial mats managed to coexist, even cooperate, with their metazoan invaders. Jensen et al. (Chapter 5) follow this up with a much more focused and critical look at the record of Ediacaran trace fossils, which have long been accepted as the definitive evidence for early triploblastic metazoans. Somewhat disturbingly, most of these structures turn out not to be trace fossils at all, though there is a residue of bona fide traces that squeaks the record of macroscopic motility back into the terminal Ediacaran.
Not very much is said in this volume about why animals might be more generally important, but Erwin (Chapter 6) provides an enormously useful primer of the developmental biology that goes in to building one, and two alternative models for what the last common ancestor of bilaterians might have looked like – only one of which is likely to express itself in the fossil record. The potential for much deeper, palaeontologically invisible, roots is also taken up by Hedges et al. (Chapter 7) who doggedly defend the results of their earlier molecular clock analyses. In explaining the pervasive absence of billion-year-old animal fossils Hedges et al. are happy to fall back on ‘permissive environments’, invoking insufficient oxygen as a barrier to large size, biomineralization and cuticularization. No attempt is made to explain the profound stasis of the acritarch record through this interval, however – compelling positive evidence for the absence of even small, non-preservable eumetazoans (Peterson & Butterfield, Reference Peterson and Butterfield2005).
Chapters 8 and 9 return to the geological record, with a view to gaining a finer temporal resolution of Neoproterozoic stratigraphy and events. Halvorsen presents a refreshingly frank account of what is known and not known from chemostratigraphic correlation, primarily δ13C, while Corsetti & Lorentz focus on the radiometric age constraints and global correlation within the mid-Neoproterozoic Cryogenian. In both instances both the data and the interpretations are in an obvious state of flux, though the case for recognizing two globally synchronous Cryogenian glaciations is looking decidedly shaky.
So is the volume successful? In principal, absolutely. This is very much the kind of multidisciplinary approach that is necessary to appreciate when and how biology revolutionized the surficial Earth system, yielding our modern, uniformitarian world. In practice it is not so clear. The range of topics covered is far from exhaustive and, as intimated above, falls somewhat short of reflecting the full range of recent opinion. Inevitably, the volume will also suffer from its own success, with most of the constituent fields advancing much more rapidly than can be tracked in a one-off, hardcover book. Its real value, then, lies in synthesis, and the focus it brings to one of the most fascinating intervals in Earth history. I look forward to the next edition.
