Geotimes
Highlights
Invertebrates
Warren Allmon

Invertebrate paleontology, at least in some areas, appears to be in a period between revolutions: it is a time of growing consolidation of new observations around existing hypotheses, and a time of strengthening syntheses of previously disparate ideas and data. Conspicuous examples are mass extinctions and the Cambrian-Precambrian boundary. Whether or not this will in retrospect be seen as an interval of Kuhnian "normal science" in these areas or as simply temporary plateaus of consensus, for the moment at least, some things are becoming clearer. In other areas, such as marine evolutionary paleoecology and Phanerozoic diversity patterns, there appear to be fewer clear points of consensus and directions are harder to discern.

The Cambrian-Precambrian boundary used to be a virtual black hole of paleontological insight, but no longer. A combination of stunning new fossils, entry into the field of numerous young researchers, and synthesis of data from geology, paleontology and biology has at last yielded a plausible and detailed scenario for events at this most crucial of stratigraphic boundaries. Instead of accumulating as lists of exceptions, many new observations are falling into place. As Andrew Knoll says in his new book Life on a Young Planet. The first three billion years of evolution on Earth (released earlier this year), the field has matured spectacularly over the past decade.

The Snowball Earth hypothesis continues to be taken very seriously by many as a potentially crucial environmental perturbation in the interval leading up to the Cambrian Explosion. (Writing in the October 2002 Geology, however, Jonathan Leather and colleagues offer evidence they say contradicts the Snowball Earth theory.) The link between evolution and this global deep-freeze may be indicated by the amazing phosphatic fossils from the Doushantuo Formation of China, which date to either just after or just before the final Neoproterozoic glaciation (about 600 million years ago). These tiny beautifully preserved fossils appear to represent algae and non-bilaterian animals, such as sponges, according to a report by Schuhai Xiao and colleagues at the Annual Meeting of the Geological Society of America (GSA) in Denver in November 2002. (In related work, Derek Martin and colleagues, writing in Geology, January 2003, v.31, n. 1, p. 39, showed experimentally that invertebrate eggs can be mineralized rapidly by phosphorus or calcium, verifying the mode of fossilization of the Doushantuo fossils.) The Doushantuo fossils, in other words, are not "The Explosion", but are succeeded by the much more complex Ediacara biota, which may mark the explosion's beginning.

After decades of uncertainty about their age, Ediacara fossils are now known to extend from 565 to 543 million years ago, but a new report by Guy M. Narbonne and James G. Gehling (Geology, January 2003, v.31, n.1, p. 27) of Ediacaran fossils from Newfoundland extends this range downward, closer to the end of the Snowball. Other presentations at the GSA meeting (as well as a comprehensive review of molecular clock data by Andrew Smith and Kevin Peterson (Annual Review of Earth and Planetary Sciences, 30: 65, 2002) bolster a synthesis that has been gaining support for several years: the Cambrian-Precambrian boundary marks a real evolutionary event, a major diversification of multicellular animals within phyletic lines that had existed for perhaps several hundred million years before the boundary.

One major element of this renaissance in understanding the early Phanerozoic has been the synthesis of paleobiology and evolutionary developmental biology into the thriving subfield known as "evo-devo." Paleontologists are now fully seated at this biological high table, as evidenced by increasingly frequent papers published in journals such as Development (see e.g., Douglas H. Erwin and Eric Davidson, 2002, v. 129, p. 3021). The reasons for this collaboration are clear: paleontologists have unique insights and data to offer to developmental biologists. Amidst the 50th anniversary of James Watson and Francis Crick's description of the double helix structure of DNA, this collaboration also applies to molecular biology (see e.g., Mary Higby Schweitzer, Palaeontologica Electronica, 2002, n. 2); the terms "molecular paleontology" and "paleogenomics" are even being bandied about in some geology departments now.

Mass Extinctions. Research continues to be active on all of the "Big 5" mass extinctions, as well as other smaller events, but one has the impression that researchers are tidying up, rather than making major new breakthroughs or addressing major new controversies. The Eocene-Oligocene event is the subject of a newly published volume edited by Donald R. Prothero and colleagues, From Greenhouse to Icehouse: the marine Eocene-Oligocene transition (2003, Columbia University Press), a book that resulted from a 1999 Penrose Conference that emphasized climatic cooling as a major cause. A symposium at the 2002 GSA meeting considered the Chesapeake Bay impact structure, which appears to date to around the Eocene-Oligocene boundary, as a possible "smoking gun" for an extraterrestrial cause for the event. A report by Mark Boslough and Peter Fawcett (Journal of Geophysical Research Atmospheres, v. 107, n. D15, p. ACL 2) suggests that such an impact may have formed a Saturn-like ring of debris around Earth, contributing to cooling.

Based on the redating of a crater in the Ukraine, a report in Meteoritics and Planetary Science (v. 37, n. 8, p. 1031) suggests multiple impacts at the end of the Cretaceous, in addition to the famous one at Chicxulub, Mexico. As a measure of the state of the art, the latest in the series of GSA special papers on impacts and mass extinction (Catastrophic events and mass extinctions: impacts and beyond, 2002, Christian Koeberl and Kenneth G. MacLeod, eds.) is the thickest yet (746 pages!), and completely takes for granted the occurrence and importance of extraterrestrial impacts in the history of life.
Disagreements continue on other mass extinction episodes, but direction of the conversation is hard to discern. Based on carbon and nitrogen isotopic disturbances, Mark A. Sephton and colleagues (Geology, December 2002, v. 30, n. 12, p. 1119) argue for a multistep event at the end of the Triassic, tied primarily to ocean anoxia. An analysis by P. McAllister Rees (Geology, September 2002, v. 30, n. 9, p.827) of fossil plant data before and after the Permian-Triassic boundary points away from a single extraterrestrial impact as a likely cause.

Marine Evolutionary Paleoecology. A symposium in honor of Richard Bambach at the Denver GSA meeting featured a number of speakers endorsing a view expounded by Bambach and others in recent years that nutrient supply to the oceans has increased during the Phanerozoic, and that this has played a major role in the evolution of marine communities. The much-discussed idea of "coordinated stasis", which suggests that many or most taxa in communities persist for long periods and then are abruptly replaced by others, continues to generate controversy. A study by Nicole Bonuso and colleagues (Geology, v. 30, n. 12, p. 1055) argues that there are significant changes — rather than stability — in the taxonomic composition and ecologic structure of macrofaunal communities in the type area for the theory — the Middle Devonian Hamilton Group of New York. This report appears likely to generate rejoinders from coordinated stasis supporters in the near future.

Phanerozoic diversity. The path ahead is murky on the topic of the history of biological diversity. The iconic Sepkoski curve that has served as a paleobiological paradigm for more than two decades continues to be scrutinized by researchers concerned about sampling strategies and problems of scale, geography and phylogeny. Contributors to a special section of an issue of Paleobiology (v. 29, n. 1) debated all of these aspects of the subject. Phanerozoic diversity is likely to be an ongoing topic of research (and probably uncertainty) for some time to come.

Biggest and Oldest. Amidst consideration of the big picture, the particulars of the field continue to fascinate. The largest known trilobite, found in northern Canada and figured on the cover of Geology in October 2000, finally has an appropriate name for its size (an incredible 70 cm): Isotelus rex. It was described by David M. Rudkin and colleagues in Journal of Paleontology (v. 77, n. 1, p. 99). The oldest known arthropod tracks from a Cambrian-Ordovician sandstone in Ontario were described by Robert B. MacNaughton and colleagues (Geology, May 2002, v. 30, n. 5, p. 391). The oldest terrestrial life may have been microbial mats preserved in 1 billion year old rocks in Scotland, according to a report by A.R. Prave in Geology (v. 30, September 2002, n. 9, p. 811).

Milestones. It was also a year of sadness over the death in May 2002 of Stephen J. Gould. Among his many other accomplishments, Steve was a great invertebrate paleontologist who fundamentally changed how we view and pursue our field. His 1,400 page magnum opus and career summation appeared just two months before his death (The Structure of Evolutionary Theory, 2002, Harvard Univ. Pres). Two other volumes (one on baseball and one on the relationship between science and the humanities) have been published posthumously. See Journal of Paleontology (v. 76, n. 6, p. 937), Geotimes, May 2002, and www.stephenjaygould.org.

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Allmon is Adjunct Professor Of Earth and Atmospheric Sciences of paleontology at Cornell University and Director of the Paleontological Research Institution. E-mail him at wda1@cornell.edu.

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