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1 May 2007 ONE STEP AT A TIME
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Amniote Paleobiology: Perspectives on the Evolution of Mammals, Birds, and Reptiles is a Festschrift honoring Jim Hopson, recently retired from his long-time post as a vertebrate paleontologist at Chicago University. From both the introductory remarks by the editors and the final chapter eulogizing Jim, it is clear that he has inspired great affection and intellectual respect among his students and collaborators, 24 of whom have contributed to the volume. In these days of intense pressure to acquire research grants, and competition to publish in the most prestigious journals, it is refreshing to have such a reminder of the overarching value of teaching. Much university research is frankly more of an enjoyable personal indulgence than a direct means of significantly addressing the needs of society. But using one's active research as an integral part of the process of teaching students how to explore problems, handle information, think about complex issues, and develop habits of intellectual rigor seems to me the most important, if underrated, aspect of the work of a university academic. How many more of Jim Hopson's former students no longer work on fossil vertebrates, but are nevertheless all the better equipped by his example for pursuing whatever other career they chose?

Sandwiched between the first and last chapters lies a collection of 13 specialist papers. It has to be said that the publisher's blurb verges on the disingenuous by seeming to imply that the book is a far more comprehensive study of fossil amniotes than it is. In fact, like Festschriften in general, it is akin to an issue of an academic journal in the subject, and it will be of direct value only to those who might subscribe to such an organ. To be precise, lest a purchaser be disappointed by the highly selective nature of the contents, there are four papers on mammal-like reptiles, four on mammals, three on dinosaurs, one on plesiosaurs, and one on the early Carboniferous Watcheria, a basal tetrapod (which was indulgently allowed into an amniote volume).

Of these papers, let me turn at once to the area over which the recipient of the Festschrift and I most overlap, the “non-mammaliaform synapsids,” as we are supposed to call the mammal-like reptiles of old. This is the extraordinary range of animal fossils that combine ever fewer primitive, reptilian characters with ever more mammalian ones, offering what is by far the best paleontological window onto how a new higher taxon arises. In chapter 5, Hans-Dieter Sues and Farish A. Jenkins Jr. provide a welcome description of the postcranial skeleton of the tritylodontid Kayentatherium, an event we have been awaiting for two or three decades. The tritylodontids are the most contentious of all the synapsid groups, illustrating as effectively as any taxon the problem of inferring phylogenetic relationships from fossil material. On the one hand, they have a number of mammalian characters, such as loss of the postorbital bar. On the other, they have characters of the highly specialized, herbivorous diademodontoid cynodonts, such as enlarged, multicusped postcanine teeth, suggesting that this group is not closely related to the lineage that led to the mammals.

Current cladistic practice necessarily requires that morphological characters be treated as independent of one another, and as having equal probabilities of evolving. This avoidance of a priori weighting of characters creates an illusion of objectivity. Yet both assumptions are counterintuitive of the real world, and worse still, the principle of objectivity is immediately violated by the selection of what an author takes to be the unit characters. Is the postcanine tooth structure of a tritylodontid a single character—an enlarged, multicusped tooth—or several characters, one for each dimension, each cusp, each crest? Both views of tritylodontid relationships are current, and depend respectively on such decisions as this. The call is always for yet more characters to help resolve the dispute, and Sues and Jenkins provide a number of these. Several of the previously acknowledged mammal-like characters of the tritylodontid postcranial skeleton are noted but, on the basis of this material, are promptly dismissed as “only superficial in nature,” and the authors conclude that “scoring them as representing the same character-state in phylogenetic analyses obscures the real structural differences in these features between tritylodontids and basal mammals.” Well, maybe so or maybe not—the argument will continue.

Richard Blob's contribution attacks another category of paleobiological problem, that of inferring the function and physiology of organisms represented by no more than their bare bones. Specifically, he addresses the question of whether the cynodonts had actually achieved a significant degree of endothermic temperature physiology, as most commentators believe. One way to test this is to find correlations between skeletal features and temperature physiology strategies in living organisms and apply them to the fossils. In this case, Blob notes an allometric relationship in mammals, in which the rate of increase in limb diameter is less than the rate of increase in body mass. In ectothermic reptiles, these two dimensions scale isometrically. He finds that the relationship within a growth series of cynodonts matches the reptilian pattern, implying that cynodonts were ectothermic. This is a rather disconcerting result in light of the evidence for enhanced feeding, ventilation, and locomotory functions that most of us take to be clear signs of an elevated metabolic rate. Blob is commendably cautious in evaluating the result, but nevertheless we should not lose sight of the fact that many a new insight began life as an anomalous result.

Fred Grine and three colleagues offer an admirably detailed biometric, species-level analysis of the South African specimens of Lystrosaurus. This genus was the most abundant, widespread mammal-like reptile of all time and has the distinction of being one of the very few amniote genera to have survived the great end-Permian mass extinction.

The third paper on mammal-like reptiles concerns evolution at the lowest taxonomic level, a level not often amenable to study in fossil amniotes, but of fundamental importance if community evolution and the rates and controls of extinction and speciation are ever to be understood. Fred Grine and three colleagues offer an admirably detailed biometric, species-level analysis of the South African specimens of Lystrosaurus. This genus was the most abundant, widespread mammal-like reptile of all time and has the distinction of being one of the very few amniote genera to have survived the great end-Permian mass extinction. Grine and colleagues reduce the existing plethora of ill-defined species to five, and define their respective stratigraphic occurrences. Only one species, Lystrosaurus curvatus, occurred during the latest Permian, while the others are all first found in the subsequent lowermost Triassic, which generates interesting thoughts about patterns of survival during and radiation after the end-Permian crisis. For example, was it the evident burrowing ability that allowed Lystrosaurus both to avoid extinction and to undergo speciation before any other terrestrial taxon, once the crisis was over?

The final contribution on mammal-like reptiles is a reminder of what lies at the very heart of paleobiology—new fossil discoveries. Christian Sidor and Bruce Rubidge describe Herpetoskylax hopsoni, a new member of the most basal therapsid taxon, Biarmosuchia. Studying this species gives us new anatomical details about the skull and jaws of one of amniote phylogeny's most interesting groups. The ancestral therapsids were the first animals to show serious evidence—including changes in their jaws, teeth, limbs, girdles, ears, and undoubtedly many more features—of setting off down the evolutionary lineage that eventually ended up as mammals. The more we know of this grade of amniote evolution, the closer we may come to understanding just how and why mammals evolved, complete with their amazing potential to radiate into all the great variety found today.

Other contributions to the volume address a similar range of issues, but in other taxa. Dinosaur buffs will appreciate Matthew Carrano's demonstration that most—though not quite all—dinosaur lineages really did evolve increasing body size, and Michael Parrish's analysis of the relationship between neck length and body size in sauropods. For me, the pick of the mammal papers is Paul Sereno's investigation of the longstanding problem of how and when the parasagittal forelimb gait evolved.

Attending a contemporary vertebrate paleontology conference, one is immediately struck by a revolutionary change that is currently affecting the subject, in the form of new techniques for studying old problems. We can now create detailed three-dimensional reconstructions of extinct organisms, using CT (computerized tomography) and laser scanning to reveal the detailed internal structure of fossils and associated computer algorithms to correct for distortion and damage. Sequences of hypothetical evolutionary stages can be drawn by the computer rather than merely imagined. The engineer's methods of finite element analysis of stress patterns in skulls and limb bones lead to new biomechanical hypotheses about how and why inferred morphological changes occurred. Molecular-based phylogenetic trees may not be directly applicable to fossil material, but examples of taxa that have living members, such as placental mammals, are nevertheless warning us about how unreliable traditional morphological-based methods can be, and are therefore stimulating a search for independent guides to relationships such as paleobiogeography and functional analysis. Stable isotopes can reveal the diets and habitats of animals that lived tens or hundreds of millions of years ago.

Amniote Paleobiology stands at the threshold of this exciting new future for its subject. It contains little that is based on these new methods of investigation, but offers instead a snapshot of the range of paleobiological problems for which provisional solutions have been developed over the last half-century—solutions whose further refinement awaits new insights based on such new techniques. A historian of science 50 years hence will undoubtedly find this attractive, well-produced volume a timely portrait of the subject as it stood at the start of the 21st century.

TOM KEMP "ONE STEP AT A TIME," BioScience 57(5), 452-454, (1 May 2007). https://doi.org/10.1641/B570514
Published: 1 May 2007
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