Review: Carnivoran Evolution

Published in the 81st newsletter of the Palaeontological Association

This banana-yellow fist full is the first volume in Cambridge University Press’ new “Studies in Morphology and Molecules: New Paradigms in Evolutionary Biology” series which promises to ‘address the interface between morphological and molecular studies in living and extinct organisms’. But rather than presenting an historical perspective on this shift (as in Sepkoski and Ruse’s extensive The Palaeobiology RevolutionCarnivoran Evolution’s structure here offers a collection of new science (following a symposium at SVP 2007 in Austin, Texas) representing current zeitgeist – be that in methodology, understanding or academic popularity – constrained within a taxonomic group.

Due to the specialism-led academic model that pervades within our community and the fact that zoologists, like children, still have favourite species and families (I don’t mean to throw stones within a glasshouse, here: in my opinion, you can’t beat the lowland streaked tenrec), CUP have guaranteed these brilliant overviews of contemporary techniques will be bought – or at least borrowed – by any researchers working on the family concerned (if for no other reason than to have a fairly accurate who’s who on their shelf). So why not start with one of the most iconic mammalian families?

Carnivora, as the editors’ synopsis and preface are quick to point out, have invaded niches as disparate as ‘bamboo-eating pandas, clam-eating walruses and… flesh-eating sabre-toothed cats” and have left an excellent fossil record (there are 3 times as many extinct carnivora genera as there are extant). Goswami puts this diversity into startling context herself: “the deepest diving carnivoran, the northern elephant seal, can reach depths of over a kilometre, while its distant relative, the cheetah, can cross that distance on land in less than a minute”. They are ideal for studying convergence, ecomorphology, macroevolutionary patterns & life history evolution. So where to begin?

All contemporary zoological investigations depend on our understanding of the evolutionary relatedness of species. Flynn, Finarelli and Spaulding write in the second chapter that “phylogenetic reconstruction within a clade forms the fundamental evolutionary frame of reference for further analysis into the evolution of character transformation and correlations within that group”. Convergence has dogged the reconstruction of Carnivora phylogenies for the past 100 years, as is summarised by a detailed historical sketch. Large hypercarnivorous forms have evolved several times, large, cat-like forms have evolved in at least 6 families, wolf-like forms at least 5 times and bone-cracking at least twice. The sub-title of Phylogeny, Form and Functionpresents the logical structuring of the book, following the progression of our ability to perform science: the first third underpins everything else.

As well as naming and proposing a phylogenetic definition and diagnosis for a newly recovered major clade within Carnivoramorpha: the Carnivoraforms, Flynn et al. provide the backbone of the book with their phylogeny cited throughout the text. The chapter itself highlights both the importance of fossil data (in order to break up long branches and to “provide temporal context for the evolution of living clades”), and the total evidence method  of analyses, on methodological and philosophical grounds.

The text moves on through the state of Carnivora phylogeny, into a suite of chapters concerned with more specific questions: beginning with the recent revision of the viverrids (Veron) which presents the difficulties encountered due to convergent osteological traits, a theme that is continued into the next chapter. Morlo and Peigné’s work on the Ailuridae proves to be especially fascinating (if for nothing more than the general loveliness of the only remaining extant member of the family; the red panda) and includes a very detailed historical review of all the molecular and morphological data and a natural history of the family including the systematic palaeontology of all known extinct forms.

One of the strengths of the collection becomes extremely apparent over the course of the next few chapters in that the reader is exposed to fresh methodologies, technology and techniques. Goswami and Polly investigate the correlated evolution of characters within carnivorans in order to assess the influence of character correlation on phylogenetic analyses. The authors present the morphometric analysis of 44% of extant genera and 15% of extant species. The resulting methodology (including P.C. space, character distance matrices, 3D landmark data and monte carlo simulations) goes almost as far as to be a protocol and the authors report that character correlations may well have affected morphological phylogenetic analyses of Carnivora.

Holliday uses the carnivoran fossil record to investigate the macroevolutionary question of whether hypercarnivory can lead to a dead end, a theme Goswami has spread at conferences far and wide in terms of the constraints of developmental and functional limitations. As well as defining the concept of modules and the evolutionary stable system, Holliday attempts to better isolate the cause of disparity through the comparative methodology, concluding that the macroevolutionary racheting (a literal phrase, brilliantly capturing the physicality of the concept) means hypercarnivores are strongly limited in their ability to respond to environmental change and that “the lack of reversals to a more generalized condition has a greater effect on the evolution of hypercarnivory than does directional selection towards specialization” (a theme that is revisited in Friscia and Van Valkenburgh’s ecomorphology chapter on creodonts).

Werdelin et al. and Wesley-Hunt et al.’s work bring biogeography into the mix and the following chapters consider disparity across carnivora (an excellent choice of study group within which to study morphological diversity: the Carnivora has the greatest range of body mass of any mammalian order). Returning back to a more conservatively palaeobiological thread, Morlo et al. present an analysis of ecospace and guild structure in Laurasia from the Eocene through to the Miocene.

The morphology theme continues into Jones and Goswami’s work on the secondarily aquatic carnivores, the pinnipeds. This elegant morphometric study presents the apparent convergence in adaptations related to diet and mating displays. Their use of independent contrasts as a method of removing phylogenetic inertia seemed a little stale, but the otherwise excellent method and natural history is a worthy addition to the literature on the oddly understudied mammals.

In the final chapters of the book the focus becomes fixed first on the postcranial skeleton and locomotion through time and then to the most iconic members of the Carnivora: the sabretoothed Machairodontidae cats. Wroe’s final FEA-heavy chapter benefits from (and indeed would have been incomprehensible without) the 19 colour plates bisecting Morlo et al’s work.

Following the chapters the reader is offered an index but no glossary. This omission is an important one: CUP are attempting to present encapsulations not just of current knowledge but also current techniques. However, a book arranged by order rather than specialization limits the ability of the reader to understand all the concepts covered and, to pick one example, those not up to scratch on their statistics (Akaike Information Criterions abound) would benefit greatly from a glossary – although the generation of a glossary from a collection of papers from multiple authors would not be easy. My only other irritation with the collection was the occasional heavy-handed ladling-on of loosely based ‘conservation’ concerns. Issues were discussed as in Veron’s chapter: “current distribution and genetic diversity of population of the red panda are the result of habitat fragmentation…”, but the idea that palaeontologists studying the macroevolutionary ratchet of dental specialization had any salient conclusions for conservation biologists was, as is always the case when palaeontologists feel some odd need to over-justify themselves, unnecessary and embarrassing.

No volume for ten years has collected papers containing state-of-the-art phylogenetic, macroevolutionary analyses for Carnivora and this volume certainly plugs the gap admirably. The Studies in Morphology and Molecules series is set to continue this year with the publication of similar volumes on bat and house mouse evolution. This condensation of evo-devo, molecular systematics, new approaches and techniques in vertebrate palaeobiology and evolutionary functional morphology within one text would have been extraordinary 10 years ago, with readers firmly rooting for either molecules or morphology whilst only a few individuals on either side were bothering to read the technical literature of the other. But we have all moved on. I found this encapsulated in Goswami’s chapter wherein the discussion of modularity and integration within the mammalian cranium illustrates the beautiful level of understanding that can be reached when morphologists, evolutionary developmentalists and geneticists visit each other’s seminars. Carnivoran Evolution stands as a celebration of effective communication and the successes evolutionary biologists, be they palaeobiologists or not, have had in the last decade as a result of profitable interactions with each other in order to listen, understand and, ultimately, discover the answers to their questions by what ever technical means possible, rather than hiding behind their academic specialisms. This book, as a few have before it, made me wonder. Are the words ‘palaeontologist’ and ‘neontologist’ really going to stick around for much longer?

Red panda image from Harlequeen on Flickr.

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