The elephant shall lie down with the elephant-shrew: Placental Mammals: Origins and Relationships of the Major Clades edited by Kenneth D. Rose and J. David Archibald. John Hopkins University Press, 2005. US$95.00 hbk (272 pages) ISBN 0 8018 8022 X

Abstract

For many years, the main groups of placental mammals (the orders, in formal terms) were each well known, but there was a lack of consensus about where the orders fitted in relationship to one another [1,2]. Biological colleagues outside the immediate field tended to assume that, partly perhaps because we are ourselves placental mammals, those relationships had long ago been worked out and were a stable component of received knowledge, no longer contended and, therefore, no longer exciting as a research area. That is not so, and I read through this large collection of chapters on the individual orders and their interrelationships with an increasing sense of optimism. Although it has mostly been there in the recent primary literature, being brought up against the collected evidence is exhilarating. Much recent progress has occurred as a result of comparative molecular analyses [3–5], although these have not been unanimous [6]. Placental Mammals achieves a balance between molecular work, on the one hand, and anatomical and palaeontological work, on the other. Influential figures of 20th-century studies of placental mammalian phylogenetics are fulsomely acknowledged, particularly W.K. Gregory and G.G. Simpson. It is indeed impressive just how much the pre-cladistic, pre-molecular figures of Gregory (a major contribution of his was published in 1910) and Simpson (a major contribution published in 1945) were able to contribute. Nevertheless, only during the past eight years or so has a fuller picture of the interrelationships of the orders of placental mammals come into focus. The previous multi-author volume [7] to consider a similar subject area was published (as the editors note in their introductory chapter to this new volume) in 1993. Perhaps there are those who would be more grudging abouttheprogressmadesincethen,but itdoesseemthatthis timely volume somehow represents that biological cliche: a new synthesis. What are the reasons for the excitement and optimism? Well, first, the recent comparative molecular studies have partly confirmed and consolidated groupings of placental orders previously suspected on anatomical and palaeontological grounds. However, there have been surprises. I know of at least three doctoral students who began their projects under the convenient view that the order Insectivora (or Lipotyphla) was, as only quite recently constituted, monophyletic, and were galvanized to learn of a challenge to this assumption halfway through their work. Two families of lipotyphlans, the tenrecs of Madagascar and the golden moles of mainland Africa, were (delightfully) found to unite with a collection of superficially improbable bedfellows (aardvark, sirenians, hyraxes, elephants and elephant-shrews) in a grouping that has come to be known as the Afrotheria. The second major contribution of the recent molecular work was the support for a few superordinal bundlings of placental orders, with the reassignment of several individual orders to superorders that were different from the ones to which they had previously been tentatively assigned. For example, the bats (Chiroptera) had long been members of the Superorder Archonta, along with Primates, Dermoptera (flying lemurs) and Scandentia (tree shrews). A series of molecular studies, however, agreed in transferring the bats to a different collection of orders, together now named the Laurasiatheria. Four superordinal groupings have emerged: Xenarthra, Afrotheria, Laurasiatheria and Euarchontaglires. Only the Xenathra could really be described as ‘traditional’. One of the encouraging features of these groupings is that they also begin to make some zoogeographical sense [8]. Placental Mammals emerged as a result of a symposium that was part of the 2002 meeting of the Society of Vertebrate Paleontology. That such a complete treatment of uniformly high quality has emerged so swiftly is a tribute to the vision and dynamism of the editors, and a vindication of their choice of contributors. Each of what the editors refer to as ‘the standard 18 anatomically based orders of placental mammals’ is represented in the volume, either by a dedicated single chapter or in a chapter dealing with another, or several, orders to which it is related. Tensions are still palpable between the new molecular synthesis described here and the necessarily narrower, order-based, interests of several contributors. For example, in her chapter about the bats, Nancy Simmons notes the 17 morphological evolutionary innovations that appear to unite bats with flying lemurs, as a grouping within the Archonta, but she acknowledges that ‘Regardless, there is no molecular support for placing bats within Archonta’. Adjustment to the new ideas is going to involve simultaneous pain and pleasure for most of those involved.