The Beginning of the Age of Mammals

  • Kenneth D. Rose
Johns Hopkins University Press: 2006. 428 pp. $150 0801884721 | ISBN: 0-801-88472-1
Fossil skeletons of Middle Eocene mammals such as Kopidodon have been found in Messel, Germany. Credit: FORSCHUNGINST. SENCKENBERG, FRANKFURT

Just looking at The Beginning of the Age of Mammals by Kenneth Rose, with its Henri Rousseau-style jacket picture, catapults the reader right into enchanted Early Eocene life. The book is a scholarly treatment of an important period in the evolution of mammals. Mammals originated some 225 million years ago, and for about 160 million years they were small nocturnal creatures living in the shadow of the dinosaurs. Most of them would have been size of a shrew or rat, with only a few being as big as a fox. When the dinosaurs died out at the end of the Cretaceous, 65 million years ago, mammals changed lifestyle and started to increase their size, with many entering niches abandoned by the extinct giants. This was the age of mammals.

Kenneth Rose's book The Beginning of the Age of Mammals is devoted to this explosion of mammals, particularly their evolution during the first half of the Cenozoic period. Rose highlights in particular the development of placental mammals in the Palaeocene and Eocene (between 65 million and 33 million years ago).

One of the charms of this book is the abundance of figures, with many attractive reconstructions of Eocene mammals. Some of these have been made possible by the unusual preservation of complete mammalian skeletons from the Middle Eocene in Messel, Germany. Rose had an opportunity to collaborate with German colleagues when describing some of these mammals. As well as line drawings and black-and-white photographs, there are eight full-page colour plates with photographs of the best-preserved fossils and reconstructions.

Although the bulk of the book is devoted to this mammalian explosion, Rose starts at the beginning of the mammals' tale. Two introductory chapters are followed by four on the origin of mammals and their early evolution in the Mesozoic.

The author no doubt faced a problem when dividing the rest of the book into chapters. Details about the roots of many groups are hazy, and the earliest representatives of most orders are unknown, so it would be impossible to restrict the contents of chapters to strictly monophyletic units. Nowadays, taxonomists have to deal with two different taxonomies: a traditional one, based on morphology, and a modern one, based on molecular analysis. Each chapter begins with an introduction describing our changing views on the relationships of the animals to be discussed. These introductions show the perspective and knowledge that Rose uses when examining the data. When available, he provides two alternative cladograms showing the relationships among the studied forms, one based on morphology, the other on molecular studies. His preference is decidedly morphological, as is evident from his attitude to the clade Afrotheria.

The notion of Afrotheria arose around 1990, based on DNA analyses demonstrating that six living orders of African placental mammals form a monophyletic clade. Although Rose often mentions the Afrotheria, he states: “No morphological evidence supporting Afrotheria has been found.” However, the Cretaceous representatives of modern mammalian orders are unknown, and the lack of an early fossil record for most groups may be responsible for any morphological gaps between them. Among the groups assigned to Afrotheria, Rose, on the basis of morphology, accepts the monophyly of Hyracoidea, Elephantoidea and Sirenia. But, against the molecular evidence, he assigns Tenrecidae and Chrysochloridae not to Afrosoricida but to Insectivora; Macroscelidea to the relatives of the Anagalida; and finally Tubulidentata (not described because of a lack of fossils) to the archaic Ungulata.

Each chapter contains a systematic table using linnaean taxa and a hierarchy of systematic units following Malcolm McKenna and Susan Bell's book Classification of Mammals (Columbia University Press, 1997). But the PhyloCode (http://www.ohio.edu/phylocode) — a set of rules governing phylogenetic nomenclature that in recent years has gained many proponents among zoologists and palaeontologists — is not mentioned in the book. In the final chapter, Rose provides a summary of the idea of a sudden explosion of mammals after the Cretaceous–Tertiary boundary.

This beautiful and thorough book will be an essential tool for all those who work on fossil and extant mammals, and for both advanced undergraduate and graduate students. It is a 'must buy' for palaeontological libraries.