The Evolution of Mammalian Characters

Abstract

INTRODUCTION This book is not intended to give a full and comprehensive account of the Mesozoic mammals, and nor is it intended as a handbook for research workers studying pre-Tertiary mammals. Our intention is to give an account of the origin and evolution of certain of the characters of the Mammalia. We have tried to portray the fossils we describe as the living animals they once were, not as dead bones. Our account ends with the end of the Lower Cretaceous, since by that time the major characters of the mammals had become established. There exist a number of characters which, at the present day, are confined to the Mammalia. These include: (1) a jaw articulation formed by the squamosal and the dentary; (2) a chain of three bones, malleus, incus and stapes connecting the tympanic membrane to the inner ear; (3) the presence of hair or fur; (4) the presence of milk-glands in the female; (5) the left aortic arch is the systemic arch; (6) the phalangeal formula in both manus and pes is 2.3.3.3.3; (7) some of the teeth have more than one root. Of these characters (1) or (2) are sufficient by themselves to define a mammal; characters (6) and (7) are known to have been already in existence in some of the mammal-like reptiles - the ancestors of the mammals. Characters (3)*, (4) and (5) leave no impression on the skeleton, which by and large is what comprises the fossil record, so that we do not know if they were also present in the mammal-like reptiles. Character (5), however, on strong indirect grounds of inference, almost certainly existed in all the mammal-like reptiles as well as in the mammals. Other characters, which are not confined to the mammals but which we always associate with them are: (8) warm-bloodedness or homiothermy (also found in birds); (9) viviparity (not found in monotremes, found in some representatives of all other living classes of gnathostomes except birds); (10) growth by epiphyses (found in some lizards). The only one of these which could appear in the fossil record is (10), and, quite unexpectedly, it does not. It probably did not evolve until the mammals had come into existence, when the remarkably poor record of post-cranial skeletal material of Mesozoic mammals would explain its absence from the fossil record. This book will deal with the interpretation of the facts as they are known from the fossil history of the mammals and the mammal-like reptiles; and it will only discuss soft parts when their nature can be unequivocally inferred from the skeleton (for example: the position of the tympanic membrane). In particular homiothermy and viviparity will not be discussed, nor will mammary glands. There is a real danger that our ignorance of the evolution of the soft parts and physiological mechanisms will lead us to forget their importance to the living animal. A simple example of this, taken from a group alive at the present day, may be salutary. On their skeletal features cats are the most specialised of the living carnivores (Fissipedia). The cats were also the first group of the living fissipedes to differentiate: perfectly good cats were in existence at the beginning of the Oligocene, if not earlier. Dogs are less specialised skeletally than cats, and typical dogs do not become conspicuous until the Miocene. Cats are rather unspecialised physiologically, while dogs show great physiological specialisations for their cursorial mode of life; notably by being able to run fast for long periods without going into oxygen debt - a thing quite impossible to a cat. But this highly important physiolo- gical specialisation of the dogs leaves no record on the skeleton, and if the fissipedes were an entirely extinct group we would have no record that this specialisation ever existed. Thus, by studying evolution by studying vertebrate palaeontology we inevitably get an incomplete and to some extent biased picture. We can only study the evidence we have, incomplete though it may be. But we should beware of the danger of so doing, even if we can do little about it. We have concentrated on those aspects of the study of mammalian origins where we ourselves have made contributions to knowledge. This is a personal book. Jaw articulations, teeth and hearing consequently figure prominently; and in dealing with the fossil forms we have given most prominence to those upon which we have worked. In the references we have tried to include all p.apers on the subject published within the last ten years. We think that the book will be useful to those people who are interested in our ultimate ancestors, as well as to those carrying out research in the field. The illustrations in the book are all by Mr A. J. Lee. The majority of them have been drawn either from the actual specimens or by redrawing and sometimes combining drawings in the original papers describing the material. We think that the illustrations will add greatly to the value of the book. Finally we should like to express our thanks to our colleagues Mrs Frances Mussett and Miss P. M. Lees, without whose cooperation much of the research mentioned in this book would never have come to fulfilment. K. A. Kermack, Department of Zoology, University College, London, Gower Street, London WCIE 6BT D. M. Kermack, Department of Pure & Applied Biology, Imperial College, Prince Consort Road, London SW7 2BB