Extinction and radiation: how the fall of the dinosaurs led to the rise of mammals



Extinction and radiation: how the fall of the dinosaurs led to the rise of mammals by J. David Archibald . Baltimore : The Johns Hopkins University Press , 2011 . 108   pp. Hardback. ISBN 13: 978-0-8018-9805-1 ISBN 10: 0-8018-9805-6. £39 .

It's more than 40 years since I listened to undergraduate lectures on this topic, so I thought I might get myself updated. I was also interested in the whole issue of evolutionary transitions – how does a reptile become a mammal when every stage of the process has to be a functionally coherent and fully adapted to its ecological environment? This book is nicely discursive and readable, but doesn't seem to address that question.

From the start it does make two good points: first that palaeontologists tend to focus on a single geological era and not the transition between them and second, that there were mammals living alongside dinosaurs (albeit not the familiar taxa that we see today). Even a six year-old knows the dinosaurs and their long names, even if they can't spell anything else, but nobody seems very aware of the contemporary mammals. Eutherians do not appear in the North American fossil record until the late Cretaceous, but recent studies on fossils from central Asia apparently extend their existence back much earlier and I recall lectures about Morganucodon from even earlier still. This was a ‘missing link’, a mammal with remnant reptilian features, but it is not mentioned here. Nor is the recent discovery of ‘the first mammal’, Juramaia, or its discoverer, so I feel slightly confused rather than updated on the origin of fossil mammals, although this book is focussed on the later Cretaceous-Tertiary transition.

What caused the extinction of dinosaurs is discussed of course. The various challenges (meteorites, acid rain, vulcanism, climate change, overexploitation by aliens, etc.) killed off dinosaurs, large and small, terrestrial and aquatic, yet so many other taxa survived, including mammals, but why? It's not clear what was actually wrong with being a dinosaur. Moving swiftly on, it is firmly (and reasonably) asserted that their removal opened the way to a rapid expansion of mammals into the groups we know today. However, ‘the molecular clock model’ apparently suggests that diversification actually occurred before the extinction of dinosaurs. The author explains away this awkward anomaly by suggesting that the cataclysms occurring at the end of the Cretaceous must have somehow caused the molecular clock to speed up. Exactly how is not explained.

What actually is a mammal? This surely is a central question. The presence of mammae to produce of milk has to be the prime feature, but is not evident in fossils. Fur and homeothermy are also difficult to study in the fossil record. They are what enabled mammals to colonise many extreme environments, but only at the expense of increased energy consumption. The relevance and significance of all this is not discussed. Two slightly incongruous pages at the end suggest that a single species (our own) achieving a population of 9 billion in the next 40 years will constitute a major extinction threat for many extant mammals (and much else). Good point, it took millions of years to see off the dinosaurs.

Overall, this book forms a commentary on the author's own research and the context in which it was carried out. Its narrow focus on molars and the application of cladistics to establish relationships between fossil taxa and their immediate ancestors will appeal to the few who are similarly engrossed. But ultimately the book doesn't really answer the question posed by its subtitle. It doesn't tell us HOW the fall of the dinosaurs led to the rise of mammals, only that it did.