Extinction and Radiation: How the Fall of Dinosaurs Led to the Rise of Mammals

Despite the Earth’s immense age, only the last 600 million years have been characterized by remarkable biodiversity. If we accept that theropod dinosaurs were the ancestors of birds, and synapsids the ancestors of mammals, we can easily understand why the Triassic synapsids did not diversify to marsupial+placental mammals, and monotremes until the Jurassic, and why early placental mammals were no larger than shrews even by the end of Cretaceous. Until then, dinosaurs made up 95% of the total biomass of vertebrates through the second half of the Mesozoic; after their mass extinction, mammals radiated dramatically filling all niches of the dinosaurs and even invading new ones. This appears to be the central theme of Archibald’s book. With an extensive breadth of knowledge of recent paleontological discoveries in South America, Europe, and Asia, as well as considerable field experience in the western United States, southern Alberta, and Uzbekistan, Archibald is superbly qualified to draw on his vast experience. As evidenced by this book, he is also clearly unafraid to enter into polemics with other scientists on interpreting the value of recent world-wide mammal discoveries, the book’s central theme. Although unimpressive in length, the book is richly embellished by the information contained. Following a 2-page Preface, the 1st chapter (The Late Cretaceous Nonavian Dinosaur Record) emphasizes the relatively brief Cretaceous/Tertiary boundary, characterized by the extinction of the nonavian dinosaurs. The terms “boundary” and “nonavian” are both clearly explained in the Notes section (pp. 87–92) although there remains controversy in their use. The detailed explanation of necessary conditions and opportunities for fossilization of plants or animals are welcomed in this chapter; the example of Archaeopteryx preserved in limestone is mentioned as well as the role of cataclysmic events. Aside from pterosaurs and ornithischians, Archibald considers all North American Dinosauria. A comprehensive table (1.1) of genera and number of species from the Dinosaur Park Formation also lists those of the Lance Formation, with additions from the Judith River, Two Medicine, and Hell Creek Formations in segments averaging 2–3 million years as well as their taxonomic diversity. Noting the richness of some genera from the Dinosaur Park Formation and their paucity in the Lance Formation (on p. 8), Archibald’s analysis shows a rapid decline of nonavian dinosaurs specially in Mastrichtian. The 2nd chapter (In the Shadow of Nonavian Dinosaurs) is a brief overview of the most ancient mammals emphasizing the scarcity of knowledge on them, and some misleading ideas voiced by recent authorities. Three main ideas emerge in this chapter: 1) mammal-like reptiles originated from synapsids (ca. 300 mya); 2) marsupials did not appear before placental mammals, and hence, are not their ancestors; 3) marsupials are not more primitive than placentals. Some mandibular characteristics (e.g., degree of the dentary angle inflection) support the idea of their being close relatives rather than succession over time. Chapter 3 (In search of Our Most Ancient Eutherian Ancestors) covers the only Late Cretaceous eutherian that has been discovered in both North America and Asia. Archibald utilizes results from his expeditions to Uzbekistan to present cladograms incorporating plesiomorphic and apomorphic features to distinguish the different taxa. His cladistic analyses are meritorious and in many cases confirm results from DNA sequencing analysis. Chapter 4 (Patterns of Extinction at the K/T Boundary) is dedicated to each major clade of fossils, before and after Cretaceous/Tertiary boundary, mostly from northern parts of interior western North America. Mammaliaformes—Prototheria, Metatheria and Eutheria—survived this boundary. At this time, multituberculates flourished and in the Western interior, some species persisted up to the Middle Tertiary; others disappeared earlier. The eutherians survived in greatest percentage, and many Tertiary relatives are derived from them.

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