Dinosaurs and mammals

Key points

• After the end-Permian mass extinction event 251 Ma, diapsid reptiles diversified in the Triassic.

• Dinosaurs were a hugely successful group for 160 myr of the Mesozoic.

• Pterosaurs were key Mesozoic flyers, and the most important marine reptiles were the plesiosaurs and ichthyosaurs.

• Birds evolved from dinosaurs, and radiated particularly during the Tertiary.

• The first mammals were small insect eaters of the latest Triassic; the mammals achieved great diversity and abundance only after the extinction of the dinosaurs.

• Molecular and paleontological evidence show that modern mammals radiated in the Late Jurassic and Early Cretaceous, and that some basic splits were geographic - with major clades separated in South America, Africa, Australasia and the northern hemisphere.

• Humans arose 6-8 Ma, and fossil evidence points to repeated human migrations out of Africa.

The dinosaur's eloquent lesson is that if some bigness is good, an overabundance of bigness is not necessarily better.

Eric Johnston, President of the US Chamber of Commerce (1896-1963)

True; not true. Dinosaurs were big - and some were very big. To politicians, the word "dinosaur" is often a term of abuse, hurled at their enemies to characterize them as old-fashioned, over-blown, past it. Paleontologists (and 4-year-old kids) know better, since dinosaurs were of course one of the most successful animal groups of all time. Nonetheless, they were certainly big, and yet in their day large size was clearly a great advantage for them: after all, dinosaurs and mammals, the subject of this chapter, both arose at the same time, in the Late Triassic, and yet dinosaurs somehow dominated ecosystems in terms of diversity and size, and kept our hairy little ancestors on the fringes.

The tetrapods moved onto land some 380 Ma in the Devonian, and they diversified and occupied more and more ecospace through the Carboniferous and Permian (see pp. 442-52). The most successful tetrapod clade, the Amniota, became most diverse by the end of the Paleozoic, dominating many terrestrial habitats. Two amniote groups in particular rose to prominence. First were the synapsids, which dominated Permian lands but were then hit very hard by the end-Permian mass extinction (see p. 170). The synapsids recovered and gave rise to the mammals in the Late Triassic. The second major amniote group, the diapsids, were minor components of Permian ecosystems, but they diversified in the Triassic, giving rise to the dinosaurs in the Late Triassic. Perhaps the devastating end-Permian mass extinction gave diapsids, and especially the dinosaurs, their chance to diversify.

In this chapter, we take the evolution of vertebrates forward into the Mesozoic and Cenozoic. We explore first the rise of the diap-sids, and especially the dinosaurs and their descendants, the birds. Then, we look at how the modest evolution of mammals through the Mesozoic set the scene for their explosive radiation at the beginning of the Cenozoic, after the dinosaurs had gone.

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