Teeth: A Very Short Introduction by Peter S. Ungar

Author:Peter S. Ungar
Language: eng
Format: epub, pdf, mobi
Publisher: OUP Oxford
Published: 2014-03-18T04:00:00+00:00

Three great waves

We begin late in the Carboniferous, around 310 mya. The early amniotes, ancestors of the reptiles, birds, and mammals, have evolved an egg that can be laid, incubated, and hatched on dry land. They are freed from the shackles of an aquatic environment, and species adapted to the new opportunities and challenges of a fully terrestrial lifestyle are beginning to proliferate. Three types of amniote evolve, distinguished by the number of holes, or ‘windows’, in the sides of their skulls: 1) anapsids with no holes, 2) synapsids with one, and 3) diapsids with two. All living reptiles are diapsids (even the turtles, which have anapsid-like skulls today); and the mammals are synapsids. But the first synapsids came well before the mammals. In fact, they were among the earliest of the amniotes. Synapsids evolved in three great waves: first the pelycosaurs, then the therapsids, and finally, the mammals. Each wave developed into an extraordinary radiation of species, and each of these radiations became the dominant land vertebrates of their time.

The pelycosaurs. The pelycosaurs ruled the warm, wet equatorial ecosystems of the supercontinent Pangea during the late Carboniferous and early Permian. Some were carnivores, with sharp, cone-shaped teeth, including a couple of large, canine-like ones in each quadrant of the mouth. Others were herbivores, with blunt and sometimes leaf-shaped teeth compressed side to side and with coarse serrations on the front and back edges. Pelycosaurs came in many different shapes and sizes, but the sail-backed Edaphosaurus and Dimetrodon are both favourites in natural history museum displays the world over. Edaphosaurus had small, peg-like teeth rimming its jaws, presumably for cropping and grinding tough vegetation. Dimetrodon, in contrast, was a top predator of its day, with enlarged incisor-like and especially canine-like teeth, but small, sharp marginal teeth shaped like recurved steak knives, with serrated cutting edges on the front and back surfaces. The pelycosaurs were very successful during their heyday and thrived for tens of millions of years, but they ultimately declined as atmospheric carbon dioxide levels, global temperatures, and seasonal aridity increased. They were gone before the end of the Permian.

The therapsids. The therapsids arose from within the pelycosaur radiation and ultimately replaced it. These reptiles flourished under the changing conditions of the middle part of the Permian and at higher latitudes, likely because they were better able to control their body temperature and water balance. These were even more mammal-like, with upright limbs pulled under the body, a higher metabolic rate, and perhaps even hair and lactation. While researchers debate when they first appeared, a diverse community was in place by about 265 mya; and the therapsids thrived for the rest of the Permian. They ranged from a few centimetres to six metres long, from specialized burrowers to swimmers, and from bulk-feeding herbivores to top predators. And many had rather complex teeth. Some species developed interlocking, opposing incisors; and long, sabre-like canine tusks were common. The well-known dicynodonts often had a pair of tusks on each side of the upper jaw—hence the name, meaning ‘two dog teeth’.

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