The mammals descended from one branch of the reptiles, the synapsids. In the early Permian, they were like the ancestors of the archosaurs, with jaws constructed from five bones, peg teeth but larger canines, a sprawling gait, no hair, and no internal temperature regulation. By the end of the Permian, mammal legs had shifted from a sprawling position to a semi sprawling position. Other evolved characteristics included differentiated teeth, secondary palate, minimal thermal regulation, possible beginning of hair, and the beginnings of a mammalian jaw.

The synapsids have a single hole behind the eye on each side of the skull. The single hole was advantageous because the single muscle connecting the skull to the jaw could be larger and longer than the diapsid (archosaur) muscles, which enabled larger and stronger jaws. The first synapsid in the fossil record was Archaeothyris, which lived in the Late Carboniferous Period in what is now Nova Scotia (Figure 11‑3).

The first step toward mammalian evolution from reptiles was the edaphosaurs (Figure 11-4). They were herbivorous animals (0.5 to 3 m length) with small heads, large midsections, and long tails. They developed enormous stomachs to ferment plant material. Like the early reptiles, they only had peg teeth. Edaphosaurus fossils range from Texas to West Virginia. One edaphosaur, Edaphosaurus (Figure 11-4), had a large sail. The purpose of the sail is unknown. It might have been for thermoregulation, muscle structure, or mating. The next step in mammal evolution was the pelycosaurs, which evolved new types of teeth to chew food before swallowing it. Prior to this, edaphosaurs swallowed vegetation whole. Differentiation of teeth became a mammalian characteristic. Some pelycosaurs became carnivores. Dimetrodon was a large carnivorous pelycosaur with a large sail and large head (Figure 11-5) that lived during the Early Permian in what is now Texas and Oklahoma.[1] They had powerful jaws and two types of teeth: sharp canines and shearing teeth. At the end of the Permian Period, 70% of amniotes (animals with an amniotic egg) were pelycosaurs, ranging up to 4 m long (UCMP, 2006).[2] Pelycosaur fossils are found in multiple locations around the world with over 30 genera of pelycosaurs identified in the fossil record.

The climate dried with the formation of the supercontinent Pangaea, and the pelycosaurs went extinct. Therapsids descended from one branch of pelycosaurs. Much of the evolution from reptilian characteristics to mammalian characteristics took place in the therapsids. Early Permian Tetraceratops was a therapsid and showed further progression toward mammalian teeth and the jaw (Figure 11‑6).[3] Mammals are defined as those animals that have mammary glands and nurse their young. They are also differentiated from reptiles by bone structure, thermal regulation (endothermy), teeth, and hair. These characteristics evolved within a branch of the therapsids, the cynodonts.

Therapsids occupied a similar ecological niche as the pelycosaurs and eventually replaced the pelycosaurs during the Middle Permian (Figure 11‑7). They were the dominant land animals until the Great Permian Extinction. The legs shifted under the body, they had an open ribcage for greater breathing capacity, and they had large canines.

The therapsids diverged into four major clades prior to the Great Permian Extinction: Gorgonopsia, Anomodontia, Therocephalia, and Cynodontia. Most therapsids died in the Great Permian Extinction. Moschorhinus was a lion-sized therapsid hunter (Figure 11‑8) that survived the Great Permian extinction and lived briefly in the Triassic. Although large therapsids such as Moschorhinus lived through the Great Permian extinction, the low oxygen environment was inhospitable for these large high respiration mammals in the early Triassic, and in this degraded environment, the archosaurs must have had advantages over the therapsids and began to dominate the earth.

References

[1] UCMP website, “Introduction to the Pelycosaurs,” http://www.ucmp.berkeley.edu/synapsids/pelycosaurs.html .

[2] UCMP, Pelycosaurs.

[3] Laurin and Reisz, Synapsida. Tree of Life Web Project. <http://tolweb.org/Synapsida>

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Diorama of Permian scene with Dimetrodon, an Early Permian synapsid. Photo credit: James St John. Diorama at Carnegie Museum of Natural History. Used here per CC BY 2.0.