Synapsids

Pelycosaur-grade Synapsids

Permo-Triassic Therapsids

Large-scale Trends in Synapsid Evolution

“Pelycosaurs” represent the first radiation of synapsids. In the US, these early synapsids are mostly found in the Permo-Carboniferous rocks of Texas, Oklahoma, and the Four-Corners region. I am particularly interested in the caseids and varanopids, two “pelycosaur” families that persisted into the Middle Permian to live alongside therapsids, because they can be used to study competitive replacement in the fossil record.

Skull of the varanopid Elliotsmithia from the Middle Permian of South Africa. (See Modesto et al. 2001)

Skull of the biarmosuchian, Lemurosaurus from the Upper Permian of South Africa. (See Sidor and Welman 2003)

Lower jaws of various fossil synapsids (from Sidor 2003).

Therapsids, which are traditionally considered the second radiation of synapsids, are best known from Middle and Upper Permian strata of Russia and South Africa. For the past several years, I have been collaborating with Dr. Bruce Rubidge (BPI, Johannesburg) on a systematic revision of the biarmosuchians, the most primitive group of therapsids. We would like to understand what made early therapsids such an dominant component of their ecosystem.

Therapsids were in their heyday at the time of the Permo-Triassic mass extinction. For this reason, they will be critical to understanding the tempo and geography of the extinction on land. For example, therapsids like Thrinaxodon and Lystrosaurus have been found in the Triassic Fremouw Formation of Antarctica and can be used as biostratigraphic markers.

Mammals are the only extant synapsids. The numerous traits that distinguish mammals from living tetrapods were acquired over a very long evolutionary history. Somewhat surprisingly, however, many of these mammal-like characteristics were independently evolved in other groups of therapsids. For example, the loss of skull roofing bones was a trend in most synapsids, not just the lineage that gave rise to mammals (see Sidor, 2001). I am interested in understanding the basis of these trends as well as what they say about the adaptive advantage of ‘mammalness.’