Geology 143

Lecture #19

November 6, 2000

Protomammals: pelycosaurs and therapsids

The synapsid lineage, first seen in the Pennsylvanian and recognized by the presence of a single temporal opening on each side of the skull below the suture between the postorbital and squamosal bones, includes pelycosaurs, therapsids, and mammals. The earliest of the these, the pelycosaurs, were ectothermic ("cold-blooded") as evidenced by their sprawling, lizard-like posture. Nevertheless, before the radiation of their descendants, the therapsids, during the Late Permian, pelycosaurs were the most abundant group of large terrestrial herbivores and carnivores. The most famous pelycosaur, a carnivorous form called Dimetrodon, was a "sail-back." Very tall nueral spines on the vertebrae supported a membranous "sail," which may have been useful in rapidly gaining heat from sunshine or losing heat to a cooling breeze. As compared to other pelycosaurs, Dimetrodon is advanced towards the therapsid (and hence the mammalian) morphological condition in the presence of minor tooth differentiation - with somewhat flattened incisor teeth and a sharp, stabbing canine tooth farther back in the mouth. Additionally, the angular bone (in the lower jaw) possesses a reflected lamina, a hook-shaped projection which indicates the presence of a primitive tympanic membrane ("eardrum"). The angular in living mammals is very small, free of the lower jaw, and coiled to stretch the tympanic membrane taut; for this reason, in mammals, the angular is termed the ectotympanic (or sometimes just the tympanic).

In addition to the dentary (so named because it bears the teeth of the lower jaw), accessory bones of the lower jaw present in early synapsids include the (previously mentioned) angular, surangular, and the articular, so named because it articulates the lower jaw to a socket in the quadrate (a small bone at the base of the skull). This articular/quadrate jaw joint is sometimes termed the reptilian jaw joint because it is found in anapsid and diapsid reptiles as well as early synapsids. (Mammals possess only the dentary within the lower jaw, which articulates with the squamosal bone at the base of the skull.)

More mammal-like than pelycosaurs in morphology, and presumbably physiology as well, were the therapsids, dominant large terrestrial herbivores and carnivores of the Late Permian and most of the Triassic. The most important morphological modification of pelycosaur design was the development of erect limb posture (the femur bears an inturned head, as in mammals and dinosaurs), which would have allowed for more extended periods of vigorous physical activity. Whether this indicates that all therapsids were endothermic or not is still debated. Many herbivorous groups (termed "head butters" in the video seen in class) had grotesquely shapen heads and likely solved intraspecific (within-species) conflicts by ramming their heads together.

The specific therapsid lineage most closely related to mammals, the cynodonts, had numerous skeletal modifications which brought them closer to mammalian form. These includes a reduction in size of the accessory bones making up the lower jaw to make way for a (1) larger dentary. The accessory bones are not sutured to the dentary (as in Dimetrodon), but rather are attached by ligaments. Cynodont (2) teeth are clearly differentiatied into three types (incisors, canines, and cheek teeth). Cynodonts have a (3) reduced rib cage (they lack abdominal ribs), which suggests the presence of a large diaphragm to allow for more rapid respiration, as in mammals. Another shared feature of cynodonts and mammals is the (4) zygomatic arch, an outbowing of the squamosal and jugal (cheek) bones that allows increased room for the attachment and expansion of jaw musculature. Also, the cynodont face bears (5) whisker pits. Whiskers (technically termed "vibrissae") in mammals are hairs that are modified for sensory perception; whiskers thus imply the presence of hair. Most mammalian hair provides insulation, and the inferred presence of hair is taken by many scientists to be an indication of endothermy in cynodonts, although whether cynodonts were as endothermic as modern mammals is debated.

A subgroup of cynodonts (very close to the ancestry of mammals) possessed two jaw joints, the "reptilian" articular/quadrate (characteristic of primitive amniotes as well as reptiles and non-mammalian synapsids) and a new joint arising from contact between the dentary of the lower jaw and the squamosal of the skull. In most mammals, the articular and quadrate are greatly reduced and aid sound transmission (from the tympanic membrane to the inner ear).