Synapsids include mammals and all extinct amniotes more closely related to mammals than to reptiles. Synapsids are the dominant large terrestrial animals worldwide, and they have also invaded the oceans (whales, pinnipeds) and the air (bats). The oldest known synapsid is an ophiacodontid from the Middle Pennsylvanian (320 million years ago) of Joggins, Nova Scotia. By the Lower Permian, therapsids (the group that includes mammals and most of their Upper Permian and more recent relatives) had appeared (Laurin and Reisz, 1990, 1996). However, the oldest known mammal only dates back to the Jurassic (Rowe, 1988). The fossil record of synapsids is one of the most extensive of any groups of vertebrates. This fossil record has been used to illustrate the concept of evolution (Hopson, 1987) and to test macroevolutionary patterns (Kemp, 1985). The largest gap in this fossil record is between the Permo-Carboniferous synapsids and therapsids.
The fossil record provides conclusive evidence that synapsids are the first amniotes to diversify. Synapsids quickly became the most diverse, widespread and most common amniotes in the Late Carboniferous, and they maintained this predominant position throughout the Paleozoic. Only during the Early Mesozoic are the synapsids eclipsed by the evolutionary radiation of reptiles (Benton, 1983; Charig, 1984). Within the Late Carboniferous and Early Permian, two different herbivorous and several faunivorous synapsid groups can be recognized.
The fossil record suggests that during the Carboniferous and Early Permian, synapsids and other amniotes were restricted to the paleoequatorial and subequatorial regions. During the Late Permian, the distribution of synapsids, and of some of the other amniotes becomes cosmopolitan. However, the evidence for this pattern rests on rather weak negative evidence (i. e., no Permo-Carboniferous synapsids have been found outside paleoequatorial regions, but other areas have not been prospected intensively).
Early synapsids had a sprawling posture and a small brain, like most early tetrapods. The parasagittal gait characteristic of most mammals (Fig. 1) appeared gradually because some therapsids were apparently capable of sprawling and parasagittal gait, and this character may have appeared in the hind limb before the fore limb. Early synapsids were moderately large (body length between 50 cm and 3 m) and most were carnivorous or insectivorous, but caseids and Edaphosaurus were herbivorous. Mammals include the largest tetrapods that ever lived (the blue whale is larger than any dinosaur), as well as very small species.
Most mammals are viviparous, but the platypus (a monotreme) is oviparous, and most, if not all, non-mammalian synapsids were probably oviparous. All mammals have mammary glands, but the presence of these structures cannot be determined in extinct taxa. The oldest preserved synapsid hair appears to belong to a Middle Jurassic docodontan mammaliaform (a stem-synapsid) from China (Ji et al., 2006). However, hair is infrequently fossilized and the next oldest record is from a Paleocene multituberculate (Meng and Wyss, 1997). Therefore, its real date of appearance is unknown. The position of this group has been debated, but they are probably mammals (Rowe, 1988), so the presence of hair in multituberculates was predictible.
Figure 1. An extant synapsid (a black-tailed deer). Like most of the
other 3500 extant synapsid species, it is endothermic and has fur, but these
features were certainly absent in all Paleozoic synapsids. Photograph by Roger Laurin.