Basal Synapsid Evolution

The synapsids split from the sauroposids (anapsids+di-apsids) in the Mid-Carboniferous, and they expanded in diversity enormously during the Permian, becoming the dominant land animals. The clade is generally divided into two groups. The 'pelycosaurs' (Romer and Price, 1940; Reisz, 1986) are a paraphyletic group of six families of basal synapsids that were particularly important in the Early Permian. These were succeeded in the Late Permian by the Therapsida, a diverse clade of small to large plant- and flesh-eaters (see Box 5.3).

5.5.1 Carboniferous and early Permian synapsids

The Ophiacodontidae, a group of six or seven genera, arose in the Mid-Carboniferous and survived into the Early Permian. The first ophiacodont, Archaeothyris from the Morien Group of Nova Scotia, which also yielded Paleothyris, is incompletely known, being represented by only a backbone, pelvis and partial skull (Figure 5.12(a)). Its relative Ophiacodon, from the Upper Carboniferous and Lower Permian of New Mexico (Romer and Price, 1940; Reisz, 1986), is larger than the amniotes so far described, being 1.5-3m in length

(Figure 5.12(b-d)). The skull is relatively very large. It has a long, high narrow snout region that makes up three-fifths of the total length, and the orbit and temporal fenestra are small and placed high. The limb bones are massive. Ophiacodon was a meat-eater, and it may have fed on fishes and tetrapods rather than mainly on insects.

The eothyrid Eothyris, a small animal from the Lower Permian of Texas (Figure 5.13(a)), has a low skull with a much shorter and broader snout than that of Ophiacodon.The two caniniform teeth are very large, and Eothyris was clearly a powerful predator.

The caseids, herbivorous pelycosaurs from the mid-Permian of North America and Europe, include small and large forms. Cotylorhynchus from Texas and Oklahoma (Figure 5.13(b-d)), is the largest pelycosaur, at a length of 3 m, but its disproportionately small skull looks as if it comes from an animal one-quarter of the size. The key caseid characters are seen in the skull (Figure 5.13(b, d)): greatly enlarged nostrils, a pointed snout that extends well in front of the tooth rows, reduced numbers of teeth with no caniniforms, and a very large parietal (pineal) opening. There are several indications that Cotylorhynchus was a herbivore: the teeth are spatulate in shape rather than pointed, and they have crinkled edges; the jaw joint is placed below the

Fossiler Wal Skelett

Fig. 5.11 Late Permian diapsids: (a,b) Coelurosauravus,restored skeleton in dorsal view, and lateral view of the skull; (c) Protorosaurus; (d) Youngina. [Figure (a) after Carroll, 1978; (b) after Evans and Haubold, 1987; (c) after Seeley, 1888; (d) after Gow, 1975; (a,d) courtesy of the Bernard Price Institute.]

Fig. 5.11 Late Permian diapsids: (a,b) Coelurosauravus,restored skeleton in dorsal view, and lateral view of the skull; (c) Protorosaurus; (d) Youngina. [Figure (a) after Carroll, 1978; (b) after Evans and Haubold, 1987; (c) after Seeley, 1888; (d) after Gow, 1975; (a,d) courtesy of the Bernard Price Institute.]

level of the tooth rows, an adaptation that shifts the maximum bite force to the cheek teeth; the jaw could probably have been moved fore-and-aft;and the barrel-shaped ribcage presumably contained massive guts that were necessary for digesting large quantities of rough plant food.

The varanopids, six or seven genera of small carnivores known until recently only from the Lower Permian of North America, survived into the Upper Permian in Russia and South Africa (Modesto et al., 2001). The limbs are long and the skeleton lightly built, so that they are interpreted as active and agile in their habits. Varanops (Figure 5.13(e,f)) has a long low skull, with the dentition extending unusually far back to lie below the temporal fenestra.

5.5.2 The sail-backed synapsids

Two groups of Early Permian synapsids, the edaphosaurids and the sphenacodontids, include genera that had massive 'sails' on their backs. The edaphosaurids, such as Edaphosaurus from the Lower Permian of New Mexico and Texas (Figure 5.14), were herbivores. They have enormously elongated neural spines of the cervical and dorsal vertebrae that were

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  • miika
    Are amphibians synapsids?
    8 years ago

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