Therapsids and the Origin of Mammals

The synapsid order Therapsida (Sect. 2.5) may have diverged from the pely-cosaurs during the Lower Permian, but the earliest fossils to be found were in the Upper Permian of Gondwanaland. Therapsids diversified greatly in the Triassic when they dominated the terrestrial vertebrate forms of the world; but only one suborder, Cynodontia, persisted into the Lower Jurassic. The preceding Dicynodontia had been the most successful herbivorous therapsids of the Late Permian and Triassic - a span of almost 50 my. These probably had a variety of masticatory movements, cropping with the horny beaks at the front of the mouth and grinding the food with the horn-covered palatal surfaces further back (King 1996). Triassic genera included Lystrosaurus (Fig. 74) from Antarctica which, at one time, was thought to have wallowed in shallow water like a hippopotamus, feeding on aquatic weeds. It is now believed to have browsed on more resistant plants and might even have been a burrower. This could have been an adaptation to an increasingly semi-arid climate (Palmer 1999). Lystrosaurus was one of the forms on which Alfred Wegener in 1912 based his evidence in favour of continental drift (Sect. 2.2.2).

Another Triassic dicynodont was Kannemeyeria (Fig. 74) from South Africa, India, and Argentina - further evidence for the existence of Gondwanaland. It had a massive head with unusually large openings for the eyes, nostrils and jaw

Triassic Mammals
■ Fig. 74. Dicynodonts. Above Lystrosaurus (Lower Triassic; length ca. 1 m). Below Kannemeyeria (Middle Triassic; length ca. 3 m). (After Palmer 1999)

muscles. It evidently tore up roots, stripped leaves from the vegetation with its horny break and ground them up with its toothless jaws (Palmer 1999). The dicynodonts had large abdomens which would have housed capacious gastrointestinal systems - as might be expected in animals that consumed large quantities of vegetable material, to be retained in the alimentary canal for quite a long time. This suggests that they did not digest their food as quickly as would have been the case if they had been tachymetabolic like the later therap-sids.

The dicynodonts were the first vertebrate herbivores to become really numerous. With their appearance, the primary production of vegetable matter could, at last, have been harvested directly. As a result, a completely new ecological system was set up (see King 1996).

The cynodonts were by far the most successful of the Therapsida and showed many advanced mammalian features. Even Cynognathus (Fig. 75) from the Lower Triassic of South Africa and Argentina probably had a coat of fur, and later forms almost certainly did. Cynognathus, one of the largest of the cynodonts, was a ferocious predator, strongly built and with its hind legs almost directly beneath its body. Other Triassic cynodonts included Thrinaxodon (Fig. 76a) from South Africa and Antarctica and Massetognathus (Fig. 76b) from Middle Triassic strata of Argentina. Oligokyphus (Fig. 76c) from England was a representative of the tritylodonts, the only group to persist into the Jurassic.

Thrinaxodon (Fig. 76a) was much more mammal-like than its earlier relations. It was a small, but sturdily built carnivore. Its body was long and clearly divided into thoracic and lumbar regions, the thoracic vertebrae forming a rib

■ Fig. 75. Cynognathus (Cynodontia; Lower Triassic; length ca. 3 m). (Cloudsley-Thompson 1999)

cage, a feature not previously found among vertebrates. Thrinaxodon was evidently quite speedy, thanks to its erect posture and strong hind legs. The rib cage was probably closed by a diaphragm that would have filled and emptied the lungs very efficiently, while a secondary bony palate separated the breathing passage from the mouth, thus enabling the animal to breathe whilst chewing its food into smaller pieces for easy digestion. These developments give a strong indication that Thrinaxodon was tachymetabolic. Massetognathus (Fig. 76b) and Oligokyphus (Fig. 76c) were both herbivorous - as indicated by their respective dentitions, despite the weasel-like appearance of the latter. Oligokyphus had evolved a fully upright four-legged posture - the only therapsid not to have sprawling forelimbs. Its dentition, too, was significantly different from that of other cynodonts. There were no canines and the front incisors were greatly en-


■ Fig. 76a-d. Later therapsids. a Thrinaxodon (Cynodontia; Lower Triassic; length ca. 50 cm), b Massetognathus (Cynodontia; Middle Triassic; length ca. 50 cm), c Oligokyphus (Cynodontia; Lower Jurassic; length ca. 50 cm), d Ericiolacerta (Therocephalia; Lower Triassic; length ca. 20 cm). (After Palmer 1999)

■ Fig. 76a-d. Later therapsids. a Thrinaxodon (Cynodontia; Lower Triassic; length ca. 50 cm), b Massetognathus (Cynodontia; Middle Triassic; length ca. 50 cm), c Oligokyphus (Cynodontia; Lower Jurassic; length ca. 50 cm), d Ericiolacerta (Therocephalia; Lower Triassic; length ca. 20 cm). (After Palmer 1999)

larged, resembling those of rodents. It was so mammal-like in appearance that, for many years, it was believed to have been a mammal! (Palmer 1999).

The origin of mammals presents no real scientific problem. The immediate ancestors of the class were undoubtedly cynodonts. Closely related to them, was another advanced therapsid suborder, Therocephalia. Its members ranged in size from small insectivores to large carnivores and, in the Early Triassic, some therocephalians such as Bauria, from South Africa, became successful herbivores (Benton 2004). Ericiolacerta (Fig. 76d) was an active insectivore with small teeth and relatively long limbs. The abundant vegetation of the Lower Triassic, consisting of ferns, conifers, and cycads, not only supported large numbers of dicynodonts but also provided abundant arthropods on which preyed lizard-like insectivores such as Ericiolacerta. The first mammals are believed to have been small forms such as Adelobasileus and Sinoconodon of the Upper Triassic, which escaped the attentions of predatory reptiles by their nocturnal habits.

Towards the end of the Triassic period, the Therapsida virtually disappeared and, within a comparatively brief space of time, the dinosaurs radiated until they had occupied nearly all terrestrial niches. They then ruled the land for the next 120 my. Many palaeontologists have assumed that the decline of the mammal-like reptiles was gradual and that dinosaurs and their ancestors, theco-dontians, rose to dominance during that time. Benton (1983) reviewed the subject and presented evidence suggesting that the takeover had not been a result of competition. Alan Charig (1984), however, claimed that the Triassic faunal replacement was certainly competitive, at least in the sense that of two sympatric lineages occupying the same broad adaptive zone, and subjected to the same environmental pressures, one reached total doninance, while the other waned almost to the point of extinction.

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