Early Diapsids


The Diapsida, with two temporal fenestrae, includes the lizards and snakes, cro-codilians, pterosaurs, dinosaurs and birds. The crocodilians and pterosaurs have already been discussed. Here, we consider the ancestral lizards and snakes

■ Fig. 78. Planocephalosaurus (Rhynchocephalia; Upper Triassic; length ca. 20 cm). (Based on Palmer 1999)

(Squamata) and Rhynchocephalia or Sphenodontida (Table 2), which are included with them in the infraclass Lepidosauria. The lepidosaurs first appeared in the Late Triassic - over 200 mya and diversified into numerous families during the Jurassic period. Planocephalosaurus (Fig. 78) was one of the earliest of the sphenodonts to evolve. Its skeleton, from the Upper Triassic of Europe, was almost identical to that of the modern tuataras (Sphenodon punctatus and S. guntheri) of New Zealand.

The teeth of Planocephalosaurus were fused to the jaws rather than being attached to grooves in the jaw bones as they are in advanced lizards. Although smaller than Sphenodon (length ca. 75 cm), it could have delivered a powerful bite. It must have preyed on insects, scorpions, worms, snails and even small lizards. The long, slender bones of Planocephalosaurus were very much like those of modern lizards but its snout was perhaps blunter than that of most. Later, sphenodontians included some bizarre Late Jurassic and Early Cretaceous forms from North America, with broad grinding teeth (Benton 2004). Pleurosaurs were specialised sphenodonts that evolved on land but returned to the water during Early Jurassic times. The limbs were reduced and the body greatly elongated. In some species, there were up to 57 vertebrae which is about twice the number typical of the order. Pleurosaurus (Fig. 26) is an example. It swam rapidly, like a snake, with sinuous movements, its limbs playing no part. Pleurosaurus must have been an amphibious predator, coming ashore to mate and lay its eggs (Sect. 4.5).


Lizards (Lacertilia) and snakes (Serpentes) are undoubtedly the most numerous and successful reptiles in the world today. Lizards are a much more ancient group than the snakes which evolved from them. The earliest known squamates

■ Fig. 79. Squamates. Left Ardeosaurus (Gekkota; Upper Jurassic; length ca. 20 cm). Right Pachyrachis (Serpentes; Lower Cretaceous; length ca. 1 m)

were small, insectivorous, lizard-like reptiles that inhabited southern Africa during the Permian period. The five taxa known today - Iguania, Gekkota, Am-phisbaenia, Scincomorpha, and Anguimorpha - must have arisen during the Jurassic, as did the Serpentes, although the oldest fossils known are mostly Cretaceous.

Extinct Mesozoic squamate taxa included the aerial Kuehneosauridae (Sect. 6.2) and other gliding lizards discussed in Chapter 6, as well as the Mosasauria, thalattosaurs, dolichosaurs, and other aquatic taxa (Sect. 4.5). The Gekkota (geckos) was one of the first of the modern groups of lizards to appear. It is exemplified by Ardeosaurus (Fig. 79) from Late Jurassic deposits in Germany. Ardeosaurus had a flattened head and large eyes, so it was probably a nocturnal predator that fed on insects, arachnids, and smaller lizards. However, whether it possessed adhesive pads on its feet is, of course, not known -they would not have been fossilised. Fragments of other gekkos, as well as of scincomorphs and anguimorphs, have been found in Middle Jurassic deposits of Britain. The Iguania, which includes iguanas, agamids, and chameleons, arose in the Late Cretaceous. The Amphisbaenia is not known from the fossil record of the Mesozoic.

The ancestors of the sixth squamate taxon, the Serpentes or Ophidia, are not known. Fossil snakes, such as Pachyrachis (Fig. 79),date back to the Lower Cretaceous. These early forms must have killed their prey by suffocation as do extant boas, pythons, and some Colubridae. They coiled tightly around the ribcage of their victim, tightening the grip every time it exhaled until it had been asphyxiated. Then they swallowed it whole. The Serpentes radiated greatly during the Tertiary when their mammalian prey diversified: poisonous forms did not appear until the Late Eocene. The ecology and behaviour of Mesozoic squamates must have been remarkably similar to that of extant forms (Cloudsley-Thompson 1999) and will not be discussed further here.

Early Archosauromorphs

Therapsid reptiles (Sect. 8.2) suffered badly during the great Permian extinction and, although a few of them became extremely successful and dominant in specific niches during the Triassic, a number of medium-sized carnivorous diapsids, such as Proterosuchus (Fig. 80a) and other Lower Triassic archosaurs, took over many ecological niches previously occupied by therapsids. Proterosuchus was a slender creature that preyed on therocephalians, dicynodonts and procolophonids. Chasmatosaurus (Fig. 80b) from South Africa and Asia, had robust limbs with five digits. These were set at an angle to the body and resulted in a sprawling, lizard-like gait. It may well have spent much of its time in water, preying on fishes. Its numerous sharp and backwardly-curving teeth were well adapted for gripping slippery prey. There were also teeth on the palate, a primitive feature absent from later archosaurs.

Early in the Middle Triassic, a number of archosaur lineages flourished comparatively briefly. One of these contained the proterosuchids, another the fam-

Chasmatosaurus Skeleton

■ Fig. 80a-g. Early archosauromorph carnivores. a Proterosuchus (Proterosuchidae; Lower Triassic; length ca. 1.5 m), b Chasmatosaurus (Proterosuchidae; Lower Triassic; length ca. 2 m; after Charig 1979). c Erythrosuchus (Erythrosuchidae; Lower Triassic; length ca. 4.5 m ; after Palmer 1999), d Ticenosuchus (Rauisuchidae; Middle Triassic; length ca. 3 m; after Palmer 1999), e Eupar-keria (Ornithosuchidae; Lower Triassic; length ca. 60 cm; after Palmer 1999), f Ornithosuchus (Or-nithosuchidae; Upper Triassic; length ca. 4 m; after Palmer 1999), g Lagosuchus (Ornithosuchidae; Middle Triassic; length ca. 30 cm; after Palmer 1999)

■ Fig. 80a-g. Early archosauromorph carnivores. a Proterosuchus (Proterosuchidae; Lower Triassic; length ca. 1.5 m), b Chasmatosaurus (Proterosuchidae; Lower Triassic; length ca. 2 m; after Charig 1979). c Erythrosuchus (Erythrosuchidae; Lower Triassic; length ca. 4.5 m ; after Palmer 1999), d Ticenosuchus (Rauisuchidae; Middle Triassic; length ca. 3 m; after Palmer 1999), e Eupar-keria (Ornithosuchidae; Lower Triassic; length ca. 60 cm; after Palmer 1999), f Ornithosuchus (Or-nithosuchidae; Upper Triassic; length ca. 4 m; after Palmer 1999), g Lagosuchus (Ornithosuchidae; Middle Triassic; length ca. 30 cm; after Palmer 1999)

ily Erythrosuchidae. The latter comprised a number of top predators including Vjushkovia, from Russia, which reached a length of 5 m and fed on dicyno-donts and other large herbivores. Another, Erythrosuchus (Fig. 80c) from South Africa, was among the most formidable of the Early and Middle Triassic erythrosuchids. It had powerful jaws, studded with sharp, conical teeth, and probably preyed on large herbivores such as dicynodonts and Stagonolepidi-dae (see below).

The amphibious crocodile-like family Phytosauridae has already been considered (Sect. 4.6.2), but a number of other crocodile-like families were terrestrial. One of these, the Rauisuchidae, contained the major carnivorous land animals of the Middle Triassic and was represented by fossils in Europe, Africa, and America. The limbs of these animals were held almost directly beneath the body. This can be seen when Ticinosuchus (Fig. 80d) is compared with the phytosaur Rutiodon (Fig. 30). The terrestrial ancestors of the crocodilians have already been described and illustrated (Sect. 4.6.3).

Benton (2004) gave a very clear account of the evolution of archosaur ankles and posture. At 3 m in length, Ticinosuchus was a medium-sized rauisuchid. In contrast, Saurosuchus from the Late Triassic of South America reached a length of up to 7 m and had an erect gait. Fossils of it have been found in association with a rich fauna of dicynodonts, cynodonts, stagonolepidids, small dinosaurs and other reptilians, on all of which it probably preyed. Saurosuchus was one of the largest predators to evolve before the appearance of the giant dinosaurs.

In South Africa, Euparkeria (Fig. 80e) may have been capable of walking both on all fours and bipedally. An early member of the family Ornithosuchi-dae, it came at the beginning of the first radiation of the archosaurs. Its hind legs were about a third longer than the forelegs, and its long tail would have balanced its body at the hips when it ran on two legs. This bipedal stance was characteristic of the carnivorous dinosaurs that appeared at the end of the Tri-assic period. Although small and slim, with light body plates down the centre of the back and tail, Euparkeria was a carnivore with long, sharp teeth curving slightly backward and serrated along the edges. Ornithosuchus (Fig. 80f) from the Late Triassic of Europe was considerably larger. Although it probably still moved around mainly on all fours, reserving bipedal locomotion for speedy movement when escaping from even bigger predators, it looked very much like a dinosaur. Finally, the diminutive Marasuchus (Fig. 83a) was the most dinosaur-like of all the ornithosuchids and unlike Lagosuchus (Fig. 80g) may, according to Palmer (1999), actually have been ancestral to the dinosaurs.

In contrast to all other thecodonts the Stagonolepididae or aetosaurs were almost certainly herbivorous. Stagonolepis (Fig. 81) from Scotland had a small skull suggesting that food was processed mainly in the alimentary canal rather than in the mouth. Its body was almost entirely encased in armour composed of body scutes, rather like those of crocodiles. The teeth were the same in shape throughout both jaws, and the lack of replacement teeth may imply that replacement was rapid - a necessary response to heavy wear from tough plants such as ferns, horsetails, and the recently evolved cycads. The snout was flat-

■ Fig. 81. Early archosauromorph herbivores. Above: Stagonolepis (Stagonolepididae; Upper Triassic; length ca. 3 m). Below: Desmatosuchus (Stagonolepididae; Upper Triassic; length ca. 5 m). (After Palmer 1999)

tened, extremely thin, and pointed. Although the jaws could have snapped shut very suddenly, they would not have produced much force until they had been closed, a capability of more importance to a herbivore than would a more kinetic initial bite as inflicted by carnivores (King 1996). Stagonolepis had a small head and no teeth at the front of its foreshortened jaws. The flattening of the pig-like snout would have been a useful adaptation for rooting in the undergrowth.

Desmatosuchus (Fig. 81) from the Late Triassic of North America had particularly heavy body armour (Sect. 9.2.2). Its back, tail and part of the belly were protected by heavy scutes, while pointed spines lined the sides of its back and tail. Larger spines, up to 45 cm in length, projected from the shoulders. Such armour was no doubt necessary to provide protection from the numerous thecodont predators of the time. The deep bodies of the Stagonolepididae were required to accommodate the long intestines needed for digesting vegetable food (Palmer 1999).

Although the archosaurs that we have been discussing were the main group of archosauromorphs to become dominant during the Triassic period, they were not the only ones to appear. The Trilophosauridae was a family of unusual diapsids which had lost the lower temporal opening (Fig. 2) so that their skull was euryapsid in appearance. (Euryapsid reptiles had a single temporal lobe placed high on each side of the skull.) Trilophosaurus (Fig. 82) from the Upper Triassic of Texas had a heavily-built skull with flattened three-ridged teeth suitable for shearing tough plant material. Its tail was unusually long. In many parts of the world, the Rhynchosauridae was the dominant family of herbivores in its time (Early to Late Triassic). Hyperodapedon (Fig. 82b) was a typical example. Numerous fossils have been obtained from Elgin, in Scotland. Seen from above, the skulls of rhynchosaurs were triangular in shape with the back wider than the front-to-back distance. Space was thus provided for powerful jaw muscles. Hyperodapedon had a powerful 'beak' with which to bite

■ Fig. 82. Above Trilophosaurus (Trilophosauridae; Upper Triassic; length ca. 2 m). Centre Hy-perodapedon (Rhynchosauridae; Upper Triassic; length ca. 1.3 m). Below Tanystropheus (Pro-lacertiformes; Middle Triassic; length ca. 3 m)

seed-ferns and other tough vegetation. Massive claws on the hind feet were used for scratching backwards and uncovering succulent roots and tubes (Benton 1997b).

A fourth group of archosauromorphs, the order Prolacertiformes, first appeared in the Middle Permian and radiated in the Triassic. Most of them looked like large lizards with long necks. Tanystropheus (Fig. 82c) from Europe and the Middle East was remarkable in that its neck was more than twice as long as its trunk. Composed of only 9-12 cervical vertebrae, this neck was not very flexible, and its function is a mystery. Juveniles had short necks that grew very rapidly. Wild (1978) has suggested that Tanystropheus may have lived in coastal waters, feeding on small fishes it caught by darting its head about very rapidly. Alternatively, it might have lived on the shoreline, dipping its head into the water to catch fish or molluscs that it crushed with its peg-like teeth (Morales 1997). Its competitors in the Triassic seas would have been nothosaurs, placodonts, and ichthyosaurs (Chaps. 4,5).

Considerably less is known of the ecology and behaviour of the reptilian groups discussed in the present chapter than is known of the Dinosauria. These animals will be dealt with in detail in the three following chapters.

The Dinosaurs:

Weapons, Display and Reproduction

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