Therapsids Were The Stock That Gave Rise To Mammals

90% of the genera; "other reptiles seem for the most part to have been crowded out." And despite mass extinctions at the end of the Permian, the mammal-like reptiles were still very common in the Early Triassic. Yet by the end of the Triassic, the therapsids were all but extinct and the world belonged to dinosaurs. (See Figure 1.)

Therapsids and dinosaurs belonged to two distinct subclasses of reptiles: the therapsids were synapsid reptiles and the dinosaurs were diapsids. Synapsid and diapsid reptiles were only very distantly related, having diverged from the basal reptile stock during the Pennsylvanian period 100 million years before the Triassic. The first synapsid reptiles appeared in the Early Pennsylvanian (Fig. 1) and soon became the preeminent terrestrial vertebrates. Among the earliest synapsids were the pelycosaurs, the so-called 'primitive mammal-like reptiles.' Dominant in the Late Pennsylvanian and Early Permian, the pelycosaurs were succeeded in the Late Permian by their descendants, the therapsids, which flourished into the Triassic.

The immediate ancestors of mammals, therapsids were already approaching a mammalian level of physiology and thermal control;29 they were certainly the most advanced animals of their day. But in the Middle Triassic, the therapsids began to encounter fierce competition from an assemblage of diapsid reptiles known as thecodonts, stem members of the superorder Archosauria and the direct ancestors of dinosaurs, crocodiles and pterosaurs. By the end of the Triassic, the therapsids were on the brink of extinction; their descendants, the mammals, barely survived into the Jurassic and beyond.

The diapsid lineage originated in the Late Pennsylvanian about 20 million years after the appearance of the earliest synapsids. Diapsids were rather obscure during the 80 million years of synapsid supremacy. They began to make their presence felt only in the Late Permian with the appearance of the proterosuchian thecodonts but did not begin to seriously challenge the therapsids until the Middle Triassic. Unlike their contemporaries, the mammal-like reptiles, the early diapsids do not reveal any indication of significant

27 Chang, 1979.

28 Colbert, 1965.

29 Kemp, 1982; Bennett and Ruben, 1986; Guillette and Hotton, 1986; cf. Bakker, 1971.

physiological advancement.30 Thecodonts almost certainly were less advanced physiologically than the therapsids and were on a par perhaps with their descendants, the crocodiles. Nevertheless, thecodonts proved eminently successful against the therapsids. But their sojourn was brief. Victims of their own success, thecodonts did not survive the Triassic.

Taxonomists classify reptiles according to the number and location of diagnostic openings, or fenestrations, in the skull.31 Indeed, the names 'synapsid' and 'diapsid' refer to this.32 The synapsid - 'fused arch' - skull is characterized by a single diagnostic fenestration behind the eye (postorbital) whereas the diapsid - 'twin-arched' - skull has two postorbital fenestrations. Thus, whereas the skull of the mammal-like synapsid reptiles was typically solid and thick-boned with only a single small opening behind the eye, the diapsid skull was much more open and lightweight, and perforated by two or more fenestrations. In archosaurs, the skull was further lightened by a large anteorbital fenestra; and in advanced thecodonts and primitive dinosaurs there was still another opening in the jaw - the lateral mandibular fenestra. What is the significance of these structures?

Skull design obviously cannot be attributed to any single causal factor. Skull fenestrations, in addition to reducing weight, also serve to anchor jaw muscles. Conversely, bone thickening in the roof of the skull, which occurred in both synapsids (e.g. dinocephalians) and diapsids (pachycephalosaur dinosaurs), can be attributed to head-butting behavior associated with sexual or territorial combat. But despite these complications, basic skull structure does give some indication of the osteological differences embodied in these two lines of reptiles. In general, the solid-skulled synapsid reptiles had massive and thick-boned skeletons whereas the skeletons of the open-skulled diapsids, and the dinosaurs in particular, were slender and lightly constructed.

Therapsids were typically compact with their mass concentrated in the trunk. Despite their relatively small size (compared, that is, to dinosaurs), therapsids possessed heavily constructed skeletons with short, stout limbs, broad flat feet, a short tail, massive skull, and virtually no neck. Variations from this type depended on size and scale: the large herbivores (e.g. Moschops) were the most heavily built while the smaller carnivores (e.g. Lycaenops) were leaner and more refined in structure. (Fig. 3.)

30 Cf. Bakker, 1986.

31 MacLean, 1986.

32 Osborn, 1903.

Kannemayeria
r j—acm

Figure 3. Some reptiles from the Permo-Triassic. Note the robust morphology and stout bones. A. Lycaenops, dinocephalian therapsid, Late Permian, length 1 m, from Colbert (1948). B. Thrinaxodon, cynodontian therapsid, Early Triassic, length 50 cm, from Brink (1956). C. Moschops, dinocephalian therapsid, Late Permian, length 5 m, from Gregory (1957). D. Erythrosuchus, proterosuchian thecodont, Early Triassic, length 5 m, from Heune (1936). E. Kannemayeria, dicynodontian therapsid, Early Triassic, length 3 m, from Pearson (1924). F. Scutosaurus, pareiasaur, Late Permian, length 2 m, from Gregory (1951). Figures are not drawn to scale.

The skeletal design embodied in the therapsids seems to have been established at the dawn of reptile evolution or even earlier, when the first vertebrates appeared on land. Indeed, most Paleozoic reptiles were even more robust than modern mammals and reptiles of comparable size. "[T]he limbs of early reptiles [from the Pennsylvanian] are almost invariably much shorter relative to shaft diameter than in living reptiles." For example, "medium-sized cotylosaurs... had much shorter limbs than similar-sized iguanids or agamids [lizards]; the large cotylosaurs had much shorter, stockier limbs than the largest living lizards, the big monitors."33

Nearly all of the early therapsids were heavily constructed. The dinocephalians, from the Early Permian, were "massive animals that must have frequently weighed a thousand pounds or more in life." From the dinocephalians there arose two new lines of mammallike reptiles, "with one branch, the titanosuchians, becoming large, ponderous carnivores, and the other, the tapinocephalians, becoming large and equally ponderous herbivores."34

"Thick-skulled predators," the titanosuchians were exemplified by Jonkeria, "a large lumbering carnivore" whose "body was robust and the limbs. very stout." These hunters "could not have been very fast or agile, so it is to be presumed that they preyed upon the largest and clumsiest of the vegetarians." Among their prey were the pareiasaurs, "heavy, ungainly animals with capacious bodies, strong limbs and broad feet, and thick solid skulls." Tapinocephalians, the plant-eating cousins of the titanosuchians, "were equally as large, and one might say equally as clumsy as the pareiasaurs. They rival the pareiasaurs in the heaviness and thickness of the skull, a skull in which the temporal opening behind the eye is much reduced and the bones are exceedingly thick."35

Many families of mammal-like reptiles went extinct at the end of the Permian. But those that survived retained the thick-boned morphology of their forebears. Dicynodonts (Fig. 3E.), the most abundant herbivores of the Permo-Triassic, "were slab-sided animals with short, heavily muscled limbs and short tail. Their plantigrade feet no longer sprawled so far to the sides as those of their pelycosaurian forebears, but they had lost the lizardlike gracility of pelycosaurs without gaining the mammalian slenderness of living dogs and cats. The body form of dicynodonts is roughly comparable to that of such robust mammals as beavers [and] badgers, but the limb bones of dicynodonts are more robust than those of even the most stout-bodied mammals."36 The meat-eating cynodonts - the ancestors of mammals - also survived the Permian extinctions. More gracile than the dicynodonts, the largest cynodonts possessed short erect limbs; in size and morphology, they resembled robust wolves. (Fig. 3B.)

33 Bakker, 1971a.

34 Colbert, 1965.

35 Ibid.

36 Hotton, 1986.

The Triassic Transition and the Rise of the Diapsid Reptiles

Selection pressures during the Permo-Triassic favoring stout thick-boned animals must have been very strong indeed. The morphologic pattern described above was not limited to the therapsids, or even to synapsid reptiles, but seems to have been characteristic of tetrapods in general. "A remarkable parallel to the therapsids were the pareiasaurs, common in the Mid and Late Permian."37 The pareiasaurs (Fig. 3F.), previously mentioned, were anapsid reptiles (characterized by no diagnostic skull fenestra), unrelated to the therapsids. Significantly, even the proterosuchian thecodonts, from the Late Permian and Early Triassic, had thick bones and were massively constructed (Fig. 3D). Despite their diapsid pedigree, the proterosuchians were "clumsy reptiles with stout bodies and generally short tails and frequently with large skulls."38 These "awkwardly built" animals represent a sterile side branch of the thecodonts and "are not in themselves truly ancestral archosaurs."39

In contrast to the robust and thick-boned reptiles that flourished throughout the Late Paleozoic, many of the archosaurs and other diapsid reptiles that emerged during the Early Mesozoic were very slender and lightly constructed. Ornithosuchian thecodonts, the vanguard of the archosaur revolution, burst upon the scene during the Lower Triassic. Euparkeria (Fig. 4A), one of the earliest ornithosuchians, "is in many regards an almost ideal ancestor for later archosaurian types."40

Many workers have commented on the lightweight construction of the archosaur skeleton and skull, a characteristic already evident in Euparkeria. "There is obviously a strong evolutionary trend among these early thecodonts to reduce as much as possible the amount of bone in the skull and jaw thereby cutting the weight of the structure." The reduction in bone mass was also evident in the post-cranial skeleton: "The lightness, one might say the fragility of the thecodonts is at once striking and apparent. The bones were delicately formed and many of them such as the long bones of the legs were hollow as are the bones in birds."41

The lightweight construction of the archosaur skeleton is most clearly evident in the early dinosaurs: there were all extremely gracile with long slender limbs and necks (Fig. 4). 'Coelurosaur,' the name given to the most primitive group of dinosaurs, literally means 'hollow reptile', a reference to the animals' hollow bones. "These were relatively small forms, very lightly built, with thin-walled bones. The skull was small, the orbits large, the neck relatively long and slender. Coelurosaurs of slender build were already common in the Late Triassic."42

Coelophysis, a coelurosaur from the Late Triassic, whose name literally means 'hollow form,' is typical of these early dinosaurs (Fig. 4D). "The skeleton is approximately 2.5

37 Bakker, 1971a.

38 Colbert, 1969.

39 Romer, 1968.

40 Romer, 1966.

41 Colbert, 1967.

42 Romer, 1966.

meters long and extremely lightly built. The limbs are long and slender, and the bones are hollow."43 "The neck is very long and flexible enough for the head to reach the pelvic region. The tail is enormously long and, although the individual vertebrae are themselves long, the end is again flexible."44 The skull "epitomizes the light, flexible construction of paper-thin bony sheets and slender struts."45 This basic body plan persisted despite a significant increase in size: Dilophosaurus, a coelurosaur from the Early Jurassic, was 6 m long (Fig. 4E.).

Euparkeria

Fi^ifiiifi; Lwmim - y liirtVl.ijii'fc F. HUliptaBiUrt«

Figure 4. From thecodonts to dinosaurs. Note the gracile mo rphology and slender bones. A. Euparkeria, ornithosuchian thecodont, Early Triassic, length 50 cm, from Ewer (1965). B. Lagosuchus, ornithosuchian thecodont, Middle Triassic, length 30 cm, from Bonaparte (1978). C. Staurikosaurus, very primitive saurischian dinosaur, Middle-Late Triassic, length 2.1 m, from Galton (1977). D. Coelophysis, theropod dinosaur, Late Triassic, length 2.5 m, from Colbert (1972). E. Dilophohosaurus, theropod dinosaur, Early Jurassic, length 6 m., from Welles (1984). The figures below the diagonal line are drawn to scale.

Fi^ifiiifi; Lwmim - y liirtVl.ijii'fc F. HUliptaBiUrt«

Figure 4. From thecodonts to dinosaurs. Note the gracile mo rphology and slender bones. A. Euparkeria, ornithosuchian thecodont, Early Triassic, length 50 cm, from Ewer (1965). B. Lagosuchus, ornithosuchian thecodont, Middle Triassic, length 30 cm, from Bonaparte (1978). C. Staurikosaurus, very primitive saurischian dinosaur, Middle-Late Triassic, length 2.1 m, from Galton (1977). D. Coelophysis, theropod dinosaur, Late Triassic, length 2.5 m, from Colbert (1972). E. Dilophohosaurus, theropod dinosaur, Early Jurassic, length 6 m., from Welles (1984). The figures below the diagonal line are drawn to scale.

Another group of Triassic dinosaurs, the prosauropods, were similarly constructed, though they were somewhat larger and more robust than the coelurosaurs. In Anchisaurus, "[t]he neck was probably long, slender and flexible, as was the back, giving these animals a rather long-bodied look."46 Neck elongation was especially acute in Massospondylus. (Extremely long necks also characterized the Jurassic sauropods: Mamenchisaurus, a 22-meter sauropod from the Late Jurassic, had a neck 10 meters

43 Carroll, 1988.

44 Halstead and Halstead, 1981.

45 Bakker, 1986.

46 Norman, 1985.

long.) Unlike the coelurosaurs, most prosauropods were only partially bipedal. Melanorosaurus, a ten-meter-long prosauropod from the Middle and Late Triassic, was a quadruped.

During the Late Triassic, gracility and elongated extremities (legs, neck and tail) were the norm rather than the exception. For example, the earliest (sphenosuchian) crocodiles (Fig. 5), which descended from thecodonts and appeared in the Middle Triassic, were very lightly constructed. In contrast to modern amphibious crocodiles, which are quite robust, the Triassic crocodiles "had very long slim limbs and were almost certainly agile, terrestrial forms."47 Terristrisuchus, for instance, has been characterized as "extremely gracile."48 Several primitive crocodiles, such as Hesperosuchus and Gracilisuchus (Fig. 5E), were facultatively bipedal and were originally thought to be thecodonts. To be sure, most of these early crocodiles were quite small, less than a meter in length, but they and their cousins, the dinosaurs, were almost always more lightly constructed than therapsids of similar size.

(Exceptions to this trend toward lightly built skeletons and elongated limbs and necks included the phytosaurs and aetosaurs, both of which were armored quadrupedal thecodonts, and the rhynchosaurs, relatives of modern lizards. Most of these groups were extinct by the end of the Triassic. )

Even the most distant relatives of the dinosaurs were built along similar lines. In the ancestors of the giant marine reptiles, "the one noticeable specialization is a considerable elongation of the neck vertebrae."49 Colbert found this 'specialization' especially puzzling: "Every now and then the paleontologist encounters a fossil that baffles him thoroughly, a specimen that defies all attempts to explain how it could have lived. The characters of the skeleton are so grotesque that one is hard pressed to guess what they mean in terms of adaptation to the environment. Such a fossil is Tanystropheus from the Middle Triassic of Europe."50 What Colbert found so baffling was the animal's incredibly long neck. Tanystropheus was only two or three meters long but its long serpentine neck accounted for half that length (Fig. 5D). Although its limbs, tail and skull were well proportioned, each of the ten neck vertebrae were grossly elongated. Indeed, Tanystropheus was so bizarre that early workers believed that the neck and body came from different animals - the neck belonging to a pterosaur and the body to a dinosaur. Recently, a worker has suggested "that the adult life must have been spent in the water, for it is difficult to envisage how such a long neck could be supported on land."51 Where have we heard that before?

47 Carroll, 1988.

48 Ibid.

49 Romer, 1966.

50 Colbert, 1965.

51 Paraphrased in Carroll, 1988.

Quadraped Carnivor Dinosaurs

F rsiTïsenjyfifiii

Figure 5. Some non-dinosaur diapsid reptiles from the Triassic. Note the dinosaur-lilke morphology. A. Hesperosuchus, sphenosuchian crocodile, Late Triassic, length 1.3 m, from Colbert (1952). B. Ornithosuchus, ornithosuchian thecodont, Late Triassic, length 4 m, from Walker (1964). C. Saltoposuchus, sphenosuchian crocodile, Late Triassic, length 1 m, from Huene (1936). D. Tanystropheus, protorosaur, Middle Triassic, length 3 m, from Wild (1973). E. Gracilisuchus, sphenosuchian crocodile, Late Triassic, length 30 cm, from Romer (1972). F. Terristrisuchus, sphenosuchian crocodile, length 50 cm, from Crush (1984). Figures are not drawn to scale.

F rsiTïsenjyfifiii

Figure 5. Some non-dinosaur diapsid reptiles from the Triassic. Note the dinosaur-lilke morphology. A. Hesperosuchus, sphenosuchian crocodile, Late Triassic, length 1.3 m, from Colbert (1952). B. Ornithosuchus, ornithosuchian thecodont, Late Triassic, length 4 m, from Walker (1964). C. Saltoposuchus, sphenosuchian crocodile, Late Triassic, length 1 m, from Huene (1936). D. Tanystropheus, protorosaur, Middle Triassic, length 3 m, from Wild (1973). E. Gracilisuchus, sphenosuchian crocodile, Late Triassic, length 30 cm, from Romer (1972). F. Terristrisuchus, sphenosuchian crocodile, length 50 cm, from Crush (1984). Figures are not drawn to scale.

Summary

"Vertebrate paleontology is comparative osteology in the time dimension."52 Skeletal scaling principles and biomedical space research suggest that natural selection in reduced gravity will favor bone thinning, a relative decrease in skeletal mass, and an increase in the uppermost limit to body size. These predictions are borne out in the fossil record: the Late Triassic witnessed the proliferation of gracile, long-limbed and lightly-constructed diapsid reptiles (thecodonts and dinosaurs) at the expense of the synapsid (mammal-like) reptiles, animals that were much more compact, cumbersome and massively-constructed. Giant dinosaurs, such as Melanosaurus, were already present in the Late Triassic, followed soon thereafter by the largest of all land-living animals, the sauropods.

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