Box Relationships Of The Synapsid Groups

Synapsida traditionally have been divided into 'pelycosaurs', a paraphyletic group, and therapsids, a well-characterized clade (see cladogram). Both groups together were formerly sometimes called 'mammal-like reptiles'. The pelycosaurs appear to form a sequence of outgroups to Therapsida, with the sphenacodontids being the most derived (Reisz, 1986). The Biarmo-suchia, perhaps a paraphyletic group, are the most basal therapsids. The dinocephalians, dicynodonts and gorgonopsians follow next (Hopson and Barghusen, 1986; Sidor and Hopson, 1998). Some have suggested the dinocephalians and dicynodonts should be paired as Anomodontia (King, 1988), but the dinocephalians may in fact be paraphyletic (Sidor and Hopson, 1998). The dicynodont branch is probably more extensive, i.e. the Anomodontia proper, consisting of basal taxa such as the venyukovioids from Russia and relatives from South Africa (Modesto and Rybczynski, 2000). The therocephalians are a sister group of the cynodonts, which properly include the mammals.

For more detail on the phylogeny of basal synapsids, go to http://tolweb.org/tree?group=Synapsida&contgroup= Amniota#TOC2

Phylogentic Tree Synapsids

Cladogram showing the postulated relationships of the main groups of synapsids, according to Hopson and Barghusen (1986), Reisz (1986) and Sidor and Hopson (1998). See Box 5.1 for context of Synapsida, and Figure 10.1 for relationships of Cynodontia. Synapomorphies: A SYNAPSIDA, maxilla contacts quadratojugal, caniniform maxillary teeth, lower temporal fenestra, paroccipital process contacts tabular and squamosal, trunk neural arches narrow; B, pointed snout formed by anteriorly tilted premaxilla, external nostril elongated, maxilla enters ventral margin of orbit; C, frontal forms at least one-third of dorsal margin of orbit, narrow long supratemporal located in a groove formed by parietal and squamosal; D, dorsal process of stapes articulates in a socket on the paroccipital process, cheek margin concave; E, postorbital narrow, parietal foramen well in front of occiput, stapes with blade-like shaft, ilium expanded in front and with horizontal dorsal margin; F, buttress in maxilla at root of caniniform tooth, premaxillary teeth in deep sockets; G THERAPSIDA, septomaxilla facial exposure extensive, contact between maxilla and prefrontal, external acoustic meatus in squamosal, basipterygoid articulation absent, jaw joint in line with occiput, anterior coronoid absent, serrations on teeth, 12 or fewer teeth behind caniniform, ectopterygoid teeth absent, vertebral notochordal canal absent in adult, anterior dorsal intercentra absent, cleithrum and clavicle separated, ossified sternum, acetabulum deep; H, posterior skull roof narrow, temporal fossa expanded laterally; I, pineal foramen opens flush with skull roof, lateral palatal foramen, coronoid process present and formed by dentary or dentary and surangular; J THERIODONTIA, zygomatic arch flares laterally, coronoid process on dentary, dentary masseteric fossa, postdentary bones reduced in height, atlas and axis pleurocentra fused, calcaneal tuber; K, temporal roof eliminated and temporal fossae meet in midline sagittal crest, postorbital reduced, postfrontal reduced, secondary palate on maxilla and palatine, teeth absent on palatine bone, dentary extends below angular.

Fig. 5.12 The ophiacodonts (a) Archaeothyris and (b—d) Ophiacodon: (a) partial skull and skeleton in lateral view; (b) skeleton; (c, d) skull in lateral and ventral views. (Modified from Romer and Price, 1940, and Reisz, 1986.)

Height EdaphosaurusEothyris Pictures
Fig. 5.13 Three early pelycosaurs: (a) Eothyris skull; (b-d) Cotylorhynchus skeleton and skull in lateral and dorsal views; (e,f) Varanops skull in lateral and dorsal views. (After Romer and Price, 1940.)
Edaphosaurus Fossils
Fig. 5.14 The herbivorous pelycosaur Edaphosaurus: (a) skeleton; (b-d) skull in lateral,dorsal,and ventral views. [Figure (a) after Romer and Price, 1940; (b-d) courtesy of Sean Modesto.]

probably covered by skin, hence the popular term 'sail backs'. The skull of Edaphosaurus (Modesto, 1995) is relatively small in comparison with the body size, and it shows several adaptations to herbivory: peg-like teeth, a deep lower jaw, a sliding jaw joint to allow propalinal (back-and-forwards) jaw movements, and extensive palatal teeth (Figure 5.14(d)) that are large and form a broad crushing surface, and occlude against a similar battery on the lower jaw.

The sphenacodontids (Reisz, 1986) were medium-to large-sized carnivores from the Upper Carboniferous and Lower Permian of North America and Europe. Dimetrodon from the Lower Permian of Texas and neighbouring states, as well as Germany, has a large sail, and it reaches a length of about 3 m. It has a large skull, with a small orbit and a high temporal fenestra (Figure 5.15). The powerful jaw muscles of Dimetrodon have been reconstructed (Figure 5.15(c)): the adductors were attached to the inside of the lower jaw and pulled the jaws shut, whereas the pterygoideus ran from the pterygoid to the outer face of the angular and provided a backwards jaw movement.

The pelycosaur sail has long been a puzzle. The neural spines in Dimetrodon have grooves at the base that were probably occupied by blood vessels. Further, when fossil skeletons are excavated, the neural spines generally lie in a neat fence-like array, which suggests that they were held together by a tough covering of skin in life. The 'sail' then was probably composed of heavily vascularized skin, and its function seems to have been thermoregulatory (Haack, 1986). The idea is that, early in the morning, when Dimetrodon was cold and sluggish, it would stand with its sail fully facing the sun, and would then absorb heat rapidly. This would have given it a head start over its sail-less prey. Later in the day, if it became overheated, Dimetrodon could stand in the shade and radiate heat from the sail. The weakness of this argument is that most pelycosaurs, and their contemporaries, lacked sails, and yet seemed to have survived perfectly well.

Dimetrodon Skull
Fig. 5.15 The carnivorous pelycosaur Dimetrodon: (a) skeleton; (b) skull in lateral view; (c) major jaw muscles reconstructed, in lateral view. (After Romer and Price, 1940.)
Dimetrodon Skull

5.5.3 Biarmosuchia: the basal therapsids

Derived characters of therapsids, in comparison with the 'pelycosaurs', include an enlarged temporal fenestra, loss of the supratemporal bone, a deeply notched reflected lamina on the angular bone (Figure 5.16(c)), a forwards position of the jaw joint, reduction of the palatal teeth, as well as modifications of the shoulder and pelvic girdles and of the hindlimb (Kemp, 1982; Hopson and Barghusen, 1986; Sidor and Hopson, 1998; see Box 5.3).

A synapsid from the Early Permian of Texas, USA, Tetraceratops (Figure 5.16(a, b)), may be the oldest-known therapsid (Laurin and Reisz, 1996). In many features, it seems to be intermediate between sphenacodontid pelycosaurs and later therapsids, but it shows an enlarged temporal fenestra and some reduction in the palatal teeth. Further early therapsids come from the Late Permian of Russia (Battail and Surkov, 2000). Biarmosuchus (Figure 5.16(c, d)), for example, was a small carnivore that resembled the sphenacodon-tids in most respects. The occiput slopes back rather than forwards, however, and the supratemporal bone is absent. The numbers of teeth are reduced and there is a prominent single canine, as well as a few small palatal teeth. An additional element, the septomaxilla, present within the nostril of pelycosaurs, is now exposed on the side ofthe skull.

5.5.4 Dinocephalia

The dinocephalians include 40 genera of synapsids known only from the Upper Permian of Russia and South Africa, which fall into both carnivorous and herbivorous lineages (Kemp, 1982; King, 1988; Battail and Surkov, 2000). A carnivorous form, Titanophoneus from the Upper Permian of Russia (Figure 5.16(e)), is a large animal with short limbs and a heavy skull. The incisors and canines are well developed, and presumably they were used for grasping and piercing prey.

The Tapinocephalidae includes a range of herbivorous forms, some quite bizarre in appearance. Moschops from South Africa (Figure 5.16(f)) is a large animal about 5 m long with a massive ribcage and heavy limbs, but tiny feet. The hindlimbs were held close under the

Tapinocephalidae

200 mm

Fig. 5.16 Early therapsids: (a,b) skull ofTetraceratopsin lateral and ventral views; (c,d) skull ofBiarmosuchusin lateral and dorsal views; (e) carnivorous dinocephalian Titanophoneus; (f) herbivorous dinocephalian Moschops. [Figures (a,b) modified from Laurin and Reisz, 1996; (c,d) after Sigogneau and Chudinov, 1972; (e,f) after King, 1988.]

200 mm

Fig. 5.16 Early therapsids: (a,b) skull ofTetraceratopsin lateral and ventral views; (c,d) skull ofBiarmosuchusin lateral and dorsal views; (e) carnivorous dinocephalian Titanophoneus; (f) herbivorous dinocephalian Moschops. [Figures (a,b) modified from Laurin and Reisz, 1996; (c,d) after Sigogneau and Chudinov, 1972; (e,f) after King, 1988.]

body in a derived erect posture, whereas the forelimbs still stuck out sideways in a sprawling posture. The head is also relatively small compared with the body, reminiscent of the herbivorous pelycosaurs (cf. Figures 5.13(c) and 5.14(a)). The skull of Moschops has a rounded snout, but the posterior part is elevated in a broad, square heavily-built structure. What was its function? The roofing bones of the cranium are extraordinarily thick (up to 100 mm), and it has been suggested (Barghusen, 1975) that this was an adaptation for head butting, as is observed today among sheep and goats (Figure 5.17). The main force of the butt hit the thickened dorsal shield of the skull, and was transmitted round the sides to the occipital condyle. The occiput was also thickened and placed well beneath the skull, and the occipital condyle lay in direct line with the butting point. The impact was then transmitted down the thick vertebral column of the neck to the massive shoulder region.

Fig. 5.17 Head-butting behaviour in Moschops: (a) lateral view of the skulls of two butting individuals showing the line of transmission of the impact through the occipital condyle; (b, c) dorsal and ventral views of the skull showing the broad thickened dorsal shield, and transmission of forces from it through the postorbital and post-temporal bars to the occipital condyle. (After Barghusen, 1975.)

Fig. 5.17 Head-butting behaviour in Moschops: (a) lateral view of the skulls of two butting individuals showing the line of transmission of the impact through the occipital condyle; (b, c) dorsal and ventral views of the skull showing the broad thickened dorsal shield, and transmission of forces from it through the postorbital and post-temporal bars to the occipital condyle. (After Barghusen, 1975.)

5.5.5 Dicynodontia

The dicynodonts, a group of over 70 genera, were dominant herbivores in the Late Permian (Kemp, 1982, King, 1988; Angielczyk, 2001), and nearly all died out at the end of the Permian. Late Permian dicynodonts, such as Robertia,were generally medium-sized pig-shaped animals with barrel-shaped bodies and unsatisfactory tails (Figure 5.18(a)). Dimensions ranged from rat- to hippo-sized. Dicynodonts were hit hard by the end-Permian mass extinction (see p. 133), but several new dicynodont lines radiated in the Triassic, and some were large, being 3 m or so long. These must have had an ecological role similar to large modern browsing mammals. The Triassic Kannemeyeria (Figure 5.18(b)) has a narrow pointed snout and the parietals form a high crest. The ribcage is vast and the limbs and girdles powerfully built. Dicynodont biology has been studied in some detail (see Box 5.4).

5.5.6 Gorgonopsia

The dominant carnivores in the Late Permian were the gorgonopsians (Figure 5.19(a, b)), a group of some 35

Fig. 5.18 Skeletons of dicynodonts: (a) Robertia from the Upper Permian; (b) Kannemeyeria from the Middle Triassic. (After King, 1988.)

Fig. 5.18 Skeletons of dicynodonts: (a) Robertia from the Upper Permian; (b) Kannemeyeria from the Middle Triassic. (After King, 1988.)

Robertia Permian

In the Late Permian, dicynodonts such as Pristerodon made up 80-90% of species in typical faunas (Benton, 1983a). Often, five or six dicynodont species of different sizes were present in a fauna, and they were preyed on by carnivorous dinocephalians or gorgonopsians. The huge success of these Late Permian dicynodonts may relate to their specialized jaw apparatus.

Pristerodon, a small dicynodont from the Upper Permian of South Africa, has a skull 40-60 mm long with a particularly short snout (see illustration I(a-d)). Pristerodon, unlike many dicynodonts, retains a few teeth in addition to the canines, about six postcanines in the maxilla and in the dentary. These small sets of teeth worked against each other, and they are worn to form a single grinding surface. The rest of the jaw margins are made of sharp bone, presumably covered by a horny beak in life.

Pristerodon had a highly mobile jaw joint. The articulating surface of the articular is nearly twice as long as that of the quadrate, so that the lower jaw could slide some distance back and forwards during a jaw opening cycle. Crompton and Hotton (1967) reconstructed the jaw actions of Pristerodon using a complete and undistorted skull. By manipulating the jaws and studying patterns of tooth wear, they were able to work out with some confidence how Pristerodon seized and processed food (see illustration II(a-d)). Firstly, the jaw opened fully, then moved forwards by sliding at the joint. The food was taken in between the tips of the jaws as the lower jaw closed completely, and was then pulled back firmly with the jaw joint sliding back. This last retraction phase was the most powerful and had the effect of tearing the food at the front of the mouth and slicing any food that was between the cheek teeth.

The jaw muscles of Pristerodon were also reconstructed (illustration II(e-g)) by an analysis of the shape of the jaw bones and patterns of the surface. Most of the jaw adductors ran nearly horizontally, and their contraction would have powered the retraction phase of the jaw action. These key muscles include a major lateral external adductor that ran from the outside of the squamosal and quadratojugal to a long ridge on the side of the dentary (illustration II(e)), a medial external adductor that ran

roller-like jaw joint

I The skull of the dicynodont Pristerodon in (a) lateral, (b) dorsal, and (c) ventral views, and (d) the mandible in dorsal view. (After Crompton and Hotton, 1967.)

roller-like jaw joint

I The skull of the dicynodont Pristerodon in (a) lateral, (b) dorsal, and (c) ventral views, and (d) the mandible in dorsal view. (After Crompton and Hotton, 1967.)

inside the zygomatic arch from the parietal and postorbital to the top of the dentary (illustration II(f)). Other features include a flexible sheet of tissue in the cheek region, the Mundplatt, which limited the jaw opening, and was kept taut by the levator an-gularis oris muscle (illustration II(e)), a small pterygoideus muscle that pulled the jaw forward, and the jaw opening muscle, the depressor mandibulae (illustration II(g)), that ran from the back of the squamosal to the retroarticular process, the part of the lower jaw behind the jaw pivot.

Pristerodon fed on vegetation that it snipped off with its horny beak and passed back, probably with a muscular tongue, to the cheek region for grinding and crushing before it was swallowed. The tusks of certain dicynodonts show wear striations when they are examined under high magnification, which suggests that they were used for scraping in the soil for plant material, and the diet may have consisted of roots, horsetail stems, club mosses and ferns. Dicynodont tusks may also show wear on the inside surfaces, confirming the prolapinal (back-and-forwards) jaw movements.

Synapsids Diet

II (a-d) A single chewing cycle of Pristerodon, (a) as It lowers Its jaw and moves it back, (b) moves it forward, (c) up for the bite, (d) and backwards to tear the food; (e-g) restoration of the jaw muscles of Pristerodon, drawn as if at progressively deeper levels, from (e) to (g). (After Crompton and Hotton, 1967.)

genera from southern Africa, Russia and China. Their anatomy is remarkably conservative, most forms being about 1 m long and with a skull superficially like that of the earlier carnivorous therapsids. A typical form, Arctognathus, could have opened its jaws with a gape of 90° or so in order to clear its vast canines. The jaws then accelerated shut on to the prey animal, and the large fangs passed each other but did not touch, thus effectively piercing the skin and flesh, and disabling its victim. The jaw then shifted forwards and the incisors met, thus removing swallowable chunks of flesh (Kemp, 1969). The gorgonopsians are reminiscent of sabre-toothed cats, which arose much later on and had similar enlarged canines and vast gapes (see p. 349). The gorgonopsians may have owed their success to the ability to prey on large thick-skinned dinocephalians and dicynodonts, and when these groups dwindled at the end of the Permian, so too did their predators.

Arctognathus

Fig. 5.19 The gorgonopsians (a) Lycaenops, (b) Arctognathus, and (c) Leontocephalus; (d,e) the therocephalian Theriognathus. [Figure (a) after Broom, 1932; (b, c) after Kemp, 1969; (d, e) after Brink, 1956, courtesy of the Bernard Price Institute.]

Fig. 5.19 The gorgonopsians (a) Lycaenops, (b) Arctognathus, and (c) Leontocephalus; (d,e) the therocephalian Theriognathus. [Figure (a) after Broom, 1932; (b, c) after Kemp, 1969; (d, e) after Brink, 1956, courtesy of the Bernard Price Institute.]

5.5.7 Suborder Therocephalia

The therocephalians, another group of carnivorous therapsids, survived from the Late Permian into the Tri-assic. They ranged in size from small insectivores to large carnivores, and also include some herbivores in the Early Triassic. Theriognathus, a small carnivorous form from the Upper Permian of South Africa (see Box 5.5), has a skull 75mm long (Figure 5.19(c, d)) with large orbits and temporal fenestrae. It shows several derived characters in comparison with the gorgonopsians (see Box 5.3): a reflected lamina placed near the back of the jaw, a vaulted palate made from vomer, premaxilla, maxilla and palatine (Figure 5.19(d)), and a narrow parietal crest that was extensively covered with the jaw adductor muscles.

Several lineages of therocephalians survived into the Triassic, and one group, represented by Bauria from the Lower Triassic of South Africa (Figure 5.20(e, f)), became successful herbivores. The teeth of Bauria are generally robust, and there is a solid battery ofbroad cheek teeth for cutting up fibrous plant material. Between these teeth, the palate is vaulted over with bone to form a secondary palate. This is like the secondary palate of mammals. Bauria also has another superficially mammalian character in the loss of the bar of bone between the orbit and temporal fenestra.

5.5.8 Cynodontia

The cynodonts, as a clade, include the mammals (see

Cynodont Synapsids
Fig. 5.20 The early cynodont Procynosuchus, skull in (a) lateral, (b) dorsal, (c) ventral,and (d) occipital views; (e,f) the herbivorous therocephalian Bauria, skull in dorsal and ventral views. [Figures (a-d) after Kemp, 1979; (e, f) modified from Carroll, 1987.]

BOX 5.5 THERAPSIDS OF THE KAROO

BOX 5.5 THERAPSIDS OF THE KAROO

Late Permian therapsids are best known from the Karoo basin of South Africa, and the southern Urals region of Russia. The first records of these extraordinary animals came from South Africa in the 1850s, and since then many thousands of skulls and skeletons have been collected. The Karoo basin covers a huge area, some 600,000 km2, more than half of South Africa, and the sequences of Permian to Jurassic sediments total 12 km in thickness (Smith, 1995). During the Late Permian, sediments were fed into the Karoo basin from a ring of mountains that girdled southern Gondwanaland, partly located on what is now South America and Antarctica.

The Upper Permian and Lower Triassic sediments of the Karoo basin belong to the Beaufort Group, which is subdivided into eight biozones, based on the distributions of tetrapod taxa. Each biozone is 250-450m thick. In all, the Beaufort Group has yielded about 100 therapsid genera, belonging to all major groups, as well as anapsids (pareiasaurs, procolophonids, millerettids), diapsids (Youngina), temnospondyls and palaeoniscid fishes. The fossil amniotes are found in association with mudstones and sandstones that were deposited by meandering rivers on a broad floodplain, and soil horizons that developed after flooding episodes. Skeletons are preserved most often in a partly disarticulated condition in mudstones that were laid down between the river channels.

Spectacular recent discoveries include excellently preserved dicynodont skeletons in coiled burrows (see illustration). The animals evidently constructed deep burrows near to river channels, perhaps to escape the midday sun, and occasionally the hapless animals were trapped by an unexpected flash flood.

continued

For everything on the fossils of the Karoo, the locations and the specimens, go to http://www.museums.org.za/sam/ resource/palaeo/cluver/index.html, an online reference, and http://www.wits.ac.za/science/palaeontology/bpihome.html, home page of the Bernard Price Institute for Palaeontological Research.

Cynodont Skeleton
Dicynodont burrows from the Late Permian, Karoo basin, South Africa: (a) part of a corkscrew-shaped living burrow (matchbox is 50 mm long); (b) skeleton of a curled-up dicynodont, overwhelmed by a flash flood (skeleton is 200 mm long). (Courtesy of Roger Smith.)

Box 5.3). Cynodonts arose at the end of the Permian and radiated mainly in the Triassic. The Permian forms are described here, and later cynodont evolution will be considered in Chapter 10, as a prelude to the origin of the mammals.

Procynosuchus from the uppermost Permian of southern Africa (Kemp, 1979) has a long-snouted skull with an expanded temporal region (Figure 5.20(a-d)). Procynosuchus shows a large number of features that are generally mammalian in character (Kemp, 1982; Hopson and Barghusen, 1986): the wide lateral flaring of the zygomatic arches that allowed an increased mass of jaw adductor muscles; a depression, the adductor fossa, for expanding jaw muscles on the upper part of the dentary behind the tooth row; an enlarged coronoid process of the dentary making up more than three-quarters of the length of the lower jaw; an enlarged nasal bone; the frontal excluded from the margin of the orbit; a double occipital condyle (Figure 5.20(d)); and the beginnings of a secondary palate composed largely of the maxillae and palatines (Figure 5.20(c)), rather than the vomers and maxillae, as in therocephalians. The size of Procynosuchus, and the nature of its teeth, suggest that it ate insects or small tetrapods.

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Responses

  • William
    What are the suggested functions of the dorsal sail of the pelycosaur?
    7 years ago
  • Carl
    Are amphibians part of synapsida?
    7 months ago

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