The Osteology of the Reptiles/Chapter 9

by Samuel Wendell Williston

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CHAPTER IX

THE SUBCLASS SYNAPSIDA

A single, lateral, temporal opening, bounded primarily by the squamosal, jugal, and postorbital only. About seven cervical vertebrae, amphicoelous. Dorsal ribs double-headed, articulating intercentrally more or less and with the arch. Pectoral girdle with two coracoids on each side, sometimes with vestigial cleithrum; interclavicle and clavicles always present. Pelvis more or less plate-like. Feet always pentadactylate.

Fig. 164 bis. Sphenacodon (Theromorpha). Restored skeleton.

4. ORDER THEROMORPHA

Vertebrae notochordal or deeply biconcave, the intercentra persistent throughout. Limbs and palate primitive. Propodials in locomotion horizontal.

There has been much discussion as to the rank and limits of the Paleozoic genera included under the above definition. Nor can we hope to reach a very satisfactory solution of the numerous problems till much more is known of them and especially of the later Permian and Triassic forms included here in the same subclass.

The above definition will distinguish fairly well the Lower Permian forms from the Middle and Upper ones, and the order Theromorpha may be therefore accepted for the present with these limitations. Originally the name was proposed by Cope to include not only the Cotylosauria but all of the African genera of the order Therapsida as

Fig. 165. Skeleton and life restoration of Dimetrodon (Theromorpha). About eight feet long.

 

well, and is still used often in that original sense or with the exclusion of the Cotylosauria.

There is greater diversity among the Theromorpha as thus distinguished than among the Cotylosauria, the only constant differences from which are the perforated temporal roof, the longer neck, and usually longer legs. Doubtless they were more active and agile animals, and their adaptive radiation was greater. But the primitive characters were less constant. The intertemporal bone is never

Fig. 166. Skeleton of Edaphosaurus (Theromorpha).

present; the interparietals, tabulars, and supratemporals are always smaller; some may be wanting, and the two former are always confined to the occipital surface when present. The quadratojugal is smaller; the lacrimal seldom extends to the nares. The teeth are often wanting on the prevomers; the postsplenial is never present in the mandible though there is a possibility of an additional coronoid, the posterior one of which is always present. The humerus has an ectepicondylar foramen only in the Edaphosauridae; the entepicondylar foramen is always present. The plate-like pelvis never has a large pubo-ischiatic or a true thyroid foramen. There are two or three sacral vertebrae. The fifth tarsale is rarely unossified. No dermal bones have been discovered in any member of the order, and parasternal ribs are known only in the Poliosauridae and

Fig. 167. Theromorph reptile Varanosaurus, from the Permian of Texas: A, restoration. B, skeleton. About one sixth natural size.

 

Fig. 168. Theromorph skeleton Ophiacodon, from mounted specimen in Walker Museum. About one twelfth natural size.

 

Ophiacodontidae. Permocarboniferous (Uppermost Carboniferous and Lowermost Permian).

A. Suborder Pelycosauria

Family Sphenacodontidae (Pelycosauria). Carnivorous reptiles of from four to eight feet in length, with long, often very long, dorsal spines; three sacral vertebrae.

Sphenacodon Marsh, New Mexico. Dimetrodon Cope, Texas. Clepsydrops Cope, Illinois, Texas. Tetraceratops Matthew, Texas. Bathygnathus Leidy, Prince Edward Island.

B. Suborder Edaphosauria

Family Edaphosauridae (Edaphosauria). Subaquatic or terrestrial invertebrate feeding reptiles, from six to eight feet in length. Spines of dorsal vertebrae very long, each with transverse processes. Skull small, short, high, with numerous palatal and coronoid conical teeth.

Edaphosaurus Cope, Texas, New Mexico. Naosaurus Cope, Texas, New Mexico, Ohio, Germany, Russia.

C. Suborder Poliosauria

Family Poliosauridae. Lizard-like, insectivorous, four or five feet in length. Teeth conical; spines of vertebrae short; two sacral vertebrae. Texas and New Mexico.

Varanops Williston, Varanosaurus Broili, Poliosaurus Case, Poecilospondylus Case, Arribasaurus Williston, Scoliomus Williston and Case.

Family Ophiacodontidae. About six feet in length, carnivorous. Skull narrow; teeth slender and conical or flattened; temporal opening small, an upper one also in Ophiacodon; ribs holocephalous; limbs short and stout; two sacral vertebrae. Texas and New Mexico.

Ophiacodon Marsh, Theropleura Cope, Diopeus Cope, Secodontosaurus Williston.

D. Suborder Caesasauria

Family Caseidae (Caseasauria). Thickset, crawling and probably burrowing, invertebrate-feeding reptiles about four feet long. Skull

Fig. 169. Skeleton of Theropleura (Theromorpha), from below. One sixth natural size.

 

Fig. 169 bis. A, Skeleton of Casea (Theromorpha). B, Restoration of Casea. About one sixth natural size.

 

broad, short, with large pineal opening, and palate and coronoids covered with conical teeth. Three sacral vertebrae. Texas.

Casea Williston, ? Trichasaurus Williston.

E. Suborder Uncertain

Family Paleohatteriidae. Small, slender reptiles. Twenty-seven presacral vertebrae; three sacrals. Parasternal ribs present. Intimate structure of skull unknown. Skeleton feebly ossified, probably young animals, the metacoracoids not ossified. Vertebrae notochordal, ribs holocephalous. Lower Permian.

Paleohatteria Credner, Germany. Haptodus Gaudry, France. ? Callibrachion Boule, France.

Incertae Sedis. Mycterosaurus Williston, Glaucosaurus Williston, Tomicosaurus Case, Metamosaurus Cope, Embolophorus Cope, Texas. Archaeobolis Cope, Illinois. Aphelosaurus Gervais, Autun, France. Stereorhachis Gaudry (? Sphenacodontidae), Autun, France.

Doubtfully members of the order: Ammosaurus Huene (Triassic). Datheosaurus.

5. ORDER THERAPSIDA

Less primitive, more upright-walking reptiles, the propodials more or less inclined in locomotion. Vertebrae amphicoelous, rarely notochordal, dorsal intercentra unknown. Palate and limbs less primitive; pelvis with larger pubo-ischiatic vacuity or thyroid opening.

As stated on a previous page, sharp distinctions between the members of this order and the preceding one cannot be made. The primitive characters common to both orders are largely included in the Synapsida. But the very great differences presented by the later, Triassic, forms, especially those included under the Cynodontia, differences as great as those between any other two orders of reptiles, render a division or divisions imperative, even though it may result, as is so often the case in other groups of animal and vegetable life, in the structural differences between members of the same group being greater than those limiting the groups themselves. This division, it seems to the writer, may be best made at the present time between the Lower and Middle Permian types, that is, based upon the stages of evolution chiefly. Perhaps when more is known of the various and diverse forms included in both orders, a better and more scientific division may be made on genealogical characters. But such are not available at present.

The characters, as a whole, of the Therapsida are primitive, but less so than those of the Theromorpha, and they are increasingly inconstant. The vertebrae are known to be notochordal only in the Dromasauria and Dinocephalia, and the intercentra are seldom if ever persistent throughout the column; there may be as many as seven sacral vertebrae; the boundaries of the temporal opening are less constant; in a few words, no characters seem to be more primitive than in the Theromorpha. The interparietals, when present, are fused into a single bone, which is rarely the case in the Theromorpha. The supratemporals are always, the postfrontals often, the quadratojugals usually, absent.^[1] The palate and teeth undergo many changes; the pterygoids are less free, palatal teeth are less constant. The cleithrum is seldom present and always small, etc.

But to divide the various groups into orders seems not to solve but rather to add to the difficulties. For that reason, perhaps it is better at present to consider the whole group as one order, as Broom has suggested, clearly differentiated from all others save the Theromorpha by the skull and pectoral girdle, and to treat its characters under the chief divisions. Of course the distribution of some, perhaps many, of the genera is more or less provisional, as must be the case in any order of reptiles or other organisms until everything about them is fully known, a result greatly to be wished, but never within the limits of human endeavor. The classification adopted is that of Broom and Watson in numerous publications and in literis, with but few modifications.

A. Suborder Dinocephalia

Powerful reptiles from the size of a boar to that of a rhinoceros. Skull very massive, especially in the cranial region. Temporal opening bounded by the postorbital and squamosal, the jugal sometimes intervening below. Lacrimals and quadratojugals small, the interparietal and tabulars large. No dermal bones fused in midline. Parietal opening large, opening in a protuberance or boss. Teeth more or less flattened and denticulated along their border,^[2] not more than eighteen in either jaw, subisodont or with a large caniniform tooth; no teeth on palate. Prevomers, palatines, and pterygoids united in midline, concealing the parasphenoid. Quadrate large. Vertebrae deeply concave or notochordal. Atlanto-axis as in Dimetrodon (Theromorpha); four sacral vertebrae. Ribs dichocephalous, probably no parasternals. Shoulder girdle massive; procoracoid barely entering glenoid fossa; a feeble cleithrum sometimes, if not always, present. Large clavicles and interclavicle. No acromion. Pelvis with small pubo-ischiatic vacuity. An entepicondylar foramen. Legs stout; epipodials and digits short; phalangeal formula unknown, probably primitive.

Fig. 170. Skeleton of Moschops (Dinocephalia). After Gregory. One twenty-second natural size. Skeleton in American Museum.

Family Tapinocephalidae. Middle and Upper Permian. Delphinognathus Seeley, Lamiasaurus^[3] Watson, Moschognathus Broom, Mormosaurus Watson, Moschops Broom, Moschosaurus Haughton, Phocosaurus Seeley, Pnigalion Watson, Struthiocephalus Haughton, Tapinocephalus Owen, Taurops Broom, Archaeosuchus Broom, Scapanodon Broom, Eccasaurus Broom, South Africa.

Family Deuterosauridae. Upper Permian. Deuterosaurus, Eichwald, Ural Mts.

Family Rhopalodontidae. Upper Permian. Rhopalodon, Eichwald, Ural Mts.

Family Titanosuchidae.^[4] Upper Permian. Titanosuchus Owen, South Africa. "Lamiasaurus" [snout].

B. Suborder Dromasauria

About the size of a rat. Skull short; orbits large; lacrimals continuous to septomaxilla; temporal opening bounded by postorbital, squamosal, and jugal; possibly the preparietal, and probably the interparietal, present; parietal foramen large; teeth isodont, subisodont, or absent; quadratojugals obsolete or absent; vertebrae notochordal, intercentra unknown; two or three sacrals, probably twenty-eight presacrals; parasternals present; no acromion and no cleithrum; pelvis plate-like, pubic foramen large; carpus primitive, tarsus with or without a fifth tarsale; phalangeal formula 2, 3, 3, 3, 3.

Family Galechiridae. A single row of subisodont teeth.

Middle Permian. Galechirus Broom, Galesphyrus Broom, Galepus Broom, South Africa.

Family Galeopidae. Edentulous.

Middle Permian. Galeops Broom, South Africa.

Family Macroscelesauridae. Macroscelesaurus Haughton.

C. Suborder Anomodontia

From the size of a mouse to that of a tapir, vegetable or invertebrate feeders. Large temporal opening bounded by postorbital, squamosal, and jugal. Skull typically short and wide, the face short; quadrates and squamosals large; lacrimals small; quadratojugals small or obsolete.^[5] Preparietal usually present, in front of, or surrounding, parietal foramen. An interparietal and small tabulars. Premaxillae fused and always toothless, and in life covered with horny beak. Maxilla usually with an enlarged, permanently growing canine, which, however, is absent in the females of some genera, and generally with a number of small molars often irregularly arranged in more than one series. Molars are always present on the mandible if in the maxilla, but there is never any canine present. Prevomers fused. A rudimentary false palate, no teeth on palatal bones. Stapes large. Occipital condyle tripartite. Dentary, angular, and surangular large; no coronoid. A mandibular foramen. Sclerotic plates in orbits. Vertebrae amphicoelous; no intercentra back of atlas; four to seven sacrals. No parasternals. Legs short and stout, hands and feet short; an entepicondylar foramen. Phalangeal formula 2, 3, 3, 3, 3. A thyroid foramen in pelvis; ilium projecting in front of acetabulum. An ossified sternum. The shoulder girdle has the coracoid and precoracoid well developed, and a distinct but short acromion. There is a small cleithrum known in Dicynodon and Cistecephalus, and possibly present in most other genera.

Family Dicynodontidae. Middle Permian. Dicynodon Owen, Pristerodon Huxley, South Africa.

Upper Permian. Tropidostoma Seeley, Diaelurodon Broom, Prodicynodon Broom, Eocyclops Broom, Emydops Broom, Diictodon Broom, Emydorhynchus Broom, Emyduranus Broom, Taognathus Broom, Cryptocynodon Seeley, Endothiodon Owen, Cistecephalus Owen, Chelyrhynchus Haughton, South Africa, Dicynodon Owen, South Africa and Russia.

Lower and Middle Triassic. Dicynodon Owen, Lystrosaurus Cope, Prolystrosaurus Haughton, Myosaurus Haughton, South Africa.

Upper Triassic. Kannemeyeria Seeley, Gordonia Newton, Geikia Newton, Scotland. Placerias Lucas, Brachybrachium Williston, Wyoming.

D. Suborder Theriodonta

Carnivorous Therapsida with more or less differentiated dentition, including at least one pair of upper caniniform teeth; a prominent coronoid. Vertebrae never notochordal; few or no teeth on palate bones. No cleithrum. Manus and pes, so far as known, rarely primitive.

Fig. 171. Skeleton of Lystrosaurus, as restored by Watson, slightly modified. One fourth natural size.

 

1. Tribe Gorgonopsia

Prefrontals and large postfrontals contiguous over orbit. A distinct preparietal in front of small parietal foramen. Temporal opening bounded above by united postorbital and squamosal, below by squamosal and jugal or squamosal only. Parietal region wide. A single vomer (? fused prevomers). No secondary palate; an ectopterygoid. No acromion on scapula; no cleithrum; coracoids relatively small; a large proatlas. Phalangeal formula primitive 2, 3, 4, 5, 3.

A group intermediate, according to Broom, between the Therocephalia and Anomodontia.

Middle and Upper Permian.

Family Gorgonopsidae. Gorgonops Owen, Scymnognathus Broom, Cyniscodon Broom, Cerdognathus Broom, Scymnosaurus Broom, Gorgonognathus Haughton, Scylacognathus Broom, Scylacops Broom, Galesuchus Haughton, Ictidomorphus Broom, Aloposaurus Broom, Aelurosaurus Owen, Cynodraco Owen, Tigrisuchus Owen, Arctosuchus Broom, Arctognathus Broom, Arctops Watson, Theriodesmus Seeley, Asthenognathus Broom, South Africa. Inostrancevia Amalitsky, Russia.

Family Ictidorhinidae. Middle and Upper Permian. Ictidorhinus Broom, South Africa.

Family Burnetidae. Lower Triassic. Burnetia Broom,^[6] South Africa.

2. Tribe Bauriasauria

A well-formed secondary palate; a median, unpaired vomer; single occipital condyle; the pterygoids extend to quadrates; no postfrontals; squamosal small; quadrate large; parietal foramen present or absent; strong incisors and grinding molars; large posterior palatine vacuities. No acromion on scapula.

Upper Triassic. Bauria Broom, Microgomphodon Seeley, Melinodon Broom, Sesamodon Broom, Aelurosuchus Broom, South Africa.

 

3. Tribe Therocephalia

Temporal opening large, bounded below by squamosal and jugal, above by the parietal or the connected postorbital and squamosal.^[7] No quadratojugals^[8]; quadrates small; a parietal foramen; squamosals large; no preparietal. Teeth conical, four or five in premaxilla; one or two large upper caniniform teeth, and five to nine smaller ones posteriorly; no secondary palate, or a rudimentary one (? Scaloposaurus); prevomers separated or fused (Scaloposaurus); an interpterygoidal opening; large posterior palatine vacuities; palate with few or no teeth; postfrontals small or absent; parietal region usually narrow. Mandible with loose symphysis, long dentary, and large coronoid; posterior elements not reduced. Postcranial skeleton largely unknown.

Family Scylacosauridae. Middle and Upper Permian. Alopecodon Broom, Pardosuchus Broom, Glanosuchus Broom, Scylacosaurus Broom, Pristerognathus Seeley, Ictidosaurus Broom, Alopecognathus Broom, Scylacorhinus Broom, South Africa.

Family Ictidosuchidae. Middle and Upper Permian. Ictidosuchus Broom, Arnognathus Broom, Cerdodon Broom, South Africa.

Family Lycosuchidae. Middle and Upper Permian. Lycosuchus Broom, Trochosuchus Broom, Hyaenasuchus Broom, South Africa.

Family Scaloposauridae. Middle and Upper Permian. Scaloposaurus Owen, Ictidognathus Broom, Simorhinella Broom, Icticephalus Broom, Akidnognathus Haughton, South Africa.

Family Alopecopsidae. Middle and Upper Permian. Alopecopsis Broom, Scymnopsis Broom, South Africa.

Family Whaitsidae. Whaitsia Haughton, South Africa.

Family doubtful. Middle and Upper Permian. Lycosaurus Owen, Eriphostoma Broom, Lycorhinus Broom, Scymnorhinus Broom, Alopecorhinus Broom, Scylacoides Broom, South Africa.

 

4. Tribe Cynodontia

Fig. 172. Skeleton of Cynognathus (Therapsida). After Gregory. One fourteenth natural size.

Especially characterized by a heterodont dentition, a secondary palate, reduced posterior mandibular bones, and two occipital condyles. Dentition composed of from three to five incisors, a canine, and seven to nine, rarely thirteen, molars, secodont or gomphognath or cuspidate. Temporal opening bounded by parietal and postorbital above, usually by squamosal and postorbital only below; frontals small, excluded from orbital margin by the union of the prefrontal and postorbital; postfrontals absent; parietals narrow; a small parietal foramen, but no preparietal bone; tabular large; quadrate small; stapes long, stout or slender; the pterygoids do not reach the quadrate; probably a small ectopterygoid; vomer large, unpaired. Coronoid large. A small acromion on scapula; scapula with reflected anterior border; no cleithrum. Fifth carpale unossified; phalangeal formula 2, 3, 3, 3, 3, so far as known. A thyroid foramen in pelvis. Feet imperfectly known, the digits short. Vertebrae amphicoelous; no dorsal intercentra. Twenty-eight presacrals, four sacrals.

Family Nythosauridae. Septomaxillae on face; molars less cuspidate; posterior mandibular bones less reduced.

Middle Triassic. Nythosaurus Owen, Ictidopsis Broom, Galesaurus Owen, Platycraniellus v. Hoepen, South Africa.

 

Family Cynosuchidae. ? Middle Triassic. Cynosuchus Owen, South Africa.

Family Cynognathidae. Septomaxillae within nares; molars cuspidate; [posterior] mandibular bones more reduced.

Upper Triassic. Cynognathus Seeley, Lycochampsa Broom, South Africa.

Family Diademodontidae. Upper Triassic. Diademodon Seeley, Gomphognathus Seeley, Trirachodon Seeley, South Africa. ? Upper Triassic Cynochampsa Owen, South Africa.

5. Theriodontia (?) Incertae Sedis

Upper Triassic. ? Dromotherium Emmons, North Carolina. Tribolodon Seeley, Karoomys Broom, South Africa.

Lower Jurassic. Tritheledon Broom, Pachygenelus Watson, South Africa.

[May be either primitive mammals or cynodonts—too imperfectly known to enable one to decide. Probably each is the type of a distinct family. —R. Broom.]

 

1. [The quadratojugal has recently been identified in anomodonts, gorgonopsians, and cynodonts, by Watson.—Ed.]
2. [This statement refers only to the cheek teeth; the premaxillary teeth and the first three or four in the dentary have a long conical crown, greatly expanded posteriorly at the base, and long roots.—Ed.]
3. [Cranium, Fig. 170.—Ed.]
4. [A number of new genera of South African Titanosuchidae were described by Broom in 1923 (Proc. Zoöl. Soc., London).—Ed.]
5. [See page 243, below.—Ed.]
6. [Made the type of a new suborder, Burnetiamorpha, by Broom, 1923.—Ed.]
7. [In typical Therocephalia, as described by Broom, the postorbital and squamosal do not connect with each other.—Ed.]
8. [See page 239, above.—Ed.]