Palaeos Vertebrates 400.100 Therapsida : Therapsida

*Palæos:*		Unit 400: Therapsida
*The Vertebrates*		100: Therapsida

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Therapsida

Taxa on This Page

Tetraceratops: misnomer & miscellaneous

Mathew (1908)

The generic name of Tetraceratops insignis Matthew 1908 means "four-horned face," although it has at least six protruberances, none of which are horns. That original misnomer is symptomatic of the confusion surrounding this animal. The fossil, a 95 mm unque skull, was originally recovered from the Early Permian Arroyo Formation (Clear Fork Group) of Baylor County, Texas. It was initially identified as an eothyrid pelycosaur of uncertain relationships. The only known specimen was embeded in a particularly recalcitrant rock matrix and was very difficult to prepare.

Laurin & Reisz (1996)

At least three pairs of horns are present on this animal, one on each premaxilla, prefrontal, and angular bones of the skull. These give the animal a superficial similarity to the primitive therapsid Burnetia mirabilis, a biarmosuchian. However, this does not necessarily indicate close relationships, because the horns are not located on the same bones. It is interesting to coinsder that similiar hornlets are found on a number of different (and only distantly related) theropod dinosaurs - Dilophosaurus, Cryolophosaurus, Allosaurus, Carnotaurus, Tyrannosaurus, etc). It is not unlikely that in all these instances the horns may have been brightly coloured and served as instruments of intra-specific display. Tetraceratops synapomorphies

Laurin & Reisz determined that Tetraceratops and the therapsids form a monophyletic group because they share the following seven derived features (synapomorphies) which appear in the primitive condition in sphenacodontines (see diagram at left):

(1) the precanine teeth are lost;
(2) the ectopterygoid teeth are lost;
(3) the upper margin of the lateral temporal fenestra is broadened and forms a new concave, ventro-laterally facing surface from which the jaw adductor musculature originated;
(4) the quadrate, the upper element of the jaw suspension, is reduced in size;
(5) a posteromedian flange of the pterygoid is present behind the interpterygoid vacuity;
(6) the ventral plate of the epipterygoid is much reduced in size;
(7) the interpterygoid vacuity is reduced in length.

The presence of a number of diagnostic therapsid features cannot be tested in Tetraceratops because of the fragmentary nature of the specimen. However, all other therapsids would seem to form a monophyletic group excluding Tetraceratops because they share the following derived characters that are not present in Tetraceratops and pelycosaurs:

(A) the septomaxilla is small, lacking a facial process;
(B) the maxilla contacts the prefrontal;
(C) the maxilla contacts the nasal and excludes the lacrimal from the naris;
(D) a distinct incisiform region is present; and
(E) all the incisors are equal in size.

This combination of primitive and advanced features showed that Tetraceratops is intermediate between pelycosaurs and all previously known therapsids, an could serve as pretty good ancestral type (or "sister taxon" in cladistic jargon) to the later Therapsida.

Conrad & Sidor (2001)

More recently, however, Conrad & Sidor (2001) again reviewed this specimen and concluded that Tetraceratops was neither an eothyrid nor a therapsid. Rather, it was a sphenacodontid. Their detailed findings are mentioned on the previous page. In essence, they assert that the skull had been sheared over time. as a result, the entire arch of the posterior jaw was displaced upwards. As a result, what appeared to be the skull roof above the temporal fenestra was actually bits and pieces of the zygomatic (jaw) arch. This actually affects only character #3 above. However, this was enough to change the shape of the cladogram in a subtle, but significant way, shifting Tetraceratops onto a dead-end side track, the Sphenacodontidae (like Dimetrodon), rather than being on the main line to the Therapsida. However, whether Tetraceratops is a therapsid or just a close cousin, it is plainly a reasonably good guide to the sorts of forms which connect the "pelycosaurs" with the Therapsida.

Evolution & Ecology

It is quite likely that proto-therapsids such as Tetraceratops (and many others which never became fossilised) evolved in an upland environment where they were not easily fossilised, away from the swamps and deltas frequented by the Pelycosaurs (the only upland Pelycosaur lineages were the Caeseidae and, most certainly, their eothyrid ancestors). This early Permian upland evolution proceeded parallel to the Permocarboniferous lowland fauna, and probably originally derived from reptiles that adapted to dry habitats during the long Kazimovian arid period. It was from this upland evolution, via intermediate forms like Tetraceratops, that the great therapsid evolutionary radiation emerged. MAK000802, ATW020518.

Characters: Large head with relatively rigid skull; $ long dorsal process of premaxilla [B&S, H&P] [RS01]; $ septomaxilla with posterodorsal facial process separating nasal & maxilla [B&S, H&P, contra LR96]; $ maxilla enlarged, separating nasal & lacrimal [B&S, H&P]; $ maxilla contacts prefrontal (except Tetraceratops) [LR96] [RS01]; $ enlarged temporal fenestra [B&S, H&B] [RS01]; $ upper margin of fenestra forms concave, ventrolaterally facing surface (for origin of jaw adductors?) [LR96]; $ pineal foramen raised on prominent boss [B&S, H&P] [RS01]; $ supratemporal absent [B&S, H&P] [RS01]; $ squamosal with groove on posterior [B&S, H&P]; $ quadrate reduced [LR96]; $ braincase firmly sutured to back of dermal skull roof [LR96]; dentary anteriorly expanded [RS01]; anterior coronoid absent [RS01]; $ posterior coronoid ventrally shifted & fails to form dorsal margin of jaw in medial view [RS01]; $ reflected lamina of angular bone notched dorsally [B&S, H&P] [RS01]; $ reflected lamina with pattern of ridges on lateral surface; $ vomer expanded transversely between choanae [B&S, H&P]; vomer with concave ventral surface; at least partial secondary palate; $ posteromedian flange of pterygoid present behind the interpterygoid vacuity [LR96]; $ pterygoid short & palatines meet at midline (except Tetraceratops) [LR96]; $ interpterygoid vacuity shortened [B&S, H&P, LR96]; basicranial articulation fused [RS01]; $ parasphenoid ventral plate ridged & lacks central groove (except Tetraceratops) [LR96]; $ precanine teeth absent; $ premaxillary teeth of equal size (except Tetraceratops) [LR96]; enlarged canine dentition [RS01]; $ upper canine length increased [B&S] $ no more than twelve upper postcanine teeth [LR96]; $ teeth absent from ectopterygoid [LR96]; $ palatal teeth restricted to raised denticle fields (except Tetraceratops) [LR96]; heavy body; $ intercentra absent from trunk [RS01]; more than 3 sacral vertebrae; tail reduced; limb girdles reduced and more mobile; limbs more slender, but fairly short; limbs held more vertically; $ scapular blade slender [RS01]; $ pectoral glenoid deepened & rounded [RS01]; $ humeral head rounded, not screw-shaped [RS01]; expanded iliac blade; $ acetabulum deepened & rounded [RS01]; $ inturned head of femur [RS01]; general reduction of ventral components (e.g. clavicle -- indicative of more upright posture); inflected femoral head [RS01]; development of greater trochanter on femur (with iliac blade, indicates gluteal hindlimb musculature); phalanges shorter (used as lever, rather than holdfast); possibly had hair; increased metabolic rate probable; found in terrestrial habitats with nearly world-wide distribution.

Tetraceratops: T. insignis Mathew, 1908. [CS01] argue convincingly that this is not a therapsid.

Characters: Skull high & short; $ first premaxillary tooth much larger than others [LR96]; upper tooth row slightly convex; 1 small precaniniform; $ long diastema on maxilla anterior to caniniform [LR96]; maxilla with broad alveolar shelf; postcaniniform teeth with sharp, curved crowns; $ very large teeth on transverse flange of pterygoid [LR96]; ectopterygoid teeth absent; dentary largest jaw element; $ dentary ends ventral to coronoid process [LR96]; angular with reflected lamina (contra, [CS01]); posteroventral flange of angular may have had additional horn; surangular forms coronoid; surangular with prominent knob for articular; choana long, narrow, bordered medially by vomers; palatines ling & narrow with medial denticle field; ectopterygoid small & triangular; posteromedian flange of pterygoid present; pterygoids with medial and lateromedial denticle rows; ventral plate of epipterygoid (?) reduced; ventral plate of epipterygoid excluded from basicranial articulation; interpterygoid vacuity shortened; pterygoids meet medially anterior & posteriorly to vacuities; basipterygoid articulation present (not fused); $ parasphenoid ventral plate narrow [LR96], with large medial depression; parasphenoid fused with basisphenoid; premaxilla very deep; septomaxilla small (!? also described as "massive" in same paper) with no facial process; maxilla contributes to narial margin; $ bony, possibly horn-bearing, tuberosities on premaxilla, prefrontal and angular [LR96]; tuberosities covered with fine ridges laterally & small foramina dorsally (=distally); maxilla does not contact nasal; lacrimal contacts nares briefly; nasal small, with (probable) median ridge; large orbital contribution of the lacrimal [LR96]; prefrontal with large tuberosity (horn); deep posteroventral excavation of prefrontal; long thin ventral process on orbital rim under lacrimal; $ suborbital process of jugal very narrow [LR96], terminating in blunt wedge in maxilla; postorbital ramus of jugal overlaps postorbital extensively (but not "twisted" as in later forms); postorbital overlaps & firmly sutured to squamosal, excluding parietal from temporal fenestra (questioned by [CS01]); broad, concave shelf of squamosal on upper, internal margin of temporal fenestra (origin of jaw adductors); wide tabular overlapping occipital portion of squamosal ([CS01] assert that [LR96]'s tabular is actually a fragment of the squamosal); braincase attached firmly to cheek; $ paroccipital process (?) narrow [LR96] & cylindrical, attached to tabular and squamosal; quadrate reduced; quadrate with two clearly separated condyles of equal size.

Notes: [1] Despite the name, Tetraceratops had at least 6 horns. Only 4 were exposed at the time it was named. [2] LR96 is a paradigm of what a descriptive paper should be. If everyone wrote like this, the professional literature would be half as long and twice as useful. Highly recommended. [3] [CS01] argue that much of the skull roof area is actually the zygomatic arch. Based on a revised parsimony analysis, they assert that Tetraceratops is a garden-variety sphenacodontid. ATW031019.

Therapsida

Abbreviated Cladogram

Contents

Taxa on This Page

Tetraceratops: misnomer & miscellaneous

Mathew (1908)

Laurin & Reisz (1996)

Conrad & Sidor (2001)

Evolution & Ecology