Gomphodont Cynodonts Research Articles

Skull anatomy and paleoneurology of a new traversodontid from the Middle-Late Triassic of Brazil.

Traversodontidae, a clade of gomphodont cynodonts, thrived during the Middle and Late Triassic, displaying a wide geographical distribution. During fieldwork in 2009, a new specimen was discovered in Ladinian/early Carnian stratigraphic layers in southern Brazil. Here, we describe this specimen and propose a new taxon closely related to Traversodon stahleckeri (Traversodontinae) but displaying a unique combination of traits (e.g., presence of a poorly developed suborbital process, mesiodistal length of the paracanine fossa similar to the length of the canine, short diastema between the fourth incisor and the upper canine, and coronoid process not entirely covering the distalmost lower postcanine). Furthermore, the endocranial anatomy of the new taxon was examined. The reconstruction of the cranial endocast revealed paleoneurological features consistent with non-Gomphodontosuchinae traversodontids. These features include the presence of a pineal body (but the absence of an open parietal foramen). These recent findings contribute significantly to our understanding of the evolutionary history and cranial anatomy of Middle-Late Triassic traversodontids, shedding light on the diversity and adaptations of non-mammaliaform cynodonts.

Evolution of postcanine complexity in Gomphodontia (Therapsida: Cynodontia).

Gomphodonts form a Triassic radiation of small to medium-bodied (<0.5-2.5 m in length) quadrupedal cynodonts characterized by labiolingually expanded gomphodont postcanines. They were the dominant cynodont group in Middle and Late Triassic ecosystems from the Southern Hemisphere and the first predominantly herbivorous cynodonts to evolve. Gomphodonts were also the first therapsids to develop hypsodonty and a dentition with complex occlusal patterns, and their highly diagnostic upper and lower postcanines show many different morphologies. Here, we explored dental complexity in gomphodont cynodonts through time using geographic information system analysis and orientation patch count applied on 3D crown surfaces of upper and lower gomphodont postcanines belonging to 32 gomphodont taxa. This study reveals that the peak in postcanine complexity was reached early in the evolution of gomphodonts with the emergence in the Early Triassic of omnivorous or insectivorous forms with postcanines made of well-separated cusps and cingular cuspules. Traversodontids evolved simpler postcanines via coalescence of cusps into crests and the development of large occlusal basins, and the Middle Triassic radiation of traversodontids led to a sharp decrease in mean postcanine complexity. Simplification of the postcanines in traversodontids is interpreted as being related to a gradual increase in the consumption of plant material. Interestingly, the trend of insectivory/omnivory high postcanine complexity and herbivory low dental complexity in gomphodonts is opposite to the trend of dental complexity reported in some extant mammals, with omnivorous having low dental complexity and herbivorous higher. Postcanine complexity remained relatively stable throughout the evolution of traversodontids and only slightly diminished in the Late Triassic due to the presence of minute forms with particularly simple postcanines in the Rhaetian. The major phylogenetic diversity and taxonomic richness of Gomphodontia are represented in two periods of time: at the end of the Anisian, an age in which the postcanine complexity is simplifying, and at the early Carnian when the postcanine complexity in traversodontids, the only Gomphodontia represented, is stable.

Osteohistology reveals the smallest adult Jurassic sauropodomorph.

The earliest sauropodomorphs were small omnivores (less than 10 kg) that first appeared in the Carnian. By the Hettangian, early branching sauropodomorphs (EBSMs) were globally distributed, had variable postures, and some attained large body masses (greater than 10 tonnes). Small-bodied EBSMs like Massospondylus carinatus (less than 550 kg) persist at least until the Pliensbachian at nearly all dinosaur-bearing localities worldwide but are comparatively low in alpha diversity. One potential reason for this is competition with other similarly sized contemporary amniotes, including Triassic gomphodont cynodonts, Jurassic early branching ornithischians, herbivorous theropods and potentially early crocodylomorphs. Today's herbivorous mammals show a range of body size classes (less than 10 g to 7 tonnes), with multiple species of small herbivorous mammals (less than 100 kg) frequently co-occurring. Comparatively, our understanding of the phylogenetic distribution of body mass in Early Jurassic strata, and its explanatory power for the lower thresholds of body mass in EBSMs, needs more data. We osteohistologically sectioned a small humerus, BP/1/4732, from the upper Elliot Formation of South Africa. Its comparative morphology and osteohistology show that it represents a skeletally mature individual of a new sauropodomorph taxon with a body mass of approx. 75.35 kg. This makes it one of the smallest known sauropodomorph taxa, and the smallest ever reported from a Jurassic stratum.

Guttigomphus avilionis gen. et sp. nov., a trirachodontid cynodont from the upper Cynognathus Assemblage Zone, Burgersdorp Formation of South Africa.

The Burgersdorp Formation of South Africa is a richly fossiliferous rock sequence at the top of the Permian-Triassic Beaufort Group and is known for its abundance of Early-Middle Triassic vertebrate remains, particularly cynodonts. Fossils from the Burgersdorp Formation are referred biostratigraphically to the Cynognathus Assemblage Zone (CAZ), which is further divided into three subzones: Langbergia-Garjainia, Trirachodon-Kannemeyeria, and Cricodon-Ufudocyclops. Each subzone is characterised by the presence of a distinct species of trirachodontid, a group of gomphodont cynodonts found relatively abundantly throughout the CAZ, with the lower two subzones characterised by the medium-sized trirachodontids Langbergia and Trirachodon. The uppermost part of the formation, the Cricodon-Ufudocyclops subzone, yields trirachodontids of larger size. The majority of these trirachodontid specimens have previously been referred to Cricodon metabolus, a taxon also known from the Manda Beds of Tanzania and the Ntawere Formation of Zambia. Here we identify one of the specimens (BP/1/5538) previously referred to Cricodon as a new taxon, Guttigomphus avilionis. Guttigomphus can be distinguished from other gomphodont cynodonts by features of the upper postcanine teeth, such as an asymmetric crown in occlusal view (crown narrower along the lingual margin than the labial). Our phylogenetic analysis recovers Guttigomphus as a basal member of Trirachodontidae, outside of the clade including Cricodon, Langbergia and Trirachodon.

A proposed terminology for the dentition of gomphodont cynodonts and dental morphology in Diademodontidae and Trirachodontidae.

Gomphodont cynodonts were close relatives of mammals and one of the Mesozoic lineages of cynodont therapsids that became extinct at the end of the Triassic. Gomphodonts were omnivorous to herbivorous animals characterized by labiolingually expanded postcanines, which allowed tooth-to-tooth occlusion. The morphology of the upper and lower postcanines presents important means of distinguishing among major lineages within Gomphodontia, that is, Diademodontidae, Trirachodontidae, and Traversodontidae, but the dentition of most Diademodontidae and Trirachodontidae remain poorly documented. Here, we present a comprehensive description of the dentition of each diademodontid and trirachodontid species, as well as detailed illustrations of each dental unit, after firsthand examination of material and 3D reconstructions of postcanine teeth. Based on dental morphology, Trirachodon berryi and “Trirachodon kannemeyeri,” considered as separate taxa by some authors are here interpreted as representing different ontogenetic stages of the same species. Likewise, Sinognathus and Beishanodon, thought to belong to non-cynognathian cynodonts and traversodontids by some authors, are referred to Trirachodontidae and Gomphodontia based on dental characters, respectively. Finally, we propose a standardized list of terms and abbreviations for incisors, canines, and postcanines anatomical entities, with the goal of facilitating future descriptions and communication between researchers studying the gomphodont dentition.

Tetrapod diversity and palaeoecology in the German Middle Triassic (Lower Keuper) documented by tooth morphotypes

Continued excavations during the last decade have yielded large quantities of tetrapod remains from the Middle Triassic (Ladinian) Erfurt Formation (Lower Keuper) in Germany. The temnospondyl dental morphotypes are highly variable but represent low taxonomic diversity. This is in contrast to the reptilian tooth morphotypes, which comprise a minimum of 26 distinct types, only some of which can be referred to taxa based on diagnostic skeletal material. The assemblage includes a taxonomically diverse range of semi-aquatic or aquatic faunivores, in addition to large terrestrial carnivores and many smaller-sized forms that may have subsisted on invertebrates and small vertebrates. With only two taxa known to date, tetrapods with dentitions suitable for oral processing of plant material form the least common faunal element. The Lower Keuper assemblages are dominated by diapsid reptiles, especially archosauriforms. Unlike in the more or less coeval tetrapod communities from Gondwana, gomphodont cynodonts are represented only by a single molariform tooth to date, whereas avemetatarsalian archosaurs are entirely absent. Most remarkable is the virtually total absence of medium- to large-sized herbivores (rhynchosaurs, dicynodont synapsids).

The Stapes of Gomphodont Cynodonts: Insights into the Middle Ear Structure of Non-Mammaliaform Cynodonts.

The stapes is known in several non-mammaliaform cynodonts, although it has only been cursorily studied. Here we thoroughly analyze the stapedial anatomy of several basal cynodonts in a phylogenetic framework. Our study shows that the stapedial anatomy is more variable than previously thought. The morphological variation of the stapes led to the recognition of 11 phylogenetic characters that were included in a total evidence data matrix centered in the analysis of gomphodont cynodonts. Stapes morphology does not provide evidence to suggest a direct connection between the stapes and a postquadrate tympanic membrane (if present) and the hypothesis of a dorsal process as the site of attachment of a small ligament or the stapedial muscle is supported. The re-evaluation of the theories concerning the position of the tympanic membrane in non-mammaliaform cynodonts allowed us to conclude that the hypothetical postquadrate tympanic membrane associated with the squamosal sulcus is at best relictual and most likely non-functional (not connected with the stapes). The sound waves were most likely transmitted to the stapes from a postdentary tympanic membrane through the quadrate. Our analysis results in a better understanding of the auditory system in basal cynodonts and its evolution, highlighting the variability of the stapedial anatomy.

Cynodont Teeth from the Carnian (Late Triassic) of Northern Italy

A jaw fragment with three teeth preserved, collected from the Gorno Formation (Carnian, Upper Triassic) of Lombardy (It− aly) is described. The teeth are transversely elongated, three− cusped and bear anterior and posterior cingula. Their overall morphology supports their identification as postcanines of a “eucynodontian”. The unique tooth morphology of the new specimen supports its attribution to a new genus and species, while at the same time precluding positive assignment to al− ready known gomphodont families. There is a fairly small re− cord of gomphodont cynodonts in Europe, so that the de− scribed specimen adds to the knowledge of the distribution and diversity of European gomphodonts and it also represents the first ever collected in Italy.

REDESCRIPTION OF PLATYCRANIELLUS ELEGANS (THERAPSIDA, CYNODONTIA) FROM THE LOWER TRIASSIC OF SOUTH AFRICA, AND THE CLADISTIC RELATIONSHIPS OF EUTHERIODONTS

Abstract: Platycraniellus elegans is an enigmatic Triassic cynodont from South Africa that has only been briefly described previously. New preparation of the holotype and additional unpublished material allows a detailed redescription and comparison with different cynodonts. Platycraniellus elegans is recognized as a valid species of basal cynodont. The distinct suborbital angulation of the zygomatic process, previously considered as a diagnostic character in chiniquodontid cynodonts, and more recently observed in some galesaurids, is also present in P. elegans. A larger, second specimen was initially referred to P. elegans, but most recently considered to belong to Galesaurus planiceps. Close comparison of this specimen with the holotype of P. elegans and with galesaurid specimens allows a tentative allocation to G. planiceps. A cladistic analysis of 32 taxa (two gorgonopsians, seven therocephalians and 23 cynodonts) and 96 craniodental characters places P. elegans as sister taxon of Eucynodontia. Results from the analysis favour a dichotomy between (1) Cynognathia, including the sectorial‐toothed cynodonts Ecteninion, Cynognathus and the gomphodont cynodonts, and (2) Probainognathia, including most sectorial‐toothed eucynodonts (e.g. Lumkuia, Probainognathus, Chiniquodon), tritheledontids, tritylodontids and mammaliaforms. The Late Triassic sectorial‐toothed Ecteninion is the most basal member of Cynognathia, whereas the Middle Triassic Lumkuia is the basal representative of Probainognathia. Tritylodontids (Oligokyphus and Kayentatherium) are placed among Probainognathia, forming a monophyletic group with the tritheledontid Pachygenelus, whereas Brasilitherium is the sister taxon of Mammaliaformes. The cladistic analysis also indicates paraphyly for Therocephalia, with the whaitsiid Theriognathus identified as sister taxon of Cynodontia.

A new trirachodontid cynodont from the lower levels of the Burgersdorp Formation (Lower Triassic) of the Beaufort Group, South Africa and the cladistic relationships of Gondwanan gomphodonts

We describe a new trirachodontid cynodont from the base of the Burgersdorp Formation (Subzone A fauna of the Cynognathus Assemblage Zone), of the South African Karoo Basin. Langbergia modisei gen. et sp. nov. is characterized by circular to ovoid in outline upper postcanines and the absence of a maxillary platform lateral to the postcanine series. Apart from the new taxon, we recognize two other species of this family in the Cynognathus Assemblage Zone: Trirachodon berryi and Cricodon metabolus, the latter also represented in the Tanzanian Manda Formation. A phylogenetic analysis of gomphodont cynodonts was conducted using a data matrix of 43 craniodental characters and 18 terminals. Trirachodontidae appears as a monophyletic assemblage, with Langbergia appearing as the sister taxon of Cricodon. The monophyly of trirachodontids is weakly supported, however, with one extra step breaking it. Traversodontid cynodont relationships were also inspected and compared with a recent phylogeny proposed for this group. Considering the resulting phylogeny and the sudden appearance and diversification of Cynognathia representatives, the origin of basal Cynognathia (i.e. Cynognathus, Diademodon, and trirachodontids) is suggested to predate their first appearance in the fossil record.

A Primitive Late Triassic ‘ictidosaur’ from Rio Grande Do Sul, Brazil

A primitive ‘ictidosaur’ from lower Norian beds of southern Brazil, Riograndia guaibensis gen. et sp. nov., represented by a fragmentary skull and a lower jaw bearing a complete dentition, shows a more generalized morphology than Chaliminia from the Upper Triassic of Argentina and PachygenelusDiarthrognathus from the Lower Jurassic of South Africa, Canada and Greenland. The frontal bone borders the orbit, and ventrally contacts the dorsal process of the palatine. The secondary bony palate extends back to the last postcanine. I1 and i2 are reduced, whereas I2‐3 and i1 are hypertrophied. Both PC 1–7 and pc 1–7 have blade‐like crowns without cingula and with 5–9 small sharp cuspules. The upper postcanine crowns are semicircular in labial view with the cuspules around their margins. The lower postcanine crowns are asymmetrical with most of the cuspules dorsodistally distributed. The possible origin of this peculiar dentition is interpreted as the retention of the juvenile dentition of ancestors. The hypothesis that Riograndia guaibensis and the so‐called ‘ictidosaurs’ might have been derived from gomphodont cynodonts is presented.

Gomphodont cynodonts of the Chañares Formation: the analysis of an ontogenetic sequence

ABSTRACT A revision of gomphodont cynodonts from the Chañares Formation of Argentina, is presented. Four species in two genera: Massetognathus pascuali, M. teruggii, M. major, and Megagomphodon oligodens were originally described by Romer in 1967 and 1972. Qualitative characters proposed originally as diagnostic of the species were revised, and a bivariate analysis of allometry of 17 linear skull measurements was performed upon 31 specimens. Allometry relationships account for most of the differences among specimens. There are neither size gaps nor systematic trends across the sample. The temporal region scales with a ‘positive’ allometry, whereas components of the neuro-cranium and muzzle show ‘negative’ or isometric scaling. This results in a change from short, stocky skulls with large muzzles in small specimens, to slender skulls with small muzzles and large temporal fossae characteristic of large individuals. The reduction and ontogenetic loss of the parietal foramen in a cynodont is reported for the first time. Consideration of qualitative characters together with the allometric results lead us to conclude that all forms belong to a single species in a growth series. Consequently, Massetognathus pascuali is recognized as the only valid entity, an idea previously suggested but without a justification based upon extensive analysis.

A new Triassic vertebrate fauna from Antarctica and its depositional setting

A new fauna of late Early to early Middle Triassic vertebrates has been found in the upper member of the Fremouw Formation in the Beardmore Glacier area of Antarctica. It includes Cynognathus, a kannmeyeriid, and other therapsid (mammal-like) reptiles representing new, more derived genera of carnivorous and gomphodont cynodonts. New genera of temnospondyl amphibians belonging to the capitosauroid evolutionary complex also occur. The unusual abundance of well-preserved amphibians may offer new insights concerning the evolution and distribution patterns of early Mesozoic temnospondyls. These fossils represent only the second terrestrial vertebrate fauna from the mainland of Antarctica. The fossils occur on a prominent sandstone platform, which represents part of the exhumed channel of a braided stream deposit. The platform is over 200 metres above the well-known Lystrosaurus fauna of the lower Fremouw Formation. The locality is near the axis of a major foreland basin that paralleled the present trend of the Transantarctic Mountains. Conditions of rapid subsidence and aggradation of fluvial units were ideal for the preservation of vertebrate faunas.

ON TOOTH SUCCESSION IN DIADEMODON.

Two characteristics clearly distinguish between the dentitions of typical mammals and reptiles: generally the former are heterodont and diphyodont while the latter are homodont and polyphyodont. Superficially it might seem that by progressively reducing the number of replacement teeth in an evolutionary sequence of animals, it was a simple matter to arrive at a diphyodont mammalian descendant from a polyphyodont reptilian ancestor. But data on the sequences in which teeth are initiated in mammalian and reptilian embryos suggest that the process was more complex. In most reptilian embryos teeth are probably developed in sequences which pass through alternate positions from the back to the front of the jaw (Osborn, 1971). It is not easy to relate this sequence to that observed in mammalian embryos; for instance for the upper jaw of Eremnitalpa, Osborn (1970) gives the sequence it, i2, C, m4/M1/m3/i3/M2/M3/I1, 12, C, P3, P4/ 13 (in which the lower case represents deciduous teeth and the sequence is known only where bars separate teeth). Both heterodonty and a very limited number of replacement teeth also evolved in the postcanine dentitions of the gomphodont cynodonts and their descendants, the tritylodonts of the Triassic and Jurassic (Crompton, 1973). Although these animals finally become extinct in the middle Jurassic, it seems possible that the manner in which they evolved their limited numbers of replacement teeth from the ancestral polyphyodont condition might shed light on the evolution of diphyodonty in mammals. The earliest of the well-known gomphodont cynodonts with a markedly heterodont postcanine denition and a linited number of replacement teeth was Diademodon. The purpose of the present paper is to plot the sequence in which postcanine teeth were replaced in Diademodon and to compare this with tooth replacement in a polyphyodont reptilian dentition. If both have been correctly understood it should indirectly be possible to obtain some idea of the biological controls which regulate tooth replacement. And it can be safely assumed that by varying similar biological controls, mammalian diphyodonty could also have been evolved from polyphyodont dentitions. The pattern by which Diademodon, a lower Triassic gomphodont cynodont, replaced its postcanine teeth has recently been studied by Crompton (1963), Ziegler (1969) and Hopson (1971), and is also referred to by Crompton (1973). In view of the fact that no new data is provided in the present paper, it might seem unnecessarily contentious to propose a fourth solution to this problem within eight years. However, during the last decade almost all workers have, with varying degrees of success (e.g., DeMar, 1972) attempted to fit patterns of tooth replacement into the framework of a general model known as the Zahnreike theory. This theory, which was proposed by Edmund (1960, 1962), apparently links together tooth replacement patterns encountered throughout the vertebrate kingdom. My own studies, referred to above and later, have convinced me that the Zaknreike theory as a model describing the initiation of patterns of tooth replacement is untenable in the face of the phylogenetic and embryological evidence. I have concluded that the rate of tooth replacement is partly controlled at each tooth position.

New Triassic dicynodonts from South America, their origins and relationships

Richly fossiliferous deposits have been found in the Ischigualasto region of Argentina in the last few years. The only known dicynodont from this area is the new genusIschigualastia, of which a diagnosis and fully illustrated description are given. A specimen from Brazil, which had earlier been referred to the genusStahleckeriaasS. lenzii, is shown to be very similar toIschigualastia, but not generically identical with it; this specimen is therefore placed in the new genusBarysoma. The only South American dicynodont which had previously been fully described isStahleckeria, from Brazil. A diagnosis and fully illustrated description are now given of the complete skeleton of the genusDinodontosaurus, also from Brazil. Earlier Brazilian material which had been referred to the African genusDicynodonis shown to belong toDinodontosaurus. A very large skull from the same deposits is identical withDinodontosaurus, except that it has a much more massive snout and tusks, and a wider occiput. The dicynodonts are herbivorous, and may well have lived in herds; it is suggested that the massive skull may belong to the old male of such a herd ofDinodontosaurus, and it is therefore not given separate taxonomic status. The skull ofPlacerias, the only dicynodont known from North America, had previously been restored by Camp &amp; Welles (1956) from the broken remains of about forty individuals. Comparison of the restored skull with that ofIschigualastiahas suggested various modifications in the reconstruction, and illustrations of the new reconstruction are given. The relationships of the Triassic dicynodonts are discussed. It is suggested that, excluding the specialized genusLystrosaurus, they show two main divergent adaptations, which are probably related to their mode of feeding. A pointed snout and high occiput is thought to characterize the family Kannemeyeriidae (which includes the formsKannemeyeria, ‘Kannemeyeria’ vanhoepeni, Sinokannemeyeria, Parakannemeyeria, Ischigualastia, BarysomaandPlacerias). A blunt snout and wide occiput is thought to characterize the family Stahleckeriidae (which includes the generaStahleckeriaandDinodontosaurus). A similar distinction is found today between the browsing black rhinoceros and the grazing white rhinoceros. The most primitive kannemeyeriids are found in the Lower Triassic of China, and these forms may also be ancestral to the stahleckeriids. The only other Triassic dicynodonts,Shansiodonand ‘Dicynodon’ njalilus, may be placed in a separate family, the Shansiodontidae. All these Triassic genera have two features in common: the presence of a separately ossified olecranon process on the ulna, and a shortened interpterygoid vacuity. It is possible that this may indicate a common ancestry for them all, but no Upper Permian or Basal Triassic genera now known appear to be possible ancestors for them. The lack of any Middle Triassic vertebrate fauna in the northern hemisphere makes it very difficult to date the Argentinian and Brazilian faunas, which include gomphodont cynodonts, dicynodonts, rhynchosaurs, pseudosuchians and a few saurischians. It is not felt that the presence of rhynchosaurs necessarily indicates a Middle Triassic age, as the group is known from the mid-Norian of India. It is possible that the presence of several saurischians and of a pseudosuchian closely related to the German Norian genusAëtosaurus, may indicate a Carnian age for the Argentine fauna. The Brazilian fauna is somewhat dissimilar to that of Argentina and contains no genera in common with it; it may therefore be of earlier, Ladinian, age. The fauna of the Manda Beds of East Africa is similar in composition to that of Brazil, but contains no genera in common with it. It also lacks saurischians and includes a dicynodont,Kannemeyeria, that is otherwise typical of the Lower TriassicCynognathuszone of South Africa. It may therefore be Anisian in age.

On some Triassic cynodonts from Tanganyika

1 A collection of gomphodont cynodonts obtained from the Manda Beds (Middle Trias) of the Ruhuhu valley in Tanganyika has been prepared. Two new forms, Cricodon metabolus and Aleodon brachyrhamphus, have been described, the former from an imperfect skeleton and the latter from a skull. 2 The postcanine teeth of Trirachodon angustifrons Parrington have been shown to differ sufficiently from the South African species of that genus to merit transfer to a new genus Scalenodon. A growth series of this form consisting of nine upper jaws and six dentaries has been described together with some postcranial elements. 3 Two postcanine teeth in an isolated maxilla have been shown to resemble those of Gomphodontosuchus brasiliensis von Huene from the Rio do Rasto Series of Brazil, and similarly the postcanine teeth of Scalenodon angustifrons have been shown to resemble those of the Brazilian form Traversodon stahleckeri von Huene, also from the Rio do Rasto Series. It has been concluded that the Triassic cynodonts of the Ruhuhu valley compare more closely with those of the Brazilian Rio do Rasto Series than with those of South African deposits. 4 It has been shown that there is evidence that the incisors and canines of Scalenodon angustifrons are replaced at least twice and consequently the dental succession of this advanced cynodont cannot be considered as mammalian. The replacement of the incisors appears to be alternate or “distichic”. 5 It has been shown that the ninth upper postcanine teeth of Cricodon metabolus, which are of the shearing type, ere undergoing replacement by gomphodont teeth; that the ninth and tenth upper postcanines are apparently also being replaced in two specimens of Scalenodon angustifrons; and that the sixth upper postcanine of Diademodon sp. indet. is undergoing replacement. It is concluded that the use of the terms “premolar” and “molar” is unjustified in gomphodont cynodonts. No evidence has been found of replacement of posterior postcanine in any mandible. 6 The available evidence supports the conclusion of earlier workers that the localities from which the material was collected are of Middle Triassic age.

IV - On the tooth-replacement in theriodont reptiles

In describing the very mammal-like dentitions of some of the advanced Theriodont reptiles various authors have identified premolars and molars (Broom , 1913). Though there is some evidence for this differentiation in the form of the teeth of some of the gomphodont cynodonts, there is no satisfactory evidence that only two dentitions were present or that the replacement was of a mammalian type. On the other hand Sushkin (1927) described the type of the primitive cynodont Permocynodon sushkini , Smith Woodward , as having several molariform teeth “ changing ” and says “ it may be established thus that the teeth are changing irregularly as usually in the reptiles and that there is no difference in this respect between the anterior and hindermost teeth ; thus despite the difference of the form, no premolars and molars can be distinguished” . Unfortunately he does not say which teeth are changing. Other authors, uncertain of the manner of replacement, have avoided implying a mammalian succession by the use of such terms as “ cheek ” teeth or “ post-canine” teeth.