The evolution of the dicynodont feeding system

The skull structure of dicynodonts may be regarded as a complex adaptation towards herbivorous feeding. The present work examines how and why this adaptation may have evolved. A cladogram of the dicynodonts is presented and from it a sequence of hypothetical ancestral forms is inferred. The jaw musculature of dicynodonts and other therapsids is described and in particular the early dicynodont Eodicynodon oosthuizeni is described in detail. This information is used to draw up a sequence of ancestral stages whose basic skull anatomy, jaw muscle organization and masticatory properties are described. Differences in masticatory properties between these stages are pinpointed and an explanation to account for the development of these differences is advanced. It is concluded that the changes in skull organization seen during the evolution of dicynodonts are consistent with the hypothesis that a propalinal jaw action was being improved by selection, and that this was required to permit dicynodonts to be efficient herbivores.     

The basicranium of dicynodonts (Synapsida) and its use in phylogenetic analysis

Current phylogenetic hypotheses for the dicynodonts conflict, probably because the characters used, especially those of the jaws and facial region, show considerable convergence. Characters of the braincase and basipterygoid articulation of the Late Permian–Middle Triassic dicynodonts Diictodon , Dicynodon , Kingoria, Lystrosaurus , Rechnisaurus , and 14 other genera, may have phylogenetic value. Parsimony analysis and the character compatability permutation test suggest, at the highest possible confidence level, that the data set contains significant hierarchical structure, interpreted as a result of phylogeny. The most parsimonious tree broadly agrees with all recent hypotheses on the relationships among dicynodonts. However, it conflicts with the recent suggestion that Lystrosaurus is part of a clade of Middle–Late Triassic dicynodonts, but supports the basal position of Kingoria . The use of Eodicynodon as an outgroup does not perturb the parsimonious relationship of the included taxa. Topological constraints reveal that phylogenetic hypotheses based only on basicranial characters are not robust. Characters of the basipterygoid articulation and inner braincase have high consistency and retention indices, which suggests that the main evolutionary transformations in the dicynodont basicranium occurred within these structures.     

Bite traces on dicynodont bones and the early evolution of large terrestrial predators

Nied?wiedzki, G., Gorzelak, P. & Sulej, T. 2010: Bite traces on dicynodont bones and the early evolution of large terrestrial predators. Lethaia, Vol. 44, pp. 87–92. Dicynodont (Synapsida: Anomodontia) bones from the Late Triassic (late Norian/early Rhaetian) of Poland yield characteristic tooth marks that can be attributed to three ichnotaxa (Linichnus serratus, Knethichnus parallelum and Nihilichnus nihilicus). The general shape and dimension of these traces perfectly match the dental morphology of a co‐occurring carnivorous dinosaur. It is therefore concluded that early carnivorous dinosaurs were feeding on dicynodonts. This discovery constitutes one of the oldest evidence of dinosaur predator–prey interaction. It is suggested that an evolutionary increase in the size of dicynodonts across the Late Triassic may have been driven by selection pressure to reach a size refuge from early dinosaur predators. □Bite traces, dicynodonts, dinosaurs, predation, Triassic.     

The Chañares Formation: a window to a Middle Triassic tetrapod community

The Chañares Formation is known worldwide for its diverse and well‐preserved Ladinian non‐marine tetrapod assemblage, including a wide variety of archosauriform reptiles (proterochampsids, early offshoots of the crocodilian line and dinosaurian precursors) and synapsids represented by dicynodonts and cynodonts. This tetrapod record offers an opportunity to evaluate, within a taphonomic context, the palaeoecology of this Middle Triassic fauna. The taphonomic analysis of the Chañares assemblage, under precise stratigraphical control, indicates that it is a good representation of the original faunal composition allowing us to address the palaeoecological interactions between its components. Mass estimations and morphology‐based palaeobiological inferences of Chañares tetrapods are used to reconstruct the trophic structure of the community. Chañares tetrapod fauna was numerically dominated by middle‐sized herbivorous and small faunivorous cynodonts, whereas middle‐sized faunivorous cynodonts and large dicynodonts were less common. In contrast to the therapsids, which show a low species‐richness and high abundance, the archosauriforms are less abundant, but are the most taxonomically diverse group. The large paracrocodylomorphs (estimated body masses between 350 and 500 kg) are identified as the top predators of the community, and the traversodontid cynodonts and dicynodonts (estimated body masses reaching approximately 43 and 360 kg, respectively) are indentified as the base herbivores of the trophic pyramid. We conclude that the worldwide faunal composition in the Ladinian reveals two continental assemblages: an eastern Laurasian assemblage dominated by temnospondyl amphibians; and a western Gondwanan assemblage dominated by therapsids but including a wide diversity of archosauriforms.     

Growth patterns of fossil vertebrates as deduced from bone microstructure: case studies from India

Bone microstructure is affected by ontogeny, phylogeny, biomechanics and environments. These aspects of life history of an extinct animal, especially its growth patterns, may be assessed as fossil bone generally maintains its histological integrity. Recent studies on the bone histology of fossil vertebrates from India encompass different types of temnospondyls and dicynodonts from different Permian and Triassic horizons. The examined taxa show that they had distinct bone histology and varied growth patterns. The Early Triassic trematosaurids had an overall fast growth, which contrasts with that of the Middle and Late Triassic temnospondyl taxa examined. The dicynodonts on the other hand, were characterized by an overall fast growth with periodic interruptions, variable growth rates dependent on ontogeny and indeterminate growth strategy. A comparative study encompassing several neotherapsid genera including the dicynodonts shows significant evolutionary trends towards determinate growth strategy and reduced developmental plasticity.     

Do extraordinarily high growth rates in Permo‐Triassic dicynodonts (Therapsida,Anomodontia) explain their success before and after the end‐Permian extinction?

Dicynodonts were the most diverse and abundant herbivorous therapsids of the Permo‐Triassic. They include Lystrosaurus, one of the few taxa known to survive the end‐Permian extinction and the most abundant tetrapod during the Early Triassic postextinction recovery. Explanations for the success of Lystrosaurus and other dicynodonts remain controversial. This study presents an assessment of dicynodont growth patterns using bone histology, with special focus on taxa associated with the end‐Permian extinction event. Bone histological analysis reveals a high cortical thickness throughout the clade, perhaps reflecting a phylogenetic constraint. Growth rings are absent early in ontogeny, and combined with high vascular density, indicate rapid, sustained growth up to the subadult stage. Extraordinarily enlarged vascular channels are present in the midcortex of many dicynodonts, including adults, and may have facilitated a more efficient assimilation of nutrients and rapid bone growth compared to other therapsids. Both increased channel density and enlarged vascular channels evolved at or near the base of major radiations of dicynodonts, implying that the changes in growth and life history they represent may have been key to the success of dicynodonts. Furthermore, this exceptionally rapid growth to adulthood may have contributed to the survival of Lystrosaurus during the end‐Permian extinction and its dominance during the postextinction recovery period. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 160 , 341–365.     

A new basal dicynodont from the Upper Permian of South Africa

The skull of a small anomodont therapsid, from the Tapinocephalus Assemblage Zone (Abrahamskraal Formation, Beaufort Group, Upper Permian) in Northern Cape Province, South Africa, represents a new basal dicynodont and is described in detail. Colobodectes cluveri gen. et sp. nov. is distinguished from other dicynodonts by an anteroposteriorly extensive caniniform process, parietals that were broadly overlapped posterolaterally by posterodorsal processes of the postorbitals, diverging anterior palatal ridges, and a dorsoventrally low foramen magnum. A phylogenetic analysis indicates that Colobodectes is the basalmost member of a dicynodont clade that excludes Eodicynodon . This position is not particularly strong, as two additional steps are needed to make Colobodectes and Eodicynodon oosthuizeni exchange places on the most parsimonious tree. Another discovery of the phylogenetic analysis is that there is little basis for recognizing Eodicynodon oelofseni as the closest relative of E. oosthuizeni . The former species is identified as the sister taxon of a clade that includes the latter and all other dicynodonts.     

The jaw function and adaptive radiation of the dicynodont mammal-like reptiles of the Karoo basin of South Africa

The structure and functioning of the dicynodont jaw system are described. A pivoting action of the lower jaw around the palate posteromedian to the caniniform processes is as basic to the jaw movement as is the well-known double-convex jaw articulation. The sequence of origin of the characteristic features of the dicynodonts is analysed cladistically; this demonstrates the patterns of association of these characters into functional character-complexes. The structures of the palate and lower jaw, and their functional integration in feeding, are described and illustrated in standard format. Five different lineages of dicynodont can be identified: Eodicynodon; the robertoids (including Diictodon ); the dicynodontoids (including Dicynodon and the majority of the large dicynodont genera of the Permian and Triassic); the endothiodontoids (including Prodicynodon [='Chelydontops'] and Pristerodon ), and the emydopoids (including Cistecephalus, Myosaurus and Kingoria ). Eodicynodon or a similar form could have been ancestral to the other four lineages. The robertoids probably fed upon the stems and rhizomes of equisetaleans, while the varied dicynodontoids probably fed upon the varied glossopterid seed-ferns. The endothiodontoids, too, were probably herbivorous, but many, perhaps all, of the small emydopoids were burrowing and may have been omnivorous. The dicynodonts were probably ectothermal, and the dicynodontoids may have migrated to warmer latitudes in the winter. Only c. 20 genera of Karoo dicynodont are now recognized as valid, and it is suggested that this fauna is now almost completely known. Their distribution in the Karoo biozones is reviewed and correlated with environmental changes. The Permian ancestors of the Triassic dicynodonts, including Lystrosaurus , probably lived on higher, drier ground, and were therefore already adapted to the more fibrous food that spread into the basins as the climate became drier in the Triassic.     

Permian anomodonts: Paleobiogeography and distribution of the group

In the Late Permian, higher anomodonts, dicynodonts, reached an almost global distribution. The wide distribution makes this group an important tool in Upper Permian biostratigraphy. Three paleobiogeographic hypotheses for the center of the origin and migration pathways of the major anomodont groups are analyzed. Remains of these animals are most abundant in South Africa; however, they are also widespread in Eastern Europe, China, and India; interesting materials come from Western Europe, Madagascar, Central Africa, and South America. Biogeographic distribution of anomodonts supports the hypothesis of two stages in the origin and distribution of anomodonts. At the first stage, primitive anomodonts evolved in the Northern and Southern Hemispheres, forming high-rank endemic groups. At the second stage, dicynodonts, which appeared in the Southern Hemisphere, evolved rapidly; some groups inhabiting equatorial regions gave rise to the second wave of adaptive radiation, with the emergence of oudenodontids and dicynodontids, which secondarily adapted to high latitudes of the Southern Hemisphere, penetrated into the Northern Hemisphere, and formed there new endemic groups of subfamily rank.     

A REVIEW OF CHINESE THEROCEPHALIAN REPTILES

<正> To be Compared with the abundance of dicynodonts in the continental sediments of late Permian and Triassic in China, theriodonts are relatively rare in quantity. Six genera have been recorded. Among them, however, except the cynodont Sinognathus, all are therocephalians. They came from four different stratigraphic horizons. (see table 1)Owing to the misunderstanding of the nature of most of the forms, their taxonomic positions remain obscure. This paper aims at giving a review of all these materials and an account of their relationships.     

Limb bone histology and growth in Placerias hesternus (Therapsida: Anomodontia) from the Upper Triassic of North America

Abstract: Patterns of bone deposition are reported and deduced from mid‐shaft sections of 21 limb bones of the dicynodont Placerias hesternus from the Placerias Quarry (Upper Triassic), Arizona, USA. All sampled elements of P. hesternus have a large medullary cavity completely filled with bony trabeculae surrounded by dense cortical bone. Dense Haversian bone extends from the perimedullary region to at least the mid‐cortex in all sampled bones. Primary bone in the outer cortex of limb elements of P. hesternus is generally zonal fibrolamellar with a peripheral layer of parallel‐fibred bone. These data suggest periodic rapid osteogenesis followed by slower growth. Among dicynodonts, this strategy is most similar to growth previously reported in other Triassic (Lystrosaurus, Wadiasaurus) and some Permian taxa (Oudenodon, Tropidostoma). An external fundamental system (EFS), suggesting complete or near complete cessation of appositional growth, is present in the largest tibia. This is the first report of EFS in dicynodonts and may represent the attainment of maximum size in P. hesternus. Slow‐growing peripheral bone was observed in elements of varying size in our sample and may support a differential growth pattern between P. hesternus individuals from this locality. A complete growth series of P. hesternus, analysis of Placerias specimens from other localities, and further sampling of other Upper Triassic dicynodonts are needed to better understand a more complete picture of the growth and remodelling patterns that we have initially investigated.     

Growth patterns as deduced from bone microstructure of some selected neotherapsids with special emphasis on dicynodonts: Phylogenetic implications

Osteohistological analysis of the dicynodonts Endothiodon, Diictodon, Lystrosaurus and Wadiasaurus reveals distinctly different growth patterns within a framework of an overall fast growth. The late Permian endemic taxon from India, Endothiodon mahalanobisi and the South African Diictodon feliceps had periodic fast growth. The early Triassic Lystrosaurus murrayi and the middle Triassic Wadiasaurus indicus had an initial fast growth followed by a relatively slow growth later in ontogeny as is observed from the presence of peripheral parallel fibred bone. Although all examined dicynodont genera had an indeterminate growth strategy, the bone microstructure of Wadiasaurus suggests that its growth was much slower than that of other dicynodonts examined. Mapping of osetohistological character states on a cladogram depicting the inter-relationship between available neotherapsid genera shows that fibrolamellar bone tissue, overall fast growth and indeterminate growth strategy were plesiomorphic for the neotherapsids. A considerable reduction in developmental plasticity and evolution of apparently independent growth trajectories from environmental conditions are evident within the non-mammalian cynodonts, with the advanced tritylodontids achieving almost a mammalian growth trajectory.     

Phylogenetic analysis of Russian Permian dicynodonts (Therapsida: Anomodontia): implications for Permian biostratigraphy and Pangaean biogeography

Dicynodont therapsids have been known from the Upper Permian of Eastern Europe since the beginning of the twentieth century, but the phylogenetic relationships of these taxa have not been examined cladistically. Here we present the results of a phylogenetic analysis that includes eight Permian dicynodonts from Russia as well as 18 taxa best known from southern Africa. Our results do not conflict with much of the established picture of Permian dicynodont phylogeny, but are consistent with several novel hypotheses. Most importantly, our analysis suggests that the genus Dicynodon is paraphyletic, and we question its use in correlating widely separated basins. However, we cannot strongly reject a monophyletic Dicynodon . Our results also indicate that the closest Permian relatives of Kannemeyeria lived in Russia, suggesting a Laurasian origin for the lineage that includes this important Triassic taxon. The phylogeny presented here also suggests a Laurasian origin for several other dicynodont clades, but a Gondwanan origin is equally likely given the data at hand. Regardless of where these groups originated, there appears to be some endemism among Late Permian dicynodont faunas. Although our understanding of dicynodont phylogeny is improving, this study emphasizes the disparity in sampling of the dicynodont record between Gondwana and Laurasia and the need for a large scale phylogenetic analysis of Permian and Triassic dicynodonts.  © 2003 The Linnean Society of London, Zoological Journal of the Linnean Society, 2003, 139 , 157−212.     

The postcranial skeleton of Kingoria nowacki (von Huene) (Therapsida: Dicynodontia)

A new specimen of Kingoria nowacki (von Huene) with a complete pelvic girdle and hindlimb is reconstructed and the method of locomotion analysed. It is concluded that the hindlimb was modified from the normal dicynodont pattern in a direction comparable to that of advanced mammal-like reptiles which are presumed to have given rise to mammals. The pectoral girdle also had a modified form, but the humerus was probably conservative in its morphology. The hindlimb stride relied on protraction and retraction to effect movement while the forelimb relied on long axis rotation of the humerus. Possible reasons for the difference in morphology and function of the fore-and hindlimbs are discussed, and a functional sequence for the generation of the Kingoria pelvic girdle from that of other Permian dicynodonts is suggested.     

A New Basal Lystrosaurid Dicynodont from the Upper Permian of South Africa

A new genus and species of late Permian dicynodont, Kwazulusaurus shakai , is described on the basis of a complete skull from the late Permian Dicynodon Assemblage Zone of the South African Beaufort Group. It is an advanced form which shows characters, such as the shape of the snout and the loss of the ectopterygoid, that link it to the early Triassic genus Lystrosaurus. Kwazulusaurus represents the most basal member of the lystrosaurian lineage. The phylogeny of progressive pristerodontian dicynodonts is discussed. It appears possible that the Kannemeyeriiformes and Lystrosauridae do not form a monophylum, as previously assumed. Instead a sister-group relationship between lystrosaurids and dicynodontids plus kannemeyeriiforms is suggested.     

A taxonomic revision of small dicynodonts with postcanine teeth

The validity of 18 genera of small (skull length less than 150 mm) dicynodonts with postcanine teeth has been assessed. It is concluded that only four genera, Emydops, Eodicynodon, Robertia and Pristerodon , can be defined satisfactorily on the basis of well-preserved and adequately prepared specimens. Of the remaining 14 genera, ten are not valid and may be accommodated within the four well-characterized genera. Three additional genera ( Chelyoposaurus, Koupia and Taognathus ) should become nomina nuda since they are based on inadequately diagnosed material which in the case of the first two genera is now lost. Brachyprosopus , although based on a single somewhat inadequate specimen, is also considered valid although future examination of the type (not seen by us) may require reassessment of this opinion.
We discuss briefly the nature of the taxonomic characters used, and the stratigraphic distribution of the four major genera.     

The last dicynodont: an Australian Cretaceous relict

Some long-forgotten fossil evidence reveals that a dicynodont (mammal-like reptile of the infraorder Dicynodontia) inhabited Australia as recently as the Early Cretaceous, ca. 110 Myr after the supposed extinction of dicynodonts in the Late Triassic. This remarkably late occurrence more than doubles the known duration of dicynodont history (from ca. 63 Myr to ca. 170 Myr) and betrays the profound impact of geographical isolation on Australian terrestrial faunas through the Mesozoic. Australia's late-surviving dicynodont may be envisaged as a counterpart of the ceratopians (horned dinosaurs) in Cretaceous tetrapod faunas of Asia and North America.