Dicynodontoides is a genus of small to medium-bodied, herbivorous, emydopoid dicynodonts from the Late Permian. The name Dicynodontoides references its “dicynodont-like” appearance (dicynodont = two-dog-tooth) due to the caniniform tusks featured by most members of this infraorder. Kingoria, a junior synonym, has been used more widely in the literature than the more obscure Dicynodontoides, which is similar-sounding to another distantly related genus of dicynodont, Dicynodon. Three species are recognized: D. recurvidens from South Africa, and D. nowacki and D. kubwa from Tanzania.Angielczyk K.D., Sidor C.A., Nesbitt S.J., Smith R.M.H & Tsuji L.A. 2009. Taxonomic revision and new observations on the postcranial skeleton, biogeography, and biostratigraphy of the dicynodont genus Dicynodontoides, the senior subjective synonym of Kingoria (Therapsida, Anomodontia), Journal of Vertebrate Paleontology, 29:4, 1174-1187, DOI: 10.1671/039.029.0427
Dicynodontoides is primarily known from fossil localities in South Africa and Tanzania, though several specimens unidentified to the species level are known from Zambia, Malawi, and India.Ray, S., & Bandyopadhyay, S. 2003. Late Permian vertebrate community of the Pranhita-Godavari Valley, India. Journal of Asian Earth Sciences 21:643–654.Jacobs, L. L., Winkler, D.A, Newman, K.D., Gomani, E.M., and Deino, A. 2005. Therapsids from the Permian Chiweta Beds and the age of the Karoo Supergroup in Malawi. Palaeontologia Electronica 8:1–23. Unlike several other members of the remarkably disparate emydopoid clade, Dicynodontoides did not survive into the Triassic, and its temporal distribution is restricted to the Late Permian.
History and discovery
Dicynodontoides was first described by Owen in 1876 based on a poorly preserved, but fairly complete skull and mandible, and was originally referred to the genus Dicynodon.Owen, R. 1876. Descriptive and Illustrated Catalogue of the Fossil Reptilia in the Collection of the British Museum. Trustees of the British Museum of Natural History, London, 88 pp. The specimen was found in Fort Beaufort, South Africa, in the Dicynodon Assemblage Zone of the Karoo Basin. However, it wasn't until Broom's 1940 publication including Dicynodontoides parringtoni, a junior synonym for Owen's D. recurvidens, that the genus was used.Broom, R. 1940. On some new genera and species of fossil reptiles from the Karroo Beds of Graaff-Reinet. Annals of the Transvaal Museum 20:157–192. A second species, produced by the Usili Formation, D. nowacki, was first described by von Huene in 1942 from Kingori Mountain, Tanzania, and was originally referred to as Dicynodon nowacki.Huene, F. von. 1942. Die Anomodontier des Ruhuhu-Gebietes in der Tübinger Sammlung. Palaeontographica Abteilung A 44:154–184.
Cox pointed out several features, most notably the hindlimb and girdle morphology, that differentiated this genus from other members of Dicynodon, and erected a new genus, Kingoria.Cox, C. B. 1959. On the anatomy of a new dicynodont genus with evidence of the position of the tympanum. Proceedings of the Zoological Society of London 132:321–367. Since, many researchers have attempted to place these ambiguous specimens within Dicynodontia.Broom, R. & Robinson, J.T. 1948. Some new fossil reptiles from the Karroo Beds of South Africa. Proceedings of the Zoological Society of London 118:392–407.Cluver, M. A., & Hotton, N. 1981. The genera Dicynodon and Diictodon and their bearing on the classification or the Dicynodontia (Reptilia, Therapsida). Annals of the South African Museum 83:99–146.Brink, A.S., & Keyser, A.W. 1986. Illustrated bibliographic catalogue of the Synapsida. Geological Survey of South Africa Handbook 10: J212A251A. Not until the last decade has significant light been shed upon the matter, solidifying the place of the senior synonym, Dicynodontoides, and affirming the presence of only two species, D. recurvidens and D. nowacki.
Description
Skull
In dorsal view, the skull is oval in shape with a broad snout, and reaches its widest point posterior of the pineal foramen, which is slightly raised. Its intertemporal bar is narrower than the interorbital bar. Although belonging to the infraorder Dicynodontia, the caniniform tusks may be present or absent in the genus. When present, they are fairly gracile. Post-caniniform keels on the maxilla are present even in specimens lacking tusks. However, pre and post-canine teeth are always absent in this genus.
Dicynodontoides features a jaw symphysis that narrows anteriorly, tapering to a blunt point, and forming a shovel-shaped snout, which contrasting with the normally flattened area present in other dicynodont forms. Its palatine bone is smooth and significantly reduced to the lateral border of the internal nostril, having important implications for food processing (see below).Hotton, N. 1986. Dicynodonts and their role as primary consumers. In: Hotton, N., MacLean, P.D., Roth, J.J., & E.C. Roth, eds. The Ecology and Biology of Mammals-like Reptiles. Washington and London: Smithsonian Institution, 71-82.
Post-cranial skeleton
Dicynodont evolution is often observed through changes in skull morphology due to the generalized post-cranial morphology in a majority of dicynodonts.King, G.M., 1993. Species longevity and generic diversity in dicynodont mammal-like reptiles. Palaeogeography, Palaeoclimatology, Palaeoecology 102:321 332. However, Dicynodontoides is an exception to this statement. Although its size (small-medium) is unexceptional, aspects of the post-cranial morphology are very specialized and have been studied thoroughly.King, G. M. 1985. The postcranial skeleton of Kingoria nowacki (von Huene) (Therapsida: Dicynodontia). Zoological Journal of the Linnean Society 84:263–289.
Most notable of this specialization is the hindlimb morphology. The pelvic girdle consists of a small pubis and an ilium with anteriorly extensive but posteriorly rudimentary processes. The femoral head is offset from the bone, forming an s-shape, and the attachments for the ilio-femoralis muscles are significant. The foot is elongate with pointed claws and does not appear to be particularly specialized.
The shoulder girdle and forelimbs are more representative of Dicynodontia as a whole than the hindlimbs. The girdle is high and narrow, reflecting a reduction in the backward-forward pulling muscles, which would have been situated above and below the humerus. The humerus suggests an emphasis of long-axis rotation, a much more conservative morphology than that of the hindlimb structure.
In sum, the structure of the palate, the lower jaw, and the sacrum distinguish the morphology of Dicynodontoides from its Permo-Triassic dicynodont counterparts.
Paleobiology
Feeding ecology
The dicynodont feeding mechanism, though conservative, is often a variation of a generalized pattern. All members of this infraorder were likely herbivorous, though both the exact nature of this dietary pattern (i.e. the inclusion of cones, roots, etc.) and the possible degree of omnivory or insectivory is not fully understood.King, G. M. 1990. The Dicynodonts: A Study in Palaeobiology. Chapman and Hall, London.
Based on the habitual downward orientation of its skull, Dicynodontoides was likely a substrate-targeted feeder, or grazer, rather than a browser. The narrow anterior portion of the jaw could have allowed highly mobile movement of the tongue for the collection of surface vegetation, though other explanations for this feature are equally possible (see below).
In most members of Dicynodontia, both the reduced dentition and sharp cutting edge around the anterior end of the lower jaw suggest a scissor-like mode of food collection. After collection, mastication would have occurred via a back-and-forth grinding process. However, Dicynodontoides strays from this general pattern of food processing. Its caniniform blades, though periodically absent in this genus, are likely to have functioned as a paper cutter. However, the short mouth, blunted edges of the lower jaw, and the lack of a tough palatal surface against which the jaw could grind downplay the significance of this apparent shearing component. The morphology of the jaw hinge prevents the anterior end of the lower jaw from meeting the palate, only allowing palatal contact with the more posterior portion of the dentary. While there is little possibility of any transverse movement in the lower jaw, a crushing function is possible, and consistent with the feeding mechanism observed in other Emydopoids.Cox, C. B. 1998. The jaw function and adaptive radiation of the dicynodont mammal-like reptiles of the Karoo Basin of South Africa. Zoological Journal of the Linnean Society 122:349–384.
Unlike other members of the infraorder, the front of the lower jaw is not flattened, but curved and tapering anteriorly. Cox suggests this feature, as well as the strong jaw musculature, indicated by the large lateral wing on the dentary, may point towards grubbing in the dirt for food. However, subsequent analyses of other specimens have not featured the same degree of bluntness of the anterior end of the lower jaw, and call this theory into question. Nevertheless, the significant reduction of the tough, horny covering of the palate in Dicynodontoides suggests that whatever it may have grubbed up and consumed would have been both small, soft, and required minimal preparation. Roots or small invertebrates could provide the answer to this problem. Nevertheless, the exact nature of the feeding ecology of Dicynodontoides continues to elude researchers.
Locomotion
While most dicynodonts fall under a generalized body shape, Dicynodontoides departs from the standard. This genus was likely adapted to fast, agile locomotion.Ray, S. 2006. Functional and evolutionary aspects of the postcranial anatomy of dicynodonts (Synapsida, Therapsida). Palaeontology 49:1263–1286.
Muscle restoration of the acetabular-femoral articulation reveals the diverging pattern of locomotion of Dicynodontoides from the typical sprawling gait of most Permian dicynodonts. The hindlimb would have been retracted by a simple rotation of the femoral head, playing a more significant role in the retraction component of the gait than in most other dicynodonts. This feature, though rare in Permian dicynodonts, becomes increasingly more common in the Triassic forms, and Dicynodontoides represents one of many incremental transitions toward upright hindlimb posture in the dicynodont locomotor pattern.
However, the glenohumeral articulation suggests a more conservative forelimb and girdle morphology than that of the pelvic girdle. Evidence points toward a sprawling position of the forelimbs, with an emphasis on long-axis rotation. This likely allowed for manoeuvrability, while the hindlimbs powered the animal.
Metabolism and thermoregulation
Members of Dicynodontia were most likely ectothermic. However, inertial homeothermy may have been possible, though less likely. These ectothermal dicynodonts would have had lower feeding demands than extant endothermal mammalian herbivores.
The large pineal foramen apparent in the skull roof in dicynodonts, including Dicynodontoides, is also found in lizards. This light-sensitive organ played a crucial role in increasing the rate of digestion in these herbivores. The pineal foramen would have monitored solar intensity, allowing the correlation of the dicynodont's daily cycle of activity with the cycling availability of solar radiation throughout the day. As Cox points out, this would have enabled the animal to function nearer the upper end of its optimal temperature range.
Histology