Robertia is an extinct genus of small herbivorous dicynodonts from the Middle to Late Permian of South Africa, between 260 and 265 million years ago.[1] It is a monospecific genus,[1] consisting of the type-species R. broomiana, which was classified by Lieuwe Dirk Boonstra in 1948 and named in honor of Robert Broom for his study of South African mammal-like reptiles.[2]
Robertia had characteristic caniniform tusks and few, small teeth on the maxillary and dentary table.[3] Its beak and the propalinal movement of the jaw, as with other dicynodonts, allowed for efficient cutting of plant matter.[1] The solid, barrel-bodied creatures had a sprawling stance with a flexible backbone, which likely gave them a lizard-like appearance as they moved.[4] They were about 15 cm in length.[4]
Robertia is a member of the family Pylaecephalidae, which includes other small dicynodont therapsids with tusks such as Diictodon, Prosictodon, and Eosimops.
History and discovery[]
Anomodonts and dicynodont subclade members were the most common species of the Permian and Triassic periods and were the first fossil vertebrates uncovered in the South African Karoo.[5] The discovery of these animals was especially important as they exhibited mammal-like traits outside of the Mammalia taxon.[5]
Keen fossil collector and amateur paleontologist A. G. Bain found the first anomodont in South Africa.[6] Noticing the two prominent canines, he assigned it to a new genus, “Bidental.”[6] Skull specimens were referred to Sir Richard Owen at the British Museum of Natural History, who placed them under the designation Dicynodon in the 1840s.[5][6] Comparable specimens, but without tusks were placed in a new genus Oudenodon.[6] As more of these mammal-like specimens were discovered during the early twentieth century, hundreds of species began to be described and amassed under the Dicynodon designation.[5][6] In 1954, Haughton and Brink alone uncovered 54 dicynodont genera in the Karoo Basin and characterized 111 species under the single Dicynodon genus.[7] Poor extraction and preparation of the Dicynodon type fossils and the minute differences that were used to distinguish its species contributed to the problem.[5] At this point, taxa were described through dorsal or lateral sketches of the skull, suture patterns, proportions of the skull, and notation of the presence or absence of teeth and tusks.[7] Further studies examining the lower jaw, postcanine teeth,[3] and other characteristics have reduced the large amount of dicynodont taxa into fewer, more valid genera.[7] The new group Pylaecephalinae (later Pylaecephalidae), within which Robertia lies, was established in 1934.[5] Species of this family contains those of Diictodon and its closest relatives, having a characteristic intertemporal region and pineal foramen located in the pre-parietal.[5]
Reexamining over a hundred skulls in the South African Museum designated Dicynodon jouberti, L.D. Boonstra separated out new taxa that fell outside the group.[5] Robertia was characterized by Boonstra in 1948.[3][6][8] The fossil specimens were discovered in the lower part of the Tapinocephalus Zone in the west part of the Beaufort Group.[6]
In the 1950s, Toerien worked to further characterize and refine the species under the Dicynodon designation based on criteria beyond the features of the dorsal skull.[6] Toerien specifically used the presence of a small palatine bone to further classify species.[6] In 1953, he defined the species Dicynodon schroederi, which was later said to be synonymous with Diictodon feliceps in the 1980s.[5] However, more recent characterization has recognized the species as Robertia broomiana, based on the specimen’s wide intertemporal bar, extensive exposure of the parietals, a narrow postorbital bar, and the presence of postcanine teeth.
Geology and Paleoenvironment[]
The South African Karoo Basin expands about 300,000 km2 and contains the 145,000 km2 Beaufort Group of the Late Permian and Early Triassic.[7] The Abrahamskraal Formation of the Beaufort group consists of the Eodicynodon, Tapinocephalus, and Pristerognathus Assemblage Zones, all of which are characterized by the prevalence and high diversity of dicynodonts.[9][10] Robertia is found in the 1441 meter thick[11] Tapinocephalus Assemblage Zone in the northern region of the Abrahamskraal Formation.[12] According to Jirah, Robertia’s range is 200 m below the Teekloof Formation, however other sources claimed it spans into the Pristerognathus Assemblage Zone.[9] This inconsistency is due to the fact that Rubidge and Angielczyk misidentified Eosimops in this range as Robertia.[11] Robertia has not been identified in other Mid-Permian continental deposits.[9]
Scarcity of fossils in the stratigraphic levels before the appearance of Robertia and its close relatives Eosimpos and Diictodon prevents accurate delineation of where members of each genus begin relative to each other.[9] Dipping in the strata may have resulted in poor exposure of well defined divisional planes.[11] In addition, many of the best-preserved specimens that can accurately be classified as Robertia were collected without precise location documentation.[10] As a result, the range of Robertia may also extend further than is currently known.[1]
Robertia fossil specimens have been uncovered in mudstone and sandstone,[9] which have been formed by river flow across the alluvial plains.[7] It is thought that the paleoenvironment of the Beaufort Karoo consisted of large rivers around 350 meters wide and 11 meters deep running into a system of lakes that were no more than 50 meters deep.[7] Late Permian South Africa was likely warm to hot, with average temperatures ranging from 16 to 20 °C and experienced seasonal rainfall, about 50 to 70 cm yearly, disrupting the semi-arid climate.[7] There would have been occasional flash-flooding.[1]
Along the Permian Karoo Basin riverbanks, the vegetation included woody deciduous Glossopteris and the bamboo-like Phyllotheca.[1] The lowland areas likely gave rise to a variety of ferns, mosses, and lycopods.[1] This would have formed the basis of Robertia’s diet. There is some suggestion of the presence of stretches of savanna, but others doubt this, since ferns do not make up modern savannas.[1] The region’s hot, semi-arid climate dependent on intermittent rainfall may have placed pressure on the herbivorous dicynodonts of the time, turning them towards digging for rhizomes below the ground surface.