Robertia

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Robertia
Temporal range: Middle Permian-Late Permian, 265–260  Ma
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Robertia1DB.jpg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Therapsida
Suborder:† Anomodontia
Clade:† Dicynodontia
Family:† Pylaecephalidae
Genus:† Robertia
Boonstra, 1948
Type species
†R. broomiana
Boonstra, 1948
Synonyms

†Dicynodon schroederiToerien, 1953
†Broilius antjiesfonteinensisToerien, 1953 [1]

Contents

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. [5]

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 genus Dicynodon. [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. [5]

Geology and paleoenvironment

The South African Karoo Basin expands to 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. [13]

Description

Skull

Skull of Robertia in right lateral view. Note the large caniniform tusk. Robertia.jpg
Skull of Robertia in right lateral view. Note the large caniniform tusk.

Robertia’s skull reached a length of 130 mm, large compared to other small dicynodonts. [3] [7] Some specimens have minor grooves on the facial surface. [10] It has a characteristic relatively wide intertemporal region, which exposes the parietal bones in the midline. [3] A low dorsal ridge of the premaxilla with a narrow groove along its midline runs between the nasals. [10] Robertia has two, large caniniform tusks and about three, [7] small irregularly placed maxillary teeth posteromedial to the tusks. [3] The anterior edge of the tusks also have a sharp edge. [7] Anteromedial to the tusks, the maxilla bears a sharp edge. [3] A sharp palatal notch and a maxillary notch are located behind the rear edge of the maxilla and upper anterior region of the tusks, respectively. [7]

Whether the presence of tusks is a sexual dimorphism in Robertia is questioned, as the best preserved specimens all have tusks, but it is more difficult to determine if they are evident in the poorly preserved fossils. [10] Some studies determined that tusks were variable in Robertia, [10] [6] however more recent accounts have stated they are consistently present across Robertia. [3]

The palatines are distinctive, forming an anterior gap and not meeting the vomers. [3] Robertia has smaller palatines compared to close relative Pristerodon, but larger than the palatines of Emydops. [3] The pterygoids are slightly curved and have fairly high, thin triangular-shaped flanges halfway down their length. [3] The interpterygoid vacuities are longer and tear-drop shaped compared to other pylaecephalids. [12] Robertia has a short secondary palate, with the choana anterior and at the same level as the tusks. [6]

The dentary shelf does not protrude as much as in Emydops, and the concave dentary tables hold five to six pointed teeth medially. [3] Robertia is one of the pylaecephalids with the most dentary teeth, [10] which occlude with the palatine pad (a ridged region posterolateral to the main secondary palate) upon jaw retraction. [7] A beak is located anterior to the tusks and the outer side of the dentary. [6]

Postcranial skeleton

Robertia is described as “solidly built, barrel-bodied animals.” [1] It had developed postural limb musculature, a trochanter on the femur, diminished pre-acetabular iliac expansion relative to the post-acetabular, an anteriorly expanded pubis, and an abducted femur, which differentiate it from Diictodon. [13]

The radius and ulna are thin and about three-quarters the length of the humerus, articulating at right angles to the humerus. [4] The antebrachium was also positioned at a right angle relative to the humerus, indicating a sprawling posture of the forelimb. [4] This suggests the necessity of strong postural muscles, which would prevent collapse under the weight of gravity. [4] The appropriate attachment sites for muscles such as the ventral adductor, biceps, brachialis, coracobrachialis, and pectoralis are accordingly well developed. [4] Characteristic of other dicynodonts, both ends of the humerus are expanded. [4] The head of this bone faces slightly medially and dorsally. [4] Robertia has blunt claws on the end of each phalanx, with a protuberance on the undersides. [4] On one fossil specimen, the metacarpal and the phalanges of the longest finger are the same length as the radius. [4]

The S-shaped femur similarly articulates in a right-angled, sprawling position. [4] All dicynodonts had a parasagittal hindlimb posture, besides Robertia. [13] The femur head is more pronounced on the dorsal surface and is not offset from the main bone shaft. [4] Well-developed regions of attachment for muscles such as the pubo-ischio-femoralis externus, ventral adductor, femorotibialis, and gastrocnemius provide support for the sprawling gait. [4] The feet also have claws, which are rounder than those of the hand and have a dorsal ridge. [4]

The thoracic and lumbar ribs are long and straight, suggesting a backward-angled orientation for clearance of the abdomen from the ground. [4] Robertia had either two or three sacral vertebrae. [10] [13] The vertebral column was flexible, the pre-zygapophyses being flat and wide and articulating horizontally with the post-zygapophyses. [4]

The tail is only slightly muscular and is about one-eighth the length of the body. [4]

Paleobiology

Feeding system

Robertia and other dicynodonts had a particularly specialized jaw. [7] A forward-backward motion of the lower jaw allowed them to effectively breakdown vegetation. [3] Robertia’s small, fragile teeth may not have played a direct role in chewing, despite their ability to run along the dentary table. [3] Shredding from movement of the dentary along the caniniform tusks and up past the premaxilla and maxilla occurred as the lower jaw motioned propalinally. [7] The sharp blades of the dentary and along the tusks provide cutting action. [7] The front end of the jaws and the anterior notch aligned the vegetation in the mouth, and as the lower jaw moves backward, it pulled the plant matter past the caniniform tusks, cutting it into bite-size pieces. [7] The food was further processed by the dentary blades and the edges of the tusks and crushed on the palatine pad. [7]

The horned beak may have allowed small dicynodonts such as Robertia to pick out individual leaves, seeds, and buds, however it is suggested that they preferred stems and rhizomes over leafy vegetation. [1] Their claws may have been utilized for tearing or digging in the search for food. [4] Different times of the year may have called for different selections of food sources in the Permian. [1]

It has been suggested that the less developed masticatory system may have led to Robertia's selection of certain foods and may have limited its ability to persist in comparison to the more sophisticated systems of groups such as Emydops and Pristerodon, which may have been more generalist feeders. [3]

Locomotion

Robertia had a sprawling gait. [4] The position and rounding of the dorsal articulation area of the femur allowed for a longer stride compared to earlier sprawling animals. [4] The pectoral girdle muscles had a postural function rather than locomotory function and thus provided less thrust than the muscles of the hindlimb. [4] The flexibility of the vertebrae allowed for extensive side to side movement as Robertia moved, similar to a modern lizard. [4] Their long hands may have provided a platform of stability, however they may not have been wide enough for proper support. [4] Robertia had a short tail, which may have helped with maneuverability at high speeds and over uneven ground. [13]

Metabolism and thermoregulation

Robertia was likely ectothermic. [7] Robertia and its dicynodont relatives have a large pineal foramen on their skull, suggesting a light-sensitive pineal organ was used to track and take advantage of solar intensity cycles. [7] Locating optimal temperatures would have helped in digestion. [7]

Related Research Articles

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Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivores that typically bore a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typically toothless beak, unique amongst all synapsids. Dicynodonts first appeared in Southern Pangaea during the mid-Permian, ca. 270–260 million years ago, and became globally distributed and the dominant herbivorous animals in the Late Permian, ca. 260–252 Mya. They were devastated by the end-Permian Extinction that wiped out most other therapsids ca. 252 Mya. They rebounded during the Triassic but died out towards the end of that period. They were the most successful and diverse of the non-mammalian therapsids, with over 70 genera known, varying from rat-sized burrowers to elephant-sized browsers.

<i>Diictodon</i> Extinct genus of dicynodonts

Diictodon is an extinct genus of pylaecephalid dicynodont. These mammal-like synapsids lived during the Late Permian period, approximately 255 million years ago. Fossils have been found in the Cistecephalus Assemblage Zone of the Madumabisa Mudstone of the Luangwa Basin in Zambia and the Tropidostoma Assemblage Zone of the Teekloof Formation, Tapinocephalus Assemblage Zone of the Abrahamskraal Formation, Dicynodon Assemblage Zone of the Balfour Formation, Cistecephalus Assemblage Zone of the Middleton or Balfour Formation of South Africa and the Guodikeng Formation of China. Roughly half of all Permian vertebrate specimens found in South Africa are those of Diictodon. This small herbivorous animal was one of the most successful synapsids in the Permian period.

<i>Emydops</i> Extinct genus of dicynodonts

Emydops is an extinct genus of dicynodont therapsids from the Middle Permian to Late Permian of what is now South Africa. The genus is generally small and herbivorous, sharing the dicynodont synapomorphy of bearing two tusks. In the following years, the genus grew to include fourteen species. Many of these species were erected on the basis of differences in the teeth and the positioning of the frontal and parietal bones. A 2008 study narrowed Emydops down to two species, E. arctatus and the newly described E. oweni.

<i>Eodicynodon</i> Extinct genus of dicynodonts

Eodicynodon is an extinct genus of dicynodont therapsids, a highly diverse group of herbivorous synapsids that were widespread during the middle-late Permian and early Triassic. As its name suggests, Eodicynodon is the oldest and most primitive dicynodont yet identified, ranging from the middle to late Permian and possessing a mix of ancestral Anomodont/therapsid features and derived dicynodont synapomorphies.

<i>Tapinocaninus</i> Extinct genus of therapsids

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<i>Endothiodon</i> Extinct genus of dicynodonts

Endothiodon is an extinct genus of large dicynodont from the Late Permian. Like other dicynodonts, Endothiodon was an herbivore, but it lacked the two tusks that characterized most other dicynodonts. The anterior portion of the upper and lower jaw are curved upward, creating a distinct beak that is thought to have allowed them to be specialized grazers.

<i>Theriognathus</i> Extinct genus of therapsids from late Permian South Africa and Tanzania

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<i>Myosaurus</i> Extinct genus of dicynodont from the lower Triassic

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<i>Dicynodontoides</i> Extinct genus of dicynodonts

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<i>Eosimops</i> Extinct genus of dicynodonts

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<i>Kawingasaurus</i> Extinct genus of dicynodonts

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<i>Odontocyclops</i> Extinct genus of dicynodonts

Odontocyclops is an extinct genus of Dicynodonts that lived in the Late Permian. Dicynodonts are believed to be the first major assemblage of terrestrial herbivores. Fossils of Odontocyclops have been found in the Karoo Basin of South Africa and the Luangwa Valley of Zambia. The phylogenetic classification of Odontocyclops has been long under debate, but most current research places them as their own genus of Dicynodonts and being very closely related to Rhachiocephalus and Oudenodon.

<i>Pelanomodon</i> Extinct genus of dicynodonts

Pelanomodon is an extinct genus of dicynodont therapsids that lived in the Late Permian period. Fossil evidence of this genus is principally found in the Karoo Basin of South Africa, in the Dicynodon Assemblage Zone. Lack of fossil record after the Late Permian epoch suggests that Pelanomodon fell victim to the Permian-Triassic extinction event.

<span class="mw-page-title-main">Chainosauria</span> Extinct clade of therapsids

Chainosauria is a large and speciose clade of anomodont therapsid that includes the highly diverse dicynodonts and a small number of closely related basal genera —although the total composition and taxonomic scope of Chainosauria is in flux. Chainosauria was named in 1923 to group together the dicynodonts and their close relatives, namely three small anomodont genera from South Africa that made up the now defunct group 'Dromasauria'. The name soon fell into disuse, however, as it was functionally replaced by Anomodontia. Chainosauria was later revived cladistically in 2009, preserving the association of dicynodonts and the 'dromasaurs' and has since served in effect as both a cladistic and a biogeographic counterpart to the Laurasian venyukovioids, with early chainosaurs appearing to have been a Gondwanan radiation.

<span class="mw-page-title-main">Pylaecephalidae</span> Extinct family of dicynodonts

Pylaecephalidae is a family of dicynodont therapsids that includes Diictodon, Robertia, and Prosictodon from the Permian of South Africa. Pylaecephalids were small burrowing dicynodonts with long tusks. The family was first named in 1934 and was redefined in 2009. Diictodontidae and Robertiidae are considered junior synonyms of Pylaecephalidae; although Pylaecephalus itself is considered a junior synonym of Diictodon, the name Pylaecephalidae predates these names and therefore takes priority.

<span class="mw-page-title-main">Abrahamskraal Formation</span> Geological formation of the Beaufort Group in South Africa

The Abrahamskraal Formation is a geological formation and is found in numerous localities in the Northern Cape, Western Cape, and the Eastern Cape of South Africa. It is the lowermost formation of the Adelaide Subgroup of the Beaufort Group, a major geological group that forms part of the greater Karoo Supergroup. It represents the first fully terrestrial geological deposits of the Karoo Basin. Outcrops of the Abrahamskraal Formation are found from the small town Middelpos in its westernmost localities, then around Sutherland, the Moordenaarskaroo north of Laingsburg, Williston, Fraserburg, Leeu-Gamka, Loxton, and Victoria West in the Western Cape and Northern Cape. In the Eastern Cape outcrops are known from Rietbron, north of Klipplaat and Grahamstown, and also southwest of East London.

<i>Bulbasaurus</i> Extinct genus of dicynodonts

Bulbasaurus is an extinct genus of dicynodont that is known from the Lopingian epoch of the Late Permian period of what is now South Africa, containing the type and only species B. phylloxyron. It was formerly considered as belonging to Tropidostoma; however, due to numerous differences from Tropidostoma in terms of skull morphology and size, it has been reclassified the earliest known member of the family Geikiidae, and the only member of the group known from the Tropidostoma Assemblage Zone. Within the Geikiidae, it has been placed close to Aulacocephalodon, although a more basal position is not implausible.

<i>Thliptosaurus</i> Extinct genus of dicynodonts

Thliptosaurus is an extinct genus of small kingoriid dicynodont from the latest Permian period of the Karoo Basin in KwaZulu-Natal, South Africa. It contains the type and only known species T. imperforatus. Thliptosaurus is from the upper Daptocephalus Assemblage Zone, making it one of the youngest Permian dicynodonts known, living just prior to the Permian mass extinction. It also represents one of the few small bodied dicynodonts to exist at this time, when most other dicynodonts had large body sizes and many small dicynodonts had gone extinct. The unexpected discovery of Thliptosaurus in a region of the Karoo outside of the historically sampled localities suggests that it may have been part of an endemic local fauna not found in these historic sites. Such under-sampled localities may contain 'hidden diversities' of Permian faunas that are unknown from traditional samples. Thliptosaurus is also unusual for dicynodonts as it lacks a pineal foramen, suggesting that it played a much less important role in thermoregulation than it did for other dicynodonts.

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