Titanogorgon

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Rubidgea
Dorsal and lateral view of the holotype (RC 13)
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Therapsida
Clade: †Gorgonopsia
Family: †Gorgonopsidae
Tribe: †Rubidgeini
Genus: †Rubidgea
Type species
†Rubidgea atrox
Broom, 1938
Synonyms

Genus-level

  • Broomicephalus Brink & Kitching, 1953
  • Titanogorgon Maisch, 2002

Species-level

  • Rubidgea kitchingi Broom, 1938
  • Rubidgea laticeps Broom, 1940
  • Gorgonognathus maximus Huene, 1950
  • Broomicephalus laticeps Brink & Kitching, 1953
  • Rubidgea majora Brink & Kitching, 1953
  • Dinogorgon (Broomicephalus) laticeps Watson & Romer, 1956
  • Titanogorgon maximus Maisch, 2002
  • Clelandina laticeps Gebauer, 2007
  • Rubidgea platyrhina

Rubidgea is an extinct genus of carnivorous therapsid from the Late Permian of South Africa and Tanzania. The genus consisted of one species, Rubidgea atrox.[1][2][3] The generic name Rubidgea honours fossil hunter Sidney Rubidge. Its species name, atrox, is derived from Latin, meaning "fierce, savage, terrible". Rubidgea is part of the subfamily Rubidgeinae, a derived group of gorgonopsians.[4][5] Gorgonopsians were a clade of saber-toothed predators that first appeared in the Middle (or Early) Permian,[6] and were the first group of synapsids to have specialized saber-like canines.[7] The largest specimen, BP/1/699, had a skull measuring 47.5 cm (18.7 in), making Rubidgea was one of the largest gorgonopsians.[8] Rubidgea was likely one of the top predators of its time and likely competed with large gorgonopsians such as Inostrancevia.[9]

Rubidgea would go extinct shortly before the end of the Permian following a turnover event that separated the lower and upper Daptocephalus Assemblage Zone. Following its extinction, it was replaced by Inostrancevia in South Africa for the remainder of the Permian.[8]

Taxonomy

Discovery

The first Rubidgea fossil was discovered by C. J. M. "Croonie" Kitching, the father of renowned paleontologist James Kitching, on the farm Doornberg outside the small town Nieu-Bethesda during the early 1930s. In a paper published in 1938, Robert Broom named the fossil Rubidgea kitchingi.[10] Broom noted the large size of the new gorgonopsid fossil, stating that it was a new species.[11] Subsequent discoveries in the following decades confirmed Broom's suspicions that a new subfamily of gorgonopsians should be categorized, and the Rubidgeinae was erected.[12]

Classification

The Rubidgeinae are a subfamily of derived gorgonopsians that have only been found in Africa. They are composed of six genera and 17 species. The Rubidgeinae are distantly related to the subfamily Inostranceviinae, which have only been mostly been found in Russia.[13] Out of the gorgonopsian clade, the systematics of the Rubidgeinae is the best resolved due to their distinctive character traits. The systematics of other gorgonopsian subfamilies remain chaotic due to a high degree of cranial homomorphism between taxa, making it difficult to distinguish different taxa effectively.[14][15][16][17][18][19][20][21][22][23]

Within his diagnostic analysis of the genus, in his 2016 paper, Kammerer recovered R. majora, R. kitchingi, R. platyrhina, and R. laticeps as to be specimens of R. atrox, suggesting the genus consisted of one species. Additionally, previously described gorgonopsians, such as Titanogorgon maximus, were recovered as junior synonym of R. atrox due to the lack of autapomorphies distinguishing it from South African specimens of Rubidgea.[3] Phylogenetic analysis recovered Rubidgea as a sister taxon of Clelandina, with both taxa being the most derived genera in the subfamily.[3][24]

The cladogram below (Kammerer and Rubidge 2022) displays the currently accepted systematic relationships of the Gorgonopsia.[13]

Evolution

Gorgonopsians are a major group of carnivorous therapsids, the oldest known definitive specimen coming from the Mediterranean island of Mallorca, dating to at least Wordian stage of the Middle Permian, with the possibility of dating to the Early Permian.[6] The clade was thought to have split off from Eutheriodontia either during the Kungurian stage of the Early Permian or during the Roadian stage of the Middle Permian.[25][26][27][6] During the Middle Permian, the majority of representatives of the clade were quite small and their ecosystems were mainly dominated by dinocephalians, large therapsids characterized by strong bone robustness.[28] The most common predators within the Middle Permian were basal therocephalians, a group of sabertoothed predators and may have limited the diversity of gorgonopsians.[22] However, some genera, such as Phorcys and Jirahgorgon, are relatively larger in size and already occupy the role of apex predator in one of the oldest geological strata of the Karoo Supergroup.[29][24] Gorgonopsians were the first group of carnivores to develop saber teeth, before the appearance of dinosaurs and mammals.[7] This feature later evolved independently multiple times in different predatory mammal groups, such as machaeroidines, nimravids, thylacosmilids, and machairodonts.[30] After the Capitanian extinction, gorgonopsians began to occupy ecological niches abandoned by anteosaurs and basal therocephalians, and adopted an increasingly imposing size, which very quickly gave them the role of apex predators.[3][23][29][31]

Geographically, gorgonopsians are mainly distributed in the present territories of Africa and European Russia,[32] with, however, an indeterminate specimen having been identified in the Turpan Depression, in northwestern China,[33] as well as a possible fragmentary specimen discovered in the Kundaram Formation, located in central India.[34] The geographic distribution of rubidgeine was restricted to Africa,[3] with their northernmost record being within the Moradi Formation of Niger.[35] Rubidgeines first appeared in the Cistecephalus Assemblage Zone, although they became more diverse in the following assemblage zones.[3]

Description

Size

Size comparison of R. atrox compared to a 1.8 m (5.9 ft) tall human

Rubidgea was the largest African gorgonopsian, as well as one of the largest known gorgonopsians, only being rivaled by Inostrancevia.[8][3] However, the sizes of the genus has varied between studies.[3][9][36] GPIT K46 was a gigantic partial skull recovered from the Usili Formation,[3] Maisch (2002) estimated the specimen had a total skull length of 50 cm (20 in).[37][9] However, other studies have suggested a lower skull length for the largest specimens.[3][36][8] Antón (2013) estimated Rubidgea had a skull length of 45 cm (18 in) and a total body length of 3 metres (9.8 ft).[36] Kammerer (2016) estimated the holotype, RC 13, had skull measuring 40.2 cm (15.8 in), similar in size to the largest Dinogorgon and Leontosaurus specimens. In the same paper, Kammerer estimated the largest Rubidgea specimens had skulls measuring 45 cm (18 in) with BP/1/699 and BP/1/195 having the most massive skulls among the specimens analyzed.[3] Kammerer et al. (2023) recovered BP/1/699 as the largest known rubidgeine, with an estimated skull length of 47.5 cm (18.7 in). The authors noted the specimen was similar in size to the holotype and paratype of Inostrancevia africana.[8]

The only gorgonopsian that exceeded Rubidgea in size were Inostrancevia alexandri and Inostrancevia latifrons, with a skull length of 50 cm (20 in) and 60 cm (24 in) respectively.[38][39]

Skull and dentition

Skull of R. atrox

The skull of Rubidgea was noted to have approached the length of Inostrancevia, but was significantly more robust.[36][3] It can be distinguished from other rubidgeines by an elongated boss present on the edge of dentary, posterior flange of postorbital bar in form of a rounded boss, and a jugal broadly exposed dorsal to squamosal in the subtemporal bar. Additional distinguishes included the combination of 1-2 upper postcanines and the absence of lower postcanines, reduced dentition palatine boss, well-developed supraorbital bosses, and a bulbous snout.[3] The incisors and canines of Rubidgea were noted to have been very large, with the canines being longer than the teeth of Tyrannosaurus.[40]

Paleobiology

Life reconstruction of R. atrox

Rubidgea was thought to have been heavily-built, large-bodied apex predator, and sporting a thick skull with long, sabre-like canines.[40][41] Lautenschlager et al. (2020) estimated that Rubidgea had a maximum jaw gape of 78.01°, with an effective gape of 45°. Their model size also suggested that Rubidgea was capable of producing a bite force of 715 newtons; the analysis found that massive gorgonopsians, such as Rubidgea, possessed a more powerful bite than other saber-toothed predators. Despite having a more powerful bite than other saber-toothed predators, gorgonopsians likely specialized on prey similar in size or smaller as their actual jaw gapes were below 80°, in addition to effective jaw gapes below 60°.[42] However, a gorgonopsian specimen (SAM-PK-11490) was noted to have an embedded tooth in its snout, with the attacker being another gorgonopsian. This suggests despite the inability to crush bone, the canines of gorgonopsians were capable of puncturing bone despite their weak bite force.[43] The biting method of gorgonopsians was different from mammalian sabertooths as they relied on kinetic-inertial jaw-closing system similar to modern crocodilians, to deliver fast and powerful jaw closures.[42][40]

Paleoecology

Numerous therapsid species, including rubidgeine gorgonopsians, are used as biostratigraphic markers in other African Basins, such as the Upper Madumabisa Mudstone Formation of Zambia, and the Chiweta Beds of Malawi.[37]

Usili Formation

Reconstruction of a gorgonopsian pursuing a herd of Endothiodon, based on the Late Permian Tanzanian Usili Formation

Rubidgea remains have been recovered within Usili Formation of Tanzania, which yielded a diversity of therapsids.[9][44][37] Gorgonopsians were represented by rubidgeines such as Ruhuhucerberus, Aelurognathus, Dinogorgon, as well as non-rubidgeine gorgonopsians included Scylacops, Gorgonops, Lycaenops, and the inostranceviine Inostrancevia africana.[44][9][45] Therocephalians present were represented by Theriognathus and Ictidosuchoides,[46] with cynodonts being solely represented by Procynosuchus delaharpeae.[44] Dicynodonts were highly diverse consisting of several genera including Compsodon,[47] Daptocephalus, Dicynodon, Dicynodontoides, Endothiodon, Geikia locusticeps and Rhachiocephalus.[44] Non-synapsid tetrapods included pareiasaurs Anthodon serrarius and Pareiasaurus serridens and the temnospondyl Peltobatrachus.[44]

Rubidgea fossils have been recovered from the Usili Formation of Tanzania, indicating biostratigraphic correlation with upper Permian-aged deposits in South Africa.[48] The formation likely correlated to the upper Cistecephalus Assemblage Zone to lower Daptocephalus Assemblage Zone.[46] The depositional environment of the formationw as thought to have been alluvial plain, which had numerous small meandering streams passing through well-vegetated floodplains. The basement of this formation would also have housed a generally high phreatic zone.[49]

Extinction

Most rubidgeines went extinct early died out shortly after the start of the lower Daptocephalus Assemblage Zone, with only Rubidgea persisting.[8] Originally, Rubidgea was thought to have persisted into the Lystrosaurus maccaigi-Moschorhinus subzone, going extinct during the Permian-Triassic extinction event.[50] However, recent analysis by Kammerer et al. (2023) found that the supposed upper Daptocephalus AZ records of Rubidgea to be problematic. RS 19, one of the supposed upper Daptocephalus Assemblage Zone specimens, has four upper incisors (an autapomorphy of Inostrancevia), a narrow skull and the absence of pachyostosis. This may suggest that RS 19 may be another specimen of Inostrancevia africana, although more confirmation is needed. RC 598, another specimen that had been attributed to Rubidgea, showed no signs of being a rubidgeine other than its large size. Its skull has been noted to have been narrower than expected for a rubidgeine, although this could be due to distortion. The youngest reliable record of Rubidgea was TM 2002, from the Ripplemead Member of the Balfour Formation, part of the lower subzone of Daptocephalus AZ. This suggests that Rubidgea went extinct during a turnover event that separated the lower and upper subzones.[8]

This turnover event also saw the extinction of many dicynodonts such as Diictodon feliceps and Aulacephalodon bainii,[8] the gorgonopsian Lyaencops,[8] and therocephalians such as Theriognathus.[51][50] Following its extinction, Rubidgea would be replaced by Inostrancevia for the remainder of the Permian.[8]

References

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