Euchambersia is an extinct genus of therocephalian therapsids that lived during the Late Permian in what is now South Africa and China. The genus contains two species. The type species E. mirabilis was named by paleontologist Robert Broom in 1931 from a skull missing the lower jaw. A second skull, belonging to a probably immature individual, was later described. In 2022, a second species, E. liuyudongi, was named by Jun Liu and Fernando Abdala from a well-preserved skull. It is a member of the family Akidnognathidae, which historically has also been referred by as the synonymous Euchambersiidae (named after Euchambersia).
Euchambersia was a small and short-snouted therocephalian, possessing large canines as is typical of the group. However, it is notable among therocephalians for possessing ridges on its canines and a large indentation in the side of the skull. It has been proposed that these structures supported a venom delivery mechanism. If this statement turns out to be true, then it would be one of the oldest known tetrapods to have this characteristic. In 2017, the internal structure of the skull of E. mirabilis has been used as stronger evidence in favour of the hypothesis that it was venomous; other possibilities, such as the indentation supporting some sort of sensory organ, still remain plausible.
Discovery and naming
The genus Euchambersia, more specifically the type species E. mirabilis, was first described in 1931 by Robert Broom based on a distorted fossil skull that he had previously discovered in the South African farm of Vanwyksfontein, owned by a Mr. Greathead, near the town of Norvalspont. This specimen, which constitutes the holotype, is now housed in the Natural History Museum, London, under the number NHMUK R5696. Describing the taxon as "the most remarkable therocephalian ever discovered", Broom named it after the eminent publisher and evolutionary thinker Robert Chambers, whose Vestiges of the Natural History of Creation was considered by Broom to be "a very remarkable work" though "sneered at by many". In 1966, a second, smaller skull of the same species, was discovered at the farm of Waschbank, located about from the type locality. It was first figured in 1977 by James Kitching, although he provided no description. This specimen is now catalogued as BP/1/4009 by the Evolutionary Studies Institute of the University of the Witwatersrand, where it is stored. The two known skulls of E. mirabilis, both lacking their lower jaws, originated from the same general layer of rock, in the upper Cistecephalus Assemblage Zone of the Beaufort Group within the Karoo Supergroup. The Cistecephalus AZ has been dated to the Wuchiapingian stage of the Late Permian, between 256.2 and 255.2 million years old.
In 2022, Jun Liu and Fernando Abdala described a second species, E. liuyudongi, based on a well-preserved skull with an associated lower jaw, catalogued as IVPP V 31137 in the collections of the Institute of Vertebrate Paleontology and Paleoanthropology. Few postcranial remains, including six vertebrae and some rib fragments, also come from this specimen, but they are not described by the two authors. The specific epithet is named in honor of Liu Yu-Dong, the technician who discovered the holotype specimen in 2020. This species originated from the Naobaogou Formation of Inner Mongolia, which is dated more broadly to the Lopingian epoch (which contains the Wuchiapingian). The formation is divided into three members based on cycles of sedimentation, numbere as members I, II, and III from oldest to youngest; E. liuyudongi originates from member I. Liu and colleagues had previously described a number of other new species from the middle portion of the Naobaogou Formation, which were among the 80 specimens that had been excavated from at least three field seasons after 2009.
Description
E. mirabilis was small and short-snouted (the snout being about half of the skull length) for a therocephalian, with the type skull having a reconstructed length of approximately , accounting for crushing and deformation in the fossil. The second known skull belonged to a smaller individual, with a length of ; it was probably immature, judging by the lack of fusion in the skull. The type skull of E. liuyudongi measures in length and has a shorter snout (less than 40% of the skull length).
According to the initial description, the eye socket of E. mirabilis was rather small. The branches of the postorbital and jugal that usually surround the back and bottom of the eye socket in therocephalians appear to be either very reduced or absent entirely. Meanwhile, the top of the eye socket is formed by the prefrontal, and the frontal is also small. The skull does not bear a pineal foramen. Like Whaitsia, the pterygoid and palatine of the palate are not separated from the transpalatine, further to the side of the jaw, by any sort of opening. E. liuyudongi differs from E. mirabilis in several details of these bones: the frontal bone separates the prefrontal from contacting the postorbital, and the postorbital fenestrae at the back of the skull are slit-like instead of rounded. Additionally, the epipterygoid and prootic of the braincase are disconnected in E. liuyudongi.
Teeth
Although the skulls of E. mirabilis are incompletely preserved, CT scanning suggests that each premaxilla held five incisors, with the sockets becoming progressively larger from the first to the fifth incisor. Like other theriodonts, the crowns of the incisors are conical; they also lack serrations, unlike gorgonopsians and scylacosaurian therocephalians. The interior edge of the incisors seems to be slightly concave, and the back edge appears to have a ridge. The smaller specimen has a displaced incisor preserved within its nasal cavity; it is more strongly recurved and has wear marks on its top edge, suggesting that it is probably a lower incisor. Its fourth incisor also has a replacement tooth growing behind it, accompanied by resorption of the root.
The type specimen of E. mirabilis preserves the right canine. Like other therocephalians, its canine was very large, resulting in a specialized predatory lifestyle that incorporates a sabertooth bite into prey killing. It is round in cross-section, and bears a prominent ridge on the side of its front surface. Immediately beside this ridge is a shallow depression that becomes wider near the top of the tooth, which is probably the same structure as the groove interpreted by some authors. Unlike E. mirabilis, however, the canines of E. liuyudongi had neither ridges nor grooves. Theriodonts usually replace their teeth in an alternating (or distichial) pattern, such that the canine tooth is always functional; both skulls of E. mirabilis show no sign of any replacement canines developing, suggesting that it was reliant on having both canines present and functional simultaneously.
Maxillary fossa and associated canals
Behind the incisors and canines, there were no additional teeth in both the upper and lower jaws (as confirmed by E. liuyudongi). Where teeth would be located in therocephalians that do have teeth behind the canines, there was instead a large depression, or fossa, on the side of the maxilla, which was also bounded below by part of the lacrimal and possibly part of the jugal. This fossa is 48% the length of the jaw in the type specimen of E. mirabilis, and 38% in the second skull. In both skulls, this fossa is divided into two parts: a shallower ridge on top, and a larger and deeper depression on the bottom. A wide furrow beginning behind the canine contacts the bottom of the fossa and then passes into the interior of the mouth. The bottom portion of the fossa is strongly pitted and bears a small opening, or foramen, on both the front and back surfaces. In E. liuyudongi, this fossa is deeper still; a bar of the maxilla caps the top of the fossa and contacts the jugal, and the inner wall of the fossa has a large opening to the nasal cavity. Its fossa nearly reaches the mid-height of the snout.
CT scanning shows that the openings of E. mirabilis lead to canals that connect to the trigeminal nerve, which controls facial sensitivity. The forward-directed canal also splits into the three main branches of the infraorbital nerve, all of which connect to the socket of the canine; the junction occurs about along the canal, another point of variation between the two skulls. The top branch, the external nasal ramus, splits into four branches in the type skull, but it does not split in the second skull. In other therapsids like Thrinaxodon, Bauria, and Olivierosuchus, the external nasal ramus generally splits into three or more branches. All of these canals would have brought nerves and nutrient-rich tissue to the root of the canines and the rest of the upper jaw.
Classification In 1934, Euchambersia was assigned to the newly named family Euchambersiidae by Lieuwe Dirk Boonstra. Boonstra initially misspelt the name as Euchambersidae (which is improper Latin), and was subsequently corrected by Friedrich von Huene in 1940. Euchambersiidae was initially considered to be separate from the families Moschorhinidae and Annatherapsididae; in 1974, Christiane Mendez recognized these groups as closely related subfamilies (renamed Annatherapsidinae, Moschorhininae and Euchambersiinae) within the wider group of her redefined Moschorhinidae (although she also referred to it as Annatherapsididae).
The 1986 phylogenetic analysis of James Hopson and Herb Barghusen supported Mendez's hypothesis of three subfamilies within Moschorhinidae, but they elected to use the name Euchambersiidae. In 2009, Adam Huttenlocker argued that the names Annatherapsididae, Moschorhinidae, and Euchambersiidae are junior synonyms of Akidnognathidae, since Akidnognathus (which also belongs in the same family) was named first before any other member of the family. This name has reached wider acceptance among researchers. Huttenlocker and Christian Sidor also later redefined Moschorhininae as all of Akidnognathidae save for Annatherapsidus and Akidnognathus.
In 2008, Mikhail Ivakhnenko included the Akidnognathidae (as the Euchambersiidae) as the sister group of the family Whaitsiidae in the superfamily Whaitsioidea. However, other researchers do not include the Akidnognathidae in the Whaitsioidea. Phylogenies by Huttenlocker and Sidor found that the Akidnognathidae was instead closest to the Chthonosauridae, with the two forming the sister group to the group containing the Whaitsioidea and the Baurioidea. Liu and Abdala performed a new phylogenetic analysis in 2022 for the description of E. liuyudongi. They found that the two species form a unified group within the Akidnognathidae, with the rest of the topology being similar to the one found by Huttenlocker and Sidor. The topology recovered by their analysis is shown below, with group labels following Huttenlocker and Sidor.
Paleobiology
Venom