| Euchambersia | |
|---|---|
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| Skull ofE. liuyudongi from the top (a), sides (c, e), and back (f), and skulls ofE. mirabilis from the top (b; BP/1/4009), side (d; NHMUK R5696), and back (g; NHMUK R5696) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Clade: | †Therocephalia |
| Family: | †Akidnognathidae |
| Genus: | †Euchambersia Broom,1931 |
| Type species | |
| †Euchambersia mirabilis Broom, 1931 | |
| Other species | |
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Euchambersia is anextinctgenus oftherocephaliantherapsids that lived during theLate Permian in what is nowSouth Africa andChina. The genus contains two species. Thetype speciesE. mirabilis was named by paleontologistRobert 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 familyAkidnognathidae, which historically has also been referred by as the synonymous Euchambersiidae (named afterEuchambersia).
Euchambersia was a small and short-snouted therocephalian, possessing largecanines 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 avenom delivery mechanism. If this statement turns out to be true, then it would be one of the oldest knowntetrapods to have this characteristic. In 2017, the internal structure of the skull ofE. 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.
The type specimen ofEuchambersia mirabilis and ofEuchambersia overall was found byRobert Broom on theSouth African farm of Vanwyksfontein, owned by a Mr. Greathead, near the town ofNorvalspont. It consists of a single, distorted skull, catalogued asNHMUK R5696, which was described by Broom in 1931.[1] A second, smaller skull, with the specimen numberBP/1/4009, was found in 1966[2] and described byJames Kitching in 1977. Both specimens are missing thelower jaw. They originated from the same general layer of rock, in the upperCistecephalus Assemblage Zone of theBeaufort Group within theKaroo Supergroup.[3] TheCistecephalus AZ has been dated to theWuchiapingian stage of theLate Permian,[4] between 256.2 and 255.2 million years old.[5]
Broom named the genusEuchambersia, which he considered "the most remarkable therocephalian ever discovered", after the eminentScottish publisher andevolutionary thinkerRobert Chambers, whoseVestiges of the Natural History of Creation was considered by Broom to be "a very remarkable work" though "sneered at by many".[1]
The second species,E. liuyudongi, was named by Jun Liu and Fernando Abdala in 2022 based on a well-preserved skull with an associated lower jaw, catalogued asIVPP V 31137. Few postcranial remains, including sixvertebrae and somerib fragments, also come from this specimen, but they are not described by the two authors. Thespecific epithet is named in honor of Liu Yu-Dong, the technician who discovered the holotype specimen in 2020. This species originated from theNaobaogou Formation ofInner Mongolia, which is dated more broadly to theLopingian epoch (which contains the Wuchiapingian). The formation is divided into threemembers based oncycles of sedimentation, numbere as members I, II, and III from oldest to youngest;E. liuyudongi originates from member I.[6] 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.[7][8]
E. mirabilis was small and short-snouted (the snout being about half of the skull length) for atherocephalian, with the type skull having a reconstructed length of approximately 11.6 cm (4.6 in), accounting for crushing and deformation in the fossil. The second known skull belonged to a smaller individual, with a length of 8 cm (3.1 in); it was probably immature, judging by the lack of fusion in the skull.[2] The type skull ofE. liuyudongi measures 7 cm (2.8 in) in length and has a shorter snout (less than 40% of the skull length).[6]
According to the initial description, the eye socket ofE. mirabilis was rather small. The branches of thepostorbital andjugal 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 theprefrontal, and thefrontal is also small. The skull does not bear apineal foramen. LikeWhaitsia, thepterygoid andpalatine of thepalate are not separated from the transpalatine, further to the side of the jaw, by any sort of opening.[1]E. liuyudongi differs fromE. mirabilis in several details of these bones: the frontal bone separates the prefrontal from contacting the postorbital, and thepostorbital fenestrae at the back of the skull are slit-like instead of rounded. Additionally, theepipterygoid and prootic of thebraincase are disconnected inE. liuyudongi.[6]
Although the skulls ofE. mirabilis are incompletely preserved,CT scanning suggests that eachpremaxilla held fiveincisors, with thesockets becoming progressively larger from the first to the fifth incisor. Like othertheriodonts, the crowns of the incisors are conical; they also lack serrations, unlikegorgonopsians andscylacosaurian 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 haswear 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 byresorption of the root.[2]
The type specimen ofE. mirabilis preserves the rightcanine.[2] Like other therocephalians, its canine was very large, resulting in a specialized predatory lifestyle that incorporates a sabertooth bite into prey killing.[9] It is round in cross-section,[3] 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.[2][10] UnlikeE. mirabilis, however, the canines ofE. liuyudongi had neither ridges nor grooves.[6] Theriodonts usually replace their teeth in an alternating[11] (or distichial) pattern,[12][13] such that the canine tooth is always functional; both skulls ofE. mirabilis show no sign of any replacement canines developing, suggesting that it was reliant on having both canines present and functional simultaneously.[2]
Behind the incisors and canines, there were no additional teeth in both the upper and lower jaws (as confirmed byE. liuyudongi).[6] 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 themaxilla, which was also bounded below by part of thelacrimal and possibly part of the jugal.[1] This fossa is 48% the length of the jaw in the type specimen ofE. 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.[2] InE. 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.[6]
CT scanning shows that the openings ofE. mirabilis lead to canals that connect to thetrigeminal nerve, which controls facial sensitivity. The forward-directed canal also splits into the three main branches of theinfraorbital nerve,[14] all of which connect to the socket of the canine; the junction occurs about 3–6 millimetres (0.12–0.24 in) 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 othertherapsids likeThrinaxodon,Bauria, andOlivierosuchus, 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.[2][14]
In 1934,Euchambersia was assigned to the newly named family Euchambersiidae byLieuwe Dirk Boonstra.[15][16] Boonstra initially misspelt the name as Euchambersidae (which is improper Latin), and was subsequently corrected byFriedrich 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).[17]
The 1986 phylogenetic analysis ofJames 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 ofAkidnognathidae, sinceAkidnognathus (which also belongs in the same family) was named first before any other member of the family.[17] This name has reached wider acceptance among researchers.[17][18][19] Huttenlocker andChristian Sidor also later redefined Moschorhininae as all of Akidnognathidae save forAnnatherapsidus andAkidnognathus.[20]
In 2008,Mikhail Ivakhnenko included the Akidnognathidae (as the Euchambersiidae) as the sister group of the familyWhaitsiidae in the superfamilyWhaitsioidea.[16] However, other researchers do not include the Akidnognathidae in the Whaitsioidea. Phylogenies by Huttenlocker and Sidor found that the Akidnognathidae was instead closest to theChthonosauridae, with the two forming thesister group to the group containing the Whaitsioidea and theBaurioidea.[20] Liu and Abdala performed a new phylogenetic analysis in 2022 for the description ofE. 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.[6]
| Therocephalia |
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The large maxillary fossae ofEuchambersia have been continual subjects of debate regarding their function. However, most researchers agree that they held some sort of secretory gland. While Broom initially argued that the fossae may have contained theparotid salivary glands,[1] this proposal was rejected by Boonstra andJean-Pierre Lehman, who noted that the parotid glands tend to be placed behind the eye; they respectively suggested that the fossae held modifiedlacrimal glands andHarderian glands.[2] However, the latter is also unlikely because Harderian glands are usually placed inside the eye socket.Franz Nopcsa suggested that the maxillary fossae housed venom glands (which may have been derived from lacrimal glands), with the ridged canines and the notches behind the canines allowing the venom to flow passively into the victim's bloodstream.[21] This hypothesis was widely accepted throughout the 20th century[18][22][23][24] and the characteristic morphology ofEuchambersia was used to support possible venom-bearing adaptations among various other prehistoric animals,[10][25][26] including the related therocephaliansMegawhaitsia[16] andIchibengops.[27]
Much of this acceptance has been based on the erroneous assumption that the canines are grooved instead of ridged;[3] grooved canines inEuchambersia would parallel the fangs of various venomous snakes as well as the venom-delivering incisors of the livingsolenodons.[24] This interpretation, which has consistently appeared in literature published after 1986, was determined by Julien Benoit to be the result of the propagation of Broom's overly reconstructed diagram of the skull, without the context of the actual specimens. He thus considered it necessary to re-evaluate the hypothesis of a venomous bite inEuchambersia.[3] Additionally, Benoit argued that grooved and ridged canines are not necessarily associated with venomous animals either, as shown by their presence inhippopotami,muntjacs, andbaboons, in which they play a role in grooming or sharpening the teeth;[3][24][28] in the latter two, ridged canines are also accompanied by a distinct fossa in front of the eye, which is entirely unconnected with venom.[24][29] Furthermore, grooved and ridged teeth in non-venomous snakes are used to reduce suctional drag when capturing slippery prey like fish or invertebrates.[30]
CT scanning of the known specimens ofEuchambersia by Benoit and colleagues was subsequently used to provide more concrete support in favour of the venom hypothesis. The canals leading into and from the maxillary fossae, as revealed by the scans, would primarily have supported the trigeminal nerve as well as blood vessels.[31][32][33] However, the fact that the canals also directly lead to the root of the canines would suggest that they had a secondary role in venom delivery. In all,Euchambersia seems to have had a venom gland (housed in the maxillary fossae), a delivery mechanism of the venom (the maxillary canals), and an instrument by which a wound for venom delivery can be inflicted (the ridged canines), which satisfy the criteria of a venomous animal as defined by Wolfgang Bücherl.[34] Benoitet al. noted that this does not conclusively demonstrate thatEuchambersia was actually venomous, especially given the previously stated objections. Additionally, there are no living animals with a delivery system analogous to the proposed system forEuchambersia (most deliver venom through the lower jaw,[35][36] while snakes have specialized ducts.[2][37]
An alternate hypothesis suggested by Benoitet al. involves some kind of sensory organ occupying the maxillary fossa. Uniquely among therapsids,[14] the canal within the maxilla is exposed on the back side of the maxillary fossa, which implies that the canal, carrying the trigeminal nerve, would probably have extended across the fossa, outside of the outline of the skull. Benoitet al. hypothesized that the fossa may have supported a specialized sensory organ analogous to the pit organ ofpit vipers and some other snakes,[38] or alternatively aganglion of nerve cells. It is also possible that this organ functioned as a replacement for theparietal eye inEuchambersia, like the pit organ does in pit vipers.[39] However, such an expanded sensory organ would be unprecedented amongtetrapods, and the few other therocephalians that also lack a parietal eye do not have a maxillary fossa either.[40] Thus, Benoitet al. considered the venom hypothesis as being more plausible.[2]
However, in the well-preserved specimen of the second species,E. liuyudongi, neither thesnout nor theorbit showed signs of the venomous gland. Only the preorbital (scent) glands are found, supporting the "scent gland hypothesis," although CT scans are required for more knowledge regarding the new species' dentition and skull.[6]
TheCistecephalus Assemblage Zone, from whereE. mirabilis is known, represents afloodplain that was covered in many small, relatively straight streams. The water level in these streams was probably seasonally dependent.[4] Judging from pollen preserved in theCistecephalus AZ, thepollen taxonPityosporites (which probably originated from a plant similar toGlossopteris) was very common, forming some 80% to 90% of the pollen discovered (although the prevalent sediments would not have been ideal for pollen preservation).[41]
In theCistecephalus AZ, other co-occurring therocephalians includedHofmeyria,Homodontosaurus,Ictidostoma,Ictidosuchoides,Ictidosuchops,Macroscelesaurus,Polycynodon, andProalopecopsis. More numerous, however, were the gorgonopsians, which includedAelurognathus,Aelurosaurus,Aloposaurus,Arctognathus,Arctops,Cerdorhinus,Clelandina,Cyonosaurus,Dinogorgon,Gorgonops,Lycaenops,Leontocephalus,Pardocephalus,Prorubidgea,Rubidgea,Scylacops,Scymnognathus, andSycosaurus.[4]
By far the most abundant herbivore was thedicynodontDiictodon, with over 1900 known specimens from theCistecephalus AZ. Other dicynodonts includedAulacephalodon,Cistecephalus,Dicynodon,Dicynodontoides,Digalodon,Dinanomodon,Emydops,Endothiodon,Kingoria,Kitchinganomodon,Oudenodon,Palemydops,Pelanomodon,Pristerodon, andRhachiocephalus. ThebiarmosuchiansLemurosaurus,Lycaenodon,Paraburnetia, andRubidgina were also present, along with thecynodontsCynosaurus andProcynosuchus. Non-synapsids included thearchosauromorphYounginia; theparareptiliansAnthodon,Milleretta,Nanoparia,Owenetta, andPareiasaurus; and thetemnospondylRhinesuchus.[4]
The Naobaogou Formation, from whichE. liuyudongi is known, is part of a series of Late Permian river and lake deposits in Inner Mongolia, which were deposited bybraided rivers, floodplains, and floodplain lakes.[42] Therocephalians had been reported from the Naobaogou Formation as early as 1989,[43] but these fossils were later lost. Subsequently, Liu and Abdala confirmed their presence in the formation by describing two other akidnognathids besidesE. liuyudongi,Shiguaignathus[7] andJiufengia,[44] as well asCaodeyao, a non-akidnognathid therocephalian closely related to the RussianPurlovia.[45] Unlike the more specializedE. liuyudongi, Liu and Abdala's 2022 phylogenetic analysis foundShiguaignathus andJiufengia to be less specialized (basal) members of Akidnognathinae, while simultaneously originating from the younger member III of the formation. Thus,E. liuyudongi provides evidence of both a therocephalian genus existing in both southern and northPangaea and of a specialized akidnognathid genus in northern Pangaea.[6]
Like theCistecephalus AZ and other Permian palaeoenvironments, dicynodonts were the most commonly preserved animal of the Naobaogou Formation.[8]Daqingshanodon was described in 1989.[43] Subsequently-discovered specimens consist of at least seven different types that may belong to separate species, with one described asTurfanodon jiufengensis, two related toDaqingshanodon, and three or four related toJimusaria.[8] Non-synapsids included thecaptorhinidGansurhinus;[46] the parareptilianElginia wuyongae;[47] and thechroniosuchianLaosuchus hun.[48]
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