| "Dixeya"nasuta | |
|---|---|
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Clade: | Theriodontia |
| Clade: | †Gorgonopsia |
| Genus: | |
| Species: | |
| Binomial name | |
| †"Dixeya"nasuta | |
| Synonyms | |
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"Dixeya"nasuta is anextinctspecies ofgorgonopsian (predatorytherapsids, related to modernmammals) that lived during theLate Permian ofEast Africa, known from fossils found in what is nowTanzania. The species has a complicatedtaxonomic history, it was originally named as a second species of thegenusDixeya which is now considered ajunior synonym ofAelurognathus. "D."nasuta itself, however, was not moved toAelurognathus, and although it was instead tentatively referred toArctognathus at first it has since been recognised to not belong to this genus either. This situation leaves "Dixeya"nasuta without a formalgenus name. It was proposed to belong to a new distinct genus, named"Njalila", that was informally proposed for the species in aPhD thesis, but this name has not yet been formally published and is currently anomen nudum. "D."nasuta has been characterised from other gorgonopsians by a combination of its straight snout profile, upturned and "pinched" nose, and curved jaw margin. The fossil record of theUsili Formation shows that thetaxon was contemporary with many other gorgonopsians, even alongside large representatives such asInostrancevia andrubidgeines.
"Dixeya"nasuta was originally named by GermanpaleontologistFriedrich von Huene in 1950 from two skulls collected from theUsili Formation (formerly known as K6 or the Kawinga Formation) in theRuhuhu Basin of Tanzania;[1] GPIT/RE/7118 (designated theholotype specimen) and GPIT/RE/7119.[2] Von Huene had erected these specimens as second species ofDixeya, a genus coined in 1927 bySidney H. Haughton for itstype speciesD. quadrata.[1] In 1970, French paleontologistDenise Sigogneau-Russell re-assignedD. quadrata toAelurognathus (asA. quadrata) in her systematic revision of gorgonopsian taxonomy, thussynonymising the two genera. However, she did not consider von Huene'sD. nasuta to belong toAelurognathus, and instead tentatively referred the species toArctognathus asArctognathus?nasuta.[3] Furthermore, Sigogneau (1970) only considered the holotype ofD. quadrata fromMalawi to belong toAelurognathus, and she did not consider two additional specimens referred toD. quadrata by von Huene in 1950 from Tanzania (GPIT/RE/7120 and GPIT/RE/7121[4]: 147 ) to belong to the same species. Instead, she suggested that they may also be referable toArctognathus?nasuta—in addition to three other Tanzanian specimens (MZC 886, MZC 887, and MZC 876[4]: 148 ) referred toD. quadrata byFrancis Rex Parrington in 1955.[5][6] Nonetheless, Sigogneau was cautious in the referral of "D."nasuta toArctognathus,[3] and had previously acknowledged that the referral was not a resolved matter, especially in her opinion that the roof of the skull of "D."nasuta was not well preserved enough for comparison.[7]
Later researchers agreed with Sigogneau's doubts and have acknowledged that"D." nasuta does not compare well toArctognathus. In Eva Gebauer's unpublished 2007Ph.D. thesis, she argued that "D."nasuta was distinct from other gorgonopsians and belonged to a new genus for which she informally proposed thenomen nudum "Njalila" (named after the Njalila, a tributary of theRuhuhu River in Tanzania). Gebauer further proposed a novel second species of this genus, "N. insigna", based on a skull previously referred to another gorgonopsian,Scylacops capensis. Gebauer differentiated "N. insigna" from "N."nasuta by possessing thicker arches between its skull openings, a posteriorly wider skull, and a slightly more rounded snout profile.[4]: 136–156 In 2015, paleontologist Christian F. Kammerer also agreed that "D."nasuta was not a species ofArctognathus, as well as that the Tanzanian specimens ofD. quadrata likely belonged to this same species. However, he was more cautious regarding their taxonomy, noting that Gebauer's proposal of a novel genus required further study of the material and needed to be more rigorouslyphylogenetically tested first. From this, he urged that the taxon should be referred to as "Dixeya"nasuta until its taxonomy and relations could be resolved (reportedly under study as of 2015).[2]
"Dixeya"nasuta is only known by its skull and jaws, which measured roughly 18 centimetres (7.1 in) in length (mid-sized for a gorgonopsian) and had a relatively short and compact snout. Compared with similarly short-snouted gorgonopsians (such asArctognathus andEriphostoma) the skull is not as wide at the rear, with only weakly flaringzygomatic arches and little constriction of the snout behind thecanines. As such, its skull appears much more straight-sided when viewed from above, and is also generally wider than it is tall. Similarly, the profile of the skull along the top of the snout is also largely straight, although the tip is characteristically turned up in a sharp point above the nostrils, which were positioned far-forwards on the snout. The snout is also distinctive for its unusual 'pinched' appearance. Thenasal bones along the top of the snout are broad, but are constricted along the middle. Furthermore, theseptomaxilla (a small bone found in and around the nostrils of therapsids) bulges strongly outwards under and behind the nostrils but then rapidly hollows out just behind them on either side, giving the bridge of the nose the pinched appearance.[4]: 151–152 [5]
Behind the snout, the roof of the skull is slightly concave and the rims of theorbits are noticeably raised above it. The orbits themselves are proportionately large and rounded, and face laterally out to the sides.[5] Thetemporal fenestra, a hole in the skull behind the eye socket for jaw muscle attachment, is also very large. Subsequently, the bony arches surrounding and separating these openings (e.g. the suborbital arch,postorbital bar) are proportionately slender and thin. Theparietal foramen ("third eye" opening) on top of the skull is large and surrounded by a raised boss of bone, and is positioned at the very back of the skull right above theocciput.[2] The occiput itself (the back face of the skull) is tall and roughly rectangular in shape, slightly concave and only gently sloping.[4]: 151–152
As in other gorgonopsians, "D."nasuta has large blade-like caniniform teeth. Theincisor teeth (five in eachpremaxilla), however, are smaller than those of related gorgonopsians, and it only had four to five small postcanine teeth.[4]: 151–152 The jawline of themaxilla in the upper jaw is notably convex, with a much more exaggerated curve of the toothrow than in other gorgonopsians except forArctognathus. This exaggerated curvature is due to the post-canine teeth being housed in a raised bony flange of the maxilla behind the canines.[2] The maxilla also has an unusual groove over the postcanine teeth, starting shallowly above the first postcanine and running down to the edge of the bone behind the 5th postcanine, deepening along its length. Like other gorgonopsians "D."nasuta also possessed palatal teeth, three on eachpalatine and two on eachpterygoid bones, with only weakly developed bosses supporting them.[5] Thevomer on the roof of the mouth is very broad at the front, but narrows rapidly to a constricted splint halfway down its length. This more resembles the vomer of the derived rubidgeines than the narrower vomer of earlier gorgonopsians. The vomer sports three ridges, one down its middle and two running along each edge.[2]
Thedentary bone of the lower jaw is comparatively slender, with a slopingmandibular symphysis that nonetheless bears the characteristic 'chin' of gorgonopsians. The reflected lamina of theangular bone towards the back of the jaw is only moderately ridged, in comparison to other gorgonopsians.[4]: 151–152
Thephylogenetic relationships of "Dixeya"nasuta (as "Njalila") were analysed by Gebauer in 2007 in her unpublished PhD thesis, and was the first computerised phylogenetic analysis of gorgonopsians ever conducted. Gebauer found "D."nasuta as a member of thefamily Gorgonopsidae, which in her classification excluded the mostbasal genera of gorgonopsians in her tree that she regarded asplesiomorphic (i.e. representing the ancestral condition) for the group. Within Gorgonopsidae, "D."nasuta was a relatively derived member but outside of the clade including the giant and derivedRubidgeinae andInostrancevia, occupying part of an evolutionary grade between them and more ancestral gorgonopsids. The results of Gebauer (2007) are depicted in thecladogram below.[4]: 242–244 Two genera mentionned in this cladogram, i. e.Scylacognathus andEoarctops, have since been synonymized withEriphostoma since 2015.[8]
The analysis of Gebauer (2007) was the first major attempt to perform a phylogenetic analysis of gorgonopsians, however its results have not been borne out by subsequent independent analyses. Namely, Kammerer (2016) regarded Gebauer's analysis as "unsatisfactory", citing that many of the characters used by her analysis were based upon skull proportions that are variable within taxa, both individually and ontogenetically (i.e. traits that change through growth). As an example of a potential problem created by this, he highlighted the basal position ofAloposaurus (awastebasket taxon of various immature gorgonopsians) compared to thestratigraphically older and morphologically basalEoarctops (now a junior synonym ofEriphostoma) being found in a relatively more derived position.[9][10]
"D."nasuta has yet to be included in any later phylogenetic analyses of gorgonopsians, and in 2015 Kammerer commented that both its generic status and phylogenetic relationships amongst other gorgonopsians needed further study pending a full re-description before a generic assignment could be made.[2]
All knownfossil specimens of "Dixeya"nasuta have been identified in the Usili Formation, Ruhuhu Basin, southern Tanzania.[2] This formation, dating from theUpper Permian, is known to provide a fairly considerable number of fossils of varioustetrapods. During this period, this formation would have been analluvial plain which would have had numerous small meandering streams passing through well-vegetatedfloodplains. The basement of this formation would also have housed a generally highphreatic zone.[11]
"D."nasuta was contemporary with many other gorgonopsians. These includeCyonosaurus,Gorgonops,Inostrancevia,Lycaenops, "Sauroctonus"parringtoni,Scylacops and the rubidgeinesAelurognathus,Dinogorgon,Rubidgea,Ruhuhucerberus andSycosaurus[a][12][11][9] The othertheriodonts present are represented by thetherocephaliansSilphictidoides andTheriognathus as well as by thecynodontProcynosuchus.[11]
The most numerous tetrapods in the formation are thedicynodonts, among which areCompsodon,[13]Daptocephalus,Dicynodon,Dicynodontoides,Endothiodon,Euptychognathus,Geikia,Katumbia,Kawingasaurus,Oudenodon,Pristerodon,Rhachiocephalus and an indeterminatecryptodont. An undeterminedbiarmosuchian similar toBurnetia is also known.Therapsids are not the only tetrapods present in the Usili Formation. Indeed,sauropsids such as thearchosauromorphAenigmastropheus[14] and thepareiasaursAnthodon andPareiasaurus are known. The onlytemnospondyl recorded isPeltobatrachus.[11]
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