| Volaticotherini | |
|---|---|
.jpg) | |
| Holotype ofVolaticotherium | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | †Eutriconodonta |
| Clade: | †Volaticotherini Menget al., 2006 |
| Type species | |
| †Volaticotherium antiquum Menget al., 2006 | |
| Genera | |
| |
Volaticotherini is aclade ofeutriconodontmammals from theMesozoic. In addition to thetype genusVolaticotherium, it includes the generaArgentoconodon,Ichthyoconodon, and potentiallyTriconolestes.
Since most remains are primarily teeth, they are foremostly diagnosticated by their highly distinctivemolars. However, the remains of one species,Volaticotherium antiquum, show that at least some members of this clade were capable ofgliding.[1] andArgentoconodon shares similar post-cranial features that also indicate aerial locomotion. As such, this clade contains some of the oldest known aerial mammals, alongside the various glidingharamiyidans.
Volaticotherini is phylogenetically defined as the clade derived from the most recent common ancestor ofArgentoconodon,Ichthyoconodon, andVolaticotherium.[2]
Ichthyoconodon was the first described member of this group, back in 1995, previously usually ranked amongeutriconodonts, albeit tentatively due to its atypical teeth.Volaticotherium, described in 2006, provided a fairly complete skeleton and led to the erection of a distinct family, Volaticotheridae, and order, Volaticotheria, to house the genus, and allowedIchthyoconodon to be recognized as a potential relative.[1] Volaticotheria was considered the sister taxon of a clade comprisingeutriconodonts,multituberculates, andtrechnotheres. However, not long after, an eutriconodont identity was suspected, and Volaticotheria has since fallen into disuse.[3]
Subsequent analyses have consistently recoveredArgentoconodon as the sister taxon ofVolaticotherium, withIchthyoconodon as the sister taxon of that clade and thus the basalmost volaticotherin.[2][4] The North American genusJugulator may be the sister taxon of Volaticotherini. These were moved to theeutriconodont familyTriconodontidae, as part of thealticonodontine assemblage, and the clade was renamed Volaticotherini accordingly.[2] However, other sources consider the clade to be a separate family from Triconodontidae, as Volaticotheridae.[5] This may be supported by other, more recent analyses, which find the clade to be more basally placed within Eutriconodonta.[4]
Since most volaticotherian remains are based on teeth, the diagnostic characteristic of the group is its molar morphology. Though classified as "triconodont" in shape, volaticotherian molars are highly atypical, possessing high, curved, backwards facingcusps aligned anteroposteriorly, lacking acingulum.[1][2] Canines and incisors tend to be fairly large.
In the two forms that do possess postcranial remains,Argentoconodon andVolaticotherium, we see a highly specialisedfemur, lacking a femoral neck.[2]
Volaticotherium is rather well preserved, bearing a mostly complete skeleton and soft-tissue impressions such ashair andpatagia.
One genus of volaticotherin,Volaticotherium, has clear evidence of being capable of gliding. It was the first gliding Mesozoic mammal discovered and lived at least 70 million years before the appearance of the first flying and glidingtherians.[1] It preserved a large,fur-coveredpatagium, extending not only between the limbs and tail, but also to the digits, "sandwiching" them. The limbs were proportionally longer than those of other Mesozoic mammals, fitting the standards in flying and gliding mammals, and the femur is uniquely specialised, allowing the leg to be extended laterally and remain steady during gliding. The tail is dorsoventrally flattened, and supports evidence ofuropatagia in at least the proximal vertebrae.[1]Argentoconodon shares similar femur characteristics, suggesting that it too may have been capable of gliding.[2]
Volaticotherini was a relatively widespread and long-lived clade, with occurrences known from theToarcian ofSouth America,Oxfordian ofChina, andBerriasian ofMorocco. The presence of volaticotherins inGondwana is unusual, as they are among the few known Gondwanan triconodonts (and, if aligned with triconodontids, the only representatives of the group in Gondwanna), withArgentoconodon occurring as far back as theEarly Jurassic in otherwiseaustralosphenidan dominated faunas.[2][6]
Though highly unusual and possibly indicating atypical occlusion patterns,[1][2] volaticotherian molars are thought to have had a shearing motion as in other eutriconodonts.[7][6] Combined with long canines, this seems to indicate that, like their relatives, they were probably carnivorous.Ichthyoconodon was fairly large by Mesozoic mammal standards, and were probably capable of tackling vertebrate prey.[7] In a study about Mesozoic mammal dietsArgentoconodon ranks among carnivorous species, whileVolaticotherium ranks among insectivorous taxa.[8] This same result is provided almost identically in a posterior study, albeit withVolaticotherium closer to the carnivore space.[9]
It has been noted that most gliding mammals are predominantly herbivorous,[10][11] which would make volaticothere carnivory truly exceptional. In particular,Volaticotherium itself has been compared to insectivorebats.[1]
Volaticotherins, as is typical for gliding animals, were adapted for an arboreal lifestyle.[1] One volaticotherin,Ichthyoconodon, was recovered from marine facies and consequently initially interpreted as an aquatic animal.[12] However, many terrestrial mammals are preserved in aquatic environments, so it is unclear how strong this evidence is regarding its life habits,[7] though its teeth appear to not have undergone long aquatic transportation.[12]
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