Volaticotherini
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Volaticotherini is a
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
of
eutriconodont Eutriconodonta is an order (biology), order of early mammals. Eutriconodonts existed in Asia (including Insular India, pre-contact India), Africa, Europe, North America, North and South America during the Jurassic and the Cretaceous periods. The ...
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s from the
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
. In addition to the
type genus In biological taxonomy, the type genus (''genus typica'') is the genus which defines a biological family and the root of the family name. Zoological nomenclature According to the International Code of Zoological Nomenclature, "The name-bearin ...
'' Volaticotherium'', it includes the genera '' Argentoconodon'', '' Ichthyoconodon'', and potentially '' Triconolestes''. Since most remains are primarily teeth, they are foremostly diagnosticated by their highly distinctive
molars The molars or molar teeth are large, flat tooth, teeth at the back of the mouth. They are more developed in mammal, mammals. They are used primarily to comminution, grind food during mastication, chewing. The name ''molar'' derives from Latin, '' ...
. However, the remains of one species, '' Volaticotherium antiquum'', show that at least some members of this clade were capable of
gliding Gliding is a recreational activity and competitive air sports, air sport in which pilots fly glider aircraft, unpowered aircraft known as Glider (sailplane), gliders or sailplanes using naturally occurring currents of rising air in the atmospher ...
. and '' Argentoconodon'' 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 gliding
haramiyida Haramiyida is a possibly Paraphyly, paraphyletic order of Mammaliaformes, mammaliaform cynodonts or mammals of controversial taxonomic affinites. Their teeth, which are by far the most common remains, resemble those of the multituberculates. Howe ...
ns.


Definition

Volaticotherini is phylogenetically defined as the clade derived from the most recent common ancestor of ''Argentoconodon'', ''Ichthyoconodon,'' and ''Volaticotherium''.


History

'' Ichthyoconodon'' was the first described member of this group, back in 1995, previously usually ranked among
eutriconodont Eutriconodonta is an order (biology), order of early mammals. Eutriconodonts existed in Asia (including Insular India, pre-contact India), Africa, Europe, North America, North and South America during the Jurassic and the Cretaceous periods. The ...
s, 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 allowed '' Ichthyoconodon'' to be recognized as a potential relative. Volaticotheria was considered the sister taxon of a clade comprising
eutriconodont Eutriconodonta is an order (biology), order of early mammals. Eutriconodonts existed in Asia (including Insular India, pre-contact India), Africa, Europe, North America, North and South America during the Jurassic and the Cretaceous periods. The ...
s, multituberculates, and trechnotheres. However, not long after, an eutriconodont identity was suspected, and Volaticotheria has since fallen into disuse. Subsequent analyses have consistently recovered ''Argentoconodon'' as the sister taxon of ''Volaticotherium'', with ''Ichthyoconodon'' as the sister taxon of that clade and thus the basalmost volaticotherin. The North American genus '' Jugulator'' may be the sister taxon of Volaticotherini. These were moved to the
eutriconodont Eutriconodonta is an order (biology), order of early mammals. Eutriconodonts existed in Asia (including Insular India, pre-contact India), Africa, Europe, North America, North and South America during the Jurassic and the Cretaceous periods. The ...
family Triconodontidae, as part of the alticonodontine assemblage, and the clade was renamed Volaticotherini accordingly. However, other sources consider the clade to be a separate family from Triconodontidae, as Volaticotheridae. This may be supported by other, more recent analyses, which find the clade to be more basally placed within Eutriconodonta.


Characteristics

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 facing
cusp A cusp is the most pointed end of a curve. It often refers to cusp (anatomy), a pointed structure on a tooth. Cusp or CUSP may also refer to: Mathematics * Cusp (singularity), a singular point of a curve * Cusp catastrophe, a branch of bifu ...
s aligned anteroposteriorly, lacking a cingulum. Canines and incisors tend to be fairly large. In the two forms that do possess postcranial remains, '' Argentoconodon'' and '' Volaticotherium'', we see a highly specialised
femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...
, lacking a femoral neck. '' Volaticotherium'' is rather well preserved, bearing a mostly complete skeleton and soft-tissue impressions such as
hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and ...
and
patagia The patagium (: patagia) is a membranous body part that assists an animal in obtaining lift when gliding or flying. The structure is found in extant and extinct groups of flying and gliding animals including bats, theropod dinosaurs (includin ...
.


Aerial locomotion

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 gliding
theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...
ns. It preserved a large,
fur A fur is a soft, thick growth of hair that covers the skin of almost all mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture from reaching the skin; the underfur acts as an ...
-covered
patagium The patagium (: patagia) is a membranous body part that assists an animal in obtaining lift when gliding or flying. The structure is found in extant and extinct groups of flying and gliding animals including bats, theropod dinosaurs (inclu ...
, 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 of uropatagia in at least the proximal vertebrae. '' Argentoconodon'' shares similar femur characteristics, suggesting that it too may have been capable of gliding.


Distribution

Volaticotherini was a relatively widespread and long-lived clade, with occurrences known from the
Toarcian The Toarcian is, in the International Commission on Stratigraphy, ICS' geologic timescale, an age (geology), age and stage (stratigraphy), stage in the Early Jurassic, Early or Lower Jurassic. It spans the time between 184.2 Megaannum, Ma (million ...
of
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...
, Oxfordian of
China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
, and
Berriasian In the geological timescale, the Berriasian is an age/ stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 143.1 ±0.6 Ma and 137.05 ± 0.2 (million years ago) ...
of
Morocco Morocco, officially the Kingdom of Morocco, is a country in the Maghreb region of North Africa. It has coastlines on the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to Algeria–Morocc ...
. The presence of volaticotherins in
Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...
is unusual, as they are among the few known Gondwanan triconodonts (and, if aligned with triconodontids, the only representatives of the group in Gondwanna), with '' Argentoconodon'' occurring as far back as the
Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassicâ ...
in otherwise
australosphenida The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. Although they have often been suggested to have acquired tribosphenic molars independently from those ...
n dominated faunas.


Diet

Though highly unusual and possibly indicating atypical occlusion patterns, volaticotherian molars are thought to have had a shearing motion as in other eutriconodonts. 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. In a study about Mesozoic mammal diets '' Argentoconodon'' ranks among carnivorous species, while '' Volaticotherium'' ranks among insectivorous taxa. This same result is provided almost identically in a posterior study, albeit with ''Volaticotherium'' closer to the carnivore space. It has been noted that most gliding mammals are predominantly herbivorous, which would make volaticothere carnivory truly exceptional. In particular, ''Volaticotherium'' itself has been compared to insectivore
bats Bats are flying mammals of the order Chiroptera (). With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most birds, flying with their very long spread-out ...
.


Paleoecology

Volaticotherins, as is typical for gliding animals, were adapted for an arboreal lifestyle. One volaticotherin, ''Ichthyoconodon'', was recovered from marine facies and consequently initially interpreted as an aquatic animal. However, many terrestrial mammals are preserved in aquatic environments, so it is unclear how strong this evidence is regarding its life habits, though its teeth appear to not have undergone long aquatic transportation.


References

{{Taxonbar, from=Q21447982 Eutriconodonta Gliding animals Fossil taxa described in 2006 Jurassic first appearances Early Cretaceous extinctions Taxa named by Jin Meng Taxa named by Yaoming Hu Taxa named by Yuanqing Wang Taxa named by Xiaolin Wang (paleontologist) Taxa named by Chuankui Li