Volaticotherium is the epitome
of a roadkill fossil (Fig. 1). It needs to be put together (Fig. 2) like a puzzle.
Earlier I made mistakes
in a reconstruction (somewhat repaired here, Fig. 2) and the large reptile tree (LRT, 1158 taxa, subset Fig. 3)) was missing taxa that were close to Volaticotherium: Morganucodon and Ukhaatherium.
(Late Cretaceous late survivor of a Triassic radiation) is basal to both Morganucodon and Volaticotherium in the LRT.
Note that, like Volaticotherium,
Morganucodon also has only two large upper molars and a similar nasal shape. Also note that simpler, linear shape of the Volaticotherium cusps is derived relative to the ancestral Ukhaatherium.
Ukhaatherium nessovi (Novacek et al. 1997; Campanian, Late Cretaceous, 80 mya) is known from eight partial skeletons. Wikipedia reports, 4 premolars and 3 molars, but the break in dental shape happens between premolar 3, typically the largest and last one, and the next tooth, molar 1 of 4. The lumbar region is curved. Traditionally considered an Asioryctidae, close to placentals, but with epipubic bones, here it nests at the base of the Metatheria (see below), between Juramaia+ kin and Eomaia + kin. In size and shape the skull resembles that of Morganucodon.
Morganucodon watsoni (Kühne 1949; Late Triassic, 200–164mya; 2–3cm skull length) is known from fissue fills (abundant, 3D and completely disarticulated) fossils from Wales and China. This insectivore was like a shrew. The anterior teeth had a single replacement. Traditionally considered a mammaliaform, here Morganucodon nests with Ukhaatherium at the base of the Theria, close to the base of the Mammalia (= Prototheria). The jaw retains some tiny rear jaw elements, so the hearing was not perfected yet.
Volaticotherium antiquus (Meng et al. 2006; Middle to Late Jurassic, 164 mya; 5 cm skull length; IVPP V14739) was described a few years back as a gliding mammal of uncertain affiintiy. It is based on a preserved patagium, or gliding membrane, complete with short hair and skin. Here, derived from a sister to Ukhaatherium and Morganucodon, this is the last of its kind, at present. The molars resemble rotary saw blades, the external naris is divided by an ascending process of the premaxilla (rare among higher cynodonts and mammals), proximally the femur has no ‘neck’ and not much of a ‘head’, and the tail is extraordinarily long. We also see a deeper mandible medial to sabertooth fangs in Thylacosmilus, and this may be the reason for the oddly deeper chin here.
Kuehne WG 1949. On a triconodont tooth of a new pattern from a fissure-filling in South Glamorgan. Proceedings of the Zoological Society of London 119:345-350
Meng J, Hu YM, Wang YQ, Wang XL and Li CK 2006. A Mesozoic gliding mammal from northeastern China. Nature 444:889-893.
Novacek MJ, Rogier GW, Wible JR, McKenna MC, Dashzev g D and Horovitz I 1997. Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia Nature 389: 483-486.