Dermotherium is a genus of fossil mammals closely related to the living colugos, a small group of gliding mammals from Southeast Asia. Two species are recognized: D. major from the Late Eocene of Thailand, based on a single fragment of the lower jaw, and D. chimaera from the Late Oligocene of Thailand, known from three fragments of the lower jaw and two isolated upper molars. In addition, a single isolated upper molar from the Early Oligocene of Pakistan has been tentatively assigned to D. chimaera. All sites where fossils of Dermotherium have been found were probably forested environments and the fossil species were probably forest dwellers like living colugos, but whether they had the gliding adaptations of the living species is unknown.
Some features of the teeth differentiate Dermotherium from both living colugo species, but other features are shared with only one of the two. The third lower incisor, lower canine, and third lower premolar at least are pectinate or comblike, bearing longitudinal rows of tines or cusps, an unusual feature of colugos (the first two lower incisors are unknown in Dermotherium). The fourth lower premolar instead resembles the lower molars. The front part of these teeth, the trigonid, is broader in D. chimaera than in D. major, which is known only from the second and third lower molars. The two species also differ in the configuration of the inner back corner of the lower molars. The upper molars are triangular teeth bearing several distinct small cusps, particularly on the second upper molar, and with wrinkled enamel.
Taxonomy
Colugos are a small group of Southeast Asian gliding mammals closely related to the primates. Their fossil record is exceptionally poor. Although Paleogene groups such as the Plagiomenidae are considered by some to be closely related to colugos, no fossils undoubtedly referable to the living colugo family, Cynocephalidae, had been reported until 1992. In that year, Stéphane Ducrocq and colleagues described a jaw fragment from the Eocene of Thailand as a new genus and species of colugo, Dermotherium major. In 2000, however, Brian Stafford and Frederick Szalay argued that Dermotherium might not be a colugo, since the fossil is so poorly preserved that few traits can be unambiguously recognized and some purported dermopteran features of the fossil are also seen in other placental groups. Mary Silcox and colleagues reaffirmed the colugo affinities of Dermotherium in 2005 on the basis of detailed similarities in molar morphology.
In 2006, Laurent Marivaux and colleagues described a second species of Dermotherium, D. chimaera, from material from the Oligocene of Thailand. They gave it the specific name chimaera (Latin for "chimera") because it shares characters with both the Philippine colugo (Cynocephalus volans) and Sunda colugo (Galeopterus variegatus), the two living colugo species. In addition, they tentatively identified a fossil from the Oligocene of Pakistan as Dermotherium chimaera and regarded some fossils from the Eocene of Myanmar as indeterminate dermopterans. According to a phylogenetic analysis carried out by Marivaux and colleagues, D. chimaera, D. major, and the Myanmar dermopteran are successive sister groups of the two living colugos.
Description
Known material of Dermotherium includes a handful of jaw fragments and isolated teeth. Dermotherium major is known only from a fragment of the left lower jaw bearing the third lower molar (m3) and a poorly preserved second lower molar (m2). The holotype of Dermotherium chimaera is a lower jaw fragment in which remnants of the deciduous third lower premolar are visible. X-ray microtomography reveals the unerupted lower third incisor (i3), canine (c1), third premolar (p3), and fourth premolar (p4) still inside the jaw. In addition, this species is known from two other jaw fragments, one bearing m1 and m2 and the other bearing m2 and m3, and two isolated molars, an upper first and second molar (M1 and M2). The tentatively referred material of this species from Pakistan includes a single M2.