Ctenodactyloids, or Comb Rats are a group of Rodents found today only in Africa. They are characterised by the combination of a protrogomorphous or hystricomorphous skull and a sciurognathous mandible. The earliest known Ctenodactyloid Rodent is Cocomys, reported from China during the Early Eocene, and Ctenodactyloids were widely distributed in East and Central Asia during the Eocene and Oligocene. The four genera of living Ctenodactyloids are restricted to areas in North and East Africa. The many known Palaeogene Ctenodactyloids are based on fragmentary specimens, while only fragmentary skulls are known for Tataromys, Yindirtemys and Bounomys in Oligocene Ctenodactyloids, which provide only limited information.
In a paper published in the journal Vertebrata PalAsiatica in October 2021, Xu Ran-Cheng of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, the Center for Excellence in Life and Paleoenvironment, and the University of the Chinese Academy of Sciences, and Li Qian, also of the Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and the Center for Excellence in Life and Paleoenvironment, report four complete skulls with articulated jaws of Ctenodactyloids that were found from the Early Oligocene of Ulantatal, Inner Mongolia, China. These specimens were recovered from the light brownish yellow clayey siltsand of the 12-meter level of the Ulantatal Main Section. Based on the teeth morphology, the new specimens are referred to Bounomys ulantatalensis. The new materials described by Xu and Li add important data on the skull morphology of Bounomys and provide new and reliable evidence for the understanding of the skull evolution of the Ctenodactyloid Rodents.
Bounomys ulantatalensis is a medium-sized ctenodactyloid, skull length is 37–43 mm, with hystricomorphous skull and sciurognathous mandible. Differs from the Eocene Ctenodactyloids Cocomys or Exmus in having a hystricomorphous skull; the anterior end of the nasal situated behind the anterior edge of the upper incisors; the posterodorsal process of premaxilla long and narrow; the dorsal branch of zygomatic process of maxilla more posterior than the ventral one; the palate comparatively narrow; the postorbital process weak; the postorbital constriction not prominent; possessing ossified external acoustic canal; the auditory bulla, mastoid and epitympanic recess more inflated, and the longitudinal length of the auditory bulla about ⅓ of the skull length; the head of malleus swollen and extending forward; the anterior process of malleus absent; the dorsal part of the supraoccipital more extended; the laminated paracondylar process of exoccipital attached to the auditory bulla posteriorly; the premaxilla possessing a laterodorsal crest; the slightly S-shaped temporal crest situated on the parietal; the nuchal crest more developed; infraorbital foramen enlarged; the incisive foramen larger, about ⅔-¾ length of the upper diastema; the anterior margin of choana distinctly behind M3, more posteriorly; the sphenopalatine foramen above M2 and M3, enlarged and slightly posteriorly; the lacrimal foramen completely situated in the lacrimal; the optic canal merged with the sphenorbital fissure; the coronoid process of mandible reduced; the ventral masseteric crest extending forward to below ml; single mental foramen below p4; medial pterygoid fossa large and deep. Differs from the other Oligocene Ctenodactyloids Tataromys or Yindirtemys in the palate comparatively wider; the major palatine foramen not as narrow as in Tataromys or Yindirtemys; the anterior margin of sphenopalatine foramen at the level of junction of M1 and M2, comparatively anteriorly; the buccinator foramen closer to the masticatory foramen. Differs from extant Ctenodactylids in malleus and incus not fused.
Differs from Bounomys bohlini in the teeth being significantly larger, about 1.5 times of Bounomys bohlini; the main cusps of the cheek teeth swollen, and lophs relatively higher and developed; on the upper molars anterior cingulum joining protoloph, mesosinus curving posterolabially; on M3 metacone usually possessing crochet connected to paracone, and metaloph tortuous; sinus separated from posterosinus; on lower molars usually having a large central basin closed by longitudinal anterior arm and transverse posterior arm of entoconid; hypoconulid more swollen, and arm of hypoconulid relatively developed.
From the Eocene to the Oligocene, the Ctenodactyloids skull had undergone significant changes, and the most distinct of these changes is the zygomasseteric structure’s transition from the protrogomorphous to hystricomorphous, which is mainly manifested as follows: the infraorbital foramen is enlarged; the forming of the premaxillary laterodorsal crest; the dorsal branch of the zygomatic process of the maxilla moving posteriorly; the coronoid process of mandible is reduced; the ventral masseteric crest extending forward. These changes probably reflect the enhancement of the masticatory function of the Oligocene Ctenodactyloids. In addition, the auditory bulla, mastoid and epitymanic recess are significantly inflated; the head of the malleus is swollen and extends forward, and the anterior process of the malleus is absent. The change in the structure of the auditory region may be beneficial to enhance their low-frequency hearing.
The Eocene–Oligocene transition was one of the most pronounced climatic events of the Cainozoic era, marking a dramatic shift from a ‘greenhouse’ to an ‘icehouse’ world, with vegetation changing from warm-humid forest to dry-temperate forest-steppe. Related studies have shown that the hystricomorphous skull is better at grinding coarse grasses than other zygomasseteric structures, and the rodents living in arid areas are usually good at low-frequency hearing, which can improve their fitness. Therefore, the changes of the zygomasseteric structure and auditory region in Ctenodactyloids from the Eocene to Oligocene probably result from the global climate change during the corresponding period.
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