Function of pretribosphenic and tribosphenic mammalian molars inferred from 3D animation
Authors:
Schultz et al
Abstract:
Appearance of the tribosphenic molar in the Late Jurassic (160 Ma) is a crucial innovation for food processing in mammalian evolution. This molar type is characterized by a protocone, a talonid basin and a two-phased chewing cycle, all of which are apomorphic. In this functional study on the teeth of Late Jurassic Dryolestes leiriensis and the living marsupial Monodelphis domestica, we demonstrate that pretribosphenic and tribosphenic molars show fundamental differences of food reduction strategies, representing a shift in dental function during the transition of tribosphenic mammals. By using the Occlusal Fingerprint Analyser (OFA), we simulated the chewing motions of the pretribosphenic Dryolestes that represents an evolutionary precursor condition to such tribosphenic mammals as Monodelphis. Animation of chewing path and detection of collisional contacts between virtual models of teeth suggests that Dryolestes differs from the classical two-phased chewing movement of tribosphenidans, due to the narrowing of the interdental space in cervical (crown–root transition) direction, the inclination angle of the hypoflexid groove, and the unicuspid talonid. The pretribosphenic chewing cycle is equivalent to phase I of the tribosphenic chewing cycle, but the former lacks phase II of the tribosphenic chewing. The new approach can analyze the chewing cycle of the jaw by using polygonal 3D models of tooth surfaces, in a way that is complementary to the electromyography and strain gauge studies of muscle function of living animals. The technique allows alignment and scaling of isolated fossil teeth and utilizes the wear facet orientation and striation of the teeth to reconstruct the chewing path of extinct mammals.
The oldest dryolestid mammal from the Middle Jurassic of Siberia
Authors:
Arerianov et al
Abstract:
Anthracolestes sergeii, gen. et sp. nov., based on three isolated lower molars and several edentulous dentary fragments from the Middle Jurassic (Bathonian) Itat Formation at Berezovsk Quarry in Krasnoyarsk Territory, Russia, is the oldest and most basal representative of Dryolestidae. It shows four unambiguous synapomorphies of Dryolestidae: lower molars with unilaterally hypsodont crowns, unequal roots, and pronounced precingulid instead of cusp f, and dentary with unequal alveolar borders. The dental formula of the new taxon is i1–4, c (double-rooted), p1–4(?), and m1–4(?). Anthracolestes sergeii, gen. et sp. nov., is plesiomorphic relative to more derived and geologically younger dryolestids in having possibly fewer lower molars, less pronounced mesiodistal compression of lower molars, oblique protocristid, short mandibular symphysis, and shallow mandibular ramus of the dentary. Asia is most likely the place of origin for Dryolestidae, but this group is so far not known from that continent after the Middle Jurassic.
