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.
Mammalian postcranial bones from the Late Jurassic of Portugal and their implications for forelimb evolution
Author:
Thomas Martin
Abstract:
Isolated bones of forelimb and pelvic girdle (two humeri, five ulnae, and an ilium) recovered from the Late Jurassic (Kimmeridgian) Guimarota coal mine in western central Portugal are attributed to the docodont Haldanodon exspectatus, dryolestoids, and a ?paulchoffatiid multituberculate. The larger of the two humeri is assigned to the dryolestid Dryolestes leiriensis based on size and shape. It clearly exhibits an incipient trochlea at the distal joint, suggesting that this derived character was well established among Late Jurassic dryolestidans, including Henkelotherium. Plesiomorphic characters are the prominent spherical radial condyle and the weakly developed ulnar condyle. An incipient medial keel is present in distal aspect of the humerus trochlea. The shallow olecranon fossa of the humerus corresponds to the small anconeal process of the radius. The smaller humerus with damaged distal joint is 50% smaller and cannot be assigned to a specific dryolestoid taxon. It is in the size range of the smaller Guimarota dryolestoids Krebsotherium, Drescheratherium, and Henkelotherium. One ulna differs from the dryolestoid ulnar shape by the nearly semicircular articular surface for the ulnar condyle of the humerus, the asymmetric olecranon with medial overhang, as well as the prominent anconeal process and is tentatively attributed to a paulchoffatiid multituberculate.
