June 21 , 2016

Here we describe the osteological structures of the lower hind limb and foot of the Late Jurassic Rugosodon eurasiaticus, the earliest-known postcranial fossil of a multituberculate mammal. This fossil extends the key cruro-tarsal apomorphies, previously known from multituberculates of the Cretaceous and Paleogene, to the paulchoffatiid multituberculates of the Jurassic. Multituberculates as a group can now be diagnosed by derived features of the astragalus, the navicular, and the entocuneiform. These features are correlated with the mobility of the upper ankle joint and pedal digit I, indicating that early multituberculates acquired new locomotor functions of the limb and foot. However, the standing pedal posture of the basal multituberculates is plantigrade, typical of primitive mammaliaforms. The digitigrade posture appeared later in derived multituberculates of the Late Cretaceous and Paleogene, and was accompanied by several derived features of the calcaneus: elongation of the calcaneal tuber, development of the peroneal process and groove, and medio-lateral compression of the proximal part of calcaneus. These derived features enhanced locomotor function by increasing the in-lever of the Achilles muscles to lift the calcaneal heel off the substrate. The derived calcaneo-cuboidal features also facilitated a wider range of movement (including abduction) of the distal pedal elements. We concur that derived multituberculates may have had digitigrade pedal posture, although only facultatively. New fossils of the Jurassic Rugosodon and Early Cretaceous Sinobaatar have revealed fresh evidence that the astragalar features of the earliest paulchoffatiids are correlated with sophisticated cruro-tarsal rotation as in other multituberculates. Rugosodon provides the ancestral condition from which more diverse calcaneal features for more versatilefunctions evolved in geologically younger and more derived multituberculates.
Luo, Z.-X., Meng, Q.-J., Liu, D., Zhang, Y.-G., and Yuan, C.-X. (2016)
Cruro-pedal structure of the paulchoffatiid multituberculate Rugosodon eurasiaticus and evolution of the multituberculate ankle.
Palaeontologia Polonica 67: 149–169.
doi: 10.4202/pp.2016.67_149