Meeting Abstract
Until recently, the fossil record of the Cretaceous to Paleogene Gondwanatheria almost exclusively consisted of isolated teeth and fragmentary jaws. An undescribed and pristinely preserved skeleton of a gondwanatherian from the Upper Cretaceous Maevarano Formation of Madagascar presents the first opportunity to study the postcranial morphology, posture, and locomotor behavior of these enigmatic early mammals. The specimen belongs to a new genus and species and represents only the fourth taxon known from articulated postcranial material for any Mesozoic mammaliaform from the southern hemisphere. The Malagasy taxon exhibits an unusually high number of trunk vertebrae for mammals (at least 19 rib-bearing/thoracic and 11 non-rib-bearing/lumbar vertebrae). The lumbar vertebrae bear anterolaterally projecting transverse processes and tall spinous processes, and the trunk vertebrae have articular facets that remain coronally oriented. These vertebral features may have facilitated lateral bending of the spine rather than parasagittal movements; in combination with other traits, this may indicate abducted placement of the hind limbs. In contrast, the forelimbs of the new Malagasy mammal exhibit adaptations for a parasagittal posture. The glenoid fossa of the scapula is ventrally oriented, the distal humerus bears a deep humeral trochlea, and the long olecranon of the ulna bends posteriorly. Rotational movements between the ulna and radius appear to have been limited; such a restriction to flexion and extension at the elbow joint was previously unknown for non-therian mammals. How the basal pattern of a sprawling hind limb and seemingly derived, therian-like arrangement of the forelimb might have impacted gait is the subject of ongoing study.
