The postcranial skeletons of the Triassic mammals Eozostrodon, Megazostrodon and Erythrotherium

Abstract

Abundant but dissociated postcranial materials assignable to Eozostrodon (from Pont Alun quarry, Glamorgan, Wales) are described together with the associated, partial skeletons of Megazostrodon rudnerae and Erythrotherium parringtoni (from the Red Beds of southern Africa). The postcranial skeletons of these Triassic triconodonts are evaluated in both comparative and functional terms. Triassic triconodonts inherited from cynodonts specializations in the atlas-axis complex (e.g. double occipital condyles, a dens, reduction of the atlanto-axial zygapophyses) which permitted extensive cranial flexion-extension at the atlanto-occipital joint and rotation at the atlanto-axial joint. Marked differentiation of the major vertebral regions appears for the first time among mammals. In Megazostrodon , post-axis cervical vertebrae bear relatively narrow laminae and pedicles, broadly spaced zygapophyses, and rod-like, posteriorly recurved spinous processes; this pattern appears to be basic to mammals, and is retained among many later, generalized groups. The structure of the cervical vertebrae reveals that a cervico-thoracic flexure and an ascending posture of the neck were present. The large size of cervical vertebral foramina is evidence of a marked enlargement of the spinal cord at levels corresponding to the origin of the brachial plexus; this relationship may be interpreted as evidence of a greater neuromuscular control of the forelimb and a freedom of neck movement typical of mammals. An ‘anticlinal’ region in the thoracolumbar vertebral series has been identified in Eozostrodon ; this musculoskeletal specialization is related to axial flexion and extension characteristic of the posture and locomotor movements in mammals. The sacrum consists of at least two (and possibly three) vertebrae. The tail is moderately long and incorporates approximately twelve vertebrae. The shoulder girdle of Eozostrodon is little modified from the basic cynodont pattern; a T-shaped interclavicle, the lack of a supraspinous fossa, and the presence of both coracoids are features retained by monotremes. Both the coracoid and the scapula participate in the narrow, semi-lunar glenoid. The humero-ulnar joint is condylar, rather than trochlear, a condition retained by monotremes and other non-therian groups. The pelvic girdle represents a fully mammalian grade of organization; there is no evidence of epipubic bones. The limb skeleton, and particularly the joints, show various features consistent with an interpretation of well-developed mobility. The claw structure is suggestive of prehensility and an ability to climb. The hallux, at least, was probably somewhat divergent, and its joint structures indicate that abduction and extension (independent of the other digits) were possible. Triassic mammals as now known were all small, insectivorous forms; Megazostrodon was approximately 10 cm in head-body length, probably weighed between 20 and 30 g, and was proportioned similarly to medium-sized shrews (e.g. Suncus murinus, Blarina blarina ). A skeletal reconstruction of a Triassic triconodont is depicted, and an interpretation of their habits is presented.