THE POSTCRANIAL SKELETON OF YANOCONODON ALLINI FROM THE EARLY CRETACEOUS OF HEBEI, CHINA, AND ITS IMPLICATIONS FOR LOCOMOTOR ADAPTATION IN EUTRICONODONTAN MAMMALS

摘要

A recent study hypothesized that Yanoconodon allini (Eutriconodonta: Jeholodentidae) from the Lower Cretaceous Yixian Formation of northeastern China was a terrestrial mammal capable of swimming. However, a detailed description and functional study of the postcranial skeleton of Yanoconodon have not yet been published. Here, we describe and analyze the functional morphology of its postcranial skeleton. Our analyses indicate that Yanoconodon has a combination of adaptive features for diverse locomotor modes. Its humerus has a spindle-shaped head, an indistinct neck, and a broad and shallow intertubercular groove, all of which resemble those of non-therian mammals and premammaliaform cynodonts. The lack of an enlarged olecranon process of the ulna and the lack of styloid processes at the distal ends of the radius and ulna would have limited the digging efficiency of Yanoconodon. The triangular scapula and the pivotal pectoral girdle of Yanoconodon resemble those in extant mammals with some climbing ability. The femur has a spherical head with a very short neck and small greater trochanter. No malleoli are present in the distal ends of the tibia and fibula to constrain the movement of the upper ankle joint in a parasagittal plane. The astragalus is partially superimposed on the calcaneus. The axial skeleton is elongate relative to the short limbs and small skull. These postcranial skeletal features imply that Yanoconodon had a sprawling posture and was a generalized terrestrial mammal, whereas its capability of swimming was inferred from multivariate ecomorphological analysis using extant small-bodied mammals as analogs.