Inferring lifestyle and locomotor habits of extinct sloths through scapula morphology and implications for convergent evolution in extant sloths.

摘要

Despite the restricted morphology and distribution of sloths today, fossils sloths show a wide variety of forms and behaviors and used to range from South America to Alaska. These extinct forms have in the past simply been lumped together as "ground sloths", separated from modern "tree sloths". However there are intermediate forms that have been posited to be semi-arboreal. In other groups such as primates the shape of the shoulder blade has been shown to vary significantly between groups with different arboreal behaviors. This study used geometric morphometrics to examine the scapulae of modern and extinct sloths to show that these three locomotor groups can in fact be distinguished by their shoulder blade shape. Juveniles of giant ground sloths also have significantly different shoulder blade shapes than the much larger adults, however they do not overlap with the smaller intermediate sloths, so may have been just as terrestrial as their parents despite their much smaller size. Finally, ontogenetic trajectories of several sloth genera do not show evidence of having different slopes. They start and end in different areas of morhospace but are all on parallel paths. This argues against the hypothesis of convergence in modern tree sloths, which despite both having an unusual suspensory lifestyle are not closely related. Rather they are both retaining an ancestral growth trajectory that all sloths have possessed.