Description of a fetal skeleton of the extinct sloth Nothrotherium maquinense (Xenarthra, Folivora): Ontogenetic and palaeoecological interpretations

A well-preserved fetus of the extinct nothrotheriid sloth Nothrotherium maquinense , recovered in situ within the skeletal remains of its mother from the karstic cave Toca da Boa Vista (Bahia, Brazil), is described and compared. The fetus is represented by numerous cranio-dental and postcranial elements, and its mother is represented by teeth and several postcranial elements. In the fetus, the exoccipitals formed the lateral and dorsal borders of the foramen magnum, from which the supraoccipital was excluded. Teeth were permanent, as in xenarthrans generally, and with growth the molariforms transformed from pyramidal to prismatic as in other ground sloths. The fetus preserves evidence of intrauterine mastication suggesting a relatively short post-natal period of lactation. Like Nothrotheriops , Nothrotherium was probably a generalist and opportunistic herbivore. The long bones are less elongated and more robust than in the adult and the claws were very powerful, perhaps having allowed the infant sloth to cling to its mother’s back after birth as in modern anteaters. The presence in the fetus of petrosal, ischium, and pubis indicates a late-stage fetus, probably near birth, as suggested by the position of the fetus, head backwards in relation to the mother. A newborn individual of this species was approximately one-third the length of its mother. This extinct nothrotheriid gave birth to a single offspring at a time, as typically occurs in the living sloths Bradypus and Choloepus . The taphonomic conditions within Toca da Boa Vista and site of recovery do not support habitual use of caves by this species.