The endocranial cast of an early miocene edentate, Hapalops indifferens Ameghino (Mammalia, Edentata, Tardigrada, Megatheriidae). Comparative study with brains of recent sloths.

A natural endocranial cast which represents a complete brain of a specimen of Hapalops indifferens is described. Comparing this cast to brains of actual Tardigrada, it shows a telencephalic morphology and a pattern of neocortical sulci that resemble more the brain of Bradypus rather than that of Choloepus. The neocortical sulci homologate the lateral or corono-lateral, suprasylvian and pseudosylvian sulci. Taking into account the studies of cortical maps in Bradypus and the notable similitude of the pattern of neocortical sulci between Bradypus and H. indifferens, the possible representation of the primary sensitive and motor somatic areas, secondary sensitive somatic area, visual and auditory areas are inferred. As in Bradypus, the primary sensitive and motor somatotopic organizations would be overlapped and would not be mirror images; they would show a predominance of the area of the forelimb. The relative brain size of H. indifferens is similar or higher than that of sloths of the genus Bradypus. The close resemblance between Bradypus and Hapalops, with respect to its brain morphology and relative brain size. is congruent with the current hypothesis of the phylogenetic relations between fossil and recent Tardigrada.