Let us take leave of our fine Order for a post and look across the tree at some intriguing new (-ish) research on the primitive stem ungulate Hyopsodus. The paper, “Digital cranial endocast of Hyopsodus (Mammalia, “Condylarthra”): A case of Paleogene terrestrial echolocation?” appeared in 2012 in PLOS One, and describes new CT scans of a well preserved Hyopsodus lepidus cranium, AMNH 143783, with a fairly complete reconstruction of the endocranium that can be compared to known endocrania of other early ungulates.
Hyopsodus is a member of the bushy radiation of Paleogene stem ungulates commonly known as “condylarths,” with scare-quotes to emphasize the fact that this group is certainly paraphyletic and probably polyphyletic. It appears to contain the earliest relatives of artiodactyls (even-toed ungulates), perissodactyls (odd-toed ungulates), and possibly the distantly related Afrotherian elephants, manatees, and hyraxes. One shouldn’t think of “condylarths” as particularly ungulate-like necessarily, as many, including Hyopsodus, were much more generalized and probably vaguely weasel- or squirrel-like in body form. It is also deeply unclear which “condylarth” group is related to which modern taxon, if any. Some, like the carnivorous mesonychids, now appear to have left no descendants, although these were once believed to be close relatives of whales (now thought to be nested within artiodactyls). Hyopsodontidae, the family to which Hyopsodus belongs, may be most closely related to perissodactyls, but this remains uncertain. The lack of phylogenetic clarity may contribute to the relative obscurity of most “condylarth” taxa, but it makes them no less interesting from an adaptive perspective.
This paper presents the first complete endocast obtained for Hyopsodus. The authors report in detail on its reconstructed neuroanatomy and apparent differences from other “condylarths.” They make particular note of the relatively high enchephalization (cranial capacity relative to body size) of Hyoposodus, which puts it in league with the most “advanced” mesonychids and early perissodactyls like Hyracotherium. Their case for echolocation in Hyopsodus rests on the development of the inferior colliculus, a structure of the midbrain important in sensory processing and orienting the body in response to stimuli. The colliculi are relatively hypertrophied in Hyopsodus and contribute to the relatively high dorsal projection of the midbrain and posterior shortening of the forebrain, or neopallium. This midbrain projection appears at first blush to be a primitive retention, but the peculiar development of the colliculi is unique among “condylarths.”
Unfortunately Hyopsodus doesn’t seem to represent some sort of hoofed bat (although, evolution, get on that), but echolocation is known from several terrestrial taxa that operate in dark and/or subterranean settings, including shrews, tenrecs, and potentially pink fairy armadillos. These taxa also share an enlargement of the inferior colliculus. Hyopsodus may have been echolocating at a similar relatively modest scale to aid in navigating low visibility environments. This is a difficult case to make conclusively, but the apparent convergence in the enlargement of this feature of the midbrain in so many taxa is certainly suggestive.
Does it seem plausible? Well decide for yourself, the paper is open-access and linked below: