Meeting Abstract

19.6  Saturday, Jan. 4 11:30  Olfactory Evidence on the Terrestrial Origins of Monotremata SIMON, R.V.*; ROWE, T.B.; The University of Texas at Austin; The University of Texas at Austin rvsimon@utexas.edu

Whether Monotremata originated as a terrestrial or aquatic clade is a hot debate that so far has revolved around the interpretation of fragmentary Early Cretaceous fossils from Australia. To test these diametrically opposed hypotheses, we used high resolution x-ray computed tomography (CT) to generate new phylogenetic characters that were added to a morphological character matrix and run in a parsimony analysis. The dataset included living and extinct monotremes, plus outgroups comprising select therian mammals and extinct mammaliaforms. Most significant was the addition of CT data on the skull of the rare and poorly studied long-beaked echidna, Zaglossus. We also included the extinct long-beaked echidna Megalibgwilia, which we scored from the literature. Our results suggest that the ancestral monotreme had a well-developed Main Olfactory System (MOS) that was designed for analyzing air-borne odorant molecules, much like the system present in mammals ancestrally and that persists in many living mammals. From this condition, the platypus clade shows progressive reduction of its MOS, including reduction of the ethmoturbinals and olfactory bulb. However its vomeronasal organ (VNO), which is sensitive to soluble odorant molecules, underwent a genomic expansion unmatched by any other known mammal. In contrast, the echidna clade shows a unique degree of hypertrophy of its ethmoid skeleton and a large olfactory bulb and pyriform cortex, with no elaboration of its VNO genome. These results suggest that the ancestral monotreme was terrestrial, that the platypus lineage is secondarily aquatic, and they contradict the controversial proposition that echidnas are secondarily terrestrial.