Eco-morphology of Mammalian Fossil Lineages: Identifying Morphotypes in a Case Study of Endemic South American Ungulates

Abstract

The notoungulates are one major clade of extinct native South American ungulate mammals. Several notoun- gulate morphotypes have been proposed by comparison with mammals from other large land masses, representing instances of convergent evolution in continental isolation. As with other entirely fossil lineages, the reconstruction of their functional biology is challenging, one major obstacle being the fragmen- tary and distorted nature of the fossil remains for which appli- cation of conventional quantitative data analysis techniques is compromised. Here we explored the patterns in morphospace of representatives of all notoungulate families on the basis of the qualitative scoring of 31 cranial characters that provided a basic description of shape and an approximation to sensory and mechanical functions of the skull. We calculated pairwise distances between all taxa with an index that proportionally corrected overall similarity by both character number and dissimilarities. The distances were used to generate a morphospace with a multivariate technique, Principal Coordinates Analysis. We recovered a tight group of basal Paleocene-Eocene notoungulates and a scattered group of Oligocene and Neogene forms. Lines drawn in that space connecting successive sister terminals clearly showed a de- parture from basal genera toward four diverging, previously proposed morphotypes, namely rabbit-, large-rodent-, small- horse-, and rhino-like forms. An extensive analysis identified the character basis of these morphotypes. Mapping additional information such as body size and degree of hypsodonty effectively aided in the interpretation of notoungulate morphofunctional evolution. We briefly discuss the utility of the qualitative morphospace as a tool for exploring the ecomorphology of fossil lineages and the evolutionary impli- cations of this reconstruction.