Relationships of the Megatheriinae, Nothrotheriinae, and Planopsinae: some skeletal characteristics and their importance for phylogeny

ABSTRACT

The characters used by Engelmann (1985) to unite various Nothrotheriinae and Megatheriinae in the Family Megatheriidae are assessed for their suitability as synapomorphies for such a clade. The elongate, slender premaxillae were cited as a synapomorphy of these subfamilies. A tendency for the posterior external opening of the mandibular canal to be located medially, together with the presence of an odontoid tibial articular process of the astragalus, were cited to unite Nothrotherium with megatheres. However, these characters are unsuited to resolving the relationships among Nothrotheriinae, Megatheriinae, and Planopsinae. The Y–shaped premaxillae of some Nothrotheriinae sensu lato may be a synapomorphy of the Plio-Pleistocene and some Santacrucian nothrotheres. These premaxillae are morphologically distinct from the V–shaped, triangular type seen in Eremotherium, which is probably the plesiomorphic condition in sloths, and the robust, quadrangular, and derived premaxillae of Megatherium. The posterior external opening of the mandibular canal may be located more medially in Nothrotheriops and some species of Hapalops, but not in Nothrotherium, Nothropus, and Pronothrotherium. The presence of an odontoid tibial articular process is not restricted to Nothrotheriinae sensu stricto and Megatheriinae. The greater separation of the astragalar ectal and tibial facets (probably a derived condition) occurs only in some mylodonts, and thus is not useful in distinguishing the significance of the presence of the process. Other evidence (size; form of the astragalus, auditory region, and tibial process for the long digital flexors) suggest a closer relationship between Megatheriinae and Planopsinae than some other combination of these subfamilies. The astragalar odontoid tibial process is usually considered to be independently derived in megatheriids and mylodontids, based on the interpretation of other morphological evidence. The alternate hypothesis, that the process is a synapomorphy of these families, is explored. Alternate polarities to those usually accepted for the dentition and dentary are suggested on the basis of Pseudoglyptodon.