Critical analysis of the alleged characters used to link afrotherian mammals and South American native ungulates

BOND M.; KRAMARZ, A.; GELFO J. N.; LOPEZ G. M.; REGUERO, M.

La Plata

Jornada; XXIV Jornadas Argentinas de Mastozoología; 2011

Sociedad Argentina para el Estudio de Los Mamíferos (SAREM)

Recent studies claimed that the South American native ungulates (SANU) grouped in the orders Litopterna Notoungulata, Astrapotheria, Xenungulata and Pyrotheria, were related to afrotherian mammals based on the alleged shared occurrence of three characters: (1) late eruption of permanent dentition relative to jaw growth, (2) presence of a cotylar fossa in the astragalus and (3) more than 19 thoracolumbar vertebrae. Here we review the presence of these characters among SANU and assess their validity as synapomorphies to ally them with afrotherians. (1) The delayed dental eruption relative to jaw growth was confused with a delayed premolar replacement, and there is no evidence of an afrotherian-like delayed eruption among SANU, except in few late diverging notoungulates. (2) The cotylar fossa is the anterior extension of the medial malleolar facet that deepens into a cup and receives the condylar articular surface of the tibial medial malleolus. Concavities on the medial side of the astragalus have been often confused with this character. A true cotylar fossa is absent in all known Pyrotheria, Xenungulata, and Litopterna; it is present in the astragali presumably assigned to Didolodontidae “condylarths” and in late diverging astrapotheres and notoungulates, but not in their earliest forms. (3) The presence of more than 19 thoracolumbar vertebrae positively occurs in Astrapotherium and in some notoungulates and litopterns, but it is variably observed in the known skeletons of †Macrauchenia patachonica and †Toxodon platensis. Other SANU interpreted as having this feature (e.g. †Protypotherium, †Pachyrukhos) have no known complete thoracolumbar series, and the occurrence of this condition was merely inferred. In sum, these characters were incorrectly identified in most SANU and, when present, they are highly variable and/or limited to the more derived lineages. We therefore conclude that these characters are insufficient to support a phylogenetic relationship between SANU and Afrotheria.