Abstract

The Cenozoic South American endemic pyrotheres, mostly characterized by a bilophodont dentition, are one of the most mysterious ungulates ever known. Even though many specialists have investigated this group, the fascinating question of its origin is still unresolved after more than a century of study. This paper provides a new description of the only known pyrothere skull, that of Pyrotherium from the Deseadan of Patagonia. Detailed comparison of the cranial anatomy indicates strong similarities with the Notoungulata, especially in the auditory region, as already mentioned by some authors. Intriguing similarities are also detected in the anterior dentition of Pyrotherium and the Casamayoran notoungulate Notostylops. These resemblances suggest a unique relationship between Pyrotheria and Notoungulata, specifically between Pyrotheria and Notostylops. These hypotheses are tested through a series of phylogenetic analyses of South American ungulate craniodental anatomy, primarily focusing on Notoungulata and Pyrotheria. These phylogenetic analyses are the first to encompass such a diversity of South American ungulate taxa. When including the bunodont taxon Proticia usually attributed to the Pyrotheria, the strict consensus of the analysis does not present much resolution. When excluding it, the analysis supports the nesting of the pyrotheres within the Notoungulata via an exclusive relationship with Notostylops. This relationship is supported by both cranial and dental anatomy. The analysis also supports the position of Astrapotheria as the sister group of the Notoungulata. It does not support, however, the monophyly of the Litopterna. The clustering of Pyrotheria, Notoungulata, and Astrapotheria supports an isolated evolutionary history of these ungulates in South America similar to that of afrotherian mammals in Africa. These results give rise to new perspectives in research on South American endemic ungulate evolution.