The Middle Jurassic triconodont Argentoconodon fariasorum: phylogeny and biogeography

GAETANO, LEANDRO C.; ROUGIER, GUILLERMO W.

Neuquén

Congreso; III Congreso Latinoamericano de Paleontología de Vertebrados; 2008

Osteological remnants of South American Jurassic mammals are known only from the outcrops of the Cañadón Asfalto Formation at the Queso Rallado quarry, Chubut Province, Argentina. The bearing strata are from the middle sector of the lower Las Chacritas Member and likely of Callovian-Oxfordian age. Three different groups of mammals are as yet known from the locality, australosphenidans, an undescribed allotherian, and Argentoconodon fariasorum (Rougier et al., 2007). Argentoconodon represents the only form with triconodontid affinities known from South America. Originally described on the basis of an upper molariform (MPEF-PV1877), A. fariasorum is now known by several specimens, including a partial skeleton which preserves most of this taxon´s lower dentition (MPEF-CC115/04). Lower incisors are small and unicuspated. The lower canines, the largest of the lower teeth, are almost conical in lateral view and bear a single root. Lower premolariforms lack mesial accessory cusps and cingula and may present one or two roots, being single rooted premolars smaller and of mesial position. Two extremely labiolingually compressed molariforms lacking well defined cingula are still in place on the left dentary. They are very similar to each other and are dominated by three strongly recumbent principal cusps (a, b and c) and an accessory one (d) projecting distally from the edge of the crown. A mesial accessory cusp (e) may be extremely reduced or absent. The main cusps are subequal in height, although cusp a is slightly taller. All lower molariforms show evidence of an ?in echelon? imbrication, where cusp d of the preceding tooth overlaps labially cusp b of the posterior one. Based on MPEF-CC115/04, Argentoconodon´s lower dental formula can be interpreted as: ?.1.3.2?. The results of a preliminary cladistic analysis dealing with the phylogenetic affinities of Argentoconodon shows that this taxon is part of a monophyletic grouping integrated by Volaticoterium antiquus Meng et al., 2006 and Ichthyoconodon jaworowskorum Sigogneau-Russell, 1995. This clade is nested within the monophyletic subfamily Alticonodontinae. The close anatomical similarities of Volaticotherium and Argentoconodon also extend to the postcranium, suggesting similar locomotory habits for both forms. These results support a triconodont ancestry for Volaticotherium, instead of representing a distinct order, and highlight the possibility of a gliding clade of triconodonts of wide distribution whose common ancestor may be tracked down before the Middle-Upper Jurassic in a Pangeic paleogeographic setting. The locomotor adaptations of the sister group, the Alticonodontinae members from the North American Cretaceous, are presently unknown.