The Homology of the Antorbital Fenestra of Metriorhynchidae (Crocodyliformes, Thalattosuchia)

LEARDI, JUAN MARTÍN; POL, DIEGO; FERNÁNDEZ, MARTA S.

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

The antorbital fenestra is a structure associated with a pneumatic feature of the skull, the paranasal cavity, diagnostic of Archosauriformes (Witmer, 1997). In metriorhynchid crocodyliforms the structure of this fenestra contrasts strongly with the condition of other crocodyliforms (as well as other archosaurs), as it involves the participation of the nasals on the edges of the fenestra, is not exposed in lateral view, and is enclosed within a deep oblique fossa. Fernandez & Herrera (2009) have recently questioned the homology of the fenestra of metriorhynchids with that of other archosaurs, based on the interpretation of exceptionally preserved natural inner casts of metriorhynchid rostra.In this contribution we test alternative homology hypotheses for these cranial openings using the dynamic homology approach (Ramirez, 2007). As the phylogenetic position of thalattosuchians is still debated, we evaluate the different homology scenarios using three alternative topologies that represent the three possible phylogenetic placements of thalattosuchians: as neosuchians, as basal mesoeucrocodylians and as basal crocodyliforms. Our results indicate the fenestra located anterior to the orbit of metriorhynchids is most parsimoniously interpreted as non-homologous to the antorbital fenestra of all other Crocodyliformes, regardless of the phylogenetic position of Thalattosuchia. This implies that metriorhynchids have internalized their antorbital cavity early in their evolutionary history and developed a neomorphic preorbital opening. These results are consistent with previous contributions and with the most parsimonious topology recovered in recent analyses for the group, as they are found as members of Neosuchia, a clade of crocodyliforms in which the antorbital fenestra is usually closed. The fenestra of basal thalattosuchians (“teleosaurids”) is instead most parsimoniously interpreted as homologous with the antorbital fenestra of other archosaurs, although further studies of the internal anatomy of their rostrum are critical to fully evaluate the homology of the small opening present in these taxa.