Pipid frogs from the Los Alamitos Formation (Campanian-Maastrichtian) of Patagonia: phylogeny and evolution.

GÓMEZ, R.O.; BÁEZ, A.M.

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Instituto y Museo de Ciencias Naturales, Universidad Nacional de San Juan, San Juan, Argentina

Extant pipids comprise three distinct clades that inhabit freshwater environments in tropical regions of northern South America (Pipa Laurenti, 1768) and sub-Saharan Africa (hymenochirines and xenopodines). Nevertheless, the South American fossil record of the group documents greater taxonomic diversity and wider geographic distribution in Cretaceous and Paleogene times. Pipid remains from the Los Alamitos Formation (Campanian-Maastrichtian) of Patagonia were formerly considered as representing a taxon related to living xenopodines on the basis of the morphology of the frontoparietal and humerus (Báez, 1987; MACN-RN 159), although it has been acknowledged that this resemblance might be symplesiomorphic rather than apomorphic. However, the taxonomic placement of this fossil form has not been tested quantitatively probably owing to the fragmentary nature of the material. Additional remains (pelvis, shenethmoid, vertebrae; MACN s/n) collected subsequently from the same unit, together with a more comprehensive phylogenetic framework for pipoids (e.g., Báez and Púgener, 2003) allow us to reconsider the evolutionary relationships of this fossil pipid. A combined phylogenetic analysis including osteological as well as molecular data places the taxon from Los Alamitos as a stem-group xenopodine, in agreement with a previous opinion (Báez and Púgener, 2003). This result implies that crown-group xenopodines and the lineage represented by the fossils from Los Alamitos might have diverged at least in the mid-Cretaceous, prior to the final breakup of western Gondwana. The sphenethmoid of the pipid from Los Alamitos, as in most other anurans and in contrast with extant pipids, forms bony margins to the frontoparietal fenestra anteriorly and remains unfused to the underlying cultriform process of the parasphenoid. These features, which are also present in other extinct South American crown pipids (Báez and Púgener, 2003), point to a chondral origin of the anterior orbital region of the braincase and not to a membrane origin. The latter condition has been described for some extant pipids (e.g., Trueb and Hanken, 1992) and considered a unique pipid condition (Trueb, 1996). According to the placement of these extinct taxa within the crown-group, two evolutionary scenarios with regards to this feature are proposed: 1) a lateral wall of the orbital braincase of mixed origin, chondral and membranous, was the ancestral condition for crown pipids or 2) the membrane bone formation in the orbital braincase appeared independently in different pipid lineages. In any case, the entire replacement of chondral bone with membrane bone in the lateral wall of the orbital braincase has occurred independently in pipines and xenopodines, constituting a remarkable example of parallel evolution in pipids.