New Anuran from the mid-Cretaceous Candeleros Formation of Patagonia: implications for pipimorph diversification in west Gondwana.

BÁEZ, A.M., L. NICOLI Y P. MUZZOPAPPA.

Mendoza, Argentina.

Conferencia; Gondwana 12 Conference; 2005

Academia Nacional de Ciencias,

The fossil record has provided some evidence on the sequence and timing of the divergence of the basal lineages of the crown-group Anura, or archaeobatrachians. Although the branching order of these lineages is still controversial (Ford and Cannatella, 1993; Haas, 2003; Roelants and Bossuyt, 2005), it is generally agreed that ascaphids, discoglossoids, pipoids, and pelobatoids originated in early splitting events in frog history. The origin of Pipoidea, a clade that includes the fossorial rhinophrynids and aquatic pipimorphs (pipids and extinct palaeobatrachids), dates back prior to the breakup of Pangea based on the record of a putative rhinophrynid in the Kimmeridgian of North America (Henrici, 1998). Fragmentation of Pangea is likely to have played a role in the divergence of stem-pipimorphs, recorded in the Lower Cretaceous in the northern fringe of Gondwana (Jones et al, 2003) and in the Iberian microcontinent (Báez, 2004), into Laurasian palaeobatrachids and Gondwanan pipids. To date, extant and extinct pipids, including putative stem-taxa (Trueb and Báez, in press), are limited to areas that were part of west Gondwana (Báez, 1996), although practically all known fossil occurrences postdate the formation of the equatorial seaway that ultimately separated Africa and South America in the Aptian. The oldest known pipimorph record in South America is from early Cenomanian red beds that accumulated in the Neuquen Basin in west-central Argentina. Dissarticulated and partially articulated remains of at least two individuals were collected in the basal section of the Candeleros Formation at Los Gigantes, province of Neuquen, and referred to a new taxon, Avitabatrachus uliana (Báez et al., 2000). Recently, a  partially articulated specimen of a small pipimorph was discovered in outcrops of the same stratigraphic unit exposed at the El Chocón locality, about 25km east of the type locality. The articulated axial skeleton and pelvic girdle are preserved, as well as the otic capsule, scapula, coracoid, and elements of the hind limb. The morphology and skeletal proportions are strongly reminiscent of corresponding elements in the holotype of Avitabatrachus, thus suggesting that these specimens represent the same, or closely related, species. As in the holotype, the transverse processes of the last three presacrals are distinctly proximally wide, recalling those of the Early Cretaceous Thoraciliacus from Israel (Trueb, 1999). The new specimen, unlike the holotype, shows the presence of a sacral vertebra, which has postzygapophyses, articulated with a post sacral vertebra bearing conspicuous transverse processes. In addition, the articulated ilia, which are incompletely preserved in the holotype, document the absence of dorsally placed interiliac tuberosities. These features support the phylogenetic placement of Avitabatrachus as a stem-pipid suggested previously (Báez et al., 2000) and provide evidence of the presence of representatives of a basal pipimorph radiation in Patagonia. From a biogeographic viewpoint it is noteworthy that these remains are associated with those of rebbachisaurids, a clade of basal diplodocoid dinosaurs (Salgado et.al. 2004) also well represented in the middle Cretaceous of west Gondwana. REFERENCES CITED Báez, A.M., 1996., The fossil record of the Pipidae, in Tisley, R.C. and Kobel, H.R., eds.,The Biology of Xenopus, Zoological Society of London, Oxford, Clarendon Press, p.329–347. Báez, A.M., 2004., Taxonomic status and relationships of Neusibatrachus wilferti Seiffert, an Early Cretaceous forg from the lithographic limestones of El Montsec, northeastern Spain. Ameghiniana, v.41 , p.5R. 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