Two new pleurodiran turtles from the Portezuelo formation (Upper Cretaceous) of Northern Patagonia, Argentina

ABSTRACT-The chelonian fauna of the Portezuelo Formation (Turonian-Coniacian), outcropping at Sierra del Portezuelo (Neuquen province, Argentina), is reported. Two new taxa of pleurodiran turtles are described. One of them is Prochelidella portezuelue new species, a short-necked chelid closely related to extinct species of the Lohan Cura (Albian), Candeleros (Cenomanian), and Bajo Barreal (Turonian) formations from northwestern and central Patagonia, and to the extant species of the genus Acanthoche

INTRODUCTION

FIELDWORK CONDUCTED by Dr. Fernando Novas from 1990 to 1998 at the outcrops of the Portezuelo Formation (Late Turonian-Early Coniacian, see Hugo and Leanza, 1998; Leanza, 1999) Neuquen Basin, in northwestern Patagonia resulted in the discovery of several fossil reptiles. The crew was from the Museo Argentine de Ciencias Naturales "Bernardino Rivadavia" (Buenos Aires), Museo "Egidio Feruglio" (Trelew, Chubut province), and Museo "Carmen Funes" (Plaza Huincul, Neuquen province). These findings notably increased the known vertebrate fauna of the Portezuelo Formation, yielding theropod maniraptorans (Patagonykus puertai Novas, 1997; Unenlagia comahuensis Novas and Puerta, 1997; Megaraptor namunhuaiquii Novas, 1998), crocodylomorphs, and turtles. The turtles are represented by three side-necked specimens of moderate size. One of them was assigned to the Family Chelidae because of the chelid-like morphology of the shell, and the fifth and eight biconvex cervical vertebrae. The two others have the shell design and cranial morphology of podocnemidoid pelomedusoid turtles.

In this article the chelonian fauna of the Portezuelo Formation is described. Chelonians are represented by two new taxa, one of a short-necked chelid and the other of a podocnemidoid turtle. These species add new information on the paleobiodiversity of the gondwanan side-necked turtles. In addition, this is the first record of an association of taxa belonging to the two main groups of pleurodiran turtles (Chelidae and Pelomedusoides) in an Upper Cretaceous horizon of Patagonia. This discovery confirms the co-existence in northwestern Patagonia of north gondwanan (Pelomedusoides) and south gondwanan (Chelidae) representatives during the Turonian-Coniacian. The paleobiogeographic significance of this discovery is discussed.

MATERIAL AND METHODS

Specimens examined for this study are deposited in the "Museo Carmen Funes of Plaza Huincul" (MCF-PVPH), Neuquen Province, Argentina; Museo Argentine de Ciencias Naturales "Bernardino Rivadavia" de Buenos Aires (MACN); and Museo Provincial "Carlos Ameghino" de Cipolletti, Rio Negro Province (MCRN). Because the cladistic analysis on the morphological characters of extant chelid species (Gaffney, 1977) was made using mostly traits of the skull, not preserved in the holotype of Prochelidella portezuelae, alfa taxonomy was used for the systematic treatment of Prochelidella portezuelae n. gen and sp. and related taxa; however, a cladistic analysis was performed to establish the phylogenetic relationships of Portezueloemys patagonica. Characters were analyzed using parsimony to elucidate Hennigian synapomorphies (Hennig, 1968). Morphological data were examined using Goloboff's parsimony based on NONA (1993). Terminal taxa included in the analysis are Notoemys, Chelidae, Araripemys, Pelomedusidae, Bothremyididae, Brasilemys, Hamadachelys, Portezueloemys, Erymnochelyinae, and Podocnemidinae. The taxa of the epifamily Podocnemidoidea (see Lapparent de Broin, 2000) are included in the ingroup, the other taxa are outgroups. The main sources of the fifty morphological characters used in this analysis were the studies of Gaffney and Meylan (1988), Gaffney et al. (1991), Meylan (1996), Lapparent de Broin and Murelaga (1999), and Lapparent de Broin (2000). Multistate characters were treated as non-additive to avoid a priori assumption of polarity. Appendix 1 includes the character description and data matrix analyzed with NONA (Goloboff, 1993). Consistency (Kluge and Farris, 1969) and retention (Farris, 1989) indices were calculated excluding autapomorphies. NONA was run using heuristic searches with random additional sequences. Optimization of characters (slow optimization) was performed using WINCLADA Beta version (Nixon, 1999-2000). This DELTRAN optimization is followed because, as Hirayama (1998) suggested, it is slightly more conservative in terms of assigning synapomorphies to clades in a data matrix with a significative amount of missing data.

SYSTEMATIC PALEONTOLOGY

Order CHELONII Brongniart, 1800

Infraorder PLEURODIRA Cope, 1864

Family CHELIDAE Gray, 1825

Genus PROCHELIDELLA Lapparent de Broin and de la Fuente, 2001

Type species.-Prochelidella argentinae Lapparent de Broin and de la Fuente, 2001; figured in Lapparent de Broin and de la Fuente, 2001, figure 3.

Emended diagnosis.-Chelid turtle having a low and wide carapace with slight cervical notch. Carapace length from small (120 mm) to moderate size (270 mm). Shell having a dense microvermiculation with rounded ridges as in the extant species of Acanthochelys. Differs from the extant taxa assigned to Phrynops sensu lato in the quadrangular neural 1, and in the narrow anterior plastral lobe. Differs from Acanthochelys in the moderate elongation of the anterior border of the carapace, in the nuchal and cervical width, the presence of neurals, and the more anteriorly placed axillary processes.