Dario G. Lazo

Marcela Cichowolski

Academic paper

Journal of Paleontology

77

784-789

PLESIOSAURS constitute a monophyletic group whose stratigraphical range is uppermost Triassic to uppermost Cretaceous (Brown, 1981). They were large predatory marine reptiles, highly adapted for submarine locomotion, with powerful paddle-like limbs and heavily reinforced limb girdles (Saint-Seine, 1955; Romer, 1966; Carroll, 1988; Benton, 1990). The Plesiosauria clade belongs to the Sauropterygia, which has recently been hypothesized as the sister-group of the Ichthyosauria. Together with that clade they form the Euryapsida (Caldwell, 1997). The Sauropterygia can be subdivided into relatively plesiomorphic stem-group taxa from the Triassic (Placodonts, Nothosauroids, and Pistosauroids), and the obligatorily marine crown-group Plesiosauria (Rieppel, 1999). Plesiosaurs are traditionally divided into two superfamilies: Plesiosauroidea, with usually small heads and long necks; and Pliosauroidea, with larger heads and shorter necks (Welles, 1943; Persson, 1963; Brown, 1981). Plesiosauroidea contains three families: Plesiosauridae, Cryptoclididae, and Elasmosauridae (Brown, 1981; Brown and Cruickshank, 1994). The validity of the Polycotylidae Cope, 1869, has long been questioned and its phylogenetic position among Plesiosauria debated, as many consider it to be related to the Pliosauridae or to be a sister-group of the Elasmosauridae (Sato and Storrs, 2000; O�Keefe, 2001). Plesiosaurs have an almost worldwide distribution in the Jurassic and Late Cretaceous, being especially abundant in Europe (Andrews, 1910; Brown, 1981; Bardet, 1995) and North America, respectively (Welles, 1943, 1952). It is worth mentioning the scarcity of these reptiles in the cosmopolitan Lower Cretaceous (Welles, 1962; Gasparini and Gon�i, 1985; Cruickshank, 1997). The only known elasmosaurid genera from this time are Brancasaurus Wegner, 1914, from the Berriasian of Germany (Wegner, 1914) and Alzadasaurus Welles, 1943, from the Aptian of Colombia (Welles, 1962; Gon�i and Gasparini, 1983). No remains of this family have been described from Valanginian or Hauterivian ages. The oldest recorded plesiosaurs of South America are from the Sinemurian of Northern Chile (Gasparini and Ferna´ndez, 1996), and the latest ones are from the Maastrichtian of Argentina (Cabrera, 1941; Gasparini and De la Fuente, 2000; Gasparini and Salgado, 2000, Gasparini et al., 2001), Chile (Casamiquela, 1969; Gasparini, 1979), and Brazil (Carvalho and Azevedo, 1998). In Argentina, the plesiosaurs range from Early Bajocian to Maastrichtian. The Jurassic plesiosaur records consist of bajocian, callovian, and tithonian pliosaurids, and callovian cryptoclidids and elasmosaurids from the Neuque´n basin (Gasparini et al., 1982, 1997, 1999; Gasparini and Spalletti, 1993; Gasparini and Ferna´ndez, 1996; Gasparini, 1997; Spalletti et al., 1999). Cretaceous elasmosaurids and polycotylids are reported from the Campanian- Maastrichtian Atlantic transgression deposits of northern and central Patagonia (Gasparini and De la Fuente, 2000; Gasparini and Salgado, 2000; Gasparini et al., 2001), but until now, there were non Lower Cretaceous records. The aim of this work is to report the first Lower Cretaceous plesiosaur remains from Argentina.

English