ARTICLE A CERATOSAURID (DINOSAURIA, THEROPODA) FROM THE LATE JURASSIC–EARLY CRETACEOUS OF URUGUAY MATÍAS SOTO *,1 and DANIEL PEREA 1 1 Departamento de Evolución de Cuencas, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, Uruguay; msoto@fcien.edu.uy, perea@fcien.edu.uy ABSTRACT—Several teeth with ceratosaurid affinities from the Tacuarembó Formation (Kimmeridgian–Hautevirian) of Uruguay (South America) are reported herein. The most diagnostic specimen is a small rostral crown (probably the first or second left premaxillary crown), the most conspicuous features of which are the presence of several, well-marked longitudinal grooves on the lingual face and the absence of mesial denticles. This combination of characters is only found in the Late Jurassic genus Ceratosaurus Marsh, 1884. The tooth was carefully compared with all potentially similar theropod teeth in order to discard non-ceratosaurid affinities. The occurrence of a South American ceratosaurid in the Tacuarembó Formation was not unexpected, given the temporal range known for the family and the probable Late Jurassic age independently proposed for the lower, fossiliferous member of this unit. Thus, the interpretation of this tooth as belonging to a Ceratosaurus-like ceratosaurid has important biostratigraphic implications, reinforcing the aforemen- tioned hypothesis. Furthermore, it indicates the presence of the ceratosaurid lineage in South America earlier than recognized, the only ceratosaurid previously identified being the Argentinean Genyodectes, of probably Aptian–Albian age. The identification of this Ceratosaurus-like ceratosaurid allows the establishment of new links with several Late Jurassic faunas. INTRODUCTION Outcrops of the Tacuarembó Formation are mainly distribut- ed in the Tacuarembó and Rivera provinces, in a N–S trend belt in NE Uruguay. Two members have been traditionally been rec- ognized. The fossiliferous lower member comprises predomi- nantly fine- to medium-grained, well-sorted, yellowish to reddish sandstones of mainly fluvial origin as well as subordinated red- dish to greenish mudstones and siltstones of lacustrine origin. Some aeolian deposits are known, mainly from the upper mem- ber of this unit. The oldest dinosaur remains from Uruguay are represented by theropod teeth from the Tacuarembó Formation, recovered by surface collecting and screen-washing. The first discoveries were described by Perea et al. (2003), but since then the sample has increased to nearly fifty teeth, almost all collected near the lo- cality called Martinote (Perea et al., 2001, 2003). One of these teeth shows distinctive characters, justifying its detailed descrip- tion. Institutional Abbreviations—FC-DPV, Colección de Verte- brados Fósiles, Facultad de Ciencias, Montevideo; MLP, Museo de La Plata, La Plata. Anatomical Abbreviations—CBL, crown base length; CBW, crown base width; CH, crown height; DC, distal mid-crown ser- ration density; DSDI  denticle size difference index. Morpho- metric terms follow the recent proposal of Smith et al. (2005). CBL, CBW and CH are roughly equivalent to FABL (fore-aft basal length), BW (basal width) or CST (cross-sectional thick- ness), and TCH (tooth crown heigth), respectively, of other au- thors. DSDI was proposed by Rauhut and Werner (1995). SYSTEMATIC PALEONTOLOGY Order SAURISCHIA Seeley, 1888 Suborder THEROPODA Marsh, 1881 Infraorder CERATOSAURIA Marsh, 1884 Family CERATOSAURIDAE Marsh, 1884 sensu Rauhut, 2004 CERATOSAURIDAE gen. et sp. indet. (Fig. 1) Type Genus—Ceratosaurus Marsh, 1884. Description—FC-DPV 1950 is a roughly conical, small (CH  15.17 mm), non-squat crown (CHR  2.34). It is slightly labio- lingually compressed (Fig. 1A, C), with a subcircular cross- sectional outline at the base (CBL  6.47 mm, CBW  5.29 mm, CBR  0.82). However, both faces are not identical, one of them (which we interpret as the lingual one) being more convex than the other. The distal margin is nearly straight, whereas the mesial margin is gently curved (Fig. 1A, D). The distal carina bears 29 preserved denticles (Fig. 1C), with 14 denticles along 5 mm mea- sured at crown mid-height (DC  2.8 denticles/mm). In the middle of the carina the denticles measure 0.36 mm long in av- erage, being almost as high as long. They are blunt, rounded and with flat tips, and slightly inclined toward the crown apex (Fig. 1D). Distal denticles decrease in size towards the crown base. However, the basalmost portion of the distal carina is not pre- served. There are no clearly visible blood-grooves or caudae. The distal carina as a whole is displaced labially (Fig. 1C). Most denticle tips are worn, and those on the basal third of the carina are obliquely truncated, giving the appearance that the denticles are leaning toward the labial face. Despite the enamel in the apical half of the mesial margin being rather badly preserved due to wear and collecting damage (Fig. 1A, B), we believe the me- sial carina would have been absent, given that there is no trace of it. The DSDI (Rauhut and Werner, 1995), consequently, cannot * Corresponding author. Journal of Vertebrate Paleontology 28(2):439–444, June 2008 © 2008 by the Society of Vertebrate Paleontology 439