Uruguaysuchus terrai Rusconi 1933

As already stated, Rusconi (1933) recognized a second species of Uruguaysuchus, U. terrai, based mainly on differences in the tooth count in the upper tooth row. Both Soto (2005) and Andrade & Bertini (2005) independently questioned the taxonomic validity of maintaining the distinction of U. terrai from U. aznarezi. U. terrai apparently possesses four maxillary teeth more than U. aznarezi: two incisiviforms, one caniniform, and ten post-caniniforms versus one incisiviform, one caniniform and seven post-caniniforms, respectively (Fig. 16). However, as already stated, it must be noted that the lack of preparation of the holotype of U. aznarezi does not allow an assessment of the total number of post-caniniform teeth (i.e. whether there are teeth beyond the ninth maxillary position).

Moreover, the maxilla of Rusconi’s specimen no. 4 (Fig. 16), a juvenile U. aznarezi according to this author, exhibits two incisiforms and one caniniform (being the third maxillary tooth), the same condition as in U. terrai. Curiously, Rusconi (1933) did not explicitly mention this relevant point. Interestingly, his fig. 20 illustrates seven post-caniniform alveoli although only six are implied in the text.

Furthermore, several measurements of U. terrai are consistently smaller than those of the only adult individual of U. aznarezi (holotype), approaching those of juveniles of U. aznarezi.

As proposed by Soto (2005), minor differences in the dental formulae can be explained by intraspecific variation in tooth count, which is rather common among crocodyliforms, either fossil or extant (C. Brochu, com. pers., 2008). In particular, if it is true that U. terrai, U. aznarezi no. 4 and FC-DPV 2320 represent juvenile individuals, it may be necessary to invoke ontogenetic loss of tooth positions to explain the fact that the adult individual of U. aznarezi (holotype) has only one incisiviform maxillary tooth instead of two. Such a phenomenon has already been recorded in several crocodylian species (e.g. Crocodylus cataphractus, C. porosus, C. siamensis, and Tomistoma schlegelii; Mook, 1921; Wermuth, 1953; Iordansky, 1973) as well as in the tyrannosaurid theropods Tyrannosaurus rex and Albertosaurus libratus (Carr, 1999).

On the other hand, the hypothesis that U. aznarezi could bear more post-caniniform teeth than recognized by Rusconi (1933) received support when FC-DPV 2320 was prepared. Indeed, the specimen described herein (Fig. 16) showed the presence of at least 12 maxillary teeth (two incisiviforms, one caniniform, and at least nine post-caniniforms), reducing the apparent gap between the tooth count of U. aznarezi and U. terrai.

In conclusion, given that no real differences in the maxillary dentition exist, we regard U. terrai as a juvenile individual of U. aznarezi. Thus, as proposed by Soto (2005) and Andrade & Bertini (2005), U. terrai must be considered a junior synonym of the latter taxon.