Cretaceous Ophidiiform Fish from North-Eastern Italy: Systematics and Evolutionary Implications

Percomorph fishes [Division Percomorphacea sensu Wiley & Johnson (2010)] represent the largest and most successful evolutionary lineage of vertebrate animals, with more than 15.000 described species characterized by astonishing morphological and ecological diversity. Percomorphs first appeared in the record at the base of the Upper Cretaceous, in the Cenomanian stage, around 99 Ma (Patterson, 1993). The first 35 million of years of history of these fishes, however, are largely unknown because of their relatively low abundance in the record, which possibly reflects their original reduced ecological relevance in the upper Mesozoic aquatic ecosystems. A recent analysis of the historical trends of morphospace occupation suggests that Cretaceous percomorphs were characterized by a very low anatomical diversity and that the origin of modern body plans occurred only in the early Cenozoic in the aftermath of the K/T mass extinction, primarily to fill vacated functional roles (Friedman, 2010). This hypothesis, however, does not appear to be consistent with the results of a study of the tempo and mode of vertebrate diversification that identified percomorphs as the fastest radiating clade of gnathostomes (Alfaro et al., 2009), with most of the modern lineages originating during the Cretaceous. The fish assemblage from the Cretaceous paralic deposits of Trebiciano (Campanian-Maastrichtian; Venturini et al., 2008), a few kilometres east of Trieste, north-eastern Italy, seem to support the latter hypothesis. Among a large number of basal neopterygians, ellimmichthyiforms, and gonorhynchiforms is a moderately diverse assemblage of derived percomorphs, thereby demonstrating that this group was represented by a relatively broad anatomical diversity before the K/T biotic crisis. Accordingly, a detailed comparative analysis of these Cretaceous percomorphs is necessary in order to properly interpret their anatomical structure and phylogenetic relationships. The high scientific value of these fossils lies in the fact that they may provide a strong interpretative key for the understanding of the origin of body plan diversity within percomorphs. One of the most interesting specimens of this assemblage is a well-preserved articulated skeleton of an ophidiiform fish. To date, the Cretaceous existence of ophidiiform fishes has been demonstrated by very rare Maastrichtian otoliths characterized by a plesiomorphic morphology. In view of that, the Cretaceous articulated skeleton from north-eastern Italy possibly represents the earliest evidence of the Ophidiiformes in the record. This Cretaceous ophidiiform exhibits a peculiar set of features (e.g., maxilla posteriorly expanded; presence of a single large supramaxilla; presence of 7 branchiostegal rays; possession of an ossified supraneural; 13 caudal-fin rays; high ratio vertebral number/dorsal-fin rays; etc.) that clearly evidence its basal phylogenetic position within the group and it is thus tentatively interpreted as a stem-group member of the Ophidiiformes basal to all other extant and extinct genera.