Almost billfish: convergent longirostry, micro-dentition, and possible glandular sinuses in a large teleost fish from the Upper Cretaceous of Northern Italy.

Abstract

A fossilized rostrum fragment was recently reevaluated from the paleontological collections of the University of Modena and Reggio Emilia (Italy). The specimen, collected from the Northern Apennines of Modena province, was previously referred to an Eocene billfish due to the presence of cylindrical paired bones and small teeth. Thanks to nannoplankton analysis of the matrix, we reassign the specimen to the Upper Cretaceous (upper Campanian-lower Maastrichtian). The morphological features of the rostrum, as well as its stratigraphic provenance, led us to assign the specimen to a longirostrine plethodid tselfatiiform (Actinopterygii, Teleostei) rather than to a billfish. The fragment is assumed to have originated from the mid-posterior portion of the rostrum, with associated upper and lower jaws. The rostrum exhibits a remarkable degree of morphological convergence with extant xiphioid billfishes, together with completely unique features. The bone surface is heavily ornamented, whereas the inner structure shows a prevalence of cancellous tissue. The suture between both premaxillae and the mesethmoid is crossed by a deep longitudinal fossa not dissimilar to that of parvipelvian ichthyosaurs, a feature never reported in tselfatiiform fishes. Abundant tiny conical teeth are found between the jaw rami, separated from their sockets. Micro-teeth can be found in both smooth-stout form or thin and crossed by apicobasal ridges but always capped by a translucent acrodin tip. Comparative analysis with swordfish (Xiphias gladius) dentition provided insights on shared similarities between the two types of micro-teeth. CT scanning of the specimen revealed a large, subtriangular, and tripartite vacuity in the upper jaw. A similar internal architecture is represented by the rostral sinus of modern billfishes, which is known to host large, globose oil-producing glands to reduce drag on the skin. We showcase these anatomical similarities with CT scan analysis of postlarval swordfish and sailfish, together with the morphological comparison with adults of these groups from the available literature. The cumulative features gathered from the specimen suggest a fast, pelagic predatory ecology. The findings further confirm the homoplastic development of a billfish-like body plan in Tselfatiiformes, with independently acquired morpho-physiological adaptation that preceded the evolution of xiphioids at least since the Late Cretaceous.