Ichthyosauria, de Blainville, 1835

Freschi, Alessandro, Morigi, Alessia, Cau, Simone, Persico, Davide, Garbasi, Francesco, Fontana, Filippo & Cau, Andrea, 2023, First biostratigraphic dating for a Cretaceous ichthyosaur from the Apennine Chain (Italy), Comptes Rendus Palevol 22 (9), pp. 143-157 : 148-149

Ichthyosauria gen. et sp. indet.

( Fig. 4)

MATERIAL. — 001-Pad1 , 001-Pad2 , 001-Pad3 , 001-Pad4 , 001- Pad5 : nine articulated anterior caudal centra and an isolated centrum fragment.

LOCALITY AND HORIZON. — Paderna (Neviano degli Arduini municipality, Parma Province), Termina valley; “ Argille Varicolori della Val Samoggia ” Formation (AVS); lower Aptian (this study).

DESCRIPTION

The specimen is represented by a series of vertebral centra ranging 32-38 mm in length and 91-97 mm in both width and height ( Fig. 4). No neural arches or ribs were found in association with the centra. The centra are very closely spaced (i.e., only a relatively thin fine-sediment matrix fills the intervertebral space) and aligned along their anteroposterior axes, a pattern which likely represents their original anatomical connection. The centra are divided into two blocks. The largest block (30 cm long) contains six centra and is crossed by a fracture. The first centrum is the most fragmentary, preserving only the right dorsal part of the bone, while the others are almost complete. The second block (15 cm long) is composed of three centra, one of which is missing its left ventral part. A single incomplete centrum was found in the same area but isolated from the two blocks. All centra appear slightly tilted to one side and shifted relative to each other. The left surfaces of the centra are better preserved than their right counterparts, a preservation artifact which might indicate that the vertebrae were partially exposed and suffered some degree of erosion prior to their discovery. As a consequence, only the left apophyses are preserved. The centra are anteroposteriorly short, displaying a distinct amphicoelous appearance. A synapophysis is present and placed in the lateroventral surface. Most centra show a distinct suture for the neurocentral pedicels on their dorsal surface, crossed longitudinally by the floor of the neural canal. The neurocentral pedicel facets appear narrower and longer in the anterior vertebrae and become more oval in the posterior ones. No subcentral foramina or haemal arch facets are present. The centrum height ( CH), centrum length (CL) and centrum width (CW) of the vertebrae decrease moderately along the anteroposterior direction of the articulated series ( Fig. 4; Table 3 ). The general shape of the articular surfaces is subcircular with the height to width ratio ( CH /CW) around c. 0.9 in all centra. The vertebrae are much shorter in length than dorsoventrally tall, with a CH /CL ratio ranging between 2.6 and 2.9.

We report three small elongate and sub-cylindrical elements, about 5 mm in diameter each, found inside the sedimentary matrix infilling the intervertebral space between two consecutive vertebral centra ( Fig. 5). These structures are oriented radially relative to the center of the adjacent intervertebral facet ( Fig. 5A), and distally they reach the rim of the intercentral facet. Internally, each structure appears as a sedimentary infilling ( Fig. 5B). We suggest that these elements are biological in origin, and interpret them as ichnological traces left by a limnivorous/saprophagous invertebrate.