Rhynchocephalia Günther, 1867

Evans, Susan. E., Raia, Pasquale & Barbera, Carmela, 2004, New lizards and rhynchocephalians from the Lower Cretaceous of southern Italy, Acta Palaeontologica Polonica 49 (3), pp. 393-408 : 399-401

Rhynchocephalia Günther, 1867

Genus indet.

Figs. 6–10 .

Description.—MPN A01/82 preserves the posterior half of a small reptile in dorsal view, from the mid−trunk through to the tip of the long tail, and including parts of the pelvis and hind limbs ( Fig. 6 ). Given the preserved remains, the original snout−vent length of this animal may be estimated as ca. 70 mm, depending on the relative length of the trunk. The specimen is of particular interest in containing part of the original gut contents, in the form of recognisable bones (see above), bone fragments, and organic matrix.

Vertebral structure ( Figs. 6A, B , 7 , 8 ).—The specimen preserves a series of large, but crushed, posterior trunk vertebrae. Each bears either a short straight rib or a long lumbar transverse process (the junction between rib and vertebral body is not always clear). These posterior vertebrae are followed by two sacrals. The first is partially obscured by the posterior gut mass, but reveals strong, distally expanded ribs. The second sacral shows one of two slightly smaller sacral ribs. The tip is again expanded but shows no bifurcation. The tail is long and slender, with at least 25 distinctive caudal vertebrae (note that the distal third of the tail is omitted from Fig. 6A ). The eight proximal caudals are relatively elongate, with long horizontal zygapophyseal surfaces and small, horizontally−directed spines. Their transverse processes are long, tapering, and angled sharply backward ( Fig. 8 ). The processes disappear around the level of caudal vertebra nine and the vertebrae become increasingly attenuated. The zygapophyses come closer to the midline, eventually forming a single midline process, and there is no neural spine. Rather, the dorsal surface of the arch bears a sulcus or depression, flanked on each side by a shallow ridge running from anterior to posterior zygapophysis Figs. 6B , 7 ). The ridge and depression are also present on more anterior caudals, but are difficult to see because the region is crushed. Ventrolateral to the zygapophysial ridge, there is a second sulcus running along the side of the neural arch, above the level of the transverse process. The first ten caudals show no trace of a fracture plane. Further distally, several of the vertebrae are grooved, but there is no consistency of position and the vertebrae do not appear to be autotomous. The haemal arches are nearly horizontal and do not increase the depth of the tail to any significant degree.

Pelvis and hind limb ( Figs. 8 , 9 ).—The pelvic girdle has been compressed and is partially obscured anteriorly by the gut contents and sacrum ( Fig. 8 ). The right ilium has fallen medially, exposing the lateral surface and the acetabulum; it had clearly not fused to the other pelvic bones. There is a small anterior iliac prominence, but the upper part of the blade has been crushed into the sacrum and its margins are not visible. The pubis lies anteroventral to the ilium. It forms a broad plate delimiting a posteromedial thyroid fenestra (the edge of which is just visible in front of the ilium). The most distinctive feature is a large, flange−like pectineal process that extends anterolaterally. From this point, the anterodorsal edge of the bone curves towards the midline rather than running forward. This suggests a short, broad pubic ramus, with a wide pubic symphysis. The ischium lies under the ilium and femur and is more clearly visible on the right side. It has a strong posterior process.

The right hind limb and most of the left femur are exposed ( Figs. 6 , 9 ). The remainder of the left limb is within the matrix, but it was not possible to prepare this region further because the matrix is hard and the bone rather fragile. There is an apparent disparity in the widths of the right and left femora, but this is an artefact of crushing and a slight smearing on the right side. On the left, the borders are intact and the bone is quite left femur gracile. Allowing for the absence of the distal femoral head and the proximal tibia, the right femur is not substantially longer than the bones of the crus, a marked difference between this specimen and MPN 19457. The tibia and fibula run in parallel, with both bones of almost constant width, although the tibia, as usual, is thicker. The right hind foot is preserved in its entirety, but, like the femur, the individual bones are crushed and somewhat smeared, so that boundaries between elements are difficult to see. Thus although the proportions of the digits shown in Fig. 9 can be taken as accurate, the lengths of individual elements, and the positions of the joints between them, are shown with less confidence. There is a single ovoid proximal tarsal close to the end of the fibula and the lateral edge of the tibia. This is probably the calcaneus, but there is no trace of the astragalus, unless it is crushed into the proximal ends of the inner metatarsals. There is at least one distal tarsal, probably the fourth, between the calcaneus and the heads of metatarsals four and five.

The foot as a whole is shorter than the crus, another significant difference with MPN 19457. Metatarsals 1–4 appear to be short and thick; metatarsal 5 is expanded proximally but the degree of hooking is unclear. The individual phalanges are also relatively shorter than those of MPN 19457. The penultimate phalanges are longest and have a distinctive shape—wide proximally, narrowing in the distal part of the shaft, and then expanding slightly at the distal articular head (mostly still cartilage). The unguals are narrow and tapering. As reconstructed, the phalangeal formula is 2: 3: 4: 4/5?: 3?.

Soft tissues ( Fig. 10B, C ).—To the left and right of the trunk vertebrae and ventral to the short ribs, there is an irregular mass made up of three distinct components ( Fig. 10B, C ). An anterior mass consists of identifiable small bones comprising the disarticulated lizard skull discussed above. Under the Scanning Electron Microscope these elements show slight acid etching. The more posterior mass comprises finely comminuted bone fragments embedded in an organic ground substance. Linking the two regions is a narrow wavy strip of stained rock containing indeterminate particulate material.

The lizard skull lies with its anterior tip directed backwards and with the ventral side uppermost. This suggests the prey may have been swallowed head first with the rest of the skeleton following (and on the missing part of the block), although it is equally possible that the lizard was decapitated or scavenged.

Figures

Fig. 6. Pietraroia specimen MPN A01/82. A. Complete specimen (but without distal one third of tail). B. Midcaudal vertebra.

Fig. 7. Pietraroia specimen MPN A01/82. Posterior caudal vertebra showing lateral zygapophyseal ridge and dorsal depression.

Fig. 8. Pietraroia specimen MPN A01/82, pelvic, sacral and caudal morphology.

Fig. 9. Pietraroia specimen MPN A01/82. Right crus and pes of MPN A01/82, digits numbered 1–5.

Fig. 10. Visceral structures. A. The gut of Sphenodon punctatus (redrawn from Guibé 1970: fig. 403); not to scale. B, C. Pietraroia specimen MPN A01/82, abdominal region; scale bars 2 mm. B. Simplified figure showing arrangement of gut parts. C. The central part of the specimen.