Pachyophis

The skull itself is slender and pointed in nature. Relation wise, the skull is about an eighteenth of the trunk length. The jaw bones are more reminiscent of lizards than snakes. The upper jaw closely relates to that of varanids, scincoids, and dolichosaurids. The maxilla specifically resembles that of lepidosaurs more so than that of snakes. The lower jaw is more resembles that of dolichosaurids.[1][4]

The upper jaw measures 17 mm (0.67 in) in length. Its shaped into a long triangular surface which is longitudinally concave and narrow in structure. Its highest point reaches 2.1 mm (0.083 in) forming a blunt tip and concave posterior outline on the shorter edge. The jawbone is approximately 1m thick. The anterior end represents the antorbital part of the jaw. A notable posterior concave outline represents the orbital opening as well as the beginning of the lacrimal. This projection then forms the connection of the jawbone with the palatine. At the anterior tip a small grove at the lower edge shows swollen character. Directly above it are two small groves with blunt lumps dividing them. This is representative of a suture and likely the location where the upper jawbone joins the premaxilla in the nasal opening region.[1]

The lower jaw measures 15.5 mm (0.61 in) in length and 1.9 mm (0.075 in) in width. The edges are elevated to produce a concavity that extends 3.5 mm (0.14 in) to a point posteriorly. The angular is the largest bone being widest anteriorly. It meets the splenial anteroventrally in a vertical joint. The splenial is preserved only at the joint, forming a bulge on the ventral margin. It is inferred then that the intramandibular joint is situated far anteriorly. This resembles that of Pachyrhachis. The tooth row extends the same length as exhibited in the upper jaw for equal alignment.[3]

In contrast to the jaw, Pachyophis teeth clearly resemble that of snakes. Their dentition show thecodont nature, their teeth fitted into bowl like alveoli lined on the jaw bones. The enamel layer of the teeth is well developed with large, hollow pulp cavities possessing small openings leading into them. The tooth row extends the entirety of the jaw with the teeth curving backwards anteriorly. The first three teeth stand 2.8 mm (0.11 in) apart, with the following becoming more crowded so the last six teeth stand only 2.4 mm (0.094 in) apart. The more spaced teeth are longer and stronger in form, with lengths up to 1.6 mm (0.063 in) and 0.5 mm (0.020 in) thickness. The closer proximal teeth in contrast are shorter with lengths of 0.8 mm (0.031 in) and widths of 0.3 mm (0.012 in).[1]

The articulation of all the vertebrae together is very snakelike. Yet the construction of the vertebrae themselves are very unlike such.

The anterior vertebrae are smaller than the more posterior. The neural arches grow to be more swollen moving anterior to posterior, indicative of pachyostotic nature. Zygosphenes presence are notable in some vertebrae. They project anteriorly from the neural arch and reach more dorsally than that of the zygapophyses located above the centrum. Neural spines rise from the zygosphene horizontally for a reach midlength of the vertebrae. This length increases to peak at the posterior end. At the anterior end, the neural arches are characteristically thin and shorter rising at an angle of 45 degrees post dorsally. Prezygapophyses are inclined at 40 degree angle anteriorly, becoming almost horizontal in the middle and then reobtaining their inclination anteriorly. Postzygapophyses width is slightly larger than that of the prezygapophyses. Both do not project very far laterally, with the anterior being more elongated that the posterior.[5]

The ribs are characterized by their needle like shape. Generally associated with the vertebrae, the ribs are technically not articulated to them. The upper third along the vertebral column are straight in shape, followed by a section of distinctive 120 degree bending, and then a return to the original straight form. The length of the ribs increases through the vertebral column. Pachyostosis also results in rib thickening, where the proximal section in each rib is around three times thicker in the posterior most than the anterior most. All exhibit a tapering to the distal tip for a consistent slender end.[5]

Exact knowledge of scalation of Pachyophis is not definitive. Scale repetition is defined in transverse rows, running parallel to one at notable right angles to the cervical vertebral column. Each bulge believed to be a scale is approximately 2.2 mm (0.087 in) in length, with minimal thickness and spacing between neighbors of about 0.8 mm (0.031 in).[1]

Pachyophis likely dwelled in shallow aquatic sea regions as evidenced by fossil location and morphology. Pre and post zygapophyses lend to undulating horizontal movement of the body. The neck in addition to this, had more flexibility and also indicates a range of vertical movement. This likely means that the Pachyophis' would hold its head above its flat bodied stature, indicative of shallow water dwelling character.[7]

Fossils were preserved specifically in calcareous mud, indicative of very little water movement in the environment. The rocks of Selišta were deposited in environments analogous to patch reef and inter reef basin systems. It is likely that Pachyophis coexisted along a diverse assemblage of marine squamates with a large diversity of fish fauna of a limited wave action lagoons.[7][8]

Pachyophis' narrowly tapered mouth and sharp backward pointing teeth indicates its carnivorous diet by action of gripping versus biting methods. The jaw-teeth structure is designed for simultaneous grabbing and swallowing, fighting back against the forward pushing force of escaping prey. The jaw's posterior elongation was also notably blocked from opening very wide, another usual characteristic of snake feeding mechanisms. Thus soft, small and slippery animals were likely the main form of prey. Specifically, marine worms.[1]