Alethinophidia Nopcsa, 1923

Alethinophidia Nopcsa, 1923

Constrictores Oppel, 1811a

(sensu Georgalis & Smith 2020)

Constrictores indet.

( Fig. 1)

MATERIAL. — Southwestern France • 1 right maxilla; UM-COS-1014 .

LOCALITY AND HORIZON. — Cos, fissure fill in the Quercy region (southwestern France); lower Eocene (MP 10-11 interval).

DESCRIPTION

Maxilla

The maxilla UM-COS-1014 is fairly preserved, although incomplete – the specimen corresponds to the anterior half of a right bone, while its posterior termination is broken off ( Fig. 1). The maximum anteroposterior length of this bone is 9.3 mm. The preserved portion of the maxilla bears nine and half tooth positions (five partly preserved teeth are still attached to the bone). It is elongated ( Fig. 1A, B) and slightly anteromedially curved in dorsal and ventral views ( Fig. 1C, D); the anterior portion of the bone is convex laterally, forming slightly bent rod-shaped projection. Its dorsal surface is pierced by a dorsal foramen (sensu Scanlon 2001), located at the level of the third tooth position (counted from anterior). Anteriorly, a distinct but short groove runs from that foramen. It continues anteriorly to the lateral side of the bone. The premaxillary process is smooth and rounded ( Fig. 1C). As typical for snakes, the maxilla lacks an articulation facet for the premaxilla. The medial margin of the anterior portion of the maxilla shows a stronger concavity, because from the sixth tooth position, the bone gradually widens posteriorly protruding into the palatine process (prefrontal process sensu Szyndlar 1984). The palatine process is broad (it occupies an area from the sixth to ninth tooth positions).It is slightly hooked posteriorly (it projects posteromedially into a short process separated posteriorly from the rest of the bone by a rounded notch; Fig. 1D). Thus, it has the shape of a “shark dorsal fin” in dorsal and ventral views ( Fig. 1C, D). The dorsal surface of the palatine process is pierced by a large posterior foramen ( Fig. 1C) of the superior dental canal (sensu Anthony & Serra 1950). This foramen is located in the anterior region of the process and is also visible in medial view. Another foramen which is called the vascular foramen (sensu Anthony & Serra 1950) is located more posteriorly on the dorsal surface of the dental portion of the maxilla (slightly anterolaterally from the posterior notch of the palatine process). The CT scan reveals that all three foramina present on the dorsal surface of the bone converge in the same internal alveolar canal in which the two supralabial foramina (see below) also open. In posterior view, the palatine process is slightly inclined medioventrally ( Fig. 1F). Thus, a shallow longitudinal depression is present between the dental portion and the process ( Fig. 1D).

In lateral view, the bone is low and presents a poorly defined suborbital margin. Note, however, that a well-visible vestige of the facial process (sensu Gauthier et al. 2012) rises at the level of the anterior dorsal foramen and disappears at roughly the level of the palatine process. The external surface of the bone is pierced by two supralabial foramina in the anterior region. The larger of these two is the anterior one and it is located at the level of the posterior area of the third tooth position (counted from anterior). The smaller posterior supralabial foramen is located at the level of the posterior area of the fourth tooth position (counted from anterior). The rest of the external surface is smooth. The posterior portion of the maxilla, i.g., the posterior process, is broken off. Thus, it is unknown if it was flared or not. The presence or absence of an ectopterygoid process cannot be assessed.

Dentition

The dentition is ankylosed-subthecodont.The teeth are mesiodistally wide, especially at their bases, however, they are also mediolaterally compressed. Although the tooth apiced are mostly broken off (the best preserved tooth is the fifth one, Fig. 1G), they are clearly recurved. The anterior teeth appear to be the largest; the tooth size gradually decreases posteriorly.

COMMENTS

The number and size of supralabial foramina is an important feature for Constrictores. The size of these foramina depends on the number of nerve fibres and size of blood vessels that serve the sensory tissue of heat-sensing circumoral epithelium in snakes. In maxilla, this tissue is innervated by the maxillary and/or ophthalmic branch of the trigeminal, and perfused by branches of the superior maxillary artery, which pass through the supralabial foramina ( Young 1988).