Pseudopus laurillardi ( Lartet, 1851 )

Rage, Jean-Claude & Bailon, Salvador, 2005, Amphibians and squamate reptiles from the late early Miocene (MN 4) of Béon 1 (Montréal-du-Gers, southwestern France), Geodiversitas 27 (3), pp. 413-441 : 422-424

Pseudopus laurillardi ( Lartet, 1851) ( Fig. 4A, B )

Anguis ? Laurillardi Lartet, 1851: 40.

Ophisaurus ? laurillardi – Estes 1983: 143.

Pseudopus laurillardi – Augé & Rage 2000: 276-278, figs 9-13.

Pseudopus laurillardi was redescribed by Augé & Rage (2000) on the basis of cranial bones from the middle Miocene (MN 6) of Sansan, France. The material from Béon 1 includes a posterior part of braincase, i.e. a skeletal element previously unknown for the species, and additional characters can be observed on the dentary. Therefore, an emended diagnosis is proposed.

MATERIAL EXAMINED. — Posterior part of a braincase (Béon 2004 LT 38), 1 right dentary (Béon 2004 LT 39), 44 trunk vertebrae (Béon 2004 LT 40-42; Béon 93 E3 SN 1; Béon 98 déblais SN 1), 3 sacral vertebrae (Béon 2004 LT 43), 82 caudal vertebrae (Béon 2004 LT 44-47; Béon 93 E3 SN 2), 16 osteoderms (Béon 2004 LT 48).

EMENDED DIAGNOSIS. — Pseudopus laurillardi differs from the three other species of the genus ( P. apodus (Pallas, 1775) , P. pannonicus (Kormos, 1911) , P. moguntinus (Boettger, 1875)) in having larger anterolateral processes of the parietal. It is distinguished from P. apodus and P. pannonicus in having a more ventrally arched dentary, a lamina horizontalis of the dentary strongly projecting medially, a broad sulcus dentalis, a weak angulation between the alveolar surface of the parapet and the subdental shelf, a thicker ventral border of the trigeminal notch, a more developed interfenestral crest, more concave posteromedial surfaces of paroccipital processes, and straight dorsal ridges (instead of curved ridges) on the posterior braincase. It further differs from P. apodus by its more weakly striated teeth and more robust dentary. Apparently, it further differs from P. moguntinus in having a short groove on the lateroventral margin of the anterior part of the dentary.

DESCRIPTION

The dentary from Béon 1 compares favorably with the neotype, i.e. a left dentary (designated by Augé & Rage 2000) and all other dentaries of P. laurillardi from Sansan. The dentary from Béon 1 is slightly smaller than the neotype but it falls within the range of variation of the species. In both specimens, the ventral border and lamina horizontalis are markedly arched ventrally, the teeth are blunt, amblyodont, and their apices are weakly striated, and the anterior inferior alveolar foramen is located beneath the 6th tooth from the rear. In addition, other features not reported in Augé and Rage’s description occur in the two specimens: the bone is robust and its lateral surface is clearly convex in cross section; the lamina horizontalis prominently extends medially, has a rounded medial border and dorsally it forms a subhorizontal subdental shelf that is slightly separated from the alveolar surface of the parapet by a weak angulation. As a consequence of the strong medial extension of the lamina horizontalis, the tooth row is clearly separated from the medial border of the lamina. A broad sulcus dentalis is present. On the lateroventral margin of the anterior part of the bone, a short and shallow groove runs anteroposteriorly. The smaller dentaries from Sansan perhaps lack the latter groove.

A posterior part of braincase is referred to P. laurillardi . The type locality, Sansan, did not yield braincases. On the whole, the specimen from Béon 1 is similar to the posterior braincase of the only living species of the genus, P. apodus , that ranges from Croatia to Central Asia and the middle East. Four features are worth mentioning. On the dorsal face of the braincase of P. laurillardi , each dorsal ridge (dr, Fig. 4B ) that joins the base of each paroccipital process to the processus ascendens of the supraoccipital is straight. The part of the prootic that is ventral to the trigeminal notch is thick and, consequently, the ventral border (vb, Fig. 4B ) of the notch forms an elongate and relatively broad subtriangular surface that faces dorsolaterally. The interfenestral ridge, between the oval fenestra and occipital recess (= round fenestra), appears to be well developed, and the posteromedial surfaces (pms, Fig. 4B ) of the paroccipital processes are clearly concave.

Vertebrae show the typical morphology of anguines; they are assigned to P. laurillardi because their size is consistent with the above skull bones, their centrum more clearly widens anteriorly than in vertebrae of Ophisaurus (see below), and their ventral surface is nearly flat. Osteoderms are referred to P. laurillardi only on the basis of their size.

COMMENTS

According to Augé & Rage (2000), the dentary of P. laurillardi differs from that of P. apodus by having weaker striae on the apices of teeth and by the more anteriorly located anterior inferior alveolar foramen. They also stated that P. laurillardi differs from P. moguntinus (= Propseudopus fraasi Hilgendorf, 1883 ) (latest Oligocenemiddle Miocene of Central Europe) and P. pannonicus (late Miocene-middle Pleistocene of Central and Eastern Europe) by its smaller size. The position of the anterior inferior alveolar foramen does not appear to be a reliable feature because in P. apodus it sometimes occurs beneath the limit between the 5th and 6th teeth (from the rear) or even entirely below the 6th tooth as in P. laurillardi ( Roček 1980: fig. 1). But, following the study of the material from Béon 1, we may add some other features that characterize the dentary of P. laurillardi . It is more robust than that of P. apodus and more arched than that of P. apodus and P. pannonicus . Moreover, in P. laurillardi the dorsal surface of the lamina horizontalis (i.e. the subdental shelf) is subhorizontal; in other words, there appears to be a weak angulation between the alveolar surface of the parapet and the subdental shelf, which is unusual in anguines; in P. apodus and P. pannonicus , the alveolar surface and the subdental shelf form a single oblique surface, without any angulation. According to the illustrations of the dentary of Propseudopus fraasi (i.e. Pseudopus moguntinus ) in Hilgendorf (1885), an angulation is perhaps present also in this species. Finally, the strong medial projection of the lamina horizontalis and the presence of a sulcus dentalis clearly distinguish P. laurillardi from P. apodus and P. pannonicus . These two features appear to be present in P. moguntinus .

Several traits of the braincase should be added to the features that characterize P. laurillardi . A conspicuous difference between the posterior braincase of P. laurillardi and those of P. cf. P. pannonicus from Montoussé 5 (late Pliocene, France; Bailon 1991), P. pannonicus , and P. apodus is that in P. laurillardi , the dorsal ridges (dr, Fig. 4B ) are straight, whereas they are medially concave in the other forms. Moreover, P. laurillardi differs from P. apodus and P. pannonicus in having thicker ventral borders of the trigeminal notches (vb, Fig. 4B ), more developed interfenestral ridges, and more concave posteromedial surfaces of the paroccipital processes (pms, Fig. 4B ). The distinction between P. laurillardi and the poorly known P. moguntinus cannot be easily established. However, aside from its larger size and smaller anterolateral processes of parietal, P. moguntinus apparently lacks the short groove that runs anteroposteriorly on the ventrolateral margin of the dentary ( Hilgendorf 1885), even on large specimens; this groove occurs in P. laurillardi , P. apodus , and P. pannonicus . Augé & Rage (2000) suggested that P. moguntinus might be referred to the synonymy of P. laurillardi , but the few above characters will have to be considered when the former species is revised.

Finally, the parietal provides a feature that markedly distinguishes P. laurillardi from the other species: the anterolateral processes of the parietal (improperly termed “prefrontal processes” in Augé & Rage 2000, partly following Fejérváry-Lángh 1923) of P. apodus , P. moguntinus , and P. pannonicus are smaller than those of P. laurillardi . However, only one parietal is known from Sansan ( Augé & Rage 2000); this bone has not been found at Béon 1. Pseudopus laurillardi has been found only at Béon 1 (MN 4) and Sansan (MN 6).

Figures

FIG. 4. — A, B, Pseudopus laurillardi (Lartet, 1851); A, right dentary (Béon 2004 LT 39); B, posterior part of a braincase (Béon 2004 LT 38); C, Ophisaurus sp., left dentary (Béon 2004 LT 49). Dorsal (d), left lateral (l), medial (m), and right lateral (r) views. Abbreviations: dr, dorsal ridge; pms, posteromedial surface of paroccipital process; vb, ventral border of trigeminal notch. Scale bars: A, B, 5 mm; C, 2 mm.