Paleorhinus undetermined

Butler, Richard J., Rauhut, Oliver W. M., Stocker, Michelle R. & Bronowicz, Robert, 2014, Redescription of the phytosaurs Paleorhinus (‘ Francosuchus’) angustifrons and Ebrachosuchus neukami from Germany, with implications for Late Triassic biochronology, Zoological Journal of the Linnean Society 170 (1), pp. 155-208 : 197-201

PALEORHINUS SP.

‘ Francosuchus sp. ’; Kuhn, 1936: 65, pl. 11, fig. 4

Material: BSPG 1931 X 503, partial skull described as Francosuchus sp. ( Kuhn, 1936: 65–67, pl. 11, fig. 4; Figs 15 , 16 ).

Locality and horizon: Bed 5 of Kuhn (1933, 1936), Ebrach quarry.

DESCRIPTION OF BSPG 1931 X 503

General: BSPG 1931 X 503 consists of a partial rostrum ( Figs 15 , 16 ) preserved largely in articulation or close association, including the complete premaxillae, most of both maxillae, probable fragments of the nasals and ‘septomaxillae’, parts of the right lacrimal and jugal, and parts of the palate including vomers, palatines, ectopterygoids, and probable fragments of the pterygoid. The specimen has undergone some dorsoventral crushing, and the dorsal part of the rostrum (above the ventral margin of the antorbital fenestra) is lost. The rostrum belongs to a moderately larger individual than the holotypes of Paleorhinus angustifrons (BSPG 1931 X 502) and Ebrachosuchus (BSPG 1931 X 501), based upon rostrum width at the level of the anterior margins of the choanae (95 mm in BSPG 1931 X 503, although this has probably been exaggerated by dorsoventral compression; 75 mm in Paleorhinus angustifrons ; 80 mm in Ebrachosuchus ) and the size of the alveoli. Despite this, the rostrum is shorter in absolute length than that of Ebrachosuchus (length of the rostrum anterior to the external nares is approximately 325 mm in BSPG 1931 X 503, and approximately 390 mm in Ebrachosuchus ). The number of alveoli (approximately 44 on each side; see below) is also fewer than in Ebrachosuchus .

The position of the posterior margin of the external nares relative to the antorbital fossa can only be determined approximately. The anterior margins of the external nares are placed above the anterior end of the eleventh maxillary tooth position as counted from the back of the tooth row, as opposed to the twelfth position in Paleorhinus angustifrons . The anterior margin of the antorbital fenestra can be established on the basis of the remnants of the ascending process of the maxilla, which is preserved on both sides. Thus, the anterior end of this fenestra was placed above the anterior end of the eighth maxillary alveolus as counted from the back of the tooth row. If the skull of Paleorhinus angustifrons is aligned with this snout so that the anterior ends of the external nares are level, the anterior end of the antorbital fenestra of BSPG 1931 X 503 is placed slightly more anterior, despite the fact that this specimen represents a larger individual. This suggests external nares that were placed proportionately slightly more posteriorly in BSPG 1931 X 503 when compared with Paleorhinus angustifrons .

Premaxilla: The premaxilla is strongly compressed and crushed, with a dorsal surface that is gently convex transversely. The external surface generally is smooth and only very weakly ornamented. As in Ebrachosuchus , the dorsal and ventral margins of the premaxilla are straight in lateral view along most of its length, but the dorsal margin becomes slightly concave and the ventral margin slightly convex posteriorly as the premaxilla arches gently upwards towards the external nares. The anterior end of the premaxilla arches first slightly upwards from the premaxillary–maxillary tooth row and the tip is then slightly downturned in lateral view. However, unlike the downturned premaxillary tip in Ebrachosuchus , the anteriormost tip of the snout is placed at approximately the same level as the pos- terior premaxillary tooth row. In dorsal and ventral views, the lateral margins of the premaxillae are straight, but diverge gently posteriorly as the snout becomes broader towards the external nares. The anterior termination of the premaxilla is rounded in dorsal and ventral views and expanded transversely to form the terminal rosette, which is as wide as long, as in Ebrachosuchus . However, in comparison with the latter taxon, the rosette is less strongly expanded relative to the width of the premaxillae posterior to it. Immediately posterior to the rosette, the premaxillae of BSPG 1931 X 503 are slightly contracted transversely.

Although the position of the premaxilla–maxilla contact is difficult to identify with certainty, 27 alveoli appear to present in the right premaxilla with 26 alveoli in the left premaxilla. Thus, the premaxillary tooth count is lower than the 34–35 estimated for Ebrachosuchus . The terminal rosette contains three large (8–10 mm diameter) ventrally and slightly posteriorly opening alveoli on its anterior and lateral margins, and one smaller alveolus (diameter 6 mm) immediately posterior to these. In contrast to Ebrachosuchus , in which the anteriormost two teeth are almost aligned transversely, the teeth in the rosette of this specimen are arranged in a gentle arch. The crown in the fourth alveolus is directed laterally in the left premaxilla, but ventrally in the right premaxilla. Posterior to this ‘tip-of-snout set’, there is a short diastema (length of one alveolus) at the level of the transverse contraction noted above, followed by the remaining 22–23 premaxillary alveoli. The premaxillary alveoli are subcircular and face ventrally and slightly laterally. There is some variation in alveolar diameter along the tooth row: the first alveolus in the right premaxilla is around 5 mm in diameter, whereas the largest alveolus (number 17) in the right premaxilla has a diameter of approximately 9.5 mm. There is a short edentulous section (probably pathological) of the left premaxilla, approximately 23 mm in length, between alveoli 14 and 15, although a shorter gap is also present on the right side between the same alveoli.

The premaxillary palate is formed by medially extending shelves that are thickened at their lateral margins, adjacent to the alveoli, into alveolar ridges. These alveolar ridges begin immediately posterior to the ‘tip-of-snout set’ and continue along the entire length of the premaxillary palate. These alveolar ridges are better developed than in Ebrachosuchus (see above). The premaxillary palate tapers in transverse width towards its posterior termination where it contacts the vomers.

Dorsally, the contacts of the premaxillae with the maxillae and with fragments of the nasals and ‘septomaxillae’ cannot be clearly identified.

Maxilla: Only the ventral parts of the maxillae are preserved. As noted above, the position of the premaxilla–maxilla contact is not certain, but approximately 16 alveoli appear to be present on the right side and at least 17 are present on the left (so that complete tooth counts for the premaxilla and maxilla combined appear to be 43 on both sides). In lateral view, the maxilla has a straight ventral margin along its anterior third and is gently convex along its posterior two-thirds.

The contacts of the maxilla with surrounding elements (premaxilla, nasal, lacrimal) cannot be determined externally. The anterior, ventral, and posterior margins of the antorbital fenestra are preserved on the right side: the fenestra is elongate (∼ 58 mm in length) and similar in size to that of Paleorhinus angustifrons , but substantially larger than that of Ebrachosuchus . An antorbital fossa is excavated shallowly into the maxilla at its anterior end but excavated deeply posteriorly. Below the antorbital fenestra, the external surface of the maxilla is flat to gently concave. The ascending process of the left maxilla is well preserved, but probably incomplete. It is a small, triangular dorsal projection positioned at about the mid-length of the maxillary body.

The maxillary alveoli have subcircular outlines. As in Paleorhinus angustifrons , the alveoli are relatively small at the anterior end of the maxilla (presumed first maxillary alveolus on the right side has a diameter of 6.5 mm) and increase in size posteriorly, reaching a maximum in the region lateral to the choanae (the presumed 10th right maxillary alveolus has an anteroposterior diameter of 12 mm). Alveolus size decreases again slightly at the posterior end of the tooth row. This morphology differs from Ebrachosuchus , in which there is less variation in alveolar size along the maxillary tooth row. Medial to the tooth row, the alveolar ridge is continuous with that of the premaxilla, but becomes narrower and less pronounced posteriorly although it is present as a distinct thickening along the entire tooth row.

The maxilla was contacted by the ectopterygoid immediately medial to the last alveolus, although the contact is disarticulated on the right side (and so the outline of the suborbital fenestra is uncertain) and the ectopterygoid is missing on the left. Lateral to the choanae the maxilla has a medially extending shelf that articulates with the palatine and excludes contact between the premaxilla and the palatine. The medial maxillary shelf lateral to the palatine is relatively narrower than in Paleorhinus angustifrons .

Only fragments of the right lacrimal, ‘septomaxillae’, and nasals are present externally, and provide little anatomical information. A flattened fragment of what is probably part of the right jugal is present, and has several low raised bumps ( Fig. 16B ) that may correspond to the nodular row present in Paleorhinus angustifrons (BSPG 1931 X 502). The jugal body ventral to the nodes is dorsoventrally high, comparable to the Polish phytosaur specimen ZPal Ab III 111, but in contrast to Paleorhinus angustifrons , although this might at least partially be due to dorsoventral crushing of the skull of the latter.

Palatine: Both palatines are preserved, the right in articulation with the maxilla and the left element disarticulated and slightly displaced posteriorly. The palatine is elongate and the main body is orientated horizontally. Medially, it possesses a sharp ridge that defines the lateral rim of the palatal vault. A dorsomedially extending flange of the palatine is present, probably overlapping the unpreserved pterygoid within the palatal vault. A dorsomedially directed flange marks the posterior end of the internal choana, which is entirely placed posterior to the external naris and is 38 mm in length. Anterior to the suborbital fenestra the palatine tapers in transverse width as it articulates with a medially extending shelf of the maxilla. In contrast to both Paleorhinus angustifrons and Ebrachosuchus , the palatine margin of the suborbital fenestra is concave, rather than convex. Consequently, the suborbital fenestra is relatively larger than in these taxa and was apparently elongate and teardrop-shaped in outline.

Vomer: Part of the right vomer is present, forming the septum separating the choanae and contacting the premaxilla anteriorly. The sutural contacts of the element cannot be established and the posterior end is probably missing.

Ectopterygoid: The right ectopterygoid is slightly disarticulated and damaged along its posterior and medial margins. A foramen cannot be identified medially, but this part of the element has broken away. Laterally the ectopterygoid is expanded broadly in an anteroposterior direction, with its posterior part drawn out into a narrow process that would have contacted the medial surface of the jugal. The anterior process for the articulation with the maxilla is long and tapering anteriorly, with the articular surface for the maxilla being set off from the main ectopterygoid body by a small step. The main transverse body of the ectopterygoid is twisted, so that the lateral part is almost horizontal, whereas the medial surface faces anteroventrally. The posterior margin, which formed the anterior rim of the subtemporal fenestra, is strongly concave.

A fragmentary bone preserved behind the end of the left maxilla might represent a remnant of the pterygoid, but cannot be identified with certainty at present.

Dentition: Parts of nine crowns, five from the anterior portion of the premaxillae, and four from the mid-toposterior portions of the maxillae are preserved, but most are damaged. Those of the anterior premaxilla particularly are preserved poorly, but appear to be relatively small, conical, not strongly recurved, and lack carinae and serrations. Those of the mid-toposterior portions of the maxilla are strongly recurved posteriorly and slightly medially towards their apices, and possess carinae along mesiolingual and distolabial surfaces. Although preservation is poor, serrations are visible along the anterior carina of the most posterior preserved crown in the right maxilla.

Taxonomic affinities: BSPG 1931 X 503 differs from Ebrachosuchus in possessing a proportionally shorter snout with lower premaxillary and maxillary tooth counts, increased heterogeneity in alveolar size, a large antorbital fenestra with an anterior antorbital fossa, well-defined alveolar ridges, and a relatively larger suborbital fenestra with a concave anterior border. It is more similar in many features to Paleorhinus angustifrons (with the exception of rostrum length which is uncertain in Paleorhinus angustifrons ), but differs in probably possessing an antorbital fenestra that is proportionally slightly smaller in size (the absolute sizes of the fenestrae are nearly identical in Paleorhinus angustifrons and BSPG 1931 X 503, but BSPG 1931 X 503 represents a moderately larger skull) and relatively closer to the external nares and a considerably larger, elongate, and teardrop-shaped suborbital fenestra. Although the bone is not well preserved, BSPG 1931 X 503 appears to possess the nodular row on the jugal that we consider diagnostic of Paleorhinus . For these reasons, we refer BSPG 1931 X 503 to Paleorhinus sp. , although the differences noted above may indicate that it represents a species different from Paleorhinus angustifrons . Since these two specimens of Paleorhinus come from different horizons within the quarry, there is a possibility that they might represent a single anagenetic lineage.

Figures

Figure 15. Paleorhinus sp., BSPG 1931 X 503. Photographs in dorsal (A) and ventral (B) views.

Figure 16. Paleorhinus sp., BSPG 1931 X 503. Interpretive line drawings in dorsal (A) and ventral (B) views. Dark grey areas represent sediment. Cross hatched areas are broken surfaces. Abbreviations: afen, antorbital fenestra; cho, choana; ect, ectopterygoid; jg, jugal; lc, lacrimal; max, maxilla; na, nasal; pal, palatine; pm, premaxilla; pt, pterygoid; sm, ‘septomaxilla’; sub, suborbital foramen; unid, unidentified bone fragment; v, vomer.