Nothosaurus winkelhorsti Klein and Albers, 2009

Klein, Nicole & Albers, Paul C. H., 2009, A new species of the sauropsid reptile Nothosaurus from the Lower Muschelkalk of the western Germanic Basin, Winterswijk, The Netherlands, Acta Palaeontologica Polonica 54 (4), pp. 589-598 : 590-595

Nothosaurus winkelhorsti Klein and Albers sp. nov.

Figs. 1–6 ; Tables 1, 2.

Etymology: In honour of Herman Winkelhorst, a dedicated collector and member of the Muschelkalk workgroup of Winterswijk who found this specimen.

Holotype: NMNHL RGM 443825 almost complete skull.

Type locality: Winterswijk quarry, Winterswijk, The Netherlands.

Type horizon: Layer 9 (see Oosterink 1986), Vossenveld Formation, Lower Muschelkalk (early Anisian, lower Middle Triassic).

Diagnosis.—A small species of Nothosaurus with a skullcondylobasal length of 46 mm; skull length to ratio of skull length/upper temporal length distinctly different to N. marchicus and N. winterswijkensis ; fused premaxillae and nasals; straight rectangular and broad parietals; short posterior extend of maxilla; fully developed massive braincase elements; an overall low number of maxillary teeth.

Material.— Holotype only.

Description.—The skull measures 46.5 mm from its snout to the occipital condyle ( Table 1). The premaxillae, nasals, frontals and parietals are fused. Sutures are well ossified and are difficult to distinguish or have been obliterated completely. The skull has been laterally compressed postmortem. In general, a distinct sculpture comprising small pits covers the entire skull. However, some elements (e.g., the posterior part of the premaxillae and the nasals show more pits and grooves than others. Several elements of the palate, the occipital region, and the left lower jaw are also preserved. However, their description is limited because these elements are distinctly compressed, broken or incomplete.

Premaxilla.—The premaxilla has a long, slender, tapering posterior process, which reaches far back along the skull midline, extending beyond the external naris. In dorsal view the anterior medial margin of the external naris is bordered by the premaxilla. The posterior medial margin of the naris, however, is separated from the premaxilla by an anterior lateral tapering process of the nasal. The premaxilla extends laterally to a level below the middle of the external naris. Its end is tapered and the tip is enclosed by the maxilla. The premaxillae are clearly divided into antero−lateral and postero−medial parts. The latter has a distinct form, which recalls a pumpkin seed, with a broad round anterior part. The posterior part ends in a tapering peak. Additionally, the bone surface is smoother. The round−oval shaped external nares measure only 40% of the longitudinal length ( Table 2) of the orbits. The snout is constricted anterior to the external nares; however, the dimensions of the constriction are rather difficult to see because of the lateral distortion of the skull. Each premaxilla bears five fangs.

Maxilla.—The maxilla extends from below the middle of the external naris to about the level of the anterior margin of the upper temporal opening. It ascends between the external naris and the orbit. The maxilla contacts the nasal with a broad suture at the level of the dorsal margin of the external naris. The maxilla limits the anterior process of the frontal and the prefrontal. The anterior part of the maxilla forms the ventral and posterior margin of the external naris. The dorsal part of the maxilla borders the anterior and entire ventral margin of the orbit but is excluded from the entire posterior part of the orbit. Ventrally, a thin tapering process of the maxilla reaches below the jugal up to the anterior edge of the upper temporal opening. At this level, the maxilla shows a pointed contact with the dorsally ascending ectopterygoid. The suture with the palatine cannot be seen in detail.

The maxillary tooth row bears teeth that are distinctly smaller than the premaxillary fangs, except in positions four and maybe five, in which (as is common in Nothosaurus ) they resemble larger fangs. None of the tooth rows is complete: on the right side 10 maxillary teeth are preserved, but on the left side only four.

Nasal.—Thin, tapering anterior processes of the nasals border the inner anterior−dorsal margin of the external naris, below the posterior processes of the premaxillae. Medially, the nasals broaden and their sutures to the maxillae are rounded. The posterior processes of the nasals taper again, and each nasal ends in a short pointed tip. The nasals are fused along the midline of the skull. The posterior processes of the nasals are broader and extend backward to a level between the anterior third of the orbits. Their suture with the frontal is interdigitated. Ventrally, the anterior part of the nasal broadly contacts the maxilla; posteriorly, it touches the anterior process of the frontal. The nasals are highly sculptured by longitudinal pits.

Prefrontal.—The left prefrontal is a rather short and slender element, lining the anterior third of the dorsal part of the orbit. On the right side it appears anteriorly broader and thicker, possibly due to the compression of the skull.

Jugal.—The massive and thick jugal is anteriorly and antero−ventrally bordered by the maxilla as well as dorsally by the postorbital. Posteriorly and postero−ventrally it contacts the squamosal and the ectopterygoid. The jugal is rectangular−shaped and deeply sculptured by grooves. It is located between the orbit and the lower temporal opening. It reaches anteriorly to the posterior margin of the orbit. Posteriorly, it meets the anterior part of the squamosal and ventrally the ectopterygoid, just in front of the anterior margin of the upper temporal opening. The jugal is separated from both openings by the postorbital. Only the anterior suture of the jugal is distinct whereas all the other sutures around the jugal are difficult to make out. The elements are nearly co−ossified in this area.

Frontal.—The tips of the antero−lateral processes of the fused frontals contact the tips of the dorsally ascending maxillae. Their medial margins meet the antero−lateral margin of the nasals, and their lateral margins run parallel to the prefrontal. The medial processes of the frontals meet the tip of the anterior nasals. The body of the fused frontals forms a relatively small interorbital bridge that separates the posterior part of the prefrontal from the anterior part of the postfrontal. The latero−posterior processes of the frontals are limited, laterally by the posterior part of the postfrontal, and dorsally by the parietals. The medial processes of the frontals are shorter than their lateral processes and contact the parietals. However, in this specimen the postero−lateral processes of the frontals are compressed together with the medial processes of the parietals and their sutures are only ill−defined.

Postfrontal.—The anterior part of the postfrontal bifurcates in a tapering dorsal and ventral part, which together form much of the posterior border of the orbit. The posterior process of the postfrontal is nearly rectangular in outline and is bound dorsally by the postero−lateral processes of the frontals and ventrally by the postorbital. The postfrontal does not enter into the margin of the upper temporal opening but meets the parietal dorsally at a level of the anterior third of the temporal opening.

Postorbital.—The postorbital is a massive bone that builds laterally nearly the entire postorbital arch. The postorbital ascends dorsally up to a level of the dorso−posterior orbit, where it deeply borders the postfrontal with a broad peak. It forms the anterior margin of the upper temporal opening before it meets the parietal posteriorly. The anterior postorbital limits the posterior edge of the orbit and slightly wedges between the maxilla and jugal. The tapering posterior part of the postorbital limits the anterior third of the ventral margin of the upper temporal opening and meet the squamosal at the temporal arch.

Parietal.—The fused parietals are more or less rectangular in outline and occupy the entire space between the upper temporal openings. The ventral part of the parietals broadens anteriorly so that a huge bulge is visible in dorsal view. Anteriorly, they have small medially running processes which interdigitate with the frontals and are coated by their postero−lateral processes. The postero−medial part of the parietals is deeply emarginated. Their dorso−lateral processes are longer and meet the squamosals at the posterior edge of the skull roof. Their ventro−lateral processes are very thin and elongated. They run postero−ventrally parallel to the squamosal, and extend up to the middle of the supraoccipital. The pineal foramen is located centrally between the upper temporal openings. It is not embedded in a distinct depression.

Squamosal.—The squamosal has a lateral process that is directed anteriorly at the skull roof and extends posteriorly in a large lobe down toward the jaw joint. The anterior part of this process is relatively thin and rectangular. Laterally, it forms the dorsal margin of the lower temporal arch. Dorsally it borders two thirds of the upper temporal opening and extends ventral to the postorbital to the anterior margin of the upper temporal opening. Its short medial process contacts the parietal. Posteriorly, the squamosal divides into a lateral process that reaches far down to the jaw joint, covering the postero−lateral edge of the quadrate. Medially is a postero−medial directed sheet, enclosing a third, short, and slender pointed process.

Quadrate.—The quadrate is a broad and massive element that forms the postero−ventral part of the skull. On its posterior side, it has two deep grooves and it ends in a prominent articular facet. The mandibular condyles are also prominent. The antero−lateral edge of the quadrate is covered by the postero−lateral processes of the squamosal. In lateral view, the quadrate defines the posterior margin of the lower temporal embayment. On the ventral inside of the lower temporal embayment the suture from the quadrate to the basisphenoid and the pterygoid are visible. No quadratojugal is present.

Pterygoid.—The posterior part of the pterygoid is divided by a distinct step into a medial part and a wing−shaped postero−lateral process. The postero−lateral process of the pterygoid contacts the quadrate underneath the posterior squamosal process, where the pterygoid supports the mandibular condyle. A prominent flange separates the medial and lateral regions of the pterygoid. This flange also forms the inner margin of the subtemporal embayment. In the anterior part of the subtemporal embayment the epipterygoid ascends dorsally from the pterygoid. The anterior pterygoids taper towards the skull midline, until they meet the vomers at a level of the anterior margin of the upper temporal openings. Laterally, the pterygoid runs parallel to the palatine.

Palatine.—The palatine is incompletely preserved and strongly compressed medio−laterally. The element appears thin and shiny compared to all the other skull elements. Sutures cannot be confidently assigned. It fills the area between maxilla, anterior pterygoid, and vomer.

Ectopterygoid.—The ectopterygoid ascends dorsally from the anterior half of the pterygoid. Dorsally it clearly broadens, and, together with the palatine, supports the postorbital region. It is directed anteriorly and dorsally supports the

KLEIN AND ALBERS—NEW BASAL NOTHOSAURID FROM MUSCHELKALK 595

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jugal. Posteriorly, a tiny tip contacts the anterior squamosal process. Anteriorly, it also has a connection to the maxilla.

Supraoccipital.—The supraoccipital, the paired exoccipitals, and the basioccipital are still articulated but are displaced ventrally from their original positions. The supraoccipital is still attached to the parietals, as well as to the medial part of the squamosal. Originally, the supraoccipital was oriented horizontally, in alignment with the parietals and the skull roof. The supraoccipital carries a sagittal crest.

Exoccipital.—Due to distortion the exoccipitals are dorsally rather than horizontally oriented. Laterally, the jugular foramina are preserved with the foramen magnum located centrally between them. All preserved foramina are distinctly compressed mediolaterally. The foramen magnum is defined ventrally and laterally by the exoccipitals as well as dorsally by the supraoccipital.

Opisthotic.—The opisthotics are large and are located lateral to the exoccipitals. They are somewhat half−rounded and fill the space between the thin, long ventro−posterior directed processes of the parietals, the squamosal, and the quadrate. Their sutures cannot be confidently assigned. Again, due to the lateral compression of the skull, the opisthotic points backwards.

Basisphenoid.—The basisphenoid is visible within the left lower temporal embayment. It has been uplifted and shifted such that its dorsal surface now faces laterally. However, the basisphenoid is still articulated with the palate and functions as a strong supporting connection between the pterygoid and the basioccipital. Its posterior end bifurcates and covers the basioccipital.

Epipterygoid.—The left and right epipterygoids are visible in the subtemporal embayments. They are broad based and extend antero−dorsally. The epipterygoid is a massive element, bracing the skull roof against the palatine. Its ventral part is expanded antero−posteriorly, forming a kind of massive foot. This “foot” joins a long suture with the posterior part of the pterygoid. The dorsal part of the epipterygoid is thick and rounded, with some thinner lateral expansions. It joins the parietals near a level adjacent to the middle of the upper temporal openings.

Dentition.—The dentition is heterodont, comprising large fangs with smaller teeth in between. The premaxilla carries five fangs, of which the fifth is distinctly the smallest. The maxillary tooth row has teeth that are noticeably smaller, except in positions four and (perhaps) five, which contain larger fangs. The teeth in positions two and three (anterior to the maxillary fangs) are clearly smaller than all other teeth. While only a few teeth are preserved on the maxillae (right side 10, left side four), the alveolar counts suggest at least 15 to 16 teeth positions, as can be determined from the right maxilla. The tooth row extends up to the anterior part of the upper temporal opening. The teeth seem to set in groups of two to three, presumably resulting from a pattern of tooth replacement in tooth rows ( Edmund 1969).

The fangs are conical, pointed and recurved. The enamel surface of the fangs is covered with longitudinal striations. The “normal” teeth are also pointed but not re−curved, and their surface is smooth. The tooth implantation is thecodont. The largest fang has a crown height of 0.375 mm, the smallest hardly 0.05 mm.

Figures

Fig. 1. Basal nothosaurid Nothosaurus winkelhorsti sp. nov., NMNHL RGM 443825 from the Lower Muschelkalk of Winterswijk, in left lateral view. A. Photograph. B. Explanatory drawing.

Fig. 2. Basal nothosaurid Nothosaurus winkelhorsti sp. nov., NMNHL RGM 443825 from the Lower Muschelkalk of Winterswijk, in right lateral view. A. Photograph. B. Explanatory drawing.

Fig. 3. Basal nothosaurid Nothosaurus winkelhorsti sp. nov., NMNHL RGM 443825 from the Lower Muschelkalk of Winterswijk, in dorsal view. A. Photograph. B. Explanatory drawing.