Orthocormus sp.
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publication ID |
https://doi.org/ 10.4202/app.00749.2020 |
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persistent identifier |
https://treatment.plazi.org/id/E94C87EE-F34F-482E-FCA5-FE1786EFFEB5 |
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treatment provided by |
Felipe |
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scientific name |
Orthocormus sp. |
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Material.— GPIT /OS/1302, skull and anteriormost postcranium in right lateral view, from the Late Jurassic of Nusplingen, Germany.
Description.— General appearance: This skull ( GPIT /OS/ 1302), from a large fish (SL estimated at ~ 1.1 m), was partially figured by Quenstedt (1856 –1857: pl. 97: 12, 1885: fig. 105) as Strobilodus giganteus and was later redescribed by Heineke (1906) as Hypsocormus macrodon following Wagner (1863). Lambers (1992) was unable to locate the material, but referred it to Pseudoasthenocormus retrodorsalis based on Heineke’s description and photograph. We located the specimen in the collections of the GPIT ( Fig. 9 View Fig ), and re-evaluated its morphology and affinities. Our description emphasizes details not adequately discussed by Heineke (1906).
The rostrodermethmoid forms two distinct processes in lateral view, separated by the antorbital: a dorsomedial portion, and a ventrolateral portion, forming a ~55° angle, making the rostrodermethmoid relatively deep rather than acutely pointed ( Fig. 9A View Fig 1 View Fig ). Anteriorly, the rostrodermethmoid forms an edentulous presymphyseal rostrum, which is flattened dorsally, and ventrally is offset from the oral margin. The dorsomedial rostrodermethmoid is strongly ornamented with tubercles. In lateral view, a large paramedial tooth is visible near the ventrolateral edge of the rostrodermethmoid.
The maxilla and suborbital fragment are as described by Heineke (1906). The premaxilla has been displaced, and is preserved ventral and medial to the maxilla. It is elongated in morphology with tubercular lateral ornamentation. The premaxilla bears two mid-sized posterior teeth and smaller anterior teeth.
The antorbital is anteroposteriorly elongate, situated between the dorsomedial and ventrolateral rami of the rostrodermethmoid anteriorly, and more posteriorly between the premaxilla and nasal ( Figs. 9 View Fig , 10A View Fig 6 View Fig ). The posterior edge contacts the lateral ethmoid. The antorbital lacks pronounced ornamentation.
The nasal is positioned lateral to the dorsomedial rostrodermethmoid on the skull roof. Posteromedially, it articulates with the frontal. The anterodorsal edge of the orbit is damaged and the dermosphenotic is not preserved, so it is unclear whether the nasal was excluded from the edge of the orbit.
The posterior parietal is tented upwards to form the anterior portion of the temporal boss. The posterior edges of the parietals are greatly thickened and laterally divergent, indicating a well-developed temporal boss ( Gouiric-Cavalli and Cione 2015), represented by several dermal bone fragments posterior to the parietals.
The braincase is not well-preserved, but some details can be discussed ( Fig. 10A View Fig 1 View Fig , A 2 View Fig ). The lateral ethmoid is highly asymmetrical along its anteroposterior axis, with the dorsal process strongly inclined posteriorly and expanded, forming a posteriorly elongate articulation with the ventral surface of the skull roof. The sphenotic is present anteriorly, largely covered by the dermal skull. Slightly more detail can be added regarding the posterior braincase. The parasphenoid remains in articulation, underlying the basi-exoccipital. The latter bears a prominent posterior semicircular extension that may represent the fusion of a posteroventral arch element. The intercalar is small and externally concave, positioned posteroventral to the opisthotic ( Fig. 10A View Fig 1 View Fig , A 2 View Fig ). Ventrally, the intercalar forms the dorsal edge of the vagus canal. The element or fragment ventral to the intercalar is of uncertain identity. The opisthotic itself is relatively poorly preserved. It is divided into dorsal and ventral portions, separated by a ridge.
The anterior edge of the quadrate forms an obtuse angle ca. 110°), and its lateral surface bears a prominent median spine ( Fig. 9 View Fig ). An overlying element is interpreted as an anteriorly displaced symplectic. The symplectic is approximately rectangular, with a weakly concave ventral edge. The lateral surface is grooved, and the anterior end is thickened and convex. The hyomandibula is slightly anteriorly inclined, forming a ~50° angle with the long axis of the lower jaw. A large opercular process is present.
The dentary, angular, and surangular form the lateral lower jaw ( Fig. 9 View Fig ). The angular is high posteriorly, and anteriorly elongated, forming a slightly concave suture with the dentary. The surangular is very small, restricted to the posterodorsal corner of the lower jaw. The mandibular sensory canal is positioned close to the ventral edge of the lower jaw, and only begins to curve dorsally near the anterior tip. The dentary bears the remnants of a large procumbent and laterally compressed tooth on its anterior edge, followed posteriorly by four to five very small teeth ( Fig. 10A View Fig 3 View Fig ); the other dentary teeth are moderate in size. The anterior dentary bears a groove on its lateral surface, extending from the oral margin ventrally. An inflated coronoid bearing a large tooth is present medial to the reduced dentary teeth ( Fig. 10A View Fig 3 View Fig ). The teeth, including the procumbent and paramedial fangs, are laterally compressed, and are oval in cross-section.
The vertebral column is aspondylous; however, neural arches and ribs are ossified. The scales, while not well preserved, are relatively large: up to 4 mm in diameter, and appear to lack ganoine. The size of the scales relative to the vertebral column is difficult to assess, but is estimated at 2:1.
The first pectoral ray is more strongly ornamented and shorter than the successive rays, and is tightly fused to the second ray, forming a compound element ( Fig. 10A View Fig 4 View Fig , A 5 View Fig ). Comparison of the right fin in dorsal view and the left fin in ventral view indicates strong asymmetry in the development of the compound ray (1+2), with ornamentation being much better developed on the dorsal surface ( Fig. 10A View Fig 4 View Fig ), and the first dorsal hemilepidotrichium being much broader than the first ventral hemilepidotrichium ( Fig. 10A View Fig 5 View Fig ). These observations suggest that the first “ray” is probably an unpaired, asymmetrical bony splint (see e.g., Arratia 2008: fig. 28) rather than a lepidotrichium, and the compound ray is formed by the fusion of the splint to the hemilepidotrichia of the first lepidotrichium.
Remarks.— GPIT /OS/1302 is a pachycormid based on the presence of a toothed rostrodermethmoid separating the premaxillae ( Liston 2008). The robust mandible and dentition, in particular the presence of large, paramedial teeth on the rostrodermethmoid, suggest GPIT /OS/1302 is most consistent with the clade of pachycormids containing Hypsocormus , Simocormus , and macrocarnivorous forms, and likely also Sauropsis and Pseudoasthenocormus ( Liston 2008; Friedman et al. 2010).
GPIT /OS/1302 differs from Hypsocormus , Simocormus , and Sauropsis in the presence of an edentulous projection of the rostrodermethmoid anterior to the mandibular symphysis. Lambers (1992) suggested the referral of this specimen to Pseudoasthenocormus based on the presymphyseal extension of the rostrodermethmoid, large angle formed between the upper jaw and the dorsal skull roof, and relatively slender fin rays. However, re-evaluation of the material indicates that there are many inconsistencies with Pseudoasthenocormus . Specifically, GPIT /OS/1302 differs from P. retrodorsalis in that (i) the anterior rostrodermethmoid is edentulous; (ii) the premaxilla is elongated relative to its depth; (iii) the antorbital portion of the dermatocranium is as long as the postorbital segment; (iv) the hyomandibula is much less steeply inclined; (v) the mandible and mandibular dentition is much more robust and the teeth are laterally compressed; (vi) the angular is reduced in lateral view relative to the dentary; (vii) mandibular ornamentation is much less prominent; (viii) the first pectoral fin ray is shorter than more posterior rays, with distinctive ornamentation, and is fused to the anterior surface of the second ray; (ix) the pectoral fin rays are more robust; and (x) the scales are smaller than those of Pseudoasthenocormus . Based on these differences, GPIT / OS/1302 is inconsistent with Pseudoasthenocormus .
The only Late Jurassic genus with an edentulous presymphyseal projection of the rostrodermethmoid, as in GPIT /OS/1302, is Orthocormus . In addition to this character, GPIT /OS/1302 shares the elongate premaxilla, long antorbital region, laterally compressed teeth, robust mandible and robust and fused pectoral fin rays with Orthocormus (see Lambers 1988). Orthocormus? tenuirostris , from the Callovian of the UK, also has a pronounced edentulous rostrum, but numerous osteological differences separate it from GPIT /OS/1302, including the shape of the premaxilla and the tubercular ornamentation completely covering the lateral surface of the lower jaw ( Woodward 1895).
Orthocormus currently contains three named Late Jurassic species: O. cornutus , O. teyleri , and O. roeperi (see Table 1). O. roeperi and O. teyleri are both relatively small fishes (<55 cm SL), whereas O. cornutus is much larger ~ 1.1 m SL); the ontogenetic status of material referred to the two smaller species is unknown. The species are differentiated primarily based on dentition and postcranial meristics. In addition, O. roeperi differs from the two other species in lacking an anterior projection of the temporal boss ( Arratia and Schultze 2013), and thus is inconsistent with GPIT /OS/1302. However, GPIT /OS/1302 cannot easily be referred to either of the other species, as both O. teyleri and O. cornutus have minute scales in a 4:1 ratio with the axial skeleton, whereas the scales in GPIT /OS/1302 appear to be considerably larger. Thus, while GPIT /OS/1302 shares several key apomorphies with Orthocormus spp. , we cannot assign this specimen to species.
A braincase referred to Orthocormus sp. , NRM P425, differs from GPIT /OS/ 1302 in a few respects ( Holmgren and Stensiö 1936; Rayner 1948). The braincase of GPIT /OS/1302 is proportionately much deeper, a feature possibly exacerbated by lateral compression. However, the position of the vagus foramen ventral to the intercalar is not shared with NRM P425. The opisthotic bears a notch in GPIT /OS/ 1302 in a similar position to the vagus foramen in NRM P425, but preservation is insufficient to assess whether this is a depression, foramen, or damage. The more posterior foramen ventral to the intercalar is absent from NRM P425 ( Holmgren and Stensiö 1936; Rayner 1948). GPIT /OS/1302 and Orthocormus sp. share the shape of the braincase in lateral view (reconstruction of Rayner 1948), in which the sphenotic and posterior edge of the braincase are roughly parallel, the posterior edge of the braincase is straight with convex projections, rather than concave, and projects posteriorly substantially beyond the posterior edge of the dorsal skull roof.
The braincases of both GPIT /OS/00836 ( Hypsocormus posterodorsalis sp. nov.) and GPIT /OS/1302 ( Orthocormus sp. ) are preserved in a similar way, permitting direct comparisons between the posterior braincases of these two genera. Both show the unexpected position of the vagus foramen ventral to the intercalar, unlike in other published descriptions. The state of preservation of both of these crania raises the possibility that the interpretation presented here may be incorrect. Both also share a well-developed opisthotic, larger than the intercalar, as do other described pachycormid braincases ( Rayner 1948; Mainwaring 1978). The two differ in some specific details: the braincase of GPIT /OS/1302 projects posteriorly further beyond the posterior edge of the dermatocranium, and lacks the concave posterior edge of the basi-exoccipital seen in H. posterodorsalis . The suture between the intercalar and opisthotic is essentially vertically oriented in Orthocormus ( GPIT / OS/1302, Rayner 1948), whereas in H. posterodorsalis it is strongly posterodorsally-anteroventrally inclined. The significance of these observations across genera requires less crushed material.
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