Figs 1A, 2D, 4D, 5 A–B
Belonostomus acutus Agassiz, 1844: 142, pl. 47a, fig. 4.
Acidorhynchus brevirostris Stensiö, 1925, partim: 176.
Acidorhynchus brevirostris – Hauff 1938, partim: pl. 22, fig. d. — Wenz 1967: pl. II, fig. A.
Anterior narial opening narrow and elongate; maxilla strongly dorsoventrally compressed and ventrally deflected suborbitally such that the contact with the premaxilla occurs at an angle; posterior dorsal skull parallel to the long axis of the skull; postorbital segment approximately equal in length to the maximum depth of the lower jaw; parasphenoid edentulous ventral to the orbit; foramen for the internal carotid and efferent pseudobranchial arteries centered within the basisphenoid (sensu Wenz 1967); lateral extrascapular fused to lateral dermopterotic and not extending dorsal to the hyomandibula; posterior edge of the mandible strongly sinusoidal; angle between the posterior and ventral edges of the mandible less than 80 degrees; subnarial laniary dentition absent; acrodin caps of the posterior laniaries straight.
Holotype
UNITED KINGDOM: Toarcian of Whitby ( NHMUK PV P 4268, Fig. 1A).
Referred material
GERMANY: all referred material consists of isolated skulls. Bisingen ( SMNS 56923, Fig. 5A); Holzmaden ( SMNS 57039 ( Fig. 5B), SMNS 88007); Holzmaden, εII 3 ( PMU 30009); Holzmaden, εII 4 ( SMNS 50924, MHH 17); Holzmaden, εII 6 ( SMNS 55324, PMU 30010); Ohmden ( NHMUK PV P 3792, NHMUK PV P 36222, NHMUK PV P 36223, SMNS 55319, SMNS 87738, GPIT /OS/133); Ohmden, εII 3 ( SMNS 51009); Ohmden, εII 4 ( SMNS 50268); Ohmden, εII 5 ( SMNS 96927, SMNS 96927); Schandelah, serpentinum Zone ( GZG.V.27932).
Saurorhynchus acutus is anatomically very similar to the Toarcian S. hauffi sp. nov., described in detail in a subsequent section. The following brief description is designed only to differentiate the two species, to clarify general osteological nomenclature within Saurorhynchus, and to describe parts of the anatomy not preserved in S. hauffi sp. nov.
No well-preserved postcranial material is available for Saurorhynchus acutus. Skull length (tip of rostrum to jaw joint, and corresponding to the length of the dermatocranium when measured along the dorsal midline) of the largest referred specimen is 125 mm ( Appendix 1). Assuming similar proportions to S. hauffi sp. nov., fork length is estimated at 44 cm for large individuals of S. acutus. All the skulls examined in the course of this study were severely compressed, making the cranial sutures difficult to differentiate.
There is no clear evidence for interrostral bones. The anterior narial opening is an elongate anteroventrally inclined slit. When the skull is viewed dorsolaterally, it is evident based on the orientation and lipping along the anterior external narial opening that it is functionally situated more dorsally than the posterior narial opening. The suborbital bar of the maxilla is strongly dorsoventrally compressed ventral to the orbit and is also ventrally deflected such that it meets the rostropremaxilla at an angle ventral to the external narial opening. This creates the impression of proportionately large, round orbits. The suture between the maxilla and premaxilla is interdigitating, differing from some of the Toarcian material of Saurorhynchus from France and Northern Germany ( Wenz 1967; Thies 1985). In some specimens, the ornamentation on the skull roof is drastically reduced and simplified, consisting mainly of pitting and being slightly more reticular only in the region of the parietals (e.g., Fig. 5B).
Unlike in S. brevirostris, the dorsal skull roof extends posteriorly without strong dorsal deflection. The element sutured to the posterolateral dermopterotic has been documented in Saurorhynchus by several authors, and has been variously interpreted as the squamosal ( Reis 1892), fused suprascapularsupracleithrum ( Gardiner 1960), or extrascapular ( Hauff 1938). It does not participate in the cranial midline, and extends slightly further posteriorly than the dermopterotic. We interpret this element as a lateral extrascapular, following the discussion about the homology in Kogan & Romano (2016). The posterior elongation of the skull roof in S. acutus relative to S. hauffi sp. nov. is partly accomplished through the posterior displacement of the lateral extrascapular relative to the dermopterotic ( Fig. 5B, D). The lateral extrascapular and the dermopterotic are fused, such that it is difficult to see the suture along parts of its length. On the ventral surface of the dermal skull roof, two ridges run longitudinally between the orbits, dividing the interorbital surface into thirds. On the lateral side of the ridges, numerous foramina are located between the ridge and the ventral surface of the frontals. Such ridges have been interpreted as part of the neurocranium in the orbitotemporal region ( Maxwell et al. 2016). The space between these ridges is filled with broken, cancellous bone tissue.
The palate is exposed in a single specimen (SMNS 50924). The dermopalatine extends as far anteriorly as the posterior external narial opening. It is convex in ventral view, broadening in a medial direction. It bears denticles; these are smaller and blunter than those of the other tooth-bearing elements. Anteriorly, it articulates with the maxilla laterally; posteriorly it articulates with the parasphenoid medially and, posterior to this, the entopterygoid. There is no lateral palatal foramen, but because the parasphenoid is not anteriorly expanded, a medial foramen exists between the anterior dermopalatine and parasphenoid. The ectopterygoid is very small and triangular. The portion of the parasphenoid posterior to the anterior edge of the orbit is edentulous; more anteriorly it bears small denticles.
The anteriormost point of the angular is located on the ventral half of the lower jaw. The ventral part of the angular-dentary suture is posteroventrally angled and straight, forming a simple ‘v’. The teeth are largest at the midpoint of the rostrum and become much smaller anteriorly. In no specimens was corrugation of the collar ganoine observed.
The opercle is preserved in a single specimen ( PMU 30010). This element is 13.6 mm high and 13.9 mm long, and ornamented with elongate pits radiating from the palatoquadrate articulation. In overall shape it is more ovate than in Saurorhynchus brevirostris.
Woodward (1895) outlined a series of characters to diagnose S. acutus, the majority of which cannot be observed in the type material and were drawn from observation of specimens from the much older Charmouth Mudstone Formation. Upon re-examination of the type material, Woodward (1899) clarified that the posterior (backwards) extension of the skull roof was the main feature uniting the Toarcian type and Sinemurian referred specimens, even though he had reservations that a species was likely to have such a long stratigraphic range.
Our observations suggest that the type specimen of S. acutus can be unambiguously differentiated from the older Sinemurian material by the short distance between the posterior edge of the orbit and the articular facet for the hyomandibula relative to the length of the orbit. These measurements fall along a single anteroposterior line, and thus are not expected to be significantly influenced by distortion. In addition, the narial opening in the holotype specimen of S. acutus is narrower than in the material from the Charmouth Mudstone. The syntype specimen figured by Agassiz (1844), NHMUK PV P 961a and its (unfigured) counterslab NHMUK PV P 36219 are non-diagnostic below family level. The holotype is consistent with a large number of specimens of Saurorhynchus from the Toarcian of Germany, and when the additional anatomical information present in this referred material is considered it becomes clear that large differences exist between the Sinemurian and Toarcian specimens. The revised diagnosis is constructed based on the referred material from Germany, as well as on the holotype.
Early Jurassic, Toarcian, Whitby ( type locality); Toarcian, tenuicostatum Zone, semicelatum Subzone to serpentinum Zone, Baden-Württemberg, Germany; serpentinum Zone, Lower Saxony, Germany (referred material).