Descriptive anatomy of the largest known specimen of Protoichthyosaurus prostaxalis (Reptilia: Ichthyosauria) including computed tomography and digital reconstruction of a three-dimensional skull

Ichthyosaur fossils are abundant in Lower Jurassic sediments with nine genera found in the UK. In this paper, we describe the partial skeleton of a large ichthyosaur from the Lower Jurassic (lower Sinemurian) of Warwickshire, England, which was conserved and rearticulated to form the centrepiece of a new permanent gallery at the Thinktank, Birmingham Science Museum in 2015. The unusual three-dimensional preservation of the specimen permitted computed tomography (CT) scanning of individual braincase elements as well as the entire reassembled skull. This represents one of the first times that medical imaging and three-dimensional reconstruction methods have been applied to a large skull of a marine reptile. Data from these scans provide new anatomical information, such as the presence of branching vascular canals within the premaxilla and dentary, and an undescribed dorsal (quadrate) wing of the pterygoid hidden within matrix. Scanning also revealed areas of the skull that had been modelled in wood, clay and other materials after the specimen’s initial discovery, highlighting the utility of applying advanced imaging techniques to historical specimens. Additionally, the CT data served as the basis for a new three-dimensional reconstruction of the skull, in which minor damage was repaired and the preserved bones digitally rearticulated. Thus, for the first time a digital reconstruction of the skull and mandible of a large marine reptile skull is available. Museum records show the specimen was originally identified as an example of Ichthyosaurus communis but we identify this specimen as Protoichthyosaurus prostaxalis. The specimen features a skull nearly twice as long as any previously described specimen of P. prostaxalis, representing an individual with an estimated total body length between 3.2 and 4 m.

251 the palate is difficult to discern, although it appears that a maxillary shelf extends medially and 252 replaces the premaxillary shelf at the level of the 18 th preserved tooth on the right side. (The 253 teeth, were reset during conservation and their positions in the jaw are not original. However, 254 their reconstructed positions act as landmarks for our description.) Except at the anterior tip of 255 the snout, the premaxillae do not meet at the ventral midline. 256 In dorsal view, the premaxillae would have contacted each other at a butt joint for much 257 of their length, although they are largely separated due to deformation (Fig. 2E). Posteriorly, the 258 nasals inserted between the premaxillae. The dorsal margin of the left premaxilla laterally and 259 dorsally overlaps the nasal from approximately the level of the 13 th premaxillary tooth to its 260 broken posterior end. In dorsal view, a small, narrow portion of the anterior process of the nasal 261 is exposed; the rest is overlapped by the premaxilla. 262 Anteriorly, the premaxilla is a laterally bowed sheet of bone in transverse cross-section; 263 at the level of the seventh preserved tooth, it develops a medial shelf that roofs the alveolar 264 groove. From this point until its articulation with the maxilla, the premaxilla consists of a ventral 265 lamina that laterally overlaps the teeth, the medial shelf, and a dorsal lamina, which is deeply 266 grooved along its margin (as preserved on the right premaxilla), presumably to receive the nasal. 267 CT scans reveal that each premaxilla encloses a branching, longitudinal canal dorsal to the tooth  (Witmer, 1997). The complex web of ramifications reported in the upper jaw of pliosaurs 285 cannot be visualized in this specimen; this may be due to their absence, preservation or scan 286 resolution. Nonetheless, it is possible these canals were also associated with pressure or electro-287 reception as seen in some extant taxa and as postulated for dinosaurs and pliosaurs (Foffa et al.  Manuscript to be reviewed 296 The alveolar groove of the maxilla is continuous with that of the premaxilla. In transverse 297 section, the anterior maxilla has a ventral lamina that extends lateral to the tooth row, a ventrally 298 curving medial shelf (forming the dorsal and medial walls of the alveolar groove) and a short 299 dorsal lamina that contacts the medial surface of the premaxilla in a scarf joint. The dorsal 300 lamina of the maxilla, which underlaps the premaxilla, is exposed slightly anterior to the middle 301 of the left maxilla due to the damaged premaxilla. Posterior to the main body, the maxilla is 302 triangular in transverse section with a ridge on its dorsomedial surface that appears to articulate 303 with the short anterior process of the lacrimal, which is poorly preserved. An articulation surface 304 on the dorsolateral surface of the posterior process of the maxilla meets the jugal in a scarf joint, 305 separating the posterior process of the maxilla from the lacrimal. 314 CT scans reveal a few short canals penetrating the nasal from its lateral surface.

315
Other fragments of the nasal were found with the specimen but not mounted on the skull 316 due to their fragile nature. Although very fragmentary, much of the right nasal is preserved 317 although the posterior end is missing and it is impossible to determine the presence of an 318 internasal foramen. It is a long and delicate element that is wide posteriorly, and tapers to a point    Postorbital. An original photograph shows that the postorbital was complete, but we have been 356 unable to locate the element (Fig. 1A). However, based on the photograph, it is clear that the   (Fig. 5A, C). Two 372 elongate depressions, one on the posterior aspect of each parietal, may represent attachment sites 373 for epaxial neck muscles (Fig. 5C).

374
In ventral view, the surface of the parietal is concave and bears impressions of structures 375 that surrounded the brain (Fig. 5B, D) Kirton, 2016, plate. 6, figs 1, 2). 416 In dorsal view, the posterior end has three wing-like projections. The medial projection, 417 which is damaged and was originally more extensive, is the largest and most robust, whereas the 418 lateral projection is slender and dorsoventrally flattened (Fig. 5E). The ventral surface is better 419 preserved, although the edge of the interpterygoid vacuity is damaged (Fig. 5F). Regardless, the 420 posterior end of the pterygoid is larger, wider, and narrows more gradually than that of 421 Ichthyosaurus (McGowan, 1973, fig. 20B). The dorsal (quadrate) wing of the posterior ramus of 422 the left pterygoid is almost certainly represented by a large but thin fragment of bone, the shape 423 of which was obscured by a large amount of wood and plaster in the original reconstruction but 424 is revealed in CT scans. 526 disappears and the dentary becomes a laterally bowed sheet of bone with a thickened dorsal 527 margin in transverse section.

528
The anterior tip of the right dentary is damaged and, as a result, the dentaries do not 529 contact each other anteriorly to form the mandibular symphysis (Figs 1, 4). As preserved, the 530 dentary and splenial do not contact each other along their entire length but this is due to 531 distortion. The anterior tip of the angular is level with the 17 th preserved tooth on the right side; 532 the angular laterally overlaps the ventral margin of the dentary in a very tight scarf joint. In 533 contrast, the suture between the dentary and surangular, which reaches the level of the 22 nd 534 preserved dentary tooth, is a loose, horizontal butt joint except at its posterior end where the 535 posterior tip of the dentary laterally overlaps the surangular.   (Figs 1, 4B). The anterior half 555 of the angular is a long, straight rod while the posterior half is both dorsoventrally and 556 mediolaterally expanded, curving dorsally and medially towards the jaw joint. In transverse 557 section, the anterior half of the angular is diamond-shaped with a dorsomedial surface that 558 contacts the ventral margin of the dentary in a tight scarf joint and a dorsolateral surface that 559 meets the ventral margin of the surangular in a loose butt joint. The ventromedial surface of the 560 anterior angular bears a shallow, longitudinal groove bounded dorsally and ventrally by low 561 ridges that presumably articulated with the splenial. Posteriorly, the angular develops a robust 562 tab or lamina that extends from its dorsomedial surface and medially laps the surangular. 563 However, immediately ventral to the jaw joint, this lamina disappears and is replaced by taller, 564 mediolaterally thin dorsolateral lamina that extensively overlaps the lateral aspect of the 565 posterior surangular. Thus, the contact between the angular and surangular is morphologically 566 simple and loose anteriorly but tighter and more complex posteriorly. In lateral view, the anterior 567 end of the surangular is broken and it appears the angular extends further anteriorly than the 568 surangular (Fig. 4B). This is similar to specimen SOMAG 12, a referred specimen of 569 Protoichthyosaurus prostaxalis . 571 Surangular. The surangular is a long, curved element forming the lateral aspect of the posterior 572 third of the lower jaw (Figs 1, 4B). The anterior half of the surangular is poorly preserved as it is 573 mediolaterally thin and is loosely joined to the dentary (dorsally) and angular (ventrally) via 574 rounded butt joints. Posterior to the dentary, the dorsal margin of the surangular thickens 575 dramatically to form the peaked coronoid process. A longitudinal lateral ridge, dorsally bounding 576 the fossa surangularis, continues to the end of the surangular and separates the thickened dorsal 577 margin from the thinner ventral lamina that articulates with the angular. The element expands 578 dorsally and medially at its rounded posterior end to laterally cup the articular.

579
In medial view, the posterior surangular bears a ridge parallel to its ventral margin that   The posterior margin is sharp while the anterior aspect is flat and broad where it contacts the 591 quadrate to form the jaw joint. The medial aspect of the bone is smooth while the lateral aspect is 592 pitted and porous. CT scans reveal several small, short canals that penetrate into the bone from