The skull of Tanystropheus hydroides is broader and flatter than that of T. longobardicus. The first five of six teeth in the premaxilla are very large and fang-like, forming an interlocking “fish trap” similar to Dinocephalosaurus and many sauropterygians such as plesiosaurs and nothosaurs. All teeth in the skull have a single cusp which is sharp, curved, and unserrated. They have an oval-shaped cross section and shallow subthecodont implantation. Like T. longobardicus, T. hydroides has a single central narial opening. Unlike T. longobardicus, T. hydroides has a nearly vertical rear edge of the premaxilla, without a postnarial process. The maxilla is low, with a large and rectangular front portion. There is a perforation near the front of the bone, which would have been penetrated by the tenth dentary tooth when the mouth was closed. Towards the rear, the maxilla develops into a concave edge overlooking a long and slender posterior process (rear branch) that projects under the rounded orbit. There are 15 teeth in the maxilla, increasing in size up to the eighth tooth, which is about as large as the premaxillary fangs. T.hydroides is not known to possess a septomaxilla, a neomorphic bone at the rear tip of the naris in some reptiles. The nasals are broad and plate-like, with a depressed central portion. The lacrimal and prefrontal, though incompletely known, were likely similar to those of T. longobardicus. T. hydroides has a particularly large nasolacrimal duct, a tubular channel opening out of the rear of the lacrimal. The frontals are quite wide and form much of the upper edge of the orbit, a condition akin to T. longobardicus. However, the paired frontals meet along a straight suture with a low ridge on the lower (internal) surface, in contrast to T. longobardicus, where the frontals meet at an interdigitating suture with a broad furrow on the underside.right|thumb|The flattened single-cusped skull of PIMUZ T 2819, a large morphotype (T. hydroides) specimenThe parietals are strongly modified in T. hydroides. They are fused into a single X-shaped bone, somewhat resembling the parietals of erythrosuchids. This shape may have resulted from fusion between the parietals’ anterolateral processes (front branches) and the postfrontals, which are separate bones in T. longobardicus but not apparent in T. hydroides. A prominent pineal foramen is positioned near the straight contact with the frontals, one of the few similarities with T. longobardicus. Strong supratemporal fossae excavate into the outer edge of the parietal and define a low sagittal crest along the midline of the skull. This trend is shared with other large archosauromorphs, like Dinocephalosaurus and Azendohsaurus. The supratemporal fenestrae (upper skull holes behind the eye) are wide and roughly triangular, exposed almost entirely from above. The postorbital has large and blocky ventral and medial processes (lower and inward branches), which meet at a sharper angle than in any other early archosauromorph. The jugal, conversely, is basically indistinguishable from that of T. longobardicus. The squamosal is deep and rectangular when viewed from the side, with little differentiation between the tall suture with the postorbital and the small suture with the quadratojugal. As a result, most of the posterior skull is clustered together, and the infratemporal fenestra is reduced to a small diagonal hole. The quadratojugal is a curved sliver which twists back alongside the quadrate. Relative to T. longobardicus, the quadrate has a larger pterygoid ramus and a strongly hooked rearward projection at its upper extent.
