Gadusaurus aqualigneus, Sousa & Roldão & Ríos & Puértolas-Pascual, 2025

Gadusaurus aqualigneus sp. nov.

Zoobank LSID: urn:lsid:zoobank.org:act:FB6DFDD0-E0D8-4FC5-B2

A4-9D319F5D6B07.

Etymology: From Latin aqua, water and lignum, wood; in reference to Água de Madeiros, which roughly translates to “water of woods”, the beach where the specimen was found.

Holotype: ML 2750 , nearly complete skull ( Fig. 2 View Fig ), in a flattened state,

the right side fully observable in lateral view, and elements of the left side observable in dorsal view.

Type locality: Praia de Água de Madeiros , São Pedro de Moel , Municipality of Marinha Grande, ( Portugal). Coordinates 39°44’28.20”N ,

°2’20.92”W.

Type horizon: Upper part of the Polvoeira Member of the Água de Madeiros Formation, upper Sinemurian (Lower Jurassic).

Diagnosis.—Small-sized ichthyosaur, with an estimated length of approximately 2 m, diagnosed by the following combination of ambiguous synapomorphies: absence of a supranarial process in the premaxilla; and the parietal foramen being entirely surrounded by the frontals. Gadusaurus aqualigneus gen. et sp. nov. is also characterized by the following potential autapomorphies within Parvipelvia: presence of a subcircular depression in the lacrimal just below the posterior region of the external naris ( Fig. 3C View Fig ); big and well-developed elliptical internasal foramen with about 5 times bigger surface area than that of the parietal foramen ( Fig. 3A View Fig ); presence of a well-marked longitudinal groove on each side of the anterolateral region of the excavatio internasalis ( Fig 3B View Fig ).

Description.—The specimen comprises a nearly complete, flattened cranium, approximately 35.2 cm in length ( Table 1) with most of the posterior elements up to the posterior edge of the orbit being absent ( Fig. 2 View Fig ). There are some disarticulated posterior elements observable, but they are too damaged to identify. The right side of the skull is fully visible in lateral view while only some elements of the left side of the rostrum and forehead are observable, the latter in dorsal view due to the flattened state of the specimen. This flattened state has also caused elements from dorsal edge of the orbit to fold into the orbital area.

Premaxilla: The right premaxilla is straight and narrows from the base to the tip of the rostrum. The supranarial process is absent, while the subnarial process is over half the length of the naris. Striations (longitudinal grooves) are observable along the tip of the premaxilla. A groove is present in the contact with the maxilla, growing increasingly shallower from the posterior to the anterior end. The left premaxilla is disarticulated and very narrow in dorsal view.

Maxilla: The left maxilla is not visible in this specimen. The right maxilla bears teeth and is reduced in size, having less than half the length of the rostrum. The bone reaches its maximum dorsoventral thickness under the external naris region, while both the premaxillary and the jugal processes of the maxilla become narrower and wedged, ending in an anterior and posterior tip respectively. Both processes extend beyond the anterior and posterior margins of the external naris, but this extension is no longer than the narial length. There is no contact with either the naris or the prefrontal since the lacrimal separates them widely. There is no ascending process (= dorsal lamina, Motani 1999; = processus postnarialis, Maisch and Matzke 2000; = postnarial process, Maxwell and Cortés 2020) posterior to the narial opening. Anteriorly it is overlapped by the premaxilla while posteriorly it contacts the jugal and the lacrimal. The jugal process of the maxilla is moderately elongated, however it does not extend to the anterior orbital margin and is considerably shorter than the suborbital process of the lacrimal. Slight striations are visible along the contacts with the premaxilla and the lacrimal.

Nasal: Both nasals are observable. There is a slight depression on the posterior end of the nasals, more observable on the left dorsal view. This is identified as the excavatio internasalis. Within the excavatio internasalis and dorsal to the naris, there is a visible wide elliptical gap between the nasals which has been identified as the internasal foramen. Both nasals are slightly separated by taphonomy and the foramen would not be as wide as it appears. If we consider them sagittally united, this foramen would still be quite large, having a surface area about 5 times larger than the parietal foramen. Considering the two nasals united sagittally, the internasal foramen would be completely surrounded by the nasals, with no involvement of the frontal bone, although it almost contacts the posterior margin of the foramen. On both nasals, next to the anterolateral margins of the internasal foramen, two parallel longitudinal elliptical grooves are observable ( Fig. 3C View Fig ). The left nasal, observable in a dorsal view due to the flattened state of the skull, has reduced anteroposterior exposure, comprising less than half the length of the prenarial rostrum, while its contact with the left premaxilla is disarticulated. The right nasal, visible in lateral view, reaches back over the orbit. It borders the naris dorsally, and has extensive contact with the postfrontal, separating the prefrontal from the frontal entirely. The difference in anterior extent between the left and right nasal prevents us from accurately determining the real anterior extension of the nasals along the snout. However, the well-articulated suture of the right nasal with the premaxilla seems to indicate that its anteroposterior extension would be short (less than 40% of the prenarial rostrum).

Lacrimal: Due to the flattened state of the specimen, the lacrimal area is very fractured, especially on the anterior end, and details of its anatomy are difficult to discern. The lacrimal contributes to the external naris anteriorly. The external naris is elongated and laterally oriented. There is a small anteroposteriorly oriented elliptical foramen that forms an arch of bone with the posteroventral margin of the external naris. The presence of an arc of bone in the narial region has also been observed in Ichthyosaurus somersetensis Lomax & Massare, 2017 (Massare et al. 2021).

Jugal: The jugal is fractured, with the posterior half being absent, and appears to have a broad anterior margin, covering the posterior end of the maxilla, which ends posterior to the anterior margin of the lacrimal, without reaching the external naris.

Prefrontal: The left prefrontal has some dorsal exposure, mainly on its contact with the nasal, but limited by the anterior process of the postfrontal. The right prefrontal comprises the anterodorsal side of orbit, and is kept from reaching the frontal by the nasal. There is no observable contact with the posterior side of the external naris.

Frontal: While both frontals are observable, only the left frontal has any significant exposure and appears to be dorsally flat, and more exposed at the nasal suture. The posterior-most region of the frontals appears to be eroded, so its relationship with the parietals and supratemporal fenestrae cannot be determined. There is a small elliptical parietal foramen on the posterior end of the frontals ( Fig 3A View Fig ). The foramen seems to be surrounded by the frontals without contacting the parietals. Next to the posterolateral margins of the foramen there are two small ridges. The frontals have a fusiform shape: starting narrow posteriorly, widening until they reach their maximum convexity at the level of the anterior edge of the parietal foramen, and becoming wedged between the nasals anteriorly. The contact with the nasals appears to be slightly disarticulated, forming a deep ridge and a medial groove leading towards the posterior margin of the internasal foramen.

Orbit: The orbit has a regularly rounded anterior shape, with its posterior margin not being preserved. Due to the flattened state of the skull, part of the pterygoid, as well as what is tentatively identified as the vomer, can be observed ventrally under the orbit. The sclerotic ring is quite fractured, and almost the entire posterior region is missing. However, at least 14 ossicles have been preserved. Applying the principle of symmetry to the ring, we can estimate that it originally consisted of approximately 18 to 20 ossicles, a relatively high number for Ichthyosauria. The size of the orbit and the sclerotic ring is similar; therefore, it would fill almost the entire orbital cavity. Although the orbit is of considerable size, there does not appear to be a marked dorsal inflection at the anterior orbital margin with the snout. Instead, this anterior narrowing is progressive.

Lower jaw: The lower jaw is well developed relative to the upper jaw, both in length and in cross-section. It has a straight ventral margin, and the dentary fossa is visible running across the length of the dentary, which has no discernible labial shelf. The surangular is visible in lateral view on the posterior end of the jaw, but it is fractured and lacking it posterior half. The angular is also visible, but it has almost no lateral exposure compared to the surangular.

Dentition: The dentition is fairly complete and well-developed, with aulacodont implantation and no discernible bony fixation. 88 teeth are observable laterally in the right side of the skull, displayed in a single row in each jaw, with a height range of 4 to 15 mm ( Table 1). Eleven are observable dorsally at the edges of the left premaxilla. The teeth are conical and round in cross-section, with a well-defined base for the enamel layer, which is thin and smooth. At the root of the teeth, there are observable grooves.

Stratigraphic and geographic range.— Type horizon and locality only.