Krabisuchus

Krabisuchus is an extinct genus of alligatoroid crocodylian that lived in what is now Thailand during the Late Eocene. It was first named by paleontologists Jeremy A. Martin and Komsorn Lauprasert in 2010, and the type species is K. siamogallicus. While originally interpreted as a relative of Allognathosuchus, later studies placed Krabisuchus in the clade Orientalosuchina, an enigmatic group of crocodilians from the Cretaceous to Paleogene of Asia with disputed affinities. Fossils have been found from the Krabi Basin of southern Thailand and include mostly cranial and mandibular elements as well as some postcranial remains. Based on these remains Krabisuchus was a relatively small animal with a short and blunt snout. During the Eocene the Krabi Basin was likely covered by dense tropical forest and featured bodies of freshwater like ponds, marshes and swamps that were home to crocodiles and turtles. It has been speculated that Krabisuchus might have been more terrestrial than modern alligators.

The fossil remains of Krabisuchus were first discovered by the Electricity Generating Authority of Thailand in three lignite pits within the Krabi Basin of Thailand, with the corresponding sediments likely dating to the Late Eocene. The material was recovered over the course of several field trips between 1989 and 1994 by a collaboration of French and Thai scientists under Varavudh Suteethorn and Eric Buffetaut. Given the overall morphology, the crocodilian fossils were initially thought to represent the genus Allognathosuchus, which at the time was known from Europe and North America. However, since then the status of several species referred to Allognathosuchus has come under scrutiny, with the genus commonly thought to be paraphyletic. This not only affected species that had previously been given distinct genera names that could be resurrected, namely Hassiacosuchus and Navajosuchus, but also the material from the Krabi Basin. Consequently, in 2010 a study was published by Jeremy E. Martin and Komsorn Lauprasert recognizing the Krabi Basin alligatoroid as a distinct genus, which the team named Krabisuchus siamogallicus. As they holotype they chose a complete skull with an attached lower jaw, with several more skull, lower jaw and even postcranial remains all referred to the species.

The genus name of Krabisuchus combines the name of its place of origin, Thailand's Krabi Basin, with the Egyptian "souchos" which means crocodile. The species name meanwhile is composed "Siam" and "Gallicus", both of which are old names for Thailand and France respectively. This means to reference the fact that collaboration between paleontologists from these two countries eventually lead to the description of this animal.

Like other orientalosuchins, Krabisuchus had a short and blunt snout, with the rostrum itself being approximately as long as the back of the head beginning with the eyesockets. The skull is furthermore described as being relatively deep, a condition known as altirostry that is also seen in Arambourgia, Procaimanoidea and Hassiacosuchus. The nares are located at the tip of the snout in typical crocodilian fashion, though they are described as facing much more towards the front rather than just upward.

The nares are surrounded almost entirely by the premaxillae, which form the very tip of the snout. The premaxillae are longer than they are wide and are sloping towards the front of the snout, which contributes to the anterior positioning of the nares. Towards the back the premaxillae meet the maxillae, forming a rather long dorsal process that extends back until the level of the third maxillary tooth. The transition from the premaxilla to the maxilla on the outer side of the snout is marked by the presence of a prominent notch that is also seen in other orientalosuchins as well as members of the genus Crocodylus. Typically, this notch would serve to receive an enlarged dentary tooth, though in the case of Krabisuchus it would appear that such a tooth would instead slide into a pit in the underside of the snout. Another prominent constriction is present behind the fifth maxillary tooth, contributing to the sinuous appearance of the element known as festooning.

The nasal bones are described as being very wide in Krabisuchus, making up an entire third of the maximum snout width. Towards the front of the skull the nasals actually form a process that reaches into the nares, a trait also seen in other orientalosuchins and in a more pronounced manner in modern alligators. Given that the preserved skulls of Krabisuchus are generally crushed, the point of contact between the nasals with the maxillae to either side is not entirely clear. Martin and Lauprasert described that the lacrimal largely prevents the two bones from meeting except for what might be a very narrow region towards the front of the snout. Consequently, in this interpretation the contact between the nasals and the lacrimals would have been especially long, stretching along 2/3rds of the length of the nasals. However, Tobias Massonne and his team later argued that what were initially interpreted as sutures between skull bones actually represented pre-orbital ridges similar to those seen in Orientalosuchus and modern saltwater crocodiles. Towards the back the elongated and tapering prefrontals insert themselves in-between the nasals and the lacrimals, and the frontal bone forms a process that extends in-between the nasals and connects the snout with the skull table.

The eyes of Krabisuchus were large and the small skull table is located much higher than the snout. This gives the skull an altirostral appearance similar to Arambourgia and Procaimanoidea. After forming the medial margins of the eyesockets, the frontal bone enters the skull table and comes into contact with the two postorbital bones and the singular parietal. The contact with the latter is described as concavoconvex and located before the supratemporal fenestrae, meaning that the frontal did not actually contribute to the edge of these openings. The postorbitals form the front corners of the skull, the back of the eyesockets and the upper part of the postorbital bar that separates the eyesockets from the infratemporal fenestrae located on the side of the skull. The squamosals attach behind the postorbitals and are elongated, ultimately making up 2/3rds of the skull table's length. Unlike in Procaimanoidea and Arambourgia, the outer edges of the squamosals run almost parallel to each other, meaning that the structure as a whole is more rectangular than trapezoid. While the squamosals form the outer edges, the parietal is the primary element of the central skull table, filling the space between the two supratemporal fenestrae. Though constricted between the openings, the width of the parietal in this region is still around the same as the space between the eyesockets, if slightly broader. The surface of the parietal varies, being raised at the very edge of the fenestrae, flat between and slightly depressed behind them near the contact with the squamosals. As in other orientalosuchins, the supraoccipital is prominently exposed on the skull table, making up the very back of the element, but does not prevent the parietal from reaching the end of the skull table as in Eurycephalosuchus or Orientalosuchus.

The jugal bone stretches from the maxilla and lacrimal in the front all the way back to the quadratojugal, contributing to the lower edge of the eyesocket and infratemporal fenestra in the process. The jugal also forms the lower part of the postorbital bar, an inset structure that separates the eyesocket and infratemporal fenestra. It has been noted that in Krabisuchus the jugal's contribution to this element is quite substantial, extending fairly high up before coming in contact with the postorbital. The jugal bears a low rim that runs across its upper surface before disappearing roughly at the level of the postorbital bar. Additionally, it's at this point that the jugal as a whole becomes more rod-like in structure, leading into the quadratojugal which contributes the rest of the infratemporal fenestra's margins and runs back along the edge of the skull in contact with the quadrate bone. The quadrate features a lateral (outer) and medial (inner) hemicondyle, with the outer hemicondyle being the larger of the two. Its surface also features a boss that receives the paroccipital process and a medially located foramen aërum.