Trilophosuchus ("triple crest crocodile") is an extinct genus of mekosuchine crocodilian from Australia. Its fossils have been found at the Ringtail Site in the Riversleigh World Heritage Area (north-western Queensland) and date to the Miocene epoch. Additional remains have also been found at the older Hiatus Site and extend its range into the Oligocene. Like the closely related Mekosuchus, it is thought to have had a short and blunt snout and large eyes that generally resembles today's dwarf crocodiles. It also shares similarities with several much older crocodylomorph groups and is commonly thought to have been more terrestrial than any crocodilian living today. Only a single species has been described, the type species T. rackhami.
History and naming Trilophosuchus is best known through holotype specimen QM F16856, which represents a partial skull missing the tip of the snout. Several other isolated bones of the skull were found as well and have been assigned to Trilophosuchus. Most material of this genus has been collected from the middle Miocene Ringtail Site of the in Riversleigh, one of Australia's most famous fossil localities. One referred specimen, a parietal, has been collected the older Hiatus Site, putting the oldest appearance of the genus within the Oligocene. This Oligocene material differs subtly and may represent a second species or could simply be the result of age or intraspecific variation. The holotype skull was uncovered in 1985 during an excavation by the University of New South Wales and is currently considered to be among the best preserved fossil crocodilians from Australia, in spite of the missing snout. The genus Trilophosuchus was erected in 1993 with the description of the remains by Paul Willis of the University of New South Wales in the Journal of Vertebrate Paleontology. In 2022 and 2023, two papers were published detailing the endocast and the skull anatomy of Trilophosuchus, greatly expanding upon the research previously published by Willis. The first of these two studies made use of high resolution μCT scan to map the shape of the crocodilian's brain. This made Trilophosuchus the second mekosuchine with available information on the neuroanatomy after Paludirex.
The name Trilophosuchus derives from Ancient Greek and translates to "triple-crested crocodile", referring to the three parallel ridges that run along the animal's skull table. The species name meanwhile honors Alan Rackham Senior, a supporter of the excavations in the Riversleigh WHA.
Description
Trilophosuchus had a short, deep skull and large eyes that face more to the sides than they would in modern semi-aquatic crocodiles. The snout, though incomplete, is thought to have been very robust, showcasing a trapezoid cross-section just before the eyes. Due to its robust and boxy skull, Trilophosuchus has been compared to a variety of other crocodylomorphs by Willis. Among these are both Osteolaemus (dwarf crocodiles) and Paleosuchus (dwarf caimans), but Willis also points at a variety of Mesozoic forms, in particular notosuchians, Theriosuchus and Protosuchus. Comparison with Mekosuchus is also common, not only due to their similar morphology but the fact that both are known to be closely related. This is evident in part due to the relation between the maxillary bone and the orbital margins. The eye sockets of Mekosuchus are in part formed by the maxilla, which is unique among all crocodilians. While this is not quite the case in Trilophosuchus, the only thing preventing the maxilla from contacting the orbital margin is a small contact between the lacrimal bone and jugal bone.
The palatal fenestrae, two openings in the underside of the skull, dominate the palate and reach up to the level of the sixth maxillary teeth. They are notably wider in this region and taper further towards the back of the skull, where they are constricted by the ectopterygoids. The palatine bone has an anterior process, or projection, that reaches to the level of the fourth maxillary tooth. Behind the palatine and the palatal fenestra, there are large pterygoids and ectopterygoids that project posteriorly and extend below the skull. The ectopterygoid is unique in that its plate bears a large and shallow cancavity. An unusual feature of the pterygoids meanwhile is that they are partially fused along the midline in the region preceding the choanae. The jugal bones, which line the side of the skull below the eyes, project laterally to form prominent ridges. The supratemporal fenestrae, two openings on the skull table, long and narrow and have been compared to the shape of an almond. The entire skull table of Trilophosuchus is somewhat moved forward, which reveals the underside of the basioccipital. This effectively creatures what Willis refers to as a "bend" in the occipital face. The upper portion is vertical, while the lower portion is inclined at a 30° angle forward.
The most distinct feature of Trilophosuchus, and the one that gives this taxon its name, is the presence of three prominent crests that extend alongside the length of the crocodilian's skull table. The central crest originates on the broad frontal bone between the eyes and continues onto the parietal bone before ending approximately at the same level as the supratemporal fenestrae. Two additional crests run parallel to this medial ridge, originating at the orbital margin and stretching from there until the end of the skull table, occupying the space between the midline and the fenestrae. Willis also notes the presence of small knobs on the outer side of the skull table, separated from the three ridges by the supratemporal fenestrae. These may represent an additional pair of parallel crests, tho not as distinct as the central three.
As no specimen of Trilophosuchus preserves the front of the skull, the precise number of teeth is currently unknown. Assuming that the largest preserved tooth socket is that of the fifth tooth in the maxilla, as in other crocodilians, it may have had between 12 and 13 maxillary teeth on either side. The individual tooth sockets, and by extension the teeth, are spaced regularly throughout most of the toothrow. However, between the sixth and seventh alveoli this even spacing is interrupted by a dentary tooth of the lower jaw that interlocks with the upper dentition and creates a prominent notch in the maxilla. Based on the known remains, Trilophosuchus shows a high degree of festooning, meaning the maxillae are distinctly wave-like in their form, visible in both side and top view. The degree of festooning is consistent with adult crocodilians, but may also be exaggerated by the shortened rostrum. The first wave of the maxilla peaks with what is thought to be the fifth maxillary tooth before ascending to the level of the dentary notch. A second wave is present behind this one, peaking around the 11th or 12th tooth. No known specimen of Trilophosuchus preserves the teeth themselves.
Endocast
Like in other crocodylomorphs, the cranial endocast does not accurately reflect the shape of the brain itself and is instead also influenced by the extensive dural envelope. The endocast regardless gives a general idea of the brain shape, which is congruent with the linear and elongated endocasts of other crocodylomorphs. One of the most unique features of Trilophosuchus is the presence of an acute dural peak, an inflation above the hindbrain. This subtriangular peak, in combination with a deeply concave midbrain, gives the endocast of Trilophosuchus a highly distinct form. Such dural inflations are generally not well developed in other crocodilians, with the only eusuchians sharing a clearly distinct dural peak being Agaresuchus and Arenysuchus, though in both of which the peaks are blunt, not acute. Instead, the endocast of Trilophosuchus compares much more favorably to the notosuchians Araripesuchus and Sebecus as well as the dyrosaurid Rhabdognathus. Among these three Araripesuchus in particular is highlighted as the most similar in the overall shape of the endocast.
The inner ear features a distinctly sub-triangular vestibular apparatus (where the semicircular canals contact the common crus), as opposed to being more rounded like in other modern crocodilians. Although this does somewhat resemble animals like Junggarsuchus and Protosuchus, the individual elements are still robust as in other eusuchians, rather than slender. A standout feature of the inner ear concerns the ratio between the height of the common crus and various other elements, in particular the height and length of the endosseous labyrinth and the height of the vestibular apparatus. In all three instances, these ratios are amongst the highest of the analyzed crocodylomorphs, often producing values similar to various basal crocodyliforms, crocodylomorphs, sebecosuchians and thalattosuchians.
In addition to elements of the brain itself, the endocast also reveals several details on the pneumaticity of Trilophosuchus' skull. While crocodilian skulls are generally pneumatized through the presence of air-filled pockets within the skull, Trilophosuchus stands out as having one of the most heavily pneumatized skulls. Among extant forms, it strongly resembles and even exceeds dwarf crocodiles (Osteolaemus) and dwarf caimans (Paleosuchus), and more broadly alligatoroids as a whole, whereas gavialoids and other crocodyloids (sans Osteolaemus) are less extensively pneumatized. While this makes Trilophosuchus one of the most highly pneumatized eusuchians, this system of air pockets is not nearly as complex as in many notosuchians.
Dwarfism Several anatomical features of Trilophosuchus have led to discussion regarding the age at which the holotype specimen died. Both the foramen for the trigeminal nerve and the foramen magnum (through which the spinal cord attaches to the brain) are large and the supratemporal fenestrae are elongated and compressed. The inclination of the occipital face also differs from most adults crocodilians. The skull table, though usually flat in adults, slopes in Trilophosuchus from the midline. While all of these features and the overall small size of Trilophosuchus could be taken as evidence that it was a juvenile, both Willis and Ristevski have argued to the contrary. Notably, both the fusion of the skull bones and the sculpting on their surface indicate that it was in fact an adult at the time of its death. They further point out that although the compression of the supratemporal fenestrae is a juvenile trait, their orientation does not match this interpretation, while the inclination of the occipital face could also be explained as being the ancestral condition. This would suggest that rather than being immature, Trilophosuchus was simply a dwarf species, with the juvenile characteristics being the result of paedomorphy. The shape of the endocast also supports this conclusion, showing morphologically mature characters such as the relatively featureless midbrain.
The skull of Trilophosuchus only measures from the back of the head to the front most preserved part of the rostrum, making it very small even when accounting for the missing material. Although no remains of the body are known, Trilophosuchus is estimated to have been around long and weighed . These values are calculated through a regression analysis using the width across the quadrates, based on data collected from modern crocodilians. This would place Trilophosuchus in the size range of many crocodilians colloquially referred to as dwarf species, including dwarf caimans, dwarf crocodiles, the Chinese alligator and Mekosuchus, all of which rarely exceed in length.
Classification