Mekosuchus
Mekosuchus is a genus of extinct Australasian mekosuchine crocodilian. Species of Mekosuchus were generally small-sized, terrestrial animals with short, blunt-snouted heads and strong limbs. Four species are currently recognized, M. inexpectatus, M. whitehunterensis, M. sanderi and M. kalpokasi, all known primarily from fragmentary remains.
Mekosuchus was a successful and widespread genus, with its earliest members being found during the Oligocene and Miocene in mainland Australia. These species coexisted with a wide variety of other mekosuchines, forming a highly diverse crocodilian fauna including terrestrial hunters, semi-aquatic ambush predators and long-snouted fish eaters. The anatomy of the neck vertebrae of M. whitehunterensis might indicate that it was quite well adapted to stripping flesh from carcasses, using blade-like teeth and violent side-to-side thrashing.
The younger two species were found on the Pacific islands of New Caledonia and Vanuatu respectively and represent some of the youngest known mekosuchines. Mekosuchus possibly died out approximately 3,000 years ago, during the Holocene, but some authors have also suggested that they may have survived until even more recently. Unlike the mainland species, M. inexpectatus is known to have had bulbous posterior teeth that may have been used to crack the shells of crustaceans and molluscs. Some researchers suggest that they were possibly nocturnal animals living in close association with rainforest streams. What caused their extinction is unclear. Although some researchers suggest a human cause, others point out that the potential overlap with human settlements is insufficiently understood and no direct signs of human involvement have been found.
History and naming
Fossils of Mekosuchus were initially recovered from various different sites across New Caledonia, with the first bone, a fragmentary quadratojugal, being collected from Kanumera Bay in 1981. Subsequent years yielded more material stemming from both the Isle of Pines and the Pindai Caves on the main island of Grande Terre. This material included various cranial and postcranial remains, ranging from the complete holotype dentary to skull fragments and isolated vertebrae. Such fossils were first reported by Eric Buffetaut in 1983 and properly described by him and Jean-Christophe Balouet four years later in 1987. Due to the strange anatomy of the material, they initially assumed the animal represented an early eusuchian and placed it in its own family, the Mekosuchidae. Ten years later, in 1997, a second species was described by Paul Willis from the Riversleigh World Heritage Area in Queensland. Named M. whitehunterensis, it was not only geographically separated from the type species but also distinctly older, dating to the Late Oligocene. This marked the first but not the last known instance of a Mekosuchus species being found on the Australian mainland, instead of on an island. The second instance came only four years later with the description of Mekosuchus sanderi, also named by Willis. The most recently described species is Mekosuchus kalpokasi, which was named in 2002 from fragmentary remains discovered at an archaeological site on Vanuatu. A 2003 expedition also yielded additional remains of M. inexpectatus, with additional fossils of M. whitehunterensis being found as well.The generic name of Mekosuchus derives from the Drehu name for Grande Terre, Mek, in combination with the suffix -suchus meaning crocodile.
Species
- M. inexpectatus
- M. kalpokasi
- M. sanderi
- 'M. whitehunterensis'''''
Description
The best known species is Mekosuchus inexpectatus, which was described as displaying a unique mix of basal and derived features of the skull. The palatine bones, which form part of the roof of the mouth, narrow towards the back. The choanae, which connect the nasal passages with the throat, are located further forward than in modern crocodiles and resemble what is seen in some Late Cretaceous crocodilians like Albertochampsa and Thoracosaurus. The wings of the pterygoid bone are well developed towards the back of the skull and the quadratojugal lacks a spine, which is a feature shared by alligatoroids but not by crocodylids. The position of the postorbital bar also differs from modern crocodilians, as it isn't displaced inward or only to a very small degree. The external nares open towards the sides and front of the skull rather than facing upwards and the opening is not contacted by the nasal bone. The eye sockets were well-developed and large and, unique among crocodilians, are in part formed by the maxilla, preventing the jugal and lacrimal bone from contacting each other. This unique contribution of the maxilla to the orbital rim is among the diagnostic features of this genus.
As in many crocodilians, the tooth row of Mekosuchus is placed in a distinct, wave-like manner also referred to as festooning. Festooning is usually the least pronounced in longirostrine forms like gharials, which have rather straight toothrows and much more prominent in short-snouted species. The maxilla displays some festooning in M. whitehunterensis and a much more extreme wave-form in M. kalpokasi. While festooning may be exaggerated in younger individuals, an analysis conducted on the material of M. kalpokasi has confirmed it to be an adult.
Other cranial features that can be used to differentiate the four species includes the extent of the palatal fenestrae. In M. sanderi and M. inexpectatus the front edge of the fenestrae extends until the 6th tooth of the maxilla, while in M. kalpokasi and M. whitehunterensis it extends only until the 7th. M. whitehunterensis further differs from all other species by possessing a longitudinal furrow beneath the eyes, while M. sanderi possesses a crest atop the squamosal bone. The extent of the shallow mandibular symphysis, the fused section at the front of the lower jaw, also differs between species. In M. inexpectatus the symphysis extends until the position of the 7th dentary tooth, while in M. whitehunterensis it ends at the 6th dentary tooth. This prevents the splenial from contributing to the symphysis, as it only extends forward to the level of the 7th dentary tooth across all species of the genus. The mandibular fenestra is strongly reduced, being almost closed in M. whitehunterensis, and the angular and surangular bones possess out-turned flanges, both of these are diagnostic for Mekosuchus.
Some postcranial remains are also known, primarily from M. inexpectatus and M. whitehunterensis. Between the two, the vertebrae of M. whitehunterensis are described in greater detail. They are procoelous and the neck vertebrae specifically were noted to be shorter than those of the extant freshwater crocodile, even when accounting for the small size of Mekosuchus. This may indicate that at least M. inexpectatus had a shortened neck. The axis vertebra displays the typical sloping neural spine of crocodilians, but bears closer resemblance to alligatorids than to crocodylids. The following neural spines follow the overall pattern expected from a crocodile, though comparably taller than in other similarly sized animals. At the same time, the neural spines are not as inclined as in today's crocodiles, especially towards the front of the neck. This has been taken as evidence that, in spite of being small, Mekosuchus had well developed and strong epaxial neck musculature. It is possible that the neck anatomy of M. whitehunterensis represents a compromise between needed mobility and enlarged musculature. Similar neck vertebrae have been described for both Mekosuchus inexpectatus as well as the genera Trilophosuchus and Volia, indicating that this anatomy may have been more widespread among derived mekosuchines.
According to Willis, the humerus was similar in form to that of modern monitor lizards and Balouet & Buffetaut make mention of well developed insertions for the musculature. In a 2013 abstract it is mentioned that the tuber of the calcaneus, the heel, is robust and unusually short.
Various parts of the osteoderms, the bony armor, are known from across the different species and were specifically mentioned for M. inexpectatus and the Oligocene mainland species. The dorsal and tail osteoderms of the continental species are described as being highly modified, which may be related to biomechanics or simply a defensive adaptation.
Dentition
The dentition of the four known Mekosuchus species varies between the taxa both in shape, number and occlusion. For instance, the lower jaw of M. inexpectatus contained 13 teeth, whereas that of M. whitehunterensis contained 16. Upper jaws on the other hand can be compared between M. kalpokensis and M. sanderi, with the former possessing 12 maxillary teeth and the latter 13.However, the differences in shape are more noticeable. The oldest species, M. whitehunterensis, was described as having smooth maxillary teeth that would display flattened sides towards the back of the jaw, making them blade-like. A similar condition can be observed in the younger mainland species, M. sanderi, in which the teeth become laterally compressed following the 5th tooth of the maxilla. The Holocene species meanwhile lack these blade-like teeth. Although only the tooth sockets are known from M. kalpokasi, these suggest that the teeth were circular to ovate in cross section, with no signs of the lateral compression seen in older forms. The teeth of M. inexpectatus are better known, but likewise fail to display the same condition as seen in the continental species. Rather than being blade-like, the posterior teeth of M. inexpectatus were bulbous molariforms, better suited for crushing than for slicing. Similar tribodont teeth are seen in many unrelated types of eusuchians, including Allognathosuchus, Bernissartia and modern dwarf crocodiles.
Similarly, the way the maxillary teeth occlude with one another also varies between these forms. This can be determined either by the form of the toothrow itself or through the presence of occlusal pits that the teeth could slide into when the jaw was closed. Generally, two states are known. Interfingering teeth as seen in modern members of Crocodylus and an overbite as seen in Alligator, however, some species of Mekosuchus also display an intermediate pattern, combining an overbite with some degree of interfingering. M. inexpectatus displays a full overbite in the maxillary toothrow and the same is the case for M. whitehunterensis. In case of the later, most maxillary teeth were simply too closely spaced to allow for interlocking dentition and towards the back of the skull, occlusal pits confirm that certain dentary teeth were positioned further inside relative to those of the upper jaw. M. sanderi and M. kalpokasi on the other hand feature a mix. In both of these species, the teeth towards the tip of the jaw and towards the back were arranged in an overbite, however, M. sanderi had an interlocking dentary tooth between the 7th and 8th teeth of the maxilla, while in M. kalpokasi the dentition interlocked between the 6th and 7th as well as the 7th and 8th maxillary teeth.