Mekosuchus is a genus of extinct Australasian mekosuchine crocodilian. Species of Mekosuchus were generally small-sized (less than long), 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 (at the time considered a suborder of Crocodylia) 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 The first discovered (and possibly youngest) member of this genus is the type species, M. inexpectatus, from the Holocene of New Caledonia. Radiocarbon dating suggests that the M. inexpectatus fossils from the Isle of Pines date to roughly 3,750 years BP. The Pindai Caves material on the other hand appears to have been younger, with some fossils possibly dating to approximately 1,720 years BP according to Balouet and Buffetaut. While survival into human times may be supported by remains found at archaeological sites, the age of some material has been disputed, with some authors suggesting a Pleistocene rather than Holocene age. The species name "inexpectatus" was chosen to reflect the unexpected appearance of a crocodilian on the isolated island of Grande Terre. Over 300 bones have been collected from the Pindai Caves alone, but the majority remains undescribed. M. kalpokasi The second Holocene species is M. kalpokasi, which lived on the island of Éfaté of Vanuatu approximately 3,200 to 2,706 years BP. Unlike with M. inexpectatus, the age of M. kalpokasi has not been disputed, making it the youngest confirmed species but also the least well understood. The remains of this species are fragmentary, consisting only of a partial maxilla and the ends of a tibia and fibula. For this reason, the poor preservation of the same area in the type species and the two species geographic and temporal range, Holt and colleagues suggest that this species may be synonymous with M. inexpectatus. This species was named after the prime minister of Vanuatu at the time of its discovery, Donald Kalpokas, who was noted for his strong support of the archaeological excavations that yielded the fossils of this crocodilian. M. sanderi M. sanderi is one of two Mekosuchus species known from the mainland of Australia, and lived during the Early Miocene in what is now Queensland. It was named by Willis based on two maxillae and various skull fragments, all stemming from the productive Riversleigh World Heritage Area, specifically the Ringtail Site within Faunal Zone C. The species name refers to Martin Sander, who supported Willis while studying in Germany. M. whitehunterensis M. whitehunterensis is the oldest known species and lived during the late Oligocene and early Miocene in Queensland. While also known from various localities of the Riversleigh, remains of M. whitehunterensis are older than those of M. sandersi and specifically found in Faunal Zones A and B, which yielded the holotype maxilla as well as referred material including vertebrae and skull remains. Besides some more subtle differences, M. whitehunterensis is most readily distinguished from the type species by the presence of blade-like posterior teeth. The name derives from the White Hunter Site, the locality where the first remains of this species were found.
Description The skull of Mekosuchus was brachycephalic or altirostral, meaning that it was notably short and raised rather than elongated and flattened as seen in most extant crocodilian species. In this regard Mekosuchus has been compared to Trilophosuchus and the modern, only distantly related genus Osteolaemus, which includes the extant dwarf crocodiles. Other researchers have also drawn comparisons between this genus and various other terrestrial crocodylomorphs including notosuchians. Two reconstructions of the skull of Mekosuchus have been published, differing greatly from one another. Following the discovery of additional remains, Holt and colleagues reconstructed Mekosuchus inexpectatus in 2007 with a skull similar to that of modern dwarf crocodiles. In 2014 on the other hand, Scanlon produced a composite skeletal for the skull of M. whitehunterensis, reconstructing it with a much more gently sloping rostrum that differed greatly from the previous depiction of the genus.
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 (near the palatine-pterygoid suture) 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 (anterolaterally) rather than facing upwards (dorsally) 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 (cervical) 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.