Mekosuchus

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. 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 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. :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. 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 (medially) 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. Size Mekosuchus is among the smallest mekosuchines and is often referred to as a dwarf species in the same fashion as Trilophosuchus. While growth is a consistent feature in crocodilians throughout their lives, the rate at which they grow each year decreases as an individual approaches maturity. Subsequently, in dwarf species like Mekosuchus this growth rate begins to decrease early on, resulting in their small body size relative to other crocodilians. The fact that Mekosuchus specimens are mature or at least almost mature can be found in the anatomy of the vertebrae. According to Christopher Brochu, maturity in crocodilians can be determined by the fusion between the neural centra and the neural spine, which progresses from the last tail vertebra to the first neck vertebra. Based on this, the vertebrae of the mainland M. whitehunterensis could clearly be identified as having belonged to an almost mature individual, despite its small size. The most complete skull of this species measures only , which may result in a total body length of only . This puts M. whitehunterensis within the lower size range of today's dwarf crocodilians, Osteolaemus and Paleosuchus, both of which typically reach lengths of over when fully grown. Estimates for other members of the genus are generally less precise, but fall into the same overall size range. M. inexpectatus for instance has been estimated to have reached a length of approximately by Balouet, while Holt and colleagues estimate members of Mekosuchus to be around in length. ==Phylogeny==

When first describing Mekosuchus, Balouet and Buffetaut struggled to determine the relationship between it and modern crocodilians, noting how the taxon displayed a variety of basal and derived traits that did not align perfectly with any of the modern groups. They ultimately determined that Mekosuchus was a Eusuchian based on the choanae and the procoelous vertebrae, and placed it in the monotypic family Mekosuchidae, which they believed to have been the sister group to all three modern crocodilian families. while Mead et al. (2002) place Mekosuchus, Quinkana and a then unnamed Volia in a large polytomy as sisters to Trilophosuchus within the clade Mekosuchini. }} The most recent analysis was performed by Ristevski et al. in 2023 and put a broader focus on not just Mekosuchines but Australasian crocodylifroms in general, which includes the extant crocodylids of Australia, Australian gavialoids as well as more basal taxa like those placed in Susisuchidae. Six out of eight analyses recovered Mekosuchinae as a monophyletic group similar to the results of Lee and Yates. These analyses recovered most mekosuchines within Mekosuchini, which in turn was split into two clades. On the one hand large, continental forms and on the other small and/or insular taxa. The latter clade somewhat resembles the previous relationships suggested for Mekosuchus, as it also contains Volia and Trilophosuchus. Notably however, "Baru" huberi was recovered as the basalmost member of this group, while Quinkana was placed in the large-bodied, continental clade. The remaining two trees deviated greatly from the traditional composition of Mekosuchinae, with Kambara and Australosuchus being recovered elsewhere in Crocodylia and Mekosuchinae also including the clade Orientalosuchina, small-bodied Cretaceous to Paleogene crocodilians from Asia. However, support for these trees is low as indicated by both phylogenetic results and morphological similarities, with many uniting characters being widespread among crocodilians. Regardless of the relationship between Mekosuchinae and Orientalosuchina, the closest relatives to Mekosuchus remain the same across the analyses, generally recovering the same small-bodied clade composed of "Baru" huberi, Volia, Trilophosuchus and Mekosuchus. Results similar to this were also recovered by Yates and Stein in their re-evaluation of Ultrastenos and "Baru" huberi. }} ==Paleobiogeography==

While fossil evidence shows that Mekosuchus originated on mainland Australia, little is known about how it dispersed throughout the South Pacific. Currently, three mekosuchines are known from the region, M. inexpectatus, M. kalpokasi and Volia. M. inexpectatus may have had the largest range in time among them, with estimates suggesting that it may have first appeared nearly 4,000 years ago. This species is known exclusively from New Caledonia, which makes it the closest geographically to mainland Australia. There is some overlap between the fauna of New Caledonia and that of Vanuatu, with the two islands sharing 12% of their native lizards. One factor possibly important to the similarities and differences among the islands of the region is the geology of the Inner and Outer Melanesian Arc. The former split from Australia during the Cretaceous, while the latter only formed during the Paleogene and Neogene. As mekosuchines first appeared during the Eocene, Mead and colleagues argue that continental drift and break up could not have played a part in their appearance in the South Pacific. Instead, it is considered more likely that the ancestors of the insular mekosuchines traveled short distances across the ocean to arrive on the islands of the Inner Melanesian Arc, before dispersing between the islands of the South Pacific from there. Although it is not known whether or not mekosuchines were tolerant to saltwater or had the same adaptations for marine dispersal as modern crocodiles (such as salt glands), it is possible that they could have actively swam between landmasses or drifted with the use of natural rafts. This process would have greatly profited from the lower sea levels present during the late Cenozoic, decreasing the distance between now isolated islands and in some instances uniting whole island chains. The presence of these significant landmasses could have served as stops or even supported populations during the dispersal of these animals. For this reason, it is believed that Mekosuchus only dispersed into the South Pacific relatively recently. Mead and colleagues name the Oligocene as the earliest possible date, though an even more recent Quaternary dispersal is deemed more likely. ==Paleobiology==

Mekosuchus, like some of its closest relatives, is believed to have been a terrestrial animal. Evidence for this may be found in several parts of its anatomy. The strong neck musculature inferred for Mekosuchus whitehunterensis has been interpreted as being an adaptation for ripping chunks of flesh from carcasses. In modern crocodilians this is achieved either through shaking the head side to side or by employing the death roll maneuver. It is noted that the small size of Mekosuchus would render the death roll maneuver less effective than in species with a body length between long, whereas headshaking is favored by small animals like juveniles. Furthermore, Stein, Archer and Hand argue that the well-developed epaxial musculature would primarily increase the force generated by headshaking whereas a death roll would bear a greater risk of the animal harming itself and damaging its limbs trying to perform it on land. Finally, M. whitehunterensis could have also used its neck musculature to strip flesh by pulling and lifting its head against a constrained or weighed down carcass, behavior that has also been inferred for more ancient archosaurs. Whether or not this mode of feeding was used to rip apart much larger prey items or utilized for scavenging is unclear, though Stein, Archer and Hand suggest that it may have been especially advantageous for the latter, allowing for even relatively small animals to consume an excess of food. How so many crocodilians could have coexisted with one another may have multiple explanations. On the one hand, the differing skull shapes between them, especially in regards to the White Hunter Site, may be enough for all taxa to fill different niches and thus not come into competition with one another. It is also possible that these assemblages were the result of thanatocoenosis and that in life, all these animals could have had different habitat preferences. However, Willis observed that the mammalian fauna of the Riversleigh WHA indicates a complex but clearly defined pattern of different ecomorphs that filled different niches. For this reason, he suggests that the Riversleigh crocodilians were truly sympatric. ==See also==