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Mekosuchinae is an extinct clade of crocodilians from the Cenozoic of Australasia. They represented the dominant group of crocodilians in the region during most of the Cenozoic. They first appear in the fossil record in the Eocene in Australia, and survived until the arrival of humans: in the Late Pleistocene in Australia and within the Holocene in the Pacific islands of Fiji, New Caledonia and Vanuatu.

Mekosuchine crocodiles are a diverse group. Another mekosuchine fossil, currently undescribed, has been found in Miocene deposits from New Zealand. One genus, Mekosuchus, managed to spread to the islands of the Pacific; it is believed to have island-hopped across the Coral Sea, moving first to a now submerged island known as Greater Chesterfield Island, then New Caledonia and onwards. In the Pleistocene, Quinkana was one of the top terrestrial predators of the Australian continent.

Mekosuchines underwent a drastic decline in post-Miocene Australia, with all genera, except for Quinkana and Paludirex (both perishing during the Quaternary extinction event) becoming extinct in Australia by the end of the Pliocene. After the demise of Quinkana and Pallimnarchus, the group survived on Vanuatu and New Caledonia until the arrival of humans, who are presumed to have driven them to extinction.

While historically considered to be true crocodiles (of the family Crocodylidae), modern research places them as an independent group within or closely related to Longirostres, which contains both crocodiles and gavialids.

Early finds
Although the family Mekosuchinae was not established until the 1990s, fossil material belonging to members of this clade was known from the Australian continent for a long time. The first material now recognized as having belonged to this group of crocodilian was described in 1886 by English zoologist Charles Walter De Vis. The fossils, discovered on the Darling Downs in Queensland, consisted of skull and postcranial fragments that De Vis dubbed Pallimnarchus pollens. De Vis himself only coined the name "out of convenience", admitting that he was too unfamiliar with the Cenozoic crocodilian fossil record to be certain that his find represented an animal distinct from any other taxa known at the time. Later research has even shown that the material belonged to not only multiple individuals, but multiple different genera, with various bones since then having been referred to Paludirex and Quinkana respectively. Regardless of De Vis' caution regarding the taxon, the name Pallimnarchus eventually came to be widely used by other authors.

Renaissance
Despite these early finds, research around members now recognized as mekosuchines would grow quiet during the first half of the 20th century, with no research focused on Australasian crocodilians published during the middle of the century, leading to a 40 year hiatus. This period would come to an end in 1977 with the publications of Max Hecht, Michael Archer and Ralph Molnar, all of which reported on fossil material collected from cave deposits in northern Queensland. This material encompased both more fragmentary remains as well as better preserved ones, including a nearly complete rostrum recovered from the Tea Tree Cave. The material was quickly noted for its distinct morphology, bearing some resemblance to terrestrial crocodylomorphs like sebecosuchians and planocraniids. Following the discovery of even more fossil fragments, the taxon came to be named Quinkana in 1982, though early interpretations linked it to the Paleogene planocraniids rather than the already established Pallimnarchus.

In 1982, just a year later, Molnar published a paper focused on Pallimnarchus, attempting to revise the genus by establishing a lectotype specimen to compensate for the fact that De Vis did not establish a holotype. Around the same time came reports of crocodilian fossil material out of New Caledonia, reports that lead to the description of Mekosuchus by French paleontologists Jean-Christophe Balouet and Eric Buffetaut. Like with Quinkana, the distinct morphology of Mekosuchus initially obscured its relationship to modern crocodilians, with the team placing it in the newly named family Mekosuchidae, which they placed as an early branching Eusuchian and the sister group to the three extant groups of crocodilians (crocodiles, gharials and alligators).

Recent works
The 1970s and 1980s set the groundwork for what would be a period of increased interest in Australasian crocodilians during the 1990s and early 2000s. In addition to Hecht, Archer and Molnar, a variety of other authors would publish on crocodile fossil material during this time, many of which going on to become quite prolific in the study of mekosuchines. These include Paul Willis, Steven Salisbury and Dirk Megirian. Following the description of Baru, scientists began to recognize shared traits among the fossil crocodiles of Australia, with Willis and colleagues proposing the presence of what they dubbed the Australian Tertiary crocodylian radiation. This concept initially included the three mainland taxa Baru, Quinkana and Pallimnarchus and was later expanded to include Australosuchus as well, named a year later in 1991.

The concept of the Australian Tertiary crocodylian radiation would come to be replaced by the subfamily Mekosuchinae in 1993, defined by Willis, Molnar and Scanlon as a subfamily of the Crocodylidae. The name was intentionally carried over from the monotypic Mekosuchidae of Balouet and Buffetaut, with Willis and colleagues meaning to preserve the seniority of the name while adapting it to more accurately reflect their supposed position among crocodilians at the time. In terms of composition, the only new taxa included were Kambara, named in the same paper, and Mekosuchus, namesake of the clade. Other mekosuchines recognized during the 1990s were Trilophosuchus, named not long after the family was described, as well as three additional species of Quinkana, two species of Baru as well as one species each of Kambara, Pallimnarchus and Mekosuchus.

This trajectory more or less carried over into the 2000s and 2010s, seeing the description of two more Kambara species, two more Mekosuchus species and even of entirely new forms such as Volia, Kalthifrons and Ultrastenos. Another spike in research occured during the late 2010s and early 2020s with the publication of multiple papers helmed by Jorgo Ristevski, whose work includes a complete overhaul of the genus Pallimnarchus, naming the genus Paludirex in its place to bring stability to the taxon, as well as a summary of Australasian crocodylian research which was co-authored by several other researchers previously involved in mekosuchine research. 2023 then saw the description of Baru iylwenpeny, previously only known as the Alcoota Baru, as well as a publication synonymizing "Baru" huberi with Ultrastenos.

Unnamed forms
In addition to the many named genera and species, scientists also recognize a plethora of distinct taxa that have not been scientifically described or named yet. Among these is the so-called "Darling Downs taxon", which is represented by multiple bone fragments that bear some resemblance to species of the genus Paludirex while also being visibly distinct at the same time. While too fragmentary to conclusively assign to the genus or erect as a new species, the material has been tentatively regarded as a third species of Paludirex, with the possibility that it might be a different genus alltogether. Another example would be the "Bullock Creek taxon", a small mekosuchine long known to share affinities with Ultrastenos and likely congeneric, but as of yet unnamed.

Unnamed species are also known for the genus Quinkana, notably in the form of the Ongeva Quinkana from Alcoota. The "Floraville taxon" meanwhile may represent a second genus of ziphodont mekosuchine rather than just another Quinkana species. Ziphodont crocodilian teeth have also been recovered from Australia's Mount Etna Caves National Park and the Otibanda Formation of Papua New Guinea, but these have so far not been figured in detail nor described, thus their true affinities remain uncertain.

Another geographically significant putative mekosuchine is the "Bannockburn Formation taxon", a crocodilian that lived in New Zealand during the Early Miocene. While the material is too fragmentary to be tested for mekosuchine affinities, field work near St. Bathans has recovered more material that could help resolve the matter. Additionally, these finds also seem to suggest that at least two different crocodilians inhabited New Zealand during this time period.

Much like the "Bannockburn Formation taxon", the "Runcorn taxon" (named after a suburb of Brisbane) is too fragmentary to be conclusively assigned to Mekosuchinae, but is regardless oftentimes speculated to have been a part of the group, with Paul Willis even suggesting it may have been a species of Kambara.

Internal relationships
Mekosuchinae is cladistically defined as a node-based taxon composed of the last common ancestor of Kambara implexidens, Mekosuchus inexpectatus, and all of its descendants. Beyond this definition, which necessitates the inclusion of Kambara and Mekosuchus in the clade, the included taxa may vary and various different interpretations of the internal relationships amongst mekosuchines exist. These different interpretations can at times differ in a multitude of ways, but also commonly share certain topologies. For instance, taxa such as Kambara and Australosuchus are typically regarded as being basally branching members of the family, sometimes joined by the much younger genus Kalthifrons. Ristevski et al., 2023, recovers Kalthifrons as the first mekosuchine to split from the clade, followed by Kambara and Australosuchus. This is contrasted by Yates, Ristevski & Salisbury, 2023, in which Kambara and Australosuchus form their own small clade at the base, with Kalthifrons diverging at a slightly later time. Meanwhile, Lee & Yates, 2018, recover Australosuchus as the basalmost mekosuchine, followed by Kambara and with Kalthifrons in a much more derived position.

Modern phylogenies commonly see more derived members of the family split among two branches that diverge from another after taxa like Kambara and Australosuchus do. Ristevski et al., 2023, see the family split into a clade containing the large bodied forms; namely Baru, Paludirex and Quinkana; and one colloquially referred to as the dwarf clade, which features Ultrastenos, Trilophosuchus, Volia and Mekosuchus. Lee & Yates, 2018, also recover Mekosuchinae as splitting into two clades, although their make up differs slightly. In their study, Baru still clades with Paludirex (although going by the name Pallimnarchus, but in place of Quinkana the clade instead includes Kalthifrons. The other clade is likewise similar to what has been recovered by later studies, featuring Ultrastenos (then known as "Baru" huberi and clading with the undescribed Bullock Creek taxon), Volia, Mekosuchus, Trilophosuchus and Quinkana. Yates, Ristevski and Salisbury did not recover this distinct branching of clades in their 2023 study, however, their phylogeny also did not include several taxa present in the other studies.

Some recent phylogenies are depicted below:

Lee & Yates, 2018

Yates, Ristevski & Salisbury, 2023

Ristevski et al. (modified by Yates & Stein, 2024)

A stark contrast to these comparably similar phylogenies is represented by Rio & Mannion, 2021. The most notable departures are the placements of Australosuchus and Quinkana, neither of which were recovered as mekosuchines in the study. Australosuchus was placed just outside of Crocodylidae and Quinkana was recovered as a proper crocodyline, sister to "Crocodylus" megarhinus. In exchange, the Chinese taxon "Asiatosuchus" nanlingensis was placed in a clade alongside a paraphyletic Kambara. Other elements are however more similar to the studies of Yates, Ristevski and colleagues. Kambara remains a basal mekosuchine and a distinct dwarf clade can be observed, formed in this case by Mekosuchus, Trilophosuchus and Ultrastenos. Notably, this study predates the 2024 reinterpretation of the taxon and thus also includes "Baru" huberi in addition to Ultrastenos (now synonyms), specifically as the sister taxon to Baru wickeni.

Orientalosuchina
In addition to the more expected topology recovered by Ristevski and colleagues, they also produced two phylogenetic trees that represent the two most novell interpretations of the group. These trees prominently feature the clade Orientalosuchina, crocodilians that lived during the Cretaceous and Palgeogene in Asia, as being deeply nested within Mekosuchinae. In both trees Mekosuchinae is divided into two clades, much as in other analysis. The more traditional of these clades includes various medium- to large-sized taxa from continental Australia, namely Kalthifrons, Quinkana, Baru and Paludirex, not dissimilar to the other trees recovered by Ristevski et al.. The other clade meanwhile includes Orientalosuchina and small-bodied as well as insular taxa, in other words Ultrastenos, Trilophosuchus, Volia and Mekosuchus.

Within this topology, Orientalosuchina largely grouped together to form their own monophyletic clade that acts as the sister-group to the small-bodied mekosuchines. This clade includes the genera Krabisuchus, Orientalosuchus, Jiangxisuchus and Eoalligator, but not Dongnanosuchus, which is closer to the traditional mekosuchine taxa. Protoalligator also wasn't part of the clade, but unlike Dongnanosuchus this was because it wasn't recovered as a mekosuchine at all but a basal eusuchian.

Another notable divergence from the traditional interpretation of Mekosuchinae concerns Australosuchus and Kambara, both recovered as non-mekosuchines. In addition to changing the internal composition of the clade, the phylogenetic trees recovering Orientalosuchina within Mekosuchinae also shuffle how the group relates to other crocodilians. While mekosuchines as a whole are more distantly related to true crocodiles in these trees, Australosuchus and Kambara remain closely allied to crocodyloids. Specifically, Australosuchus is recovered as the basalmost crocodyloid in both analysis, while Kambara was recovered as the sister-taxon to the Crocodylidae.

However, there is little that actually supports this grouping. Five possible synapomorphies were identified in the trees that feature Orientalosuchina as members of Mekosuchinae, however none of them are exclusive to these groups nor identified in all members. Various other morphological features are shared between certain mekosuchines and orientalosuchins as well, but these too are only inconsistently present or widespread amongst crocodilians, especially among alligatoroids. Additionally, the remainder of the eight analysis all recovered a more traditional Mekosuchinae while members of Orientalosuchina claded with alligatoroids. Ristevski and colleagues conclude that the evidence to support this hypothesis is weak, both phylogenetically and morphologically, but should nonetheless be explored with greater focus.

External relationships
The relationship between mekosuchines and other groups of crocodilians is another matter that has seen repeated shifts across time. When Mekosuchidae was established in 1987 for Mekosuchus, the then monotypic family was placed within Eusuchia. However, around that time crocodile phylogenetics were still poorly studied and Eusuchia was considered a subgroup of Crocodylia, unlike today. By 1993, when Mekosuchinae was coined to include all other Australian Cenozoic crocodilians known at the time, the clade was placed in the family Crocodylidae and regarded as a mere subfamily, a view that remained prominent in literature in subsequent years.

Today however this view is no longer supported, with modern phylogenetic analysis generally agreeing that the clade does not represent a subfamily of crocodylids, but rather a more basal clade. While it is not exactly agreed upon where they clade, two main hypothesis exist. In their 2018 study, which incorporates morphological, molecular (DNA sequencing), and stratigraphic (fossil age) data, Lee and Yates recover mekosuchines as the sister group to the clade Longirostres, which is formed by modern gharials (and false gharials) together with true crocodiles. Similar results were yielded by two phylogenetic trees recovered by Ristevski and colleagues in 2023, with the better resolved of the two suggesting that mekosuchines (including Orientalosuchina) split from other crocodilians at some point after planocraniids but before the split between crocodyloids and gavialoids.

However, more widespread is the notion that mekosuchines, while not members of the Crocodylidae itself, are still crocodyloids. In this hypothesis, also featured in Ristevski et al., 2023, they are nested deep within Longirostres as the immediate sister group to crocodylids, with the African "Crocodylus" megarhinus as the sister to the clade formed by crocodylids and mekosuchines. The same topology was previously reported by Rio & Mannion, 2021, and was later repeated in Yates, Ristevski and Salisbury, 2023.

Ecology
Being the dominant group of crocodilian throughout the vast majority of the Cenozoic in Australia, mekosuchines largely went without much competition from other groups and thus filled a wide range of nisches. Broadly speaking, mekosuchines may be divided into three main ecomorphs: semi-aquatic generalists, more terrestrial altirostral dwarf forms and terrestrial ziphodonts. Though a longirostrine ecomorph has been proposed at one point, later studies have proven this assumption to have been based on insufficient and missinterpreted material.

Semi-aquatic Forms
A vast majority of mekosuchines falls within the first category, generally resembling other generalist crocodilians of today in their skull shape and subsequently their inferred behavior. Australosuchus, Kalthifrons, Ultrastenos, the four species of Kambara and Paludirex all share the hallmarks of semi-aquatic ambush predators, having flattened skulls of medium length and eyes positioned high up on the skull, allowing them to keep their eyes above the water surface while most of their bodies remain hidden. Baru, although also semi-aquatic, did differ significantly in its unique skull morphology, leaning closer to altirostry. These semi-aquatic forms can further be divided by prey preference and size, although the former remains largely speculative for many of them.

For instance, Ultrastenos, the smallest mekosuchine to feature a generalized skull shape, was once speculated to have potentially fed on small reptiles and amphibians. However, this hypothesis was largely based on the now incorrect assumption that it had narrow, elongated jaws, and was established in order to explain such morphology despite the apparent lack of fish typically associated with longirostrine forms. Less is known about both Australosuchus and Kalthifrons from the Lake Eyre Basin, tho both are considered medium-sized for mekosuchines, the former reaching 3 m.

Kambara meanwhile is better studied, with researchers having spent more time on its potential feeding habits and even some direct fossil evidence. Like the afforementioned taxa, Kambara had a flattened skull indicative of semi-aquatic habits, but was surprisingly diverse in the details of the skull shape. The fact that dentition between species differs in whether or not it forms an overbite or interlocks has been suggested to be of ecological significance, with an overbite possibly useful in breaking and slicing while interlocking teeth are more apt to grip and hold. This, coupled with differently pronounced muscle attachments suggesting differences in grip strength, has been used to suggest that some species of Kambara were more adept at feeding on smaller prey while others, in particular Kambara taraina, were capable of hunting even larger mammals. However, the only direct evidence for the specific diet of Kambara comes in the form of turtle shells that bear crocodilian bitemarks. The specific patterns of bites show that Kambara engaged in behavior known as "juggling", i.e. repeated bites meant to allign the prey with the teeth or with the throat, either to crush or swallow it.

While Paludirex gracilis overlaps in size with Kambara and is assumed to be similarily generalist, the larger Paludirex vincenti is thought to have been able to tackle much larger prey by simple virtue of its size alone, reaching a length of around 5 m. This was only aided by a noticably more robust skull, leading some researchers to suggest that it could have even preyed on the larger marsupials of its time.

Though not quite as large as Paludirex, Baru may display the greatest degree of specialisation towards hunting large prey. It's skull is much deeper than that of other semi-aquatic mekosuchines and its teeth were highly elongated, recurved and in some species feature small crenulations. It is hypothesized that the cleaver-like head served to deliver an strong, incapacitating blow to its prey, possibly even piercing armour or tough hide. The curvature of the teeth would then be able to restrain the prey while the serrations may have been used to cut through flesh. Willis and colleagues speculate that through this method it may have hunted animals as large as 300 kg, while fossil evidence shows bitemarks, likely those of Baru, left on the bones of Dromornis, Emuarius and Neohelos. It is possible that this stark contrast to other semi-aquatic mekosuchines could be rooted in habitat preferences. Baru may have inhabited relatively shallow bodies of water, which would render it much more difficult to drown prey and thus may have required the animal to dispatch of prey in a quicker manner.

Altirostral dwarf taxa
In stark contrast to the medium to large sized semi-aquatic taxa stand the two genera of altirostral dwarf forms, namely Trilophosuchus and Mekosuchus.

Terrestrial ziphodonts
Perhaps the most enigmatic mekosuchine morphotype concerns those with ziphodont dentition, traditionally interpreted as terrestrial predators. The best known and only named representative of this type is Quinkana, which has been the center of much speculation ever since its discovery.