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, first appearing in the fossil record in the Eocene of Australia, and surviving until the arrival of humans: the Late Pleistocene on the Australian continent and during the Holocene in the Pacific islands of Fiji, New Caledonia and Vanuatu.
Mekosuchine crocodiles are a diverse group displaying a great variety of shapes and sizes. Some taxa, like Baru and Paludirex, were large semi-aquatic ambush hunters, though the two genera likely differed significantly in their hunting methods. The medium-sized Australosuchus may have been relatively cold-resistant and taxa like Trilophosuchus and Mekosuchus are renowned for their small size. One of the most distinct mekosuchines was Quinkana, with its altirostral (deep) skull and blade-like serrated teeth.
There is some question around the lifestyle of mekosuchines. Based on skull shape, many taxa are semi-aquatic and most mekosuchines have relatively conservative hip morphology, although other factors might indicate greater terrestrial capabilities. The humeri are straighter than in modern crocodiles, allowing them to perform the so-called "highwalk" more easily. Mekosuchus and Trilophosuchus are commonly regarded as more terrestrial, perhaps similar to dwarf caimans and dwarf crocodiles, while Quinkana displays a skull shape very similar to terrestrial crocodylomorphs like sebecids and planocraniids. In the case of Quinkana, this interpretation is mostly hindered by the near complete lack of postcranial material, with the exception of some isolated hip bones which suggest the presence of a mekosuchine with erect limbs in the Riversleigh World Heritage Area, where Quinkana was found.
Mekosuchines were historically considered to be true crocodiles (of the family Crocodylidae), but modern research favors the idea that they either diverged before the split between gharials and crocodiles or that they are a sister group to Crocodylidae. Some recent studies have even played with the idea that Orientalosuchina, a clade of small crocodilians from the Cretaceous to Paleogene of Asia, might be a part of Mekosuchinae. Regardless of their origins, mekosuchines rapidly diversified between their first appearance during the Eocene and the Oligocene-Miocene boundary, when as many as five different genera of very different morphology all inhabited the freshwater environments and forests of the Riversleigh.
Mekosuchines underwent a decline in post-Miocene Australia, with most genera believed to have gone extinct due to an especially severe period of aridification. While mekosuchines recovered during the Pliocene, the continuous decline of inland freshwater systems and the associated terrestrial biomes gradually lead to the decline of the family. By the Pleistocene only the genera Quinkana and Paludirex still inhabited mainland Australia, alongside the more recent true crocodiles. After the demise of the last mainland mekosuchines, the group survived on Fiji, Vanuatu and New Caledonia until the Holocene. In all instances the extinction of mekosuchines happens around roughly the same time as the arrival of humans, though it is unclear how much of a part, if any, they played in these events. For the mainland taxa it is argued that their disappearance was mostly related to climate change, with the pattern of extinction matching the disappearance of river basins but less so with the appearance of humans. Things are more complex for the island forms, with some researchers questioning how much Mekosuchus and humans truly overlapped. If humans played a role in the extinction of the last mekosuchines, it could have been either directly due to overhunting or more indirectly through habitat destruction and invasive species like rats.
History of discovery
Early finds
Although the family Mekosuchinae was not established until the 1990s, fossil material belonging to members of this clade had been known from the Australian continent for a long time. The first material now recognised as belonging to this group of crocodilians was described in 1886 by English zoologist Charles Walter De Vis. The fossils, discovered in 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 not only to 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. This material encompassed 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 was named Quinkana in 1981, though early interpretations linked it to the Paleogene planocraniids rather than the already established Pallimnarchus. these leading to the description of Mekosuchus by French paleontologists Jean-Christophe Balouet and Eric Buffetaut in 1987. 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 branch of Eusuchia and the sister group to the three extant groups of crocodilians (crocodiles, gharials, alligators and caiman). and a summary of Australasian crocodylian research co-authored with several other researchers previously involved in mekosuchine research. 2023 saw the description of Baru iylwenpeny, previously only known as the "Alcoota Baru",
|Late Oligocene - Early Miocene
|
|Australosuchus is known from abundant remains; it was likely a semi-aquatic generalist and had a moderately broad and flattened skull. It is among the southern-most members of its family and has been hypothesized to have been much more cold resistant than its relatives, allowing it to thrive in areas where others could not.
|frameless|Australosuchus
|-
|rowspan=3|Baru
|B. darrowi
|Middle Miocene
|
|rowspan=3|Species of Baru are among the largest and most robust mekosuchines, bearing heavily built skulls with large and finely crenulated teeth. Having reached lengths upwards of , they were the apex predators of Miocene freshwater habitats. Baru is sometimes known as the "cleaver-headed crocodile".
|rowspan=3|frameless|Baru species
|-
|B. iylwenpeny
|Late Miocene
|
|-
|B. wickeni
|Late Oligocene
|
|-
|Kalthifrons
|K. aurivellensis
|Pliocene
|
|Kalthifrons was a medium-sized mekosuchine found on the shores of Lake Palankarinna in South Australia. It is generally thought to have been a semi-aquatic generalist similar to modern crocodilians. It has been hypothesized that it might have either been outcompeted by the genus Crocodylus or that the latter filled the same niche following the extinction of Kalthifrons in the Lake Eyre Basin.
|frameless|Kalthifrons skull reconstruction
|-
|rowspan=4|Kambara
|K. implexidens
|Eocene
|
|rowspan=4|All four known species of Kambara lived during the Eocene in what is now Queensland, with at least two of them possibly having coexisted. The different species chiefly differ in how their teeth occlude, with K. murgonensis possessing an overbite akin to an alligator whereas other species appear more similar to crocodiles in possessing interlocking teeth. Although fossil remains are numerous, most material of Kambara has never been properly described.
|rowspan=4|frameless|Kambara skulls
|-
|K. molnari
|Eocene
|
|-
|K. murgonensis
|Eocene
|
|-
|K. taraina
|Eocene
|
|-
|rowspan=4|Mekosuchus
|M. inexpectatus
|Holocene
|
|rowspan=4|Among the smallest mekosuchines, Mekosuchus was a long lived genus that first appeared during the Late Oligocene on mainland Australia and may have died out as recently as 3,000 years ago, if not more recently still. Mekosuchus is among the most enigmatic members of its group, with no strong consensus on what kind of lifestyle it had. A more terrestrial lifestyle is commonly suggested and sometimes the genus is likened to modern dwarf-crocodiles, which inhabit small rainforest streams.
|Holocene
|
|-
|M. sanderi
|Early Miocene
|
|-
|M. whitehunterensis
|Late Pleistocene
|
|rowspan=2|The genus Paludirex was established as a substitute for the poorly defined genus Pallimnarchus and is represented by two species. Among these, the larger P. vincenti is the single largest mekosuchine, reaching an estimated body length of around , although the smaller P. gracilis still reached lengths comparable to those of Baru.
|rowspan=2|frameless|Paludirex species
|-
|P. vincenti
|Pliocene - Pleistocene?
|
|-
|rowspan=4|Quinkana
|Q. fortirostrum
|Pleistocene
|
|rowspan=4|Quinkana possessed an altirostral skull and ziphodont dentition, leading scientists to compare it to sebecids and planocraniids, although none of them are closely related to each other. This has led to the hypothesis that Quinkana was a primarily terrestrial animal, although no postcranial material has been confidently assigned to the genus to confirm this.
|rowspan=4|frameless|Quinkana holotype specimens
|-
|Q. babarra
|Early Pliocene
|
|-
|Q. meboldi
|Middle Miocene
|
|-
|Trilophosuchus
|T. rackhami
|Oligocene - Miocene
|
|Measuring an estimated in length, Trilophosuchus is the smallest known mekosuchine. Described from a partial skull showing three distinct crests running down the skull table, Trilophosuchus shows signs of having been a more terrestrial animal that held its head higher than most modern taxa.
|frameless|Trilophosuchus
|-
|Ultrastenos
|U. huberi
|Pleistocene
|
|Volia was a medium-sized insular mekosuchine endemic to the island of Fiji, where it lived during the Pleistocene. At in length, it was the largest predator on the island and likely fed on flightless birds and giant iguanas.
|frameless|Volia
|-
|}
Though sometimes included within Mekosuchinae,
Remains of a ziphodont crocodilian have been known from the Pliocene Otibanda Formation of Papua New Guinea since 1967,
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 often speculated to have been a part of the group regardless, with Paul Willis even suggesting it may have been a species of Kambara.
Description
Skull shape
One of the most noticeable traits of mekosuchines is the wide range of skull shapes found across the family. At their simplest mekosuchines greatly resemble modern crocodiles, with taxa such as Kambara, The teeth of two of the species, B. darrowi and B. iylwenpeny, bear fine serrations along the cutting edges of the teeth. This led some early publications to refer to them as ziphodont, although later papers prefer the term "crenulated" to distinguish them from truly ziphodont teeth.
Size
Mekosuchines come in a wide range of sizes, with the smallest known genus Trilophosuchus measuring an estimated long. pushing it into the range of the medium-sized members of the family. Some larger reports exist, but these concern remains that are poorly understood and fragmentary. Other mekosuchines in this range include Kalthifrons,
Phylogeny
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, various different interpretations of the members of the group and their internal relationships exist. These interpretations differ in a multitude of ways, but also share certain topologies. For instance, taxa such as Kambara and Australosuchus are typically regarded as being basal offshoots of the family, sometimes joined by the much younger genus Kalthifrons. Ristevski and colleagues recovered Kalthifrons as the first mekosuchine to split from the clade, followed by Kambara and Australosuchus, in 2023.
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, sees the family split into two clades: one containing the large bodied forms Baru, Paludirex and Quinkana; and one clade of insular and dwarf taxa, which features Ultrastenos, Trilophosuchus, Volia and Mekosuchus.
A stark contrast to these comparably similar phylogenies can be found in a 2021 study by Jonathan Rio & Philip D. Mannion. The most notable differences from the above phylogenies are the placements of Australosuchus and Quinkana, neither of which were recovered as mekosuchines in this 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 within an offshoot of mekosuchinae 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 Ultrastenos and thus also includes "Baru" huberi as a separate taxon, although both fall within the dwarf clade.
Orientalosuchina
thumb|right|It is possible that the [[Cretaceous and Paleogene members of Orientalosuchina were relatives of the mekosuchines.]]
In addition to the more standard topology recovered by Ristevski and colleagues, they also produced two phylogenetic trees that represent the two most novel interpretations of the group. These trees prominently feature the clade Orientalosuchina, crocodilians that lived during the Cretaceous and Paleogene in Asia, as being deeply nested within Mekosuchinae. In both trees Mekosuchinae is divided into two clades, a feature they share with other analyses. 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.. Meanwhile, the other clade includes Orientalosuchina and small-bodied as well as insular taxa, in other words: Ultrastenos, Trilophosuchus, Volia and Mekosuchus.
Extinction
Despite the fact that mekosuchines initially recovered from the aridification that caused the Miocene-Pliocene faunal turnover, the group continued to suffer from the effects of global cooling and Australia's continuous drift north throughout the Pleistocene. This gradual change in climate was felt across most groups of megafauna that inhabited Australia at the time, with the semiaquatic mekosuchines like Paludirex suffering the most from the disappearance of inland river systems. Hocknull and colleagues write that the Lake Eyre Basin was among the first systems to rapidly deteriorate some 48,000 years ago, followed shortly by other systems in the next eight thousand years. Rainfall fell to levels far below those of today, deriving the systems of their water supply. This did not change until 30,000 years ago, when it was too late for the systems to recover. While it has been speculated that members of the genus Crocodylus could have retreated to coastal waters, surviving due to their osmoregulation, while the more freshwater dependant mekosuchines like Paludirex would have died out as their habitats dried up.
This drying may have also affected more terrestrial forms like Quinkana. Despite its inferred lifestyle, members of the genus are still commonly found in close proximity to freshwater and likely inhabited more forested environments. The aridification of Australia led to the collapse of the continents rainforest systems approximately 50,000 years ago and by 44,000 years ago fires had begun to crop up more frequently than before. Even before these events, authors note a shift from vine scrubland to more open environments during the Late Pliocene and Early Pleistocene, coinciding with a decrease in known Quinkana material.
thumb|The Lapita people inhabited large parts of Melanesia and Polynesia, including islands that were home to the last mekosuchines.
While mainland mekosuchines died out during the Pleistocene, isolated insular taxa continued to survive, possibly even into the Holocene. The extinction of Mekosuchus is frequently linked to the arrival of the Lapita people in the South Pacific, supported by the fact that the range of the genus overlapped with human settlements and by the association of M. kalpokasi remains with human tools at the Arapus archaeological site on Efate. Factors contributing to the disappearance of island mekosuchines range from direct hunting of the crocodiles to habitat destruction and the introduction of invasive species like pigs and rats. On the other hand, M. kalpokasi fossils found at archaeological sites do bear the gnaw marks of rats and fossils tentatively referred to Volia were found as oven contents.
A later study focused on mekosuchines in general; examining material assigned to Kambara, Baru, the "Floraville Taxon" and what might have either been a juvenile Baru or Mekosuchus; suggests that the group as a whole had collumnar humeri than modern crocodylids with an elliptical rather than rounded cross-section. These factors significantly affect the stress the bones are put under when the animals lift their bodies of the ground, either for a sprawling gait or for the highwalk. The study suggests that the curved humeri of modern crocs serve to provide a more constant core of neutral stress regardless of what gait is assumed by them and provide greater safety when sprawling. Mekosuchines meanwhile appear to have had a lower baseline stress that does not spike as much when switching from one gait to the other, effectively making the highwalk much more viable. The anatomy also seems to support the idea that mekosuchines would have been able to perform the highwalk even after reaching a certain size, while many modern crocodiles cease to move this way after growing too large.
All these adaptations, while advantageous for walking on land, do not necessarily show that mekosuchines as a whole were terrestrial,
thumb|Some mekosuchines like Quinkana might have spent a significant amount of time on land.
In Mekosuchus the eyes are large and directed towards the side of the skull while the nostrils open more towards the front, whereas both open more towards the top of the skull in semi-aquatic crocodiles, allowing them to see above the water's surface even if otherwise submerged. and is entirely absent in recent literature including the 2023 work by Jorgo Ristevski (which Willis participated in). While the former point could still be applicable, the latter was eventually combated by Christopher Brochu, who confirmed the authenticity of that morphology. Naturally, Wroe's arguments do not fully account for Stein's "pelvic form four", described 15 years later.
This general bauplan is taken a step further by the large Paludirex, which also features a broad and flattened skull, although one that is notably more robust than those of Australosuchus and Kambara. Given that fact and the shere size of the larger of the two species, P. vincenti, it has been proposed that this genus was specialised in taking larger prey items than its relatives.
thumb|left|Quinkana may have hunted in a manner similar to Komodo dragons.
Some of the most specialised anatomy can be seen in Quinkana, suggesting that the genus had a lifestyle drastically different from other mekosuchines. The teeth of Quinkana are ziphodont, resembling those of terrestrial sebecosuchians, planocraniids and monitor lizards, although their precise function is unclear. Molnar has argued that ziphodont dentition indicates an ability to tackle larger prey, terrestrial prey or possibly other crocodilians. Murray and Vickers-Rich take a similar approach, but conclude that Quinkana could have still ambushed prey on land, possibly by lying in wait near game trails.
Possibly the most enigmatic mekosuchines in terms of ecology are the dwarf taxa Trilophosuchus and Mekosuchus. In the case of Trilophosuchus, the study of muscle attachments seems to indicate that rapid sideways movement of the head did play a part in its feeding method, but also showcases that it did not have to face the same resistances as semi-aquatic crocodilians, causing the muscles to be somewhat weaker.
There are further important differences between the Neogene mainland species like the aforementioned Mekosuchus whitehunterensis and the much more recent insular taxa like Mekosuchus inexpectatus. While the former possessed bladed teeth well suited for cutting flesh, the more recent forms appear to have developed more robust cheek teeth that appear more suited for dealing with hard-shelled prey items. This would include various crustaceans, insects and even molluscs It is thought that the climate grew wetter during Faunal Zones B and C, leading to a more prominent lowland rainforest environment being present during the deposition of the Ringtail Site (home to Trilophosuchus, Baru darrowi and Mekosuchus sanderi).
thumb|The separation of Baru wickeni and Australosuchus clarkae may reflect a prominent climate barrier across Oligocene Australia.
Other major Late Oligocene localities yielding mekosuchine material include the Etadunne, Namba and Wipajiri Formation of the South Australian Lake Eyre Basin. Like with the Riversleigh, the terrestrial environment has been described as tropical rainforest that was later replaced by dry sclerophyll forest. The aquatic environment was represented by widespread shallow lakes, which were home to a diverse assemblage of waterfowl. However, despite the environmental similarities to the Riversleigh and extensive bodies of freshwater, these southern formations only preserve a single mekosuchine: Australosuchus clarkae. Crocodilians were seemingly separated by habitat preferences, with the gavialoid Harpacochampsa found in deeper waters like parts of the meandering river and the oxbow sloughs while the mekosuchine Baru seemingly preferred slow-moving streams in accordance with the hunting style proposed by Willis and colleagues. Alcoota shares much of its fauna with Bullock Creek, but is notably younger and shows a noticeable drop in freshwater and streambank taxa, which went from making up about half of the Bullock Creek fauna to only about a fourth of Alcoota's, a decline likely tied to the continuous aridification of Australia during this time. Still, the locality is thought to have been lacustrine in nature, featuring permanent bodies of water fed by springs that in times of increased rainfall may have expanded to form an enormous but very shallow lake. The terrestrial biomes of Alcoota may have featured savannah and localized forests according to Murray and Megirian,
One thing showcased by both localities is the impact of the climate change that Australia faced during the Miocene. The presence of certain minerals, namely evaporites and lithoclastic carbonate deposits, seems to suggest that Bullock Creek underwent periods of dry, possibly even semi-arid conditions. The deposits further indicate that these minerals were not formed in a single event, but rather sequentially, possibly due to seasonally changing conditions causing droughts. Further south, the Lake Eyre Basin also continued to support mekosuchine populations, sharing Quinkana with Bluff Downs and also preserving the bones of Kalthifrons as the regions semi-aquatic crocodilian. The Tirari Formation is noticeably drier than the environment of the underlying Miocene formations inhabited by Australosuchus, but nonetheless much wetter than the region is today. It may have housed fan-deltas, lakes and floodplains, and was home to many freshwater taxa beyond the crocodilians, including ducks, flamingos, anhingas, cormorants, egrets and pelicans. Still, the Tirari Formation exemplifies the aridification of Australia, with its freshwater bodies less extensive than the lakes of the Etadunna and Namba Formations that preceded it. The Tirari Formation and Kalthifrons are furthermore key examples for the threat that aridification continued to pose to mekosuchines, with the type locality of Kalthifrons representing a watering hole that dried up in a drought, killing the aquatic taxa that lived in it.