Mekosuchus is a group of extinct crocodilian animals that lived in Australasia. These animals were generally small, measuring less than 2 meters (6 feet 7 inches) in length. They were land-dwelling creatures with short, wide heads and strong legs. Four species are currently known: M. inexpectatus, M. whitehunterensis, M. sanderi, and M. kalpokasi. Most information about these species comes from incomplete remains, such as bones and teeth.
Mekosuchus was a widespread and successful group. The earliest members lived during the Oligocene and Miocene periods in mainland Australia. These species lived alongside many other mekosuchines, creating a diverse group of crocodilians that included land hunters, semi-aquatic ambush predators, and fish-eaters with long snouts. The structure of the neck bones in M. whitehunterensis suggests it may have been adapted to tear flesh from dead animals, using sharp, blade-shaped teeth and powerful side-to-side movements.
Two of the younger species were found on the Pacific islands of New Caledonia and Vanuatu. These species are among the most recently known mekosuchines. Mekosuchus may have gone extinct around 3,000 years ago during the Holocene, though some scientists think they might have survived even more recently. Unlike the mainland species, M. inexpectatus had rounded back teeth that may have been used to break the shells of crustaceans and mollusks. Some researchers believe these animals may have been nocturnal and lived near rainforest streams. The cause of their extinction is unknown. While some scientists suggest humans may have played a role, others note that evidence of human interaction is unclear, and no direct signs of human involvement have been found.
History and naming
Fossils of Mekosuchus were first found in different areas of New Caledonia. The first fossil, a piece of a bone called a quadratojugal, was collected from Kanumera Bay in 1981. Later, more fossils were discovered on the Isle of Pines and in the Pindai Caves on Grande Terre, the main island. These fossils included parts of the skull, other bones, and even a complete lower jaw. Scientists first reported these fossils in 1983, and they were fully described in 1987 by Eric Buffetaut and Jean-Christophe Balouet. Because the fossils had unusual features, they believed the animal was an early type of crocodile and placed it in its own family, Mekosuchidae.
In 1997, a second species, Mekosuchus whitehunterensis, was found in Queensland, Australia, by Paul Willis. This species lived much earlier, during the Late Oligocene, and was the first Mekosuchus species discovered on the Australian mainland instead of an island. Another species, Mekosuchus sanderi, was described in 1998 by Willis. The most recently discovered species, Mekosuchus kalpokasi, was named in 2002 from pieces of bones found in Vanuatu. In 2003, more fossils of Mekosuchus inexpectatus and Mekosuchus whitehunterensis were found during an expedition.
The name Mekosuchus comes from the Drehu language, which is spoken in New Caledonia. "Mek" refers to Grande Terre, and "-suchus" means crocodile.
Description
The skull of Mekosuchus was short and raised, unlike the long and flat skulls of most living crocodile species. This feature has led scientists to compare Mekosuchus with Trilophosuchus and the modern, only distantly related genus Osteolaemus, which includes dwarf crocodiles. Some researchers have also compared Mekosuchus to other land-dwelling crocodile relatives, such as notosuchians. Two different reconstructions of the Mekosuchus skull have been published, showing very different shapes. In 2007, Holt and colleagues reconstructed Mekosuchus inexpectatus with a skull similar to modern dwarf crocodiles. In 2014, Scanlon created a different reconstruction of M. whitehunterensis, showing a skull with a much gentler slope than the earlier version.
The best-known species is Mekosuchus inexpectatus, which had a unique mix of old and new features in its skull. The palatine bones, which form part of the roof of the mouth, narrow toward the back. The choanae, which connect the nasal passages to the throat, are located farther forward than in modern crocodiles and resemble those of some Late Cretaceous crocodiles, like Albertochampsa and Thoracosaurus. The wings of the pterygoid bone are well developed toward the back of the skull, and the quadratojugal lacks a spine, a feature found in alligatoroids but not in crocodylids. The postorbital bar is positioned differently from modern crocodiles, as it is not moved inward much. The external nares open toward the sides and front of the skull, not upward, and are not touched by the nasal bone. The eye sockets were large and well-developed, and uniquely, part of the maxilla formed the edge of the sockets, preventing the jugal and lacrimal bones from touching. This feature is a key trait of the genus.
Like many crocodiles, the teeth of Mekosuchus are arranged in a wave-like pattern called festooning. This pattern is less noticeable in long-snouted species like gharials but more pronounced in short-snouted species. M. whitehunterensis shows some festooning in the maxilla, while M. kalpokasi has a more extreme wave pattern. While festooning might be more exaggerated in young individuals, studies on M. kalpokasi confirm that it is an adult.
Other skull features help distinguish the four Mekosuchus species. The front edge of the palatal fenestrae (openings in the roof of the mouth) reaches the 6th tooth in M. sanderi and M. inexpectatus but only the 7th in M. kalpokasi and M. whitehunterensis. M. whitehunterensis has a furrow beneath the eyes, while M. sanderi has a crest on the squamosal bone. The mandibular symphysis (the fused front part of the lower jaw) extends to the 7th dentary tooth in M. inexpectatus but ends at the 6th in M. whitehunterensis. This prevents the splenial bone from contributing to the symphysis, as it only reaches the 7th dentary tooth in all species. The mandibular fenestra is nearly closed in M. whitehunterensis, and the angular and surangular bones have outward flanges, both of which are unique to Mekosuchus.
Some postcranial remains, such as vertebrae, are known, especially from M. inexpectatus and M. whitehunterensis. The vertebrae of M. whitehunterensis are described in more detail and are procoelous. The cervical (neck) vertebrae are shorter than those of modern freshwater crocodiles, even when considering Mekosuchus's small size. This suggests that M. inexpectatus may have had a shortened neck. The axis vertebra has a sloping neural spine typical of crocodiles but resembles alligatorids more than crocodylids. The neural spines are taller than in similarly sized animals but not as steeply angled as in modern crocodiles. This has been interpreted as evidence of strong neck muscles in Mekosuchus. The neck anatomy of M. whitehunterensis may balance mobility with strong muscles. Similar vertebrae are found in M. inexpectatus, Trilophosuchus, and Volia, suggesting this trait was common among some Mekosuchus relatives.
The humerus (upper arm bone) of Mekosuchus is similar to that of modern monitor lizards, and well-developed muscle attachment points are noted. A 2013 study mentions that the calcaneus (heel bone) has a strong, unusually short tuber.
Osteoderms (bony armor) are known from various species, especially M. inexpectatus and Oligocene mainland species. The dorsal and tail osteoderms of mainland species are highly modified, possibly for defense or biomechanics.
The teeth of the four known Mekosuchus species differ in shape, number, and how they fit together. For example, M. inexpectatus had 13 teeth in the lower jaw, while M. whitehunterensis had 16. Upper jaws of M. kalpokasi and M. sanderi differ, with the former having 12 maxillary teeth and the latter 13.
The shape of the teeth also varies. M. whitehunterensis had smooth, blade-like maxillary teeth that flattened toward the back of the jaw. A similar pattern is seen in M. sanderi, where teeth become compressed after the 5th tooth. Holocene species lack these blade-like teeth. In M. kalpokasi, tooth sockets suggest circular or oval-shaped teeth with no lateral compression. M. inexpectatus had bulbous, molariform teeth (better for crushing) in the back of the jaw, similar to those in unrelated eusuchians like Allognathosuchus, Bernissartia, and modern dwarf crocodiles.
The way maxillary teeth fit together also differs. Modern crocodiles have interfingering teeth, while alligators have an overbite. Some Mekosuchus species show a mix of these patterns. M. inexpectatus
Phylogeny
When first describing Mekosuchus, Balouet and Buffetaut had difficulty figuring out its connection to modern crocodilians. They observed that the group showed a mix of basic and advanced traits that did not clearly match any modern crocodile families. They later concluded that Mekosuchus was a Eusuchian based on features like the choanae and procoelous vertebrae, placing it in the family Mekosuchidae. They believed this family was closely related to all three modern crocodile families. Later research on Australasian crocodiles expanded the family to include more species, now called Mekosuchinae. Though scientists still do not fully understand the relationships within Mekosuchinae, Mekosuchus is traditionally grouped with other altirostral forms like Trilophosuchus and Quinkana. Willis (1997) suggested Mekosuchus and Trilophosuchus were closely related, with Quinkana as their relative. Mead et al. (2002) placed Mekosuchus, Quinkana, and an unnamed Volia in a large group as relatives to Trilophosuchus within Mekosuchini. A 2018 study by Lee & Yates using physical traits, DNA data, and fossil ages found similar results, though the exact relationships within Mekosuchini varied slightly. In this study, Trilophosuchus was the closest relative of Quinkana, with Mekosuchus as their sister group, and Volia as the earliest member of Mekosuchini.
Kambara implexidens †
Kambara murgonensis †
Bullock Creek taxon †
The most recent analysis by Ristevski et al. (2023) focused on Australasian crocodiles, including living and ancient species. Six out of eight studies found Mekosuchinae to be a group with a single common ancestor, similar to earlier findings by Lee and Yates. These studies grouped most mekosuchines into Mekosuchini, which split into two groups: large, land-based species and small or island-dwelling species. The small group included Volia and Trilophosuchus, similar to earlier ideas about Mekosuchus. However, "Baru" huberi was found to be the earliest member of this group, while Quinkana was placed in the large, land-based group. The other two studies placed Kambara and Australosuchus outside Mekosuchinae and included Orientalosuchina, a group of small, ancient crocodiles from Asia, in Mekosuchinae. However, these findings had weak support due to limited evidence. Despite differences, most studies agreed that Mekosuchus is closely related to a group containing "Baru" huberi, Volia, Trilophosuchus, and itself. Similar results were also found by Yates and Stein when reviewing Ultrastenos and "Baru" huberi.
"Crocodylus" megarhinus †
Kalthifrons aurivellensis †
Australosuchus clarkae †
Trilophosuchus rackhami †
Volia athollandersoni †
Mekosuchus whitehunterensis †
Mekosuchus inexpectatus †
Paleobiogeography
Fossil evidence shows that Mekosuchus first lived on mainland Australia. However, scientists know little about how it spread to the South Pacific. Today, three mekosuchine species are known from the region: M. inexpectatus, M. kalpokasi, and Volia. M. inexpectatus may have lived for the longest time among these species, with estimates suggesting it first appeared about 4,000 years ago. This species is only found on New Caledonia, which is the closest island to mainland Australia. Some lizards live on both New Caledonia and Vanuatu, with 12% of their species being shared. The differences and similarities in the wildlife of these islands may be influenced by the geology of the Inner and Outer Melanesian Arc. The Inner Melanesian Arc separated from Australia during the Cretaceous period, while the Outer Melanesian Arc formed later during the Paleogene and Neogene periods. Since mekosuchines first appeared during the Eocene, Mead and colleagues believe that continental drift and breakup could not have caused their spread to the South Pacific. Instead, scientists think the ancestors of these animals likely swam short distances across the ocean or traveled on natural rafts to reach the islands of the Inner Melanesian Arc. From there, they may have moved between islands in the South Pacific. It is not clear if mekosuchines could tolerate saltwater or had adaptations like salt glands found in modern crocodiles. However, they may have swum or drifted between landmasses. Lower sea levels during the late Cenozoic likely helped this process by reducing the distance between islands and sometimes connecting them. These larger land areas could have acted as rest stops or supported populations during their movement. Because of this, scientists believe Mekosuchus spread to the South Pacific relatively recently. Mead and colleagues suggest the earliest possible time was the Oligocene, but a more recent Quaternary dispersal is considered more likely.
Paleobiology
Mekosuchus, like some of its closest relatives, is believed to have lived on land. Evidence for this comes from parts of its body. Its skull is altirostral, similar to extinct land-dwelling animals like Notosuchians and members of the Planocraniidae. In contrast, semi-aquatic crocodiles usually have flat, platyrostral skulls, which help them reduce drag and keep their eyes and nose above water without alerting prey. In Mekosuchus, the eyes and nares (nostrils) are not adapted for underwater life. The nares open toward the front of the skull, and the eyes face sideways, not upward. Balouet and Buffetaut also note strong muscle attachments and the absence of freshwater in the area where fossils were found. They point out that karstic environments are often linked to land-dwelling crocodylomorphs. In 1995, Australian paleontologist Paul Willis suggested that animals like Mekosuchines may have filled a role similar to modern monitor lizards, even proposing they could climb trees. However, later research dismissed this idea because monitor lizards were already in Australia longer than Willis thought, and analysis of Mekosuchus toe bones showed no signs of exceptional climbing ability.
The strong neck muscles of Mekosuchus whitehunterensis are thought to help it tear flesh from carcasses. Modern crocodiles do this by shaking their heads side to side or using the death roll maneuver. However, Mekosuchus was small, making the death roll less effective than in larger species. Head shaking is more common in small animals, like juveniles. Stein, Archer, and Hand argue that strong back muscles would improve head shaking but make the death roll riskier on land. Mekosuchus may also have pulled its head upward to strip flesh from carcasses, a behavior seen in older archosaurs. Whether this feeding method was used for hunting large prey or scavenging is unclear, but researchers suggest it was especially useful for scavenging, allowing small animals to eat large amounts of food.
Mainland Mekosuchus species are found in areas with fossils of other mekosuchines, suggesting they may have lived together. The White Hunter Site, where M. whitehunterensis was found, also has fossils of Baru wickeni, Ultrastenos, and Quinkana meboldi. The younger Ringtail Site preserves another Baru species, Mekosuchus sanderi, and Trilophosuchus. Scientists have proposed several reasons for how many crocodiles could coexist. Differences in skull shapes may have allowed them to occupy different niches without competing. It is also possible that fossil groupings (thanatocoenosis) do not reflect their actual habitats. Willis noted that mammal fossils in the Riversleigh area show distinct ecological roles, suggesting the crocodiles were sympatric (living in the same area). He highlighted Trilophosuchus, a box-headed, land-dwelling crocodile similar to Mekosuchus, which may have shared a niche with other crocodiles. However, they may have been more different than currently thought.
Unlike the sharp teeth of mainland Mekosuchus species, Mekosuchus inexpectatus had specialized back teeth suited for crushing hard-shelled invertebrates like mollusks, crustaceans, and insects. Balouet and Buffetaut suggest it may have eaten mollusks of the genus Placostylus, common in New Caledonia. Holt and colleagues, based on newer fossils and similarities to modern dwarf crocodiles, propose M. inexpectatus may have lived like modern dwarf crocodiles or caimans. They suggest it may have lived in small, slow streams in New Caledonia’s rainforests, foraging at night near water and on land.
The extinction of Mekosuchus in the South Pacific has been linked to human settlers, particularly the Lapita people. Supporters point to the overlap between Mekosuchus’ range and human settlements in Vanuatu and the association of Mekosuchus kalpokasi bones with human artifacts at the Arapus site on Efate. If humans caused their extinction, factors like invasive animals (pigs, rats), habitat loss, and hunting may have contributed. However, this idea is not universally accepted. Anderson and colleagues note that most Mekosuchus inexpectatus remains were found before humans arrived in New Caledonia, with only one jaw bone overlapping with human presence. They also state there is no evidence humans caused the crocodiles’ extinction.