Mekosuchus
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''Mekosuchus'' is a
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of
extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
Australasia Australasia is a subregion of Oceania, comprising Australia, New Zealand (overlapping with Polynesia), and sometimes including New Guinea and surrounding islands (overlapping with Melanesia). The term is used in a number of different context ...
n 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 The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...
and
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
in mainland
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
. 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 The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is bounded by the cont ...
islands of
New Caledonia New Caledonia ( ; ) is a group of islands in the southwest Pacific Ocean, southwest of Vanuatu and east of Australia. Located from Metropolitan France, it forms a Overseas France#Sui generis collectivity, ''sui generis'' collectivity of t ...
and
Vanuatu Vanuatu ( or ; ), officially the Republic of Vanuatu (; ), is an island country in Melanesia located in the South Pacific Ocean. The archipelago, which is of volcanic origin, is east of northern Australia, northeast of New Caledonia, east o ...
respectively and represent some of the youngest known mekosuchines. ''Mekosuchus'' possibly died out approximately 3,000 years ago, during the
Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...
, 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 Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of Arthropod, arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquat ...
and
molluscs Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...
. Some researchers suggest that they were possibly
nocturnal Nocturnality is a ethology, behavior in some non-human animals characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnality, diurnal meaning the opposite. Nocturnal creatur ...
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 New Caledonia ( ; ) is a group of islands in the southwest Pacific Ocean, southwest of Vanuatu and east of Australia. Located from Metropolitan France, it forms a Overseas France#Sui generis collectivity, ''sui generis'' collectivity of t ...
, 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 A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...
to skull fragments and isolated vertebrae. Such fossils were first reported by
Eric Buffetaut The given name Eric, Erich, Erikk, Erik, Erick, Eirik, or Eiríkur is derived from the Old Norse name ''Eiríkr'' (or ''Eríkr'' in Old East Norse due to monophthongization). The first element, ''ei-'' may be derived from the older Proto-Nor ...
in 1983 and properly described by him and
Jean-Christophe Balouet Jean-Christophe Balouet (12 November 1956 − 31 March 2021) was a French palaeontologist. He has collaborated extensively with Storrs Olson of the Smithsonian Institution on palaeornithological research on the extinct birds of New Caledonia in ...
four years later in 1987. Due to the strange anatomy of the material, they initially assumed the animal represented an early
eusuchian Eusuchia is a clade of neosuchian crocodylomorphs that first appeared in the Early Cretaceous, which includes modern crocodilians. Along with Dyrosauridae and Sebecosuchia, they were the only crocodyliformes who survived the K-Pg extinction. ...
(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 Riversleigh World Heritage Area is Australia's most famous fossil location, recognised for the series of well preserved fossils deposited from the Late Oligocene to more recent geological periods. The fossiliferous limestone system is located n ...
in
Queensland Queensland ( , commonly abbreviated as Qld) is a States and territories of Australia, state in northeastern Australia, and is the second-largest and third-most populous state in Australia. It is bordered by the Northern Territory, South Austr ...
. 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 Drehu (; also known as Dehu, Lifou, Lifu, qene drehu) is an Austronesian language mostly spoken on Lifou Island, Loyalty Islands, New Caledonia. It has about 12,000 fluent speakers and the status of a French regional language. This status mean ...
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 In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...
, ''M. inexpectatus'', from the
Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...
of
New Caledonia New Caledonia ( ; ) is a group of islands in the southwest Pacific Ocean, southwest of Vanuatu and east of Australia. Located from Metropolitan France, it forms a Overseas France#Sui generis collectivity, ''sui generis'' collectivity of t ...
.
Radiocarbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for Chronological dating, determining the age of an object containing organic material by using the properties of carbon-14, radiocarbon, a radioactive Isotop ...
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 Vanuatu ( or ; ), officially the Republic of Vanuatu (; ), is an island country in Melanesia located in the South Pacific Ocean. The archipelago, which is of volcanic origin, is east of northern Australia, northeast of New Caledonia, east o ...
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 A prime minister or chief of cabinet is the head of the cabinet and the leader of the ministers in the executive branch of government, often in a parliamentary or semi-presidential system. A prime minister is not the head of state, but r ...
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 The Early Miocene (also known as Lower Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages: the Aquitanian age, Aquitanian and Burdigalian stages. The sub-epoch lasted from 23.03 ± 0.05 annum, Ma to ...
in what is now
Queensland Queensland ( , commonly abbreviated as Qld) is a States and territories of Australia, state in northeastern Australia, and is the second-largest and third-most populous state in Australia. It is bordered by the Northern Territory, South Austr ...
. It was named by Willis based on two maxillae and various skull fragments, all stemming from the productive
Riversleigh World Heritage Area Riversleigh World Heritage Area is Australia's most famous fossil location, recognised for the series of well preserved fossils deposited from the Late Oligocene to more recent geological periods. The fossiliferous limestone system is located n ...
, 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 The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...
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 bone In anatomy, the palatine bones (; derived from the Latin ''palatum'') are two irregular bones of the facial skeleton in many animal species, located above the uvula in the throat. Together with the maxilla, they comprise the hard palate. Stru ...
s, which form part of the roof of the mouth, narrow towards the back. The
choanae The choanae (: choana), posterior nasal apertures or internal nostrils are two openings found at the back of the nasal passage between the nasal cavity and the pharynx, in humans and other mammals (as well as crocodilians and most skinks). They ...
, 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 ''Thoracosaurus'' (chest lizard) is an extinct genus of long-snouted eusuchian which existed during the Late Cretaceous and Early Paleocene in North America and Europe. Taxonomy ''Thoracosaurus'' had traditionally been thought to be related to ...
''. The wings of the
pterygoid bone The pterygoid is a paired bone forming part of the palate of many vertebrates, behind the palatine bone In anatomy, the palatine bones (; derived from the Latin ''palatum'') are two irregular bones of the facial skeleton in many animal specie ...
are well developed towards the back of the skull and the
quadratojugal The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. Anatomy and function In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...
lacks a spine, which is a feature shared by alligatoroids but not by crocodylids. The position of the
postorbital bar The postorbital bar (or postorbital bone) is a bony arched structure that connects the frontal bone of the skull to the zygomatic arch, which runs laterally around the eye socket. It is a trait that only occurs in mammalian taxa, such as most strep ...
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 nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Eac ...
. The eye sockets were well-developed and large and, unique among crocodilians, are in part formed by the
maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...
, preventing the
jugal The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species. Anatomy ...
and
lacrimal bone The lacrimal bones are two small and fragile bones of the facial skeleton; they are roughly the size of the little fingernail and situated at the front part of the medial wall of the orbit. They each have two surfaces and four borders. Several bon ...
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 squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral ...
. The extent of the shallow
mandibular symphysis In human anatomy, the facial skeleton of the skull the external surface of the mandible is marked in the median line by a faint ridge, indicating the mandibular symphysis (Latin: ''symphysis menti'') or line of junction where the two lateral ha ...
, 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 The splenial is a small bone in the lower jaw of reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology ...
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 The surangular or suprangular is a jaw bone found in most land vertebrates, except mammals. Usually in the back of the jaw, on the upper edge, it is connected to all other jaw bones: dentary, angular bone, angular, splenial and articular. It is o ...
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 Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...
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 The freshwater crocodile (''Crocodylus johnstoni)'', also known Common name, commonly as the Australian freshwater crocodile, Johnstone's crocodile, and the freshie, is a species of crocodile native to the northern regions of Australia. Unlike ...
, 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. According to Willis, the humerus was similar in form to that of modern
monitor lizard Monitor lizards are lizards in the genus ''Varanus,'' the only extant genus in the family Varanidae. They are native to Africa, Asia, and Oceania, and West African Nile monitor, one species is also found in south America as an invasive species. A ...
s and Balouet & Buffetaut make mention of well developed insertions for the musculature. In a 2013 abstract it is mentioned that the tuber of the
calcaneus In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel; : calcanei or calcanea) or heel bone is a bone of the Tarsus (skeleton), tarsus of the foot which constitutes the heel. In some other ...
, the heel, is robust and unusually short. 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 Biomechanics is the study of the structure, function and motion of the mechanical aspects of biological systems, at any level from whole organisms to Organ (anatomy), organs, Cell (biology), cells and cell organelles, using the methods of mechani ...
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 ''Allognathosuchus'' (meaning "other jaw crocodile") is an extinct genus of alligatorine crocodylian with a complicated taxonomic history. It was named in 1921. Description ''Allognathosuchus'' was a medium-sized predator up to 1.5 m in length. ...
'', '' 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 ''Crocodylus'' is a genus of true crocodiles in the family Crocodylidae. Taxonomy The Genus, generic name, ''Crocodylus'', was proposed by Josephus Nicolaus Laurenti in 1768. ''Crocodylus'' contains 13–14 extant taxon, extant (living) species ...
'' and an overbite as seen in ''
Alligator An alligator, or colloquially gator, is a large reptile in the genus ''Alligator'' of the Family (biology), family Alligatoridae in the Order (biology), order Crocodilia. The two Extant taxon, extant species are the American alligator (''A. mis ...
'', however, some species of ''Mekosuchus'' also display an intermediate pattern, combining an overbite with some degree of interfingering. ''M. inexpectatus'' displays a full
overbite Overbite is the extent of vertical ( superior-inferior) overlap of the maxillary central incisors over the mandibular central incisors, measured relative to the incisal ridges. The term overbite does not refer to a specific condition, nor is ...
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 Christopher is the English version of a Europe-wide name derived from the Greek name Χριστόφορος (''Christophoros'' or '' Christoforos''). The constituent parts are Χριστός (''Christós''), "Christ" or "Anointed", and φέρει ...
, 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. Since then, research on Australasian crocodilians has placed a wide range of other taxa in the family, which is now referred to by the name Mekosuchinae. Although mekosuchines are still a poorly understood group whose internal and external relationships commonly shift, ''Mekosuchus'' is traditionally allied with other altirostral forms such as ''Trilophosuchus'' and ''Quinkana''. Willis (1997) suggests a close link between ''Mekosuchus'' and ''Trilophosuchus'', with ''Quinkana'' as their sister taxon, 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. A 2018
tip dating Tip dating is a technique used in molecular dating that allows the inference of time-calibrated phylogenetic trees. Its defining feature is that it uses the ages of the samples to provide time information for the analysis, in contrast with traditio ...
study by Lee & Yates using a combination of morphological, molecular (
DNA sequencing DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, thymine, cytosine, and guanine. The ...
), and
stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...
(
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
age) data recovered broadly similar results, although the precise relations within Mekosuchini do differ. Here, ''Trilophosuchus'' was recovered as the closest relative of ''Quinkana'', with ''Mekosuchus'' being the sister taxon to their grouping and ''Volia'' as the basalmost mekosuchinin. 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 ''Kambara'' is an extinct genus of Mekosuchinae, mekosuchine Crocodilia, crocodylian that lived during the Eocene epoch in Australia. It is generally thought to have been a semi-aquatic generalist, living a lifestyle similar to many of today's ...
'' and '' Australosuchus'' being recovered elsewhere in Crocodylia and Mekosuchinae also including the clade ''
Orientalosuchina Orientalosuchina is an extinct clade of alligatoroid crocodylians from Southeast Asia, Southeast and East Asia that lived between Maastrichtian and Eocene. The clade was named as the result of a 2019 study by Massonne ''et al.'' that included se ...
'', 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 skull is altirostral, similar to extinct terrestrial forms like
Notosuchia Notosuchia is a clade of primarily Gondwanan mesoeucrocodylian Crocodylomorpha, crocodylomorphs that lived during the Jurassic and Cretaceous. Some phylogeny, phylogenies recover Sebecosuchia as a clade within Notosuchia, others as a sister group ...
ns and members of the
Planocraniidae Planocraniidae is an extinct family of eusuchian crocodyliforms known from the Paleogene of Asia, Europe and North America. The family was coined by Li in 1976, and contains three genera, '' Boverisuchus'', '' Duerosuchus'' and '' Planocrania' ...
, while semi-aquatic crocodilians typically have flattened platyrostral skulls, adapted to reduce drag and allow raising the eyes and nose out of the water without drawing the attention of potential prey items. In ''Mekosuchus'', both the eyes and nares are not built for an aquatic mode of life. Rather than opening towards the top of the skull, which would allow the animal to breathe while remaining largely submerged, the nares open towards the front of the skull, and the eyes are similarly directed towards the sides, not the top. Balouet and Buffetaut further point to the well developed muscular insertions and the absence of freshwater in the deposition area, while pointing out that karstic environments are often associated with terrestrial crocodylomorphs. In 1995 Australian paleontologist
Paul Willis Paul Willis (born 1945) is a British social scientist known for his work in sociology and cultural studies. Paul Willis' work is widely read in the fields of sociology, anthropology, and education, his work emphasizing consumer culture, sociali ...
informally suggested that animals like ''Mekosuchines'' may have filled a niche equivalent to modern
monitor lizards Monitor lizards are lizards in the genus ''Varanus,'' the only extant genus in the family Varanidae. They are native to Africa, Asia, and Oceania, and West African Nile monitor, one species is also found in south America as an invasive species. A ...
, even going as far as to suggest arboreal (climbing) habits. However, this idea has been dismissed by more recent research, as monitor lizards had been present in Australia for longer than assumed by Willis, while analysis of mekosuchine toe bones showed no significant differences to those of other crocodilians and thus not supporting the notion that they were exceptional climbers.Darren Naish, "Tetrapod Zoology": "The small, recently extinct, island-dwelling crocodilians of the south Pacific", 2006
/ref> 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 In a keel boat, a death roll is the act of broaching to windward, putting the spinnaker pole into the water and causing a crash-jibe of the boom and mainsail, which sweep across the deck and plunge down into the water. The death roll often resu ...
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 Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...
. 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. These mainland species are known from localities that have also preserved the fossilised remains of multiple other mekosuchines, which they may have coexisted with. The White Hunter Site that yielded ''M. whitehunterensis'' also preserved the broad-snouted generalist '' Baru wickeni'' and the narrower-snouted '' Ultrastenos'' as well as the terrestrial ziphodont ''Quinkana meboldi''. The younger Ringtail Site of the Riversleigh on the other hand preserves another species of ''Baru'', ''Mekosuchus sanderi'' and ''Trilophosuchus''. 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 Thanatocoenosis (from Greek language ''thanatos'' - death and ''koinos'' - common) are all the embedded fossils at a single discovery site. This site may be referred to as a "death assemblage". Such groupings are composed of fossils of organisms whi ...
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 In biology, two closely related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter each other. An initially interbreeding population that splits into two or more distinct spe ...
. Willis does take particular note of ''Trilophosuchus'', which was a box-headed terrestrial form similar to ''Mekosuchus'' and thus may have inhabited a similar niche as opposed to the much larger, semi-aquatic crocodilians of the site. It is however possible that they were morphologically and ecologically much more different than currently thought and that the similarities may simply be exacerbated by the lack of better material. Unlike the bladed teeth of the mainland species, ''Mekosuchus inexpectatus'' had specialized back teeth more suited for cracking hard-shelled invertebrates such as molluscs, crustaceans and insects. Balouet and Buffetaut suggest that it may have fed on molluscs of the genus ''
Placostylus ''Placostylus'', or flax snails, are a genus of very large, air-breathing land snails, Terrestrial animal, terrestrial pulmonate gastropod molluscs in the family (biology), family Bothriembryontidae.MolluscaBase eds. (2020). MolluscaBase. Placost ...
'', which was common on New Caledonia. Based on newer material and the previously noted similarities between ''Mekosuchus'' and modern dwarf crocodiles, Holt and colleagues suggest that ''M. inexpectatus'' could have possibly lived a similar lifestyle to the modern dwarf crocodiles (''Osteolaemus spp.'') or dwarf caimans (''Paleosuchus spp.''). According to their hypothesis, ''M. inexpectatus'' may have inhabited small, slow moving streams in the rainforests of New Caledonia and foraged at night near the waters edge and on land.


Extinction

The extinction of ''Mekosuchus'' in the South Pacific has historically been linked to the arrival of human settlers, in particular the Lapita people. Supporters of this hypothesis point at the fact that the range of ''Mekosuchus'' overlaps with human settlement of Vanuatu and the direct association between the bones of ''Mekosuchus kalpokasi'' with human artifacts at the Arapus archaeological site on the island of Efate. If the extinction of this taxon was linked to the arrival of humans, there may have been multiple factors contributing to their disappearance. These include the introduction of invasives like
pigs The pig (''Sus domesticus''), also called swine (: swine) or hog, is an omnivorous, domesticated, even-toed, hoofed mammal. It is named the domestic pig when distinguishing it from other members of the genus '' Sus''. Some authorities cons ...
and
rats Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' (pack rats), '' Bandicota'' (bandicoot ...
,
habitat destruction Habitat destruction (also termed habitat loss or habitat reduction) occurs when a natural habitat is no longer able to support its native species. The organisms once living there have either moved elsewhere, or are dead, leading to a decrease ...
and being used as a food source. However, this idea is not universally accepted and has been disputed by other researchers. Anderson and colleagues for instance note that in the case of ''Mekosuchus inexpectatus'', most remains were deposited prior to human settlement of New Caledonia, with only a single mandible overlapping with human presence. They further highlight that no evidence exists of humans contributing to the crocodilians extinction.


See also

* Biodiversity of New Caledonia *
Holocene extinction The Holocene extinction, also referred to as the Anthropocene extinction or the sixth mass extinction, is an ongoing extinction event caused exclusively by human activities during the Holocene epoch. This extinction event spans numerous families ...


References

{{Taxonbar, from=Q604542, from2=Q371172, from3=Q1918871, from4=Q117466737, from5=Q117466756 Mekosuchinae Neogene crocodylomorphs Oligocene crocodylomorphs Miocene crocodylomorphs Terrestrial crocodylomorphs New Caledonia Holocene fauna Holocene extinctions Pliocene crocodylomorphs Pleistocene crocodylomorphs Taxa named by Jean-Christophe Balouet Prehistoric pseudosuchian genera