Trilophosuchus
Encyclopedia
Trilophosuchus is an extinct genus
of the crocodilian subfamily
Mekosuchinae
. Unlike living crocodilians, it is hypothesized to have been terrestrial
. Trilophosuchus was approximately 1.5 metres (4.9 ft) in length. It had a short skull with three ridges on top and large eyes. Fossils have been found at Riversleigh
in north-western Queensland
, and are Miocene
in age. Only a single species has been described, the type species
T. rackhami.
Trilophosuchus is known from a posterior portion of the skull, QM
F16856, the holotype
specimen. Several other isolated bones of the skull were found. The material was collected from the Ringtail Site of the Gag Plateau in Riversleigh
, one of Australia's most famous fossil localities. The skull was uncovered in 1985 during an excavation by the University of New South Wales
. The deposit in which Trilophosaurus were found are Early Miocene
in age, roughly 20 million years old. The deposit, known as System C, is the youngest of the Oligocene
-Miocene sequences at Riversleigh. The genus Trilophosuchus was erected in 1993 with the description of the remains by Paul Willis of the University of Sydney
in the Journal of Vertebrate Paleontology
.
, Osteolaemus, and caiman, Paleosuchus
. It also resembles earlier crocodyliforms such as the notosuchia
ns, atoposaurid
s, and protosuchia
ns. Being a mekosuchine, Trilophosuchus was not closely related to any of these forms and likely acquired a short snout and small body through evolutionary convergence.
In addition to its small size and short snout, Trilophosuchus has several unique characteristics, or autapomorphies, that distinguish it from other crocodylians. A posterior dentary tooth of the lower jaw passes between the sixth and seventh maxilla
ry teeth of the upper jaw, leaving a deep groove in the maxilla.The palatal fenestrae, two openings in the palate
, reach up to the level of the sixth maxillary teeth. The palatine bone
has an anterior process, or projection, that reaches to the level of the fourth maxillary teeth. Behind the palatine and the palatal fenestra, there are large pterygoids and ectopterygoids that project posteriorly and extend below the skull. The jugal
bones, which line the side of the skull below the eyes, project laterally to form prominent ridges. The supratemporal fenestrae, two holes on the skull table
, are long and narrow. There are also three ridges running longitudinally on the skull table, giving the genus its name, which means "triple-crested crocodile" in Greek
.
The occipital or posterior surface of the skull has structures for the insertion of muscles that would have held the head above the body.
Willis, 1993
Mead et al., 2002
Despite its resemblace to early crocodyliforms such as protosuchians and notosuchians, Trilophosuchus is an advanced crocodylian. It possesses a postorbital bar behind the eye socket that is depressed into the skull, a derived
feature of the clade
Neosuchia
. It can be united with the eusuchians because its internal nares are contained within the pterygoid bones and the teeth extend behind the level of the anterior edges of the palatal fenestrae. Based on several distinctive characteristics and characteristics shared with other crocodylians, Trilophosuchus was initially found to be the most basal
of the known mekosuchines. More recently, older mekosuchines such as Kambara
have been found in Australia that are more basal than Trilophosuchus. Later phylogenetic studies have placed Trilophosuchus as a more derived mekosuchine, more closely related to younger forms such as Quinkana
and Mekosuchus
. It has been placed in the derived mekosuchine clade Mekosuchini to distinguish it from more basal taxa such as Kambara and Australosuchus.
radiation of mekosuchines in Australia. Other mekosuchines that lived in Australia at the time included Pallimnarchus
, Australosuchus
, and Quinkana
. Pallimnarchus and Australosuchus were broad-snouted, generalized crocodylians, while Quinkana was ziphodont, meaning it had a deep snout. Trilophosuchus is unusual among mekosuchines, and crocodilians in general, in that it had a short-deep snout and an arboreal lifestyle. It represented a third adaptive type in the early radiation of Australian mekosuchines. All of these early mekosuchines were terrestrial, unlike their semiaquatic crocodile relatives.
The paleoecology of Trilophosuchus is difficult to determine because of a poor understanding of the ecology of the living crocodilians that have the most similar appearance to the genus: Paleosuchus and Osteolaemus. Both genera are semiaquatic, and have been proposed to inhabit small streams in closed-canopy forests, fast-flowing streams, larger, slow moving rivers, and lakes. While Paleosuchus and Osteolaemus are the most similar living crocodlylians to Trilophosuchus, Trilophosuchus more closely resembles early crocodyliforms such as notosuchians and protosuchians that were entirely terrestrial.
In the first description of Trilophosaurus, Paul Willis tentatively considered the genus to be terrestrial, because most other extinct crocodyliforms with short snouts and laterally directed eyes were terrestrial. The arrangements of the neck muscles of Trilophosaurus, evidenced by the shape of the occipital portion of the skull, suggests that it held its head high, a posture more suitable for a terrestrial rather than semiaquatic lifestyle. Compared to living animals, Trilophosuchus probably had a posture more like monitor lizard
s than crocodiles. The short length of its snout, contrasting with the longer snouts of larger carnivorous crocodylians such as Pristichampsus
, suggests that Trilophosaurus restricted its diet to smaller prey. It may have consumed fish, turtles, snakes, and small mammals, all of which have been found from Riversleigh. The neck musculature of Trilophosuchus may have enabled it to make rapid movements of the head when feeding, up and down, side to side, and rotationally.
It has been suggested by Paul Willis that the mekosuchine Mekosuchus inexpectatus was arboreal, given its small size. An arboreal lifestyle is otherwise unknown in crocodyliforms. Given that Trilophosuchus is similar appearance to M. inexpectatus, it has often been suggested that it too was arboreal. Mekosuchus inexpectatus has a limb bone structure that may be compatible with an arboreal lifestyle, but there is no evidence for climbing abilities in Trilophosuchus. Because of the lack of evidence, an arboreal lifestyle is not agreed upon by other paleontologists.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of the crocodilian subfamily
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Mekosuchinae
Mekosuchinae
Mekosuchinae was a subfamily of crocodiles from Australia and the South Pacific that have now become extinct. They first appear in the fossil record in the Eocene in Australia, and survived until the Pleistocene in Australia and until the arrival of humans in the Pacific islands of Fiji, New...
. Unlike living crocodilians, it is hypothesized to have been terrestrial
Terrestrial animal
Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...
. Trilophosuchus was approximately 1.5 metres (4.9 ft) in length. It had a short skull with three ridges on top and large eyes. Fossils have been found at Riversleigh
Riversleigh
Riversleigh, in North West Queensland, is Australia's most famous fossil site. The 100 km² area has fossil remains of ancient mammals, birds and reptiles of Oligocene and Miocene age...
in north-western Queensland
Queensland
Queensland is a state of Australia, occupying the north-eastern section of the mainland continent. It is bordered by the Northern Territory, South Australia and New South Wales to the west, south-west and south respectively. To the east, Queensland is bordered by the Coral Sea and Pacific Ocean...
, and are Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
in age. Only a single species has been described, the type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
T. rackhami.
Discovery
Queensland Museum
The Queensland Museum is the state museum of Queensland. The museum currently operates four separate campuses; at South Brisbane, Ipswich, Toowoomba and Townsville.The museum is funded by the State Government of Queensland.-History:...
F16856, the holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
specimen. Several other isolated bones of the skull were found. The material was collected from the Ringtail Site of the Gag Plateau in Riversleigh
Riversleigh
Riversleigh, in North West Queensland, is Australia's most famous fossil site. The 100 km² area has fossil remains of ancient mammals, birds and reptiles of Oligocene and Miocene age...
, one of Australia's most famous fossil localities. The skull was uncovered in 1985 during an excavation by the University of New South Wales
University of New South Wales
The University of New South Wales , is a research-focused university based in Kensington, a suburb in Sydney, New South Wales, Australia...
. The deposit in which Trilophosaurus were found are Early Miocene
Early Miocene
The Early Miocene is a sub-epoch of the Miocene Epoch made up of two stages: the Aquitanian and Burdigalian stages....
in age, roughly 20 million years old. The deposit, known as System C, is the youngest of the Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
-Miocene sequences at Riversleigh. The genus Trilophosuchus was erected in 1993 with the description of the remains by Paul Willis of the University of Sydney
University of Sydney
The University of Sydney is a public university located in Sydney, New South Wales. The main campus spreads across the suburbs of Camperdown and Darlington on the southwestern outskirts of the Sydney CBD. Founded in 1850, it is the oldest university in Australia and Oceania...
in the Journal of Vertebrate Paleontology
Journal of Vertebrate Paleontology
The Journal of Vertebrate Paleontology was founded in 1980 at the University of Oklahoma by Dr. Jiri Zidek. It is a scientific journal that publishes original contributions on all aspects of the vertebrate paleontology, including vertebrate origins, evolution, functional morphology, taxonomy,...
.
Description
Trilophosuchus had a short, deep skull and relatively large eyes. Although only the skull is known, Trilophosuchus is estimated to have been around 1.5 metres (4.9 ft) in length. Because of its small size, Trilophosuchus resembles the living dwarf crocodileDwarf Crocodile
The dwarf crocodile is an African species of crocodile. It is also the smallest extant crocodile species in the world. Recent sampling has identified three genetically distinct populations...
, Osteolaemus, and caiman, Paleosuchus
Paleosuchus
Paleosuchus is a South American genus of reptile in the Alligatoridae family. They are the smallest members of the order Crocodilia in the Americas.The genus contains two species:* Cuvier's Dwarf Caiman ....
. It also resembles earlier crocodyliforms such as the notosuchia
Notosuchia
Notosuchia is a suborder of primarily Gondwanan mesoeucrocodylian crocodylomorphs that lived during the Cretaceous. Fossils have been found from South America, Africa, and Asia...
ns, atoposaurid
Atoposauridae
Atoposauridae is a family of crocodile-like archosaurs. Members of the family have been found from France, Spain, Portugal, Romania, Germany, Kazakhstan, the United Kingdom, the United States, Cameroon, and Thailand.-Phylogeny:...
s, and protosuchia
Protosuchia
Protosuchia is a group of extinct Mesozoic crocodyliforms. They were small in size and terrestrial. In phylogenetic terms, Protosuchia is considered an informal group because it is a grade of basal crocodyliforms, not a true clade....
ns. Being a mekosuchine, Trilophosuchus was not closely related to any of these forms and likely acquired a short snout and small body through evolutionary convergence.
In addition to its small size and short snout, Trilophosuchus has several unique characteristics, or autapomorphies, that distinguish it from other crocodylians. A posterior dentary tooth of the lower jaw passes between the sixth and seventh maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
ry teeth of the upper jaw, leaving a deep groove in the maxilla.The palatal fenestrae, two openings in the palate
Palate
The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...
, reach up to the level of the sixth maxillary teeth. The palatine bone
Palatine bone
The palatine bone is a bone in many species of the animal kingdom, commonly termed the palatum .-Human anatomy:...
has an anterior process, or projection, that reaches to the level of the fourth maxillary teeth. Behind the palatine and the palatal fenestra, there are large pterygoids and ectopterygoids that project posteriorly and extend below the skull. The jugal
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....
bones, which line the side of the skull below the eyes, project laterally to form prominent ridges. The supratemporal fenestrae, two holes on the skull table
Skull roof
The skull roof , or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land living vertebrates. The bones are derived from dermal bone, hence the alternative name dermatocranium...
, are long and narrow. There are also three ridges running longitudinally on the skull table, giving the genus its name, which means "triple-crested crocodile" in Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
.
The occipital or posterior surface of the skull has structures for the insertion of muscles that would have held the head above the body.
Classification
Despite its resemblace to early crocodyliforms such as protosuchians and notosuchians, Trilophosuchus is an advanced crocodylian. It possesses a postorbital bar behind the eye socket that is depressed into the skull, a derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
feature of the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
Neosuchia
Neosuchia
Neosuchia is an unranked clade within Mesoeucrocodylia that includes all modern extant crocodilians and their closest fossil relatives. It is defined as the most inclusive clade containing all crocodylomorphs more closely related to Crocodylus niloticus than to Notosuchus terrestris...
. It can be united with the eusuchians because its internal nares are contained within the pterygoid bones and the teeth extend behind the level of the anterior edges of the palatal fenestrae. Based on several distinctive characteristics and characteristics shared with other crocodylians, Trilophosuchus was initially found to be the most basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
of the known mekosuchines. More recently, older mekosuchines such as Kambara
Kambara
Kambara is an extinct genus of mekosuchine crocodylian that lived during the Eocene and Oligocene epochs in Australia.At around 55 million years old, remains of Kambara are among the oldest Tertiary fossils found in Australia . Kambara is the oldest known mekosuchine...
have been found in Australia that are more basal than Trilophosuchus. Later phylogenetic studies have placed Trilophosuchus as a more derived mekosuchine, more closely related to younger forms such as Quinkana
Quinkana
Quinkana is an extinct genus of mekosuchine crocodile that lived in Australia from ~24 million years ago to ~40,000 years ago. By the Pleistocene Quinkana had become one of the top terrestrial predators of Australia, possessing long legs and ziphodont teeth .Ziphodont teeth tend to arise in...
and Mekosuchus
Mekosuchus
Mekosuchus is a genus of extinct Australasian crocodiles within the subfamily Mekosuchinae. They are believed to have been made extinct by the arrival of man on the South Pacific islands where they lived...
. It has been placed in the derived mekosuchine clade Mekosuchini to distinguish it from more basal taxa such as Kambara and Australosuchus.
Paleobiology
Trilophosuchus is representative of an early CenozoicCenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
radiation of mekosuchines in Australia. Other mekosuchines that lived in Australia at the time included Pallimnarchus
Pallimnarchus
Pallimnarchus is a genus extinct mekosuchine crocodylian from the Pliocene and Pleistocene of Australia. Remains of this animal have been found in the Riversleigh lagerstätte of northwestern Queensland. It was medium-sized, up to 5 meters long...
, Australosuchus
Australosuchus
Australosuchus is an extinct genus of crocodilian belonging to the subfamily Mekosuchinae.The type and only known species Australosuchus clarkae lived during the Late Oligocene and the Early Miocene of southern Australia. The generic name Australosuchus means "Southern crocodile"...
, and Quinkana
Quinkana
Quinkana is an extinct genus of mekosuchine crocodile that lived in Australia from ~24 million years ago to ~40,000 years ago. By the Pleistocene Quinkana had become one of the top terrestrial predators of Australia, possessing long legs and ziphodont teeth .Ziphodont teeth tend to arise in...
. Pallimnarchus and Australosuchus were broad-snouted, generalized crocodylians, while Quinkana was ziphodont, meaning it had a deep snout. Trilophosuchus is unusual among mekosuchines, and crocodilians in general, in that it had a short-deep snout and an arboreal lifestyle. It represented a third adaptive type in the early radiation of Australian mekosuchines. All of these early mekosuchines were terrestrial, unlike their semiaquatic crocodile relatives.
The paleoecology of Trilophosuchus is difficult to determine because of a poor understanding of the ecology of the living crocodilians that have the most similar appearance to the genus: Paleosuchus and Osteolaemus. Both genera are semiaquatic, and have been proposed to inhabit small streams in closed-canopy forests, fast-flowing streams, larger, slow moving rivers, and lakes. While Paleosuchus and Osteolaemus are the most similar living crocodlylians to Trilophosuchus, Trilophosuchus more closely resembles early crocodyliforms such as notosuchians and protosuchians that were entirely terrestrial.
In the first description of Trilophosaurus, Paul Willis tentatively considered the genus to be terrestrial, because most other extinct crocodyliforms with short snouts and laterally directed eyes were terrestrial. The arrangements of the neck muscles of Trilophosaurus, evidenced by the shape of the occipital portion of the skull, suggests that it held its head high, a posture more suitable for a terrestrial rather than semiaquatic lifestyle. Compared to living animals, Trilophosuchus probably had a posture more like monitor lizard
Monitor lizard
Monitor lizards are usually large reptiles, although some can be as small as in length. They have long necks, powerful tails and claws, and well-developed limbs. Most species are terrestrial, but arboreal and semiaquatic monitors are also known...
s than crocodiles. The short length of its snout, contrasting with the longer snouts of larger carnivorous crocodylians such as Pristichampsus
Pristichampsus
Pristichampsus is an extinct genus of pristichampsid crocodylian that grew to approximately three metres in length....
, suggests that Trilophosaurus restricted its diet to smaller prey. It may have consumed fish, turtles, snakes, and small mammals, all of which have been found from Riversleigh. The neck musculature of Trilophosuchus may have enabled it to make rapid movements of the head when feeding, up and down, side to side, and rotationally.
It has been suggested by Paul Willis that the mekosuchine Mekosuchus inexpectatus was arboreal, given its small size. An arboreal lifestyle is otherwise unknown in crocodyliforms. Given that Trilophosuchus is similar appearance to M. inexpectatus, it has often been suggested that it too was arboreal. Mekosuchus inexpectatus has a limb bone structure that may be compatible with an arboreal lifestyle, but there is no evidence for climbing abilities in Trilophosuchus. Because of the lack of evidence, an arboreal lifestyle is not agreed upon by other paleontologists.