Platyognathus
Encyclopedia
Platyognathus is an extinct genus
of protosuchia
n crocodylomorph. Fossils are known from the Early Jurassic
Lower Lufeng Formation in Yunnan
, China
and belong to the type
and only species, P. hsui.
. Two teeth, dentaries 5 and 6, are enlarged into caniniforms in the lower jaw. Paired caniniform teeth are not seen in any other described crocodyliform taxa, but they have been recorded from an unidentified crocodyliform from the Lower Jurassic Kayenta Formation
and from a protosuchid from the Lower Jurassic McCoy Brook Formation
in Nova Scotia
.
jaw consisted of 11 alveoli (tooth sockets) on the left side and 12 alveoli on the right side, in addition to one broken caniniform tooth. Several diagnostic features were mentioned, including an expansion at the tip of the jaw, a tooth cross section that is octangular in outline, and a lateral constriction of the jaw behind the caniniform tooth, which is followed by an expansion further back. The anterior teeth are small and angled forward, while there is evidence in the jaw of at least two caniniform teeth that were larger and oriented obliquely and anteriorly. Further back in the jaw, the posterior teeth are about the same size and project vertically. The entire jaw is well ossified, and sutures between the bones could not be identified in the holotype. The symphysis, the point where the two sides of the jaw come together, is represented by a broad ventral groove.
Based on these features, Young found no close similarities between the jaw of Platyognathus and any other known early crocodylomorph such as Notochampsa
, Sphenosuchus
, Erythrochampsa
, and Pedeticosaurus
(at the time, these crocodylomorphs were collectively known as the "Pseudosuchia
"). The jaw of Protosuchus richardsoni
, described by Barnum Brown
in 1933 from Arizona
, is similar in size to that of Platyognathus, although it lacks many of the distinguishing features of the Chinese genus as described by Young. Because Platyognathus differed from any other known pseudosuchian, Young suggested that it may belong to its own family.
During World War II
, the holotype was either lost or destroyed. The classification of the genus continued to be debated, as descriptions of the fragmentary jaw were all that was available to study. Platyognathus continued to be referred to Pseudosuchia in the following years, and was suggested to have a close relationship with Sphenosuchus. In 1955, it was suggested to have affinities with aetosaur
s, and in 1956 Alfred Romer
tentatively assigned it to the Notochampsidae (now known as the Protosuchidae
).
The new skull revealed an additional diagnostic feature of Platyognathus that distinguishes it from other protosuchians: the distinctive curvature of the jugal
. Additional diagnostic characters were proposed by the authors, but their character-states cannot be determined in other protosuchian taxa. These characters include a width of the mandibular symphysis across the swellings of the caniniform teeth that is almost equal to the anteroposterior length of the symphysis and an anterolateral process of the ectopterygoid with an inverted V-shaped ridge on the dorsal surface. The shortness of the snout relative to the rest of the skull and the presence of a laterally and ventrally open notch between the premaxilla
and maxilla
indicate that Platyognathus is a member of Protosuchia as defined by Wu et al. (1994). Because the infratemporal fenestra of Platyognathus is very small and the mandibular symphysis extends posteriorly to the level of the seventh to ninth tooth, the genus is thought to be more derived than Orthosuchus
. The authors of the description of the neotype suggested that Platyognathus may be closely related to the clade
containing Protosuchidae
and Shantungosuchus
because it possesses a dentary tooth that fits into a notch between the premaxilla and maxilla. Because Platyognathus is known from such incomplete material, there is yet to be an adequate cladistic analyses that can determine a phylogenetic position for Platyognathus within Protosuchia.
An unnamed mesosuchia
n from the Salt Wash Member of the Late Jurassic
Morrison Formation
of western Colorado
in the United States
possesses several features that are also seen in Platyognathus. The dentition of the mesosuchian is similar to that of Platyognathus and like Platyognathus, the dentary is laterally expanded. However, the dentary of the unnamed mesosuchian is much broader and spatulate.
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 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....
n crocodylomorph. Fossils are known from the Early Jurassic
Early Jurassic
The Early Jurassic epoch is the earliest of three epochs of the Jurassic period...
Lower Lufeng Formation in Yunnan
Yunnan
Yunnan is a province of the People's Republic of China, located in the far southwest of the country spanning approximately and with a population of 45.7 million . The capital of the province is Kunming. The province borders Burma, Laos, and Vietnam.Yunnan is situated in a mountainous area, with...
, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
and belong to the type
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...
and only species, P. hsui.
Description
The skull of Platyognathus is small, being about 5 centimetres (2 in) in length. The snout is narrow and shorter than the remainder of the skull, as indicated by the anterior position of the antorbital fenestraAntorbital fenestra
An antorbital fenestra is an opening in the skull, in front of the eye sockets. This skull formation first appeared in archosaurs during the Triassic Period. Living birds today possess antorbital fenestrae, but the feature has been lost in modern crocodilians...
. Two teeth, dentaries 5 and 6, are enlarged into caniniforms in the lower jaw. Paired caniniform teeth are not seen in any other described crocodyliform taxa, but they have been recorded from an unidentified crocodyliform from the Lower Jurassic Kayenta Formation
Kayenta Formation
The Kayenta Formation is a geologic layer in the Glen Canyon Group that is spread across the Colorado Plateau province of the United States, including northern Arizona, northwest Colorado, Nevada, and Utah. This rock formation is particularly prominent in southeastern Utah, where it is seen in the...
and from a protosuchid from the Lower Jurassic McCoy Brook Formation
McCoy Brook Formation
The McCoy Brook Formation is a Mesozoic geologic formation on Nova Scotia. Dinosaur remains diagnostic to the genus level are among the fossils that have been recovered from the formation.-Synapsids:-Ornitodires:-Dinosaurs:-See also:...
in Nova Scotia
Nova Scotia
Nova Scotia is one of Canada's three Maritime provinces and is the most populous province in Atlantic Canada. The name of the province is Latin for "New Scotland," but "Nova Scotia" is the recognized, English-language name of the province. The provincial capital is Halifax. Nova Scotia is the...
.
Holotype
Platyognathus was first named by Chung-Chien Young in 1944 on the basis of a partial lower jaw found from the Dark Red Beds of the Lower Lufeng Formation in 1939. The material was first mentioned briefly four years earlier in a paper on Lufeng fossil vertebrates. The holotypeHolotype
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...
jaw consisted of 11 alveoli (tooth sockets) on the left side and 12 alveoli on the right side, in addition to one broken caniniform tooth. Several diagnostic features were mentioned, including an expansion at the tip of the jaw, a tooth cross section that is octangular in outline, and a lateral constriction of the jaw behind the caniniform tooth, which is followed by an expansion further back. The anterior teeth are small and angled forward, while there is evidence in the jaw of at least two caniniform teeth that were larger and oriented obliquely and anteriorly. Further back in the jaw, the posterior teeth are about the same size and project vertically. The entire jaw is well ossified, and sutures between the bones could not be identified in the holotype. The symphysis, the point where the two sides of the jaw come together, is represented by a broad ventral groove.
Based on these features, Young found no close similarities between the jaw of Platyognathus and any other known early crocodylomorph such as Notochampsa
Notochampsa
Notochampsa is an extinct genus of protosuchian crocodylomorph. Fossils have been found from the upper Elliot Formation and the lower Clarens Formation of the Karoo Supergroup in South Africa, dating back to the Early Jurassic. It lends its name to the family Notochampsidae, a taxon that has...
, Sphenosuchus
Sphenosuchus
Sphenosuchus is an extinct genus of sphenosuchian from the Elliot Formation of South Africa, discovered and described early in the 20th century. The skull is excellently preserved but other than elements of the forelimb and isolated parts of the hind limb, the Sphenosuchus material is incomplete...
, Erythrochampsa
Erythrochampsa
Erythrochampsa is an extinct genus of protosuchian crocodylomorph. Fossils have been found from the Red Beds of the Stormberg Group, the youngest group of strata from the Karoo Supergroup outcropping in South Africa.Material from E...
, and Pedeticosaurus
Pedeticosaurus
Pedeticosaurus is an extinct genus of sphenosuchian crocodylomorph from the Clarens Formation of South Africa. The type species is P. leviseuri....
(at the time, these crocodylomorphs were collectively known as the "Pseudosuchia
Pseudosuchia
Pseudosuchia is the name originally given to a group of prehistoric reptiles from the Triassic period. The name has been variously interpreted, and it is still sometimes, if infrequently, used in scientific literature today. A more commonly used name, Crurotarsi, is often substituted for...
"). The jaw of Protosuchus richardsoni
Protosuchus
Protosuchus is an extinct genus of carnivorous crocodylomorph from the Early Jurassic. The name Protosuchus means "first crocodile", and is among the earliest animals that resemble crocodilians...
, described by Barnum Brown
Barnum Brown
Barnum Brown , a paleontologist born in Carbondale, Kansas, and named after the circus showman P.T. Barnum, discovered the second fossil of Tyrannosaurus rex during a career that made him one of the most famous fossil hunters working from the late Victorian era into the early 20th century.Sponsored...
in 1933 from Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
, is similar in size to that of Platyognathus, although it lacks many of the distinguishing features of the Chinese genus as described by Young. Because Platyognathus differed from any other known pseudosuchian, Young suggested that it may belong to its own family.
During World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, the holotype was either lost or destroyed. The classification of the genus continued to be debated, as descriptions of the fragmentary jaw were all that was available to study. Platyognathus continued to be referred to Pseudosuchia in the following years, and was suggested to have a close relationship with Sphenosuchus. In 1955, it was suggested to have affinities with aetosaur
Aetosaur
Aetosaurs are an extinct order of heavily armoured, medium- to large-sized Late Triassic herbivorous archosaurs. They have small heads, upturned snouts, erect limbs, and a body covered by plate-like scutes. All aetosaurs belong to the family Stagonolepididae...
s, and in 1956 Alfred Romer
Alfred Romer
Alfred Sherwood Romer was an American paleontologist and comparative anatomist and a specialist in vertebrate evolution.-Biography:...
tentatively assigned it to the Notochampsidae (now known as the Protosuchidae
Protosuchidae
Protosuchidae was a family of crocodylmorph reptiles from the Late Triassic, Jurassic, and Cretaceous time periods.-Distribution:Triassic protosuchids are known from Lesotho and Arizona. Jurassic protosuchids are known from Nova Scotia, Poland, South Africa, and Arizona...
).
New material
In 1965, additional specimens from the Lower Lufeng were referred to Platyognathus. It was placed in a new pseudosuchian family, the Platyognathidae, and was considered to be intermediate between Pseudosuchia and Protosuchia. Most later studies of Platyognathus were based on one specimen of the new material known as CUP 2083 which preserved the lower jaw. Some of these studies concluded that Platyognathus was a pseudosuchian related to Sphenosuchus and Pedeticosaurus, but most considered it to be a protosuchian. One 1986 study also considered the material to be from a protosuchian, but did not consider CUP 2083 to be from Platyognathus because the mandibular symphysis was elongate and unfused, unlike the holotype described by Young which was ossified. Later that year, a new paper also concluded that the newer material did not represent Platyognathus, but considered the specimens and Platyognathus to belong to different basal crocodyliform taxa of uncertain affinities. In that paper, the validity of P. hsui was questioned on the basis of Young's description of the holotypeNeotype
In 1996, a newly described partial skull known as IVPP V8266 that was referable to Platyognathus was designated as a neotype. It was collected in 1984 from the Lower Lufeng in Yunnan. The authors of the paper that described the neotype agreed with the previous studies that the material described in 1965 does not belong to Platyognathus because CUP 2083 lacks many of the features of the holotype. In CUP 2083, the mandibular symphysis is unfused, there is a single caniniform tooth in the dentary, and there is no groove along the symphysis. This contrasts with the holotype, which has a fused symphysis, at least two caniniform teeth, and a conspicuous trough along the symphysis. The authors also mention that the holotype and CUP 2083 come from two different strata within the "Dark Red Beds" of the Lower Lufeng; the holotype is from stratum 6 of the Dahuangtian locality while CUP 2083 is from stratum 5 of the Dadi locality, about 400m southeast of the Dahuangtian locality. This shows that the two specimens are from different horizons and are likely to belong to different taxa.The new skull revealed an additional diagnostic feature of Platyognathus that distinguishes it from other protosuchians: the distinctive curvature of 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....
. Additional diagnostic characters were proposed by the authors, but their character-states cannot be determined in other protosuchian taxa. These characters include a width of the mandibular symphysis across the swellings of the caniniform teeth that is almost equal to the anteroposterior length of the symphysis and an anterolateral process of the ectopterygoid with an inverted V-shaped ridge on the dorsal surface. The shortness of the snout relative to the rest of the skull and the presence of a laterally and ventrally open notch between the premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
and 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...
indicate that Platyognathus is a member of Protosuchia as defined by Wu et al. (1994). Because the infratemporal fenestra of Platyognathus is very small and the mandibular symphysis extends posteriorly to the level of the seventh to ninth tooth, the genus is thought to be more derived than Orthosuchus
Orthosuchus
Orthosuchus is an extinct genus of protosuchian crocodyliform that lived during the Early Jurassic....
. The authors of the description of the neotype suggested that Platyognathus may be closely related to 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...
containing Protosuchidae
Protosuchidae
Protosuchidae was a family of crocodylmorph reptiles from the Late Triassic, Jurassic, and Cretaceous time periods.-Distribution:Triassic protosuchids are known from Lesotho and Arizona. Jurassic protosuchids are known from Nova Scotia, Poland, South Africa, and Arizona...
and Shantungosuchus
Shantungosuchus
Shantungosuchus is an extinct genus of Cretaceous crocodyliform found in China. It includes three species: Shantungosuchus chuhsienensis, and S. brachycephalus, which were both described by Yang Zhongjian – usually referred to as "Young" – in 1961 and 1982, and S. hangjinensis, which...
because it possesses a dentary tooth that fits into a notch between the premaxilla and maxilla. Because Platyognathus is known from such incomplete material, there is yet to be an adequate cladistic analyses that can determine a phylogenetic position for Platyognathus within Protosuchia.
An unnamed mesosuchia
Mesosuchia
"Mesosuchia" is an obsolete name for a group of terrestrial, semi-aquatic, or fully aquatic crocodylomorph reptiles. The marine crocodile Metriorhynchus had paddle-like forelimbs, Dakosaurus andiniensis had a skull that was adapted to eat large sea reptiles, and Shamosuchus was adapted for eating...
n from the Salt Wash Member of the Late Jurassic
Late Jurassic
The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago , which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age...
Morrison Formation
Morrison Formation
The Morrison Formation is a distinctive sequence of Late Jurassic sedimentary rock that is found in the western United States, which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone and limestone and is light grey, greenish...
of western Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
possesses several features that are also seen in Platyognathus. The dentition of the mesosuchian is similar to that of Platyognathus and like Platyognathus, the dentary is laterally expanded. However, the dentary of the unnamed mesosuchian is much broader and spatulate.
External links
- Platyognathus in the Paleobiology DatabasePaleobiology Database' is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms.-History:The Paleobiology Database was founded in 2000. It has been funded by the National Science Foundation and the Australian Research Council...