Rhabdognathus
Encyclopedia
Rhabdognathus is an extinct genus
of dyrosaurid
crocodylomorph
. It is known from rocks dating to the Paleocene
epoch from western Africa
, and specimens dating back to the Maastrichtian
era were identified in 2008. It was named by Swinton in 1930 for a lower jaw fragment from Nigeria
. The type species is Rhabdognathus rarus. Stéphane Jouve subsequently assessed R. rarus as indeterminate at the species level, but not at the genus level, and thus dubious
. Two skulls which were assigned to the genus Rhabdognathus but which could not be shown to be identical to R. rarus were given new species: R. aslerensis and R. keiniensis, both from Mali
. The genus formerly contained the species Rhabdognathus compressus, which was reassigned to Congosaurus
compressus after analysis of the lower jaw of a specimen found that it was more similar to that of the species Congosaurus bequaerti. Rhabdognathus is believed to be the closest relative to the extinct Atlantosuchus
.
of Rhabdognathus is as high as it is wide or higher, which distinguishes it from Hyposaurus. The mandible is dorsally directed toward the tip, and the first pair of alveoli (tooth sockets) at the very tip of the jaw are higher than the others. Another distinguishing feature is the extreme length of the mandibular symphysis
, which extends past the nineteenth mandibular alveolus. The splenial
also extends beyond this point, although the position of its symphysis varies during growth. The alveoli of Rhabdognathus are rounded and directed slightly laterally, causing the teeth to project at an angle.
The skulls of R. aslerensis and R. keiniensis possess numerous characters that distinguish Rhabdognathus from other dyrosaurids. The posterior wall of the supratemporal fenestra inclines dorsally so that it is visible when the skull is viewed in dorsal aspect. In Dyrosaurus phosphaticus, the wall is vertical and thus not visible in dorsal aspect. In both species of Rhabdognathus, the space between the occipital condyle
and the basioccipital tubera (both located in the back of the skull where the vertebrae articulate) is anteroposteriorly longer than in D. phosphaticus. In the skulls of both species of Rhabdognathus, the posterior margin is inclined so that both the occipital condyle and the basioccipital tubera below it are visible in occipital view.
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 dyrosaurid
Dyrosauridae
Dyrosauridae is a family of extinct neosuchian crocodyliforms that lived from the Late Cretaceous to the Eocene. Fossils of this group have been found in almost every continent, specifically Africa, Asia, Europe, North America and South America....
crocodylomorph
Crocodylomorpha
The Crocodylomorpha are an important group of archosaurs that include the crocodilians and their extinct relatives.During Mesozoic and early Tertiary times the Crocodylomorpha were far more diverse than they are now. Triassic forms were small, lightly built, active terrestrial animals. These were...
. It is known from rocks dating to the Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
epoch from western Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, and specimens dating back to the Maastrichtian
Maastrichtian
The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...
era were identified in 2008. It was named by Swinton in 1930 for a lower jaw fragment from Nigeria
Nigeria
Nigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...
. The type species is Rhabdognathus rarus. Stéphane Jouve subsequently assessed R. rarus as indeterminate at the species level, but not at the genus level, and thus dubious
Nomen dubium
In zoological nomenclature, a nomen dubium is a scientific name that is of unknown or doubtful application...
. Two skulls which were assigned to the genus Rhabdognathus but which could not be shown to be identical to R. rarus were given new species: R. aslerensis and R. keiniensis, both from Mali
Mali
Mali , officially the Republic of Mali , is a landlocked country in Western Africa. Mali borders Algeria on the north, Niger on the east, Burkina Faso and the Côte d'Ivoire on the south, Guinea on the south-west, and Senegal and Mauritania on the west. Its size is just over 1,240,000 km² with...
. The genus formerly contained the species Rhabdognathus compressus, which was reassigned to Congosaurus
Congosaurus
Congosaurus is an extinct genus of dyrosaurid mesoeucrocodylian. Fossils have been found from Angola and date back to the Paleocene epoch. In 1952 and 1964 Congosaurus was proposed to be synonymous with Dyrosaurus. The genus was later regarded to be synonymous with Hyposaurus in 1976 and 1980...
compressus after analysis of the lower jaw of a specimen found that it was more similar to that of the species Congosaurus bequaerti. Rhabdognathus is believed to be the closest relative to the extinct Atlantosuchus
Atlantosuchus
Atlantosuchus is an extinct genus of dyrosaurid crocodylomorph from Morocco. One defining characteristic that distinguishes it from other long-snouted dyrosaurids was its proportionally elongate snout, the longest in proportion to body size of any dyrosaurid...
.
Description
Rhabdognathus has an extremely elongated snout that makes up around 75% of the length of the entire skull. The total skull length of R. keiniensis is 73.1 centimetres (28.8 in), while the length of the skull of R. aslerensis is unknown because the front of the snout is not preserved in the only known skull, CNRST-SUNY-190. The mandibleMandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
of Rhabdognathus is as high as it is wide or higher, which distinguishes it from Hyposaurus. The mandible is dorsally directed toward the tip, and the first pair of alveoli (tooth sockets) at the very tip of the jaw are higher than the others. Another distinguishing feature is the extreme length of the mandibular symphysis
Symphysis
A symphysis is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint.1.A symphysis is an amphiarthrosis, a slightly movable joint.2.A growing together of parts or structures...
, which extends past the nineteenth mandibular alveolus. The splenial
Splenial
The splenial is a small bone in the lower jaw of reptiles, amphibians and birds, usually located on the lingual side between the angular and suprangular....
also extends beyond this point, although the position of its symphysis varies during growth. The alveoli of Rhabdognathus are rounded and directed slightly laterally, causing the teeth to project at an angle.
The skulls of R. aslerensis and R. keiniensis possess numerous characters that distinguish Rhabdognathus from other dyrosaurids. The posterior wall of the supratemporal fenestra inclines dorsally so that it is visible when the skull is viewed in dorsal aspect. In Dyrosaurus phosphaticus, the wall is vertical and thus not visible in dorsal aspect. In both species of Rhabdognathus, the space between the occipital condyle
Occipital condyle
The occipital condyles are undersurface facets of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra....
and the basioccipital tubera (both located in the back of the skull where the vertebrae articulate) is anteroposteriorly longer than in D. phosphaticus. In the skulls of both species of Rhabdognathus, the posterior margin is inclined so that both the occipital condyle and the basioccipital tubera below it are visible in occipital view.
External links
- Rhabdognathus at 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...