Russellosaurus
Encyclopedia
Russellosaurus is a basal mosasaur
oid from the Upper Cretaceous
of North America
. The genus
was described from a skull
discovered in an exposure of the Arcadia Park Shale (lower Middle Turonian
) at Cedar Hill, Dallas County in south-central Texas
. The skull (SMU 73056, Shuler Musueum of Paleontology
, Southern Methodist University
) was found in 1992 by a member of the Dallas Paleontological Society, who then donated to the museum. Other fragmentary specimens of Russelosaurus have been recovered from the slightly older Kamp Ranch Limestone at two other localities in the Dallas area.
, R. coheni, was named for the amateur fossil
collector who discovered SMU 73056, and the genus name honours paleontologist Dale A. Russell for his extensive work on mosasaurs ("Russell's lizard
"). This is the second species
of mosasaur to have been named for Russell, the first being Selmasaurus
russelli (Wright and Shannon, 1988). The type specimen of Russellosaurus is notable as being the oldest well-preserved mosasaur yet found in North America.
narrow with a length to width ratio of 1.6:1. Sixteen maxilla
ry teeth and 16 dentary teeth. Anterior premaxilla
broad and blunt, in dorsal view the premaxillary-maxillary suture is oblique. The ectopterygoid is lightly built and composed of a distinct small subrectangular pterygoid
process and a slender rod-like jugal
process. Incipient emargination of the frontal by the external nares. A pair of foramina separated by a thin median septum
in the floor of the basioccipital are interpreted as the entrance of the basilar artery, and exit the ventral surface of the basioccipital as multiple small and anteriorly placed foramina. Extreme downgrowth of the pterygoid processes of the basisphenoid. Lightly built postorbitofrontal processes. The pineal foramen is located in the centre of the triangular parietal table. The supraoccipital loose cartilaginous contact with parietal. Median cleft in posterior parietal margin in dorsoventral aspect."
of marine lizards is defined as all mosasaurs more closely related to the subfamilies Tylosaurinae
and Plioplatecarpinae
and their sister group, consisting of the taxa Tethysaurus, Yaguarasaurus, and Russellosaurus, their common ancestor and all descendants than to the subfamily Mosasaurinae
. The skull of Russelosaurus shows many characters diagnostic of the Plioplatecarpinae while retaining many plesiomorphic traits. Cladistic analysis indicates a close relationship between Russellosaurus and Yaguarasaurus columbianus
, a primitive South America
n mosasaur from the Turonian of Colombia
. Together with Tethysaurus nopcsai, another early mosasauroid from the Turonian of Morocco
, these genera
are believed to constitute a clade "basal to the divergence" of the subfamilies Plioplatecarpinae and Tylosaurinae.
Based on the lack of fusion between elements of the basicarnium and a high degree of vascularization of the bone surface, which suggests the animal was undergoing a rapid growth stage when it died, the holotype
skull of Russellosaurus coheni is believed to have come from a subadult individual. An even more primitive mosasaur, Dallasaurus turneri, was recovered from the same Cedar Hill locality as this specimen.
Mosasaur
Mosasaurs are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...
oid from the Upper Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. The genus
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...
was described from a skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
discovered in an exposure of the Arcadia Park Shale (lower Middle Turonian
Turonian
The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous epoch, or a stage in the Upper Cretaceous series. It spans the time between 93.5 ± 0.8 Ma and 89.3 ± 1 Ma...
) at Cedar Hill, Dallas County in south-central Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
. The skull (SMU 73056, Shuler Musueum of Paleontology
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
, Southern Methodist University
Southern Methodist University
Southern Methodist University is a private university in Dallas, Texas, United States. Founded in 1911 by the Methodist Episcopal Church, South, SMU operates campuses in Dallas, Plano, and Taos, New Mexico. SMU is owned by the South Central Jurisdiction of the United Methodist Church...
) was found in 1992 by a member of the Dallas Paleontological Society, who then donated to the museum. Other fragmentary specimens of Russelosaurus have been recovered from the slightly older Kamp Ranch Limestone at two other localities in the Dallas area.
Etymology
The type speciesType 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...
, R. coheni, was named for the amateur fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
collector who discovered SMU 73056, and the genus name honours paleontologist Dale A. Russell for his extensive work on mosasaurs ("Russell's lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...
"). This is the second species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of mosasaur to have been named for Russell, the first being Selmasaurus
Selmasaurus
Selmasaurus is a genus of medium-sized plioplatecarpine mosasaur from the Upper Cretaceous Mooreville Chalk Formation of western Alabama...
russelli (Wright and Shannon, 1988). The type specimen of Russellosaurus is notable as being the oldest well-preserved mosasaur yet found in North America.
Appearance
Polcyn et. Bell (2005, p. 323) diagnose Russellosaurus as follows: "Small, lightly built mosasaur, frontalFrontal bone
The frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....
narrow with a length to width ratio of 1.6:1. Sixteen 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 and 16 dentary teeth. Anterior 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....
broad and blunt, in dorsal view the premaxillary-maxillary suture is oblique. The ectopterygoid is lightly built and composed of a distinct small subrectangular pterygoid
Pterygoid
Pterygoid can refer to:* Pterygoid processes of the sphenoid bone** The Lateral pterygoid plate by it* a muscle such as Lateral pterygoid muscle or Medial pterygoid muscle* a branch of the Mandibular nerve...
process and a slender rod-like 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....
process. Incipient emargination of the frontal by the external nares. A pair of foramina separated by a thin median septum
Septum
In anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.-In human anatomy:...
in the floor of the basioccipital are interpreted as the entrance of the basilar artery, and exit the ventral surface of the basioccipital as multiple small and anteriorly placed foramina. Extreme downgrowth of the pterygoid processes of the basisphenoid. Lightly built postorbitofrontal processes. The pineal foramen is located in the centre of the triangular parietal table. The supraoccipital loose cartilaginous contact with parietal. Median cleft in posterior parietal margin in dorsoventral aspect."
Species
Polcyn et Bell (2005, p. 322) designated Russellosaurus the type genus of a new parafamily of mosasaurs, the Rullessosaurina (= subfamily Russellosaurinae of Bell, 1997). This cladeClade
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...
of marine lizards is defined as all mosasaurs more closely related to the subfamilies Tylosaurinae
Tylosaurinae
Tylosaurinae is a subfamily of mosasaurs, a diverse group of Late Cretaceous marine lizards.Russell defined the Tylosaurinae as follows: "Large rostrum present anterior to premaxillary teeth. Twelve or more teeth in dentary and maxilla. Cranial nerves X, XI, and XII leave lateral wall of...
and Plioplatecarpinae
Plioplatecarpinae
Plioplatecarpinae is a subfamily of mosasaurs, a diverse group of late Cretaceous marine squamates.Russell defined the Plioplatecarpinae as follows: Small rostrum present or absent anterior to premaxillary teeth. Cranial nerves X, XI, XII leave lateral wall of opisthotic through single foramen...
and their sister group, consisting of the taxa Tethysaurus, Yaguarasaurus, and Russellosaurus, their common ancestor and all descendants than to the subfamily Mosasaurinae
Mosasaurinae
Mosasaurinae is a subfamily of mosasaurs, a diverse group of Late Cretaceous marine squamates.Russell defined the Mosasurinae as differing from all other mosasaurs as follows: "Small rostrum present or absent anterior to premaxillary teeth. Fourteen or more teeth present in dentary and maxilla...
. The skull of Russelosaurus shows many characters diagnostic of the Plioplatecarpinae while retaining many plesiomorphic traits. Cladistic analysis indicates a close relationship between Russellosaurus and Yaguarasaurus columbianus
Yaguarasaurus
Yaguarasaurus was a genus of mosasaur from the Late Cretaceous period of Colombia, South America. The remains discovered were defined as a new genus and species of the tribe Plioplatecarpini, Yaguarasaurus columbianus, by the Colombian paleontologist María Páramo, former director of the Museo de...
, a primitive South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
n mosasaur from the Turonian of Colombia
Colombia
Colombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the...
. Together with Tethysaurus nopcsai, another early mosasauroid from the Turonian of Morocco
Morocco
Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...
, these genera
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...
are believed to constitute a clade "basal to the divergence" of the subfamilies Plioplatecarpinae and Tylosaurinae.
Based on the lack of fusion between elements of the basicarnium and a high degree of vascularization of the bone surface, which suggests the animal was undergoing a rapid growth stage when it died, 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...
skull of Russellosaurus coheni is believed to have come from a subadult individual. An even more primitive mosasaur, Dallasaurus turneri, was recovered from the same Cedar Hill locality as this specimen.