Russellosaurus (Polcyn et Bell, 2005) is a basal
mosasaurMosasaurs were serpentine marine reptiles. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...
oid from the Upper
CretaceousThe Cretaceous , Latin language for "chalky", 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 on the Jurassic period and is followed by the Paleogene period of the...
of
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
. The
genusIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
was described from a
skullThe skull is a bony structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury....
discovered in an exposure of the Arcadia Park Shale (lower Middle
TuronianThe 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
TexasTexas is the second-largest U.S. state in both area and population, and the largest state in the contiguous United States.The name had wide usage among native Americans, meaning "friends" or "allies"...
. The skull (SMU 73056, Shuler Musueum of
PaleontologyPaleontology
[from Greek: παλαιός "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought"] is the study of prehistoric life, including organisms' evolution and interactions with each other and their environments...
,
Southern Methodist UniversitySouthern Methodist University is a private, coeducational university in University Park, Texas . Founded in 1911 by the Methodist Episcopal Church, South, SMU currently operates campuses in University Park, Plano, and Taos, New Mexico...
) 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.
The
type speciesIn taxonomy, a type species is a technical phrase, involved in the application of formal names...
,
R.
Russellosaurus (Polcyn et Bell, 2005) is a basal
mosasaurMosasaurs were serpentine marine reptiles. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...
oid from the Upper
CretaceousThe Cretaceous , Latin language for "chalky", 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 on the Jurassic period and is followed by the Paleogene period of the...
of
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
. The
genusIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
was described from a
skullThe skull is a bony structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury....
discovered in an exposure of the Arcadia Park Shale (lower Middle
TuronianThe 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
TexasTexas is the second-largest U.S. state in both area and population, and the largest state in the contiguous United States.The name had wide usage among native Americans, meaning "friends" or "allies"...
. The skull (SMU 73056, Shuler Musueum of
PaleontologyPaleontology
[from Greek: παλαιός "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought"] is the study of prehistoric life, including organisms' evolution and interactions with each other and their environments...
,
Southern Methodist UniversitySouthern Methodist University is a private, coeducational university in University Park, Texas . Founded in 1911 by the Methodist Episcopal Church, South, SMU currently operates campuses in University Park, Plano, and Taos, New Mexico...
) 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 speciesIn taxonomy, a type species is a technical phrase, involved in the application of formal names...
,
R. coheni, was named for the amateur
fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
collector who discovered SMU 73056, and the genus name honours paleontologist Dale A. Russell for his extensive work on mosasaurs ("Russell's
lizardLizards are a very large and widespread group of squamate reptiles, with nearly 5,000 species, ranging across all continents except Antarctica as well as most oceanic island chains...
"). This is the second
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
of mosasaur to have been named for Russell, the first being
SelmasaurusSelmasaurus 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, frontal narrow with a length to width ratio of 1.6:1. Sixteen
maxillaThe 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 is sometimes called "upper maxilla", with the mandible being the "lower maxilla"...
ry teeth and 16 dentary teeth. Anterior
premaxillaThe 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
pterygoidPterygoid 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
jugalThe 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
septumIn anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.- In human anatomy :* Interatrial septum, the wall of tissue that separates the left and right atria of the heart...
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
cladeA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
of marine lizards is defined as all mosasaurs more closely related to the subfamilies
TylosaurinaeTylosaurinae 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
PlioplatecarpinaePlioplatecarpinae 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
MosasaurinaeMosasaurinae 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 anaylsis indicates a close relationship between
Russellosaurus and
Yaguarasaurus columianbus, a primitive
South AmericaSouth America is the southern continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere...
n mosasaur from the Turonian of
ColombiaColombia , officially the Republic of Colombia , is a constitutional republic in northwestern South America. Colombia is bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the northwest by Panama; and to the west by the Pacific Ocean...
. Together with
Tethysaurus nopscai, another early mosasauroid from the Turonian of
MoroccoMorocco, officially the Kingdom of Morocco, is a country located in North Africa with a population of nearly 32 million and an area just under . Its capital is Rabat, and its largest city is Casablanca. Morocco has a coast on the Atlantic Ocean that reaches past the Strait of Gibraltar into the...
, these
generaIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
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
holotypeA holotype is one of several possible biological types. A type is what fixes a name to a taxon. A holotype is a single physical example of an organism, known to have been used when the species was formally described...
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.