''Russellosaurus'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

of tethysaurine mosasauroid from the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

of North America. The

was described from a

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, t ...

discovered in an exposure of the Arcadia Park Shale (lower Middle

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 93.9 ± 0.8 Ma and 89.8 ± 1 Ma (million years ago). The Turonian is preceded ...

) at Cedar Hill, Dallas County in south-central

Texas Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...

, United States. The skull (SMU 73056, Shuler Museum of

Paleontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fos ...

Southern Methodist University , mottoeng = " The truth will make you free" , established = , type = Private research university , accreditation = SACS , academic_affiliations = , religious_affiliation = United Methodist Church , president = R. Gerald Turner , pr ...

) was found in 1992 by a member of the

Dallas Paleontological Society Dallas () is the third largest city in Texas and the largest city in the Dallas–Fort Worth metroplex, the fourth-largest metropolitan area in the United States at 7.5 million people. It is the largest city in and seat of Dallas County wit ...

, 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 Dallas () is the List of municipalities in Texas, third largest city in Texas and the largest city in the Dallas–Fort Worth metroplex, the List of metropolitan statistical areas, fourth-largest metropolitan area in the United States at 7.5 ...

area.

Etymology

The

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen( ...

, ''R. coheni'', was named for the amateur

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

collector who discovered SMU 73056, and the genus name honours

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of foss ...

Dale A. Russell for his extensive work on mosasaurs ("Russell's

lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia al ...

"). This is the second

species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...

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.

Description

It was a small mosasaur measuring long and weighing . Polcyn et. Bell (2005, p. 323) diagnose ''Russellosaurus'' as follows: "Small, lightly built mosasaur,

frontal Front may refer to: Arts, entertainment, and media Films * ''The Front'' (1943 film), a 1943 Soviet drama film * ''The Front'', 1976 film Music *The Front (band), an American rock band signed to Columbia Records and active in the 1980s and ea ...

narrow with a length to width ratio of 1.6:1. Sixteen

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The ...

ry teeth and 16

dentary In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

teeth. Anterior

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...

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 The pterygoid processes of the sphenoid (from Greek ''pteryx'', ''pterygos'', "wing"), one on either side, descend perpendicularly from the regions where the body and the greater wings of the sphenoid bone unite. Each process consists of a m ...

and a slender rod-like

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. Anat ...

process. Incipient emargination of the frontal by the external nares. A pair of

foramina In anatomy and osteology, a foramen (;Entry "foramen"
in

separated by a thin median

septum In biology, a septum (Latin for ''something that encloses''; plural septa) is a wall, dividing a cavity or structure into smaller ones. A cavity or structure divided in this way may be referred to as septate. Examples Human anatomy * Interat ...

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 Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck a ...

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 Russellosaurina (= subfamily Russellosaurinae of Bell, 1997). This clade of marine lizards is defined as all mosasaurs more closely related to the subfamilies

Tylosaurinae The Tylosaurinae are a subfamily of mosasaurs,Williston, S. W. 1897. Range and distribution of the mosasaurs with remarks on synonymy. ''Kansas University Quarterly'' 4(4):177-185. a diverse group of Late Cretaceous marine squamates. Members of ...

and

Plioplatecarpinae Plioplatecarpinae is a subfamily of mosasaurs, a diverse group of Late Cretaceous marine squamates. Members of the subfamily are informally and collectively known as "plioplatecarpines" and have been recovered from all continents, though the occ ...

and their

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

, consisting of the

taxa In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...

''Tethysaurus'', ''Yaguarasaurus'', and ''Russellosaurus'', their common ancestor and all descendants than to the subfamily

Mosasaurinae The Mosasaurinae are a subfamily of mosasaurs, a diverse group of Late Cretaceous marine squamates. Members of the subfamily are informally and collectively known as "mosasaurines" and their fossils have been recovered from every continent excep ...

. The skull of ''Russelosaurus'' shows many characters diagnostic of the Plioplatecarpinae while retaining many

plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, an ...

traits.

Cladistic Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived ch ...

analysis indicates a close relationship between ''Russellosaurus'' and '' Yaguarasaurus columbianus'', a primitive

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the souther ...

n mosasaur from the Turonian of Colombia. Together with '' Tethysaurus nopcsai'', another early mosasauroid from the Turonian of

Morocco Morocco (),, ) officially the Kingdom of Morocco, is the westernmost country in the Maghreb region of North Africa. It overlooks the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria ...

, these

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial ...

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 A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...

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.

References

Sources

*Bell, G. L. Jr., 1997. A phylogenetic revision of North American and Adriatic Mosasauroidea. pp. 293–332 In Callaway J. M. and E. L Nicholls, (eds.), ''Ancient Marine Reptiles''.

Academic Press Academic Press (AP) is an academic book publisher founded in 1941. It was acquired by Harcourt, Brace & World in 1969. Reed Elsevier bought Harcourt in 2000, and Academic Press is now an imprint of Elsevier. Academic Press publishes refere ...

, 501 pp. *Polcyn, M. J. and Bell, G. L., Jr. 2005. ''Russellosaurus coheni'' n. gen., n. sp., a 92 million-year-old mosasaur from Texas (USA), and the definition of the parafamily Russellosaurina. ''Netherlands Journal of Geosciences'' 84(3):321–333. *Wright, K. R. and S. W. Shannon. 1988. '' Selmasaurus russelli'', a new plioplatecarpine mosasaur (Squamata, Mosasauridae) from Alabama. ''Journal of Vertebrate Paleontology'' 8(1):102–107. {{Taxonbar, from=Q7381906 Russellosaurins Mosasaurs of North America Fossil taxa described in 2005