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''Dallasaurus'' ("
Dallas 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 w ...
lizard") is a basal
mosasaur Mosasaurs (from Latin ''Mosa'' meaning the ' Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht o ...
oid from the
Upper 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'', t ...
of
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the C ...
. Along with '' Russellosaurus'', ''Dallasaurus'' is one of the two oldest mosasauroid
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 nam ...
currently known from North America. It is also the smallest known mosasaurine, measuring up to in length and in body mass.


Specimens

The
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 no ...
is based upon two partial skeletons recovered from the Arcadia Park
Shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments ( silt-sized particles) of other minerals, especial ...
(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 ...
), approximately 15 meters above its contact with the older Kamp Ranch Limestone in Dallas County in north-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 ...
. 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 several ...
specimen (TMM 43209-1, Texas Memorial Museum, University of Texas at Austin) consists of an incomplete and disarticulated skull, along with considerable portions of the postcranial skeleton, making up about 80 percent of the animal. The second referred specimen (DMNH 8121-8125, 8143-8149, and 8161-8180, Dallas Museum of Natural History) lacks any skull material and consists entirely of disarticulated postcranial remains. The strata containing these fossils were temporarily exposed during excavations for a housing development, and both sites have now been reburied by construction. The two specimens were discovered about 100 meters from one another; the first was found by an amateur collector, Van Turner, for whom the type species (''Dallasaurus turneri'') was named. The genus is named for Dallas County, where both specimens were found.


Anatomy

Polcyn and Bell diagnose ''Dallasaurus'' as follows: "Small, plesiopedal mosasauroid possessing the following autapomorphies: posterior
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 strongly recurved posteriorly, slightly inflated at the crown and bearing only posterior carinae that is slightly offset laterally; atlas neural arch mediolaterally compressed but not flattened at its base, condylar surfaces irregularly figure-eight shaped;
cervical vertebra In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In sau ...
synapophyses protrude below the level of the
ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
edge of the centrum; short, wide fossa excavated immediately below the ventral rim of the cotyle of at least one middle cervical vertebra; hypapophysis anteroventral edge terminating in short projections of irregular length; postglenoid process capped by bony
epiphysis The epiphysis () is the rounded end of a long bone, at its joint with adjacent bone(s). Between the epiphysis and diaphysis (the long midsection of the long bone) lies the metaphysis, including the epiphyseal plate (growth plate). At the jo ...
bearing a calcified
cartilage 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 ...
apex." Bell and Polcyn use the term "plesiopedal" to indicate a "conservative ecologically adaptive grade" characterized by "small size, slightly modified swimming tail and relatively
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, and ...
limb condition" compared to more derived mosasauroids. Polcyn and Bell note that plesiopedal mosasauroids tend to be relatively small lizards possessing limbs in which the "propodial elements quals_humerus,_radius_(bone).html" "title="humerus.html" ;"title="quals humerus">quals humerus, radius (bone)">radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
, and ulna] remain elongated, generally constituting one-half or more of the full length of the osseus limb", as compared to more derived "hydropedal" mosasaurs in which the propodial elements are stout and have been substantially shortened, constituting less than one-half of the full length of the ossueus limb. While hydropedal mosasaurs were probably entirely aquatic, plesiopedal mosasauroids were still capable of terrestrial locomotion and so likely lived an amphibious lifestyle.


Classification

Bell and Polcyn have conducted a cladistic analysis of ''Dallasaurus'', concluding that this
taxon 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 ...
should be placed within 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 ...
, "well within
he family He or HE may refer to: Language * He (pronoun), an English pronoun * He (kana), the romanization of the Japanese kana へ * He (letter), the fifth letter of many Semitic alphabets * He (Cyrillic), a letter of the Cyrillic script called ''He'' ...
Mosasauridae", and that, despite its small size and the "primitive" condition of its limbs, it should not be placed within the
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
family Aigialosauridae. ''Dallasaurus'' is the
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 ...
to more derived mosasaurine mosasaurs such as ''
Clidastes ''Clidastes'' is an extinct genus of marine lizard belonging to the mosasaur family. It is classified as part of the Mosasaurinae subfamily, alongside genera like ''Mosasaurus'' and '' Prognathodon''. ''Clidastes'' is known from deposits rangin ...
'', ''
Prognathodon ''Prognathodon'' is an extinct genus of marine lizard belonging to the mosasaur family. It is classified as part of the Mosasaurinae subfamily, alongside genera like ''Mosasaurus'' and '' Clidastes''. ''Prognathodon'' has been recovered from dep ...
'', ''Mosasaurus'', and '' Plotosaurus''. In the popular press, ''Dallasaurus'' has been hailed as a " missing link" uniting fully aquatic mosasaurs with their
terrestrial Terrestrial refers to things related to land or the planet Earth. Terrestrial may also refer to: * Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on or near the ground, as opposed to ...
ancestors."Missing fossil link 'Dallasaurus' found"
''Science Daily'', 16 November 2005.


References


Sources

*G. L. Bell, Jr.; Polcyn, M. J. (2005). "''Dallasaurus turneri'', a new primitive mosasauroid from the Middle Turonian of Texas and comments on the phylogeny of the Mosasauridae (Squamata)." ''Netherlands Journal of Geoscience'' (''Geologie en Mijnbouw'') 84 (3) pp. 177–194. {{Taxonbar, from=Q2335300 Mosasaurines Transitional fossils Mosasaurs of North America Fossil taxa described in 2005