''Dallasaurus'' ("

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 ...

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 on ...

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'', th ...

of North America. Along with ''

Russellosaurus ''Russellosaurus'' is an extinct genus of tethysaurine mosasauroid from the Late 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, Dalla ...

'', ''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 nom ...

is based upon two partial skeletons recovered from 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 b ...

), approximately 15 meters above its contact with the older Kamp Ranch Limestone in

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in north-central

Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...

. 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 sever ...

specimen (TMM 43209-1, Texas Memorial Museum,

University of Texas at Austin The University of Texas at Austin (UT Austin, UT, or Texas) is a public research university in Austin, Texas. It was founded in 1883 and is the oldest institution in the University of Texas System. With 40,916 undergraduate students, 11,07 ...

) 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

, 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. T ...

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 sa ...

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 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 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 ...

Specimens

Anatomy

Classification

References

Sources