''Dallasaurus'' ("

Dallas Dallas () is a city in the U.S. state of Texas and the most populous city in the Dallas–Fort Worth metroplex, the List of Texas metropolitan areas, most populous metropolitan area in Texas and the Metropolitan statistical area, fourth-most ...

lizard") is a basal

mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains wer ...

oid from the

Upper Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent 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 ''cret ...

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

. Along with '' Russellosaurus'', ''Dallasaurus'' is one of the two oldest mosasauroid

taxa In biology, a taxon (back-formation from ''taxonomy''; : 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 name and ...

currently known from North America. It is also one of the smallest known mosasaurines, measuring approximately in length.

Specimens

The

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

is based upon two partial skeletons recovered from the Arcadia Park Shale (lower Middle

Turonian The Turonian is, in the International Commission on Stratigraphy, ICS' geologic timescale, the second age (geology), age in the Late Cretaceous epoch (geology), Epoch, or a stage (stratigraphy), stage in the Upper Cretaceous series (stratigraphy), ...

), approximately 15 meters above its contact with the older Kamp Ranch Limestone in Dallas County in north-central

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

. The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

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 university, public research university in Austin, Texas, United States. Founded in 1883, it is the flagship institution of the University of Texas System. With 53,082 stud ...

) 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 In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

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 In vertebrates, the maxilla (: maxillae ) 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 two maxil ...

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 (: 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 sauropsid s ...

synapophyses protrude below the level of the

ventral Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. 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 An epiphysis (; : epiphyses) is one of the rounded ends or tips of a long bone that ossify from one or more secondary centers of ossification. Between the epiphysis and diaphysis (the long midsection of the long bone) lies the metaphysis, inc ...

bearing a calcified

cartilage Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...

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, an ...

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 and femur">humerus.html" ;"title="quals humerus">quals humerus and femur] 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''; : 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 name and ...

should be placed within the subfamily

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

, "well within he family

Mosasauridae Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains were discovered in ...

", and that, despite its small size and the "primitive" condition of its limbs, it should not be placed within the

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

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'', ''

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 depos ...

'', ''Mosasaurus'', and ''

Plotosaurus ''Plotosaurus'' ("swimmer lizard") is an extinct genus of large mosasaurs which lived during the Upper Cretaceous (Maastrichtian) in what is now North America. The taxon was initially described by University of California, Berkeley, Berkeley pale ...

''. In the popular press, ''Dallasaurus'' has been hailed as a " missing link" uniting fully aquatic mosasaurs with their terrestrial 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