[1] The genus is based upon two partial skeletons recovered from the Arcadia Park Shale (lower Middle Turonian), approximately 15 meters above its contact with the older Kamp Ranch Limestone in Dallas County in north-central Texas.

[2] 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 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.

Polcyn and Bell diagnose Dallasaurus as follows: "Small, plesiopedal mosasauroid possessing the following autapomorphies: posterior maxillary 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 synapophyses protrude below the level of the ventral 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 bearing a calcified cartilage apex.

"[4] Bell and Polcyn use the term "plesiopedal" to indicate a "conservative ecologically adaptive grade" characterized by "small size, slightly modified swimming tail and [a] relatively plesiomorphic limb condition" compared to more derived mosasauroids.