Dallasaurus
Encyclopedia
Dallasaurus is a basal mosasaur
oid from the Upper Cretaceous of North America
. The genus
is based upon two partial skeleton
s 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
. The holotype
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. 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 fossil
s 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
was named. The genus is named for Dallas County, where both specimens were found ("Dallas lizard
"). Along with Russellosaurus
, Dallasaurus is one of the two oldest mosasauroid taxa currently known from North America. This small semi-aquatic lizard measured less than a meter in length, compared to such gigantic derived mosasaurs as Tylosaurus
and Mosasaurus
, each exceededing 15 meters.
Polcyn and Bell (2005, p. 179-180) 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
synopophyses 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."
Bell and Polcyn (2005) 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. Polcyn et Bell (2005, p. 178) note that plesiopedal mosasauroids tend to be relatively small lizards possessing limbs in which the "propodial elements [equals humerus
, radius
, 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.
Bell and Polcyn (2005, 188-190) have conducted a cladistic analysis of Dallasaurus, concluding that this taxon
should be placed within the subfamily Mosasaurinae
, "well within [the family] Mosasauridae", and that, despite its plesiopedal condition it should not be placed within the paraphyletic family Aigialosaur
idae, despite its small size and the "primitive" condition of its limbs. Dallasaurus is the sister group to more derived mosasaurine mosasaurs such as Clidastes
, Prognathodon
, Mosasaurus, and Plotosaurus
. In the popular press, Dallasaurus has been hailed as a "missing link
" uniting fully aquatic mosasaurs with their terrestrial
ancestorshttp://www.sciencedaily.com/releases/2005/11/051116173945.htm.
Mosasaur
Mosasaurs are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...
oid from the Upper Cretaceous of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. The genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
is based upon two partial skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
s recovered from the Arcadia Park Shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
(lower Middle Turonian
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 93.5 ± 0.8 Ma and 89.3 ± 1 Ma...
), approximately 15 meters above its contact with the older Kamp Ranch Limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
in Dallas County in north-central Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
. The holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
specimen (TMM 43209-1, Texas Memorial Museum, University of Texas at Austin
University of Texas at Austin
The University of Texas at Austin is a state research university located in Austin, Texas, USA, and is the flagship institution of the The University of Texas System. Founded in 1883, its campus is located approximately from the Texas State Capitol in Austin...
) consists of an incomplete and disarticulated skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
, along with considerable portions of the postcranial skeleton. 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
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
containing these fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s 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
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
was named. The genus is named for Dallas County, where both specimens were found ("Dallas lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...
"). Along with Russellosaurus
Russellosaurus
Russellosaurus is a basal mosasauroid from the Upper Cretaceous of North America. The genus was described from a skull discovered in an exposure of the Arcadia Park Shale at Cedar Hill, Dallas County in south-central Texas...
, Dallasaurus is one of the two oldest mosasauroid taxa currently known from North America. This small semi-aquatic lizard measured less than a meter in length, compared to such gigantic derived mosasaurs as Tylosaurus
Tylosaurus
Tylosaurus was a mosasaur, a large, predatory marine lizard closely related to modern monitor lizards and to snakes.-Paleobiology:...
and Mosasaurus
Mosasaurus
Mosasaurus is a genus of mosasaur, carnivorous, aquatic lizards, somewhat resembling flippered crocodiles, with elongated heavy jaws. The genus existed during the Maastrichtian age of the Cretaceous period , around 70-65 millions years ago in the area of modern Western Europe and North America...
, each exceededing 15 meters.
Polcyn and Bell (2005, p. 179-180) diagnose Dallasaurus as follows: "Small, plesiopedal mosasauroid possessing the following autapomorphies: posterior maxillary
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
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
Cervical vertebrae
In vertebrates, cervical vertebrae are those vertebrae immediately inferior to the skull.Thoracic vertebrae in all mammalian species are defined as those vertebrae that also carry a pair of ribs, and lie caudal to the cervical vertebrae. Further caudally follow the lumbar vertebrae, which also...
synopophyses protrude below the level of the ventral edge of the centrum; short, wide fossa
Fossa (anatomy)
In anatomical terminology, fossa has come to mean a depression or hollow, in general, in a bone. Other parts of the body may be involved, for example, the antecubital 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
Epiphysis
The epiphysis is the rounded end of a long bone, at its joint with adjacent bone. Between the epiphysis and diaphysis lies the metaphysis, including the epiphyseal plate...
bearing a calcified cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
apex."
Bell and Polcyn (2005) 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. Polcyn et Bell (2005, p. 178) note that plesiopedal mosasauroids tend to be relatively small lizards possessing limbs in which the "propodial elements [equals humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
, radius
Radius (bone)
The radius is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally...
, and ulna
Ulna
The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...
] 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
Aquatic animal
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...
, plesiopedal mosasauroids were still capable of terrestrial locomotion and so likely lived an amphibious lifestyle.
Bell and Polcyn (2005, 188-190) have conducted a cladistic analysis of Dallasaurus, concluding that this taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
should be placed within the subfamily Mosasaurinae
Mosasaurinae
Mosasaurinae is a subfamily of mosasaurs, a diverse group of Late Cretaceous marine squamates.Russell defined the Mosasurinae as differing from all other mosasaurs as follows: "Small rostrum present or absent anterior to premaxillary teeth. Fourteen or more teeth present in dentary and maxilla...
, "well within [the family] Mosasauridae", and that, despite its plesiopedal condition it should not be placed within the paraphyletic family Aigialosaur
Aigialosaur
Aigialosauridae is family of Late Cretaceous semi-aquatic varanoid lizards, regarded by some paleontologists to form distinct monophyletic group and by others as an adaptive grade within the basal mosasauroids....
idae, despite its small size and the "primitive" condition of its limbs. Dallasaurus is the sister group to more derived mosasaurine mosasaurs such as Clidastes
Clidastes
Clidastes is an extinct genus of mosasaur lizard from marine environments of the Late Cretaceous.Clidastes was an agile and fast swimmer that cruised the surface or shallow waters hunting for fish, flying reptiles and anything that got too close.- Appearance :Clidastes was the smallest of the...
, Prognathodon
Prognathodon
Prognathodon is an extinct genus of marine reptile belonging to the mosasaur family. It had protective bony rings surrounding its eye sockets, indicating it lived in deep water. its fossil remains have been found in the U.S.A , Canada , Belgium, New Zealand, Morocco and The Netherlands...
, Mosasaurus, and Plotosaurus
Plotosaurus
Plotosaurus is an extinct genus of mosasaur from the Upper Cretaceous of Fresno County, California. Originally named Kolposaurus by Berkeley paleontologist Charles Lewis Camp in 1942, it was changed to Plotosaurus in 1951 when Camp discovered the name had already been assigned to a type of...
. In the popular press, Dallasaurus has been hailed as a "missing link
Transitional fossil
A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...
" uniting fully aquatic mosasaurs with their terrestrial
Terrestrial animal
Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...
ancestorshttp://www.sciencedaily.com/releases/2005/11/051116173945.htm.
External links
- Move Over Dinosaurs — Here Come the Mosasaurs: Amateur Finds Prehistoric Lizard That Returned to the Sea
- Dino-Era Lizard Is Missing Link to Swimming Reptiles, Experts Say (National Geographic; November 28, 2005)
- Prehistoric N. Texas lizard helps prove evolutionary leap (Dallas Morning News; November 17, 2005)
- Missing Fossil Link 'Dallasaurus' Found (ScienceDaily; November 16, 2005)
- A Prehistoric Discovery That's as Big as Texas (WashingtonPost.com; November 18, 2005)
- 'Dallasaurus' confirmed as key evolutionary link (MSNBC; November 18, 2005)