''Habrosaurus'' (, meaning "graceful lizard") is an extinct

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 n ...

of prehistoric

salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All ten ...

s, and the oldest known member of the family Sirenidae. Two species are known, ''H. prodilatus'' from the middle

Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campani ...

Dinosaur Park Formation The Dinosaur Park Formation is the uppermost member of the Belly River Group (also known as the Judith River Group), a major geologic unit in southern Alberta. It was deposited during the Campanian stage of the Late Cretaceous, between about 7 ...

of Alberta, and ''H. dilatus'' from the late

Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interv ...

and

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...

of western North America. It is relatively common in the

Lance Formation The Lance (Creek) Formation is a division of Late Cretaceous (dating to about 69 - 66 Ma) rocks in the western United States. Named after Lance Creek, Wyoming, the microvertebrate fossils and dinosaurs represent important components of the lat ...

of Wyoming, and less common in the

Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation stretches over portions of ...

of Montana. Habrosaurus Dilatus

Description

''Habrosaurus'' was one of the largest

lissamphibia The Lissamphibia is a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia (frogs, toads, and their extinct relatives), the Caudata (salamanders, newts, and their extinct relative ...

ns of all time, comparable in length to modern

giant salamanders The Cryptobranchidae are a family of fully aquatic salamanders commonly known as the giant salamanders. They include the largest living amphibians. The family is native to China, Japan, and the eastern United States. They constitute one of two l ...

at about 1.6 meters, as estimated from its trunk vertebrae length (up to 2 cm long). ''H. prodilatus'' possessed chisel-like teeth, while ''H. dilatus'' had bulkier crowns. The teeth of ''H. dilatus'' also exhibited heavy wear facets while those of ''H. prodilatus'' had only mild wear, suggesting that ''H. dilatus'' was more adapted for durophagy than ''H. prodilatus''. ''H. dilatus'' may have preyed upon arthropods with hard carapaces as well as mollusks.

Classification

''Habrosaurus'' is a sister taxa to both '' Pseudobranchus'' and '' Siren''"Gardner" which are the only genera in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Sirenidae Sirenidae, the sirens, are a family of neotenic aquatic salamanders. Family members have very small fore limbs and lack hind limbs altogether. In one species, the skeleton in their fore limbs is made of only cartilage. In contrast to most othe ...

. Sirenids are classified by their

neotenous Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compared ...

traits (complete lack of hindlimbs and external gills in both larval and adult states).

References

Sirenoidea Prehistoric amphibian genera Prehistoric salamanders Late Cretaceous amphibians Cretaceous amphibians of North America Late Cretaceous animals of North America Campanian genus first appearances Paleocene genus extinctions Cretaceous–Paleogene boundary Cretaceous United States Hell Creek fauna Lance fauna Fossil taxa described in 1933 Taxa named by Charles W. Gilmore {{paleo-salamander-stub