''Sclerothorax'' is an extinct

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 ...

temnospondyl Temnospondyli (from Greek language, Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') or temnospondyls is a diverse ancient order (biology), order of small to giant tetrapods—often considered Labyrinth ...

from the

Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between 251.9 Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which ...

of Germany. It is distinguished from other temnospondyls by its short and very wide skull and the elongated

neural spine Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

s that form a ridge along its back. ''Sclerothorax'' is a basal member of

Capitosauria Capitosauria is an extinct group of large temnospondyl amphibians with simplified stereospondyl vertebrae. Mainly living as piscivores in lakes and rivers, the Capitosauria and its sister taxon Trematosauria were the only major labyrinthodonts ...

, a large

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

of temnospondyls that lived throughout the Triassic.

Description

Among the most unusual features of ''Sclerothorax'' are its elongated neural spines. The neural spines are tallest at the front of the spine. ''Sclerothorax'' also has a very large

pectoral girdle The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans, it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists o ...

; the

interclavicle An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals ( marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In ...

bone is longer than the skull, meaning that ''Sclerothorax'' has the proportionally largest interclavicle of any temnospondyl. The body of ''Sclerothorax'' is covered by small scale-like bony ossicles that would have been embedded in the

dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...

in life. The skull of ''Sclerothorax'' is short, wide, and roughly rectangular in shape. The snout is wider than the cheek region. The bones along the midline of the

skull roof The skull roof or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes, including land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium. In com ...

are also very wide relative to those of other temnospondyls.

Discovery

Two fossil specimens of ''Sclerothorax'' were discovered in the German state of

Hesse Hesse or Hessen ( ), officially the State of Hesse (), is a States of Germany, state in Germany. Its capital city is Wiesbaden, and the largest urban area is Frankfurt, which is also the country's principal financial centre. Two other major hist ...

in the 1920s. German paleontologist

Friedrich von Huene Baron Friedrich Richard von Hoyningen-Huene (22 March 1875 – 4 April 1969) was a German nobleman paleontologist who described a large number of dinosaurs, more than anyone else in 20th-century Europe. He studied a range of Permo-Carbonife ...

studied the remains and named the new genus and species ''Sclerothorax hypselonotus'' in 1932. 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 preserved only the vertebral column, but was identifiable as a temnospondyl by its rhachitomous vertebrae and as a new species by its tall neural spines. Huene's second specimen included a complete skull,

, and back portion of the

dorsal vertebra In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebrae of intermediate size between the cervical and lumbar ve ...

l column that was also clearly a temnospondyl. However, it was not apparent that the specimen belonged to the same species as the first until Huene exposed more of the crushed vertebrae and found that they were similar to those of his first specimen. A third specimen of ''Sclerothorax'' was discovered in 1967 that preserved the vertebral column and the lower jaw. It was described in 2004 by Michael Fastnacht, who also reported that the specimen had an impression of the

palate The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly sep ...

. The palate seemed to be long and narrow like that of another German temnospondyl called ''

Trematosaurus ''Trematosaurus'' is an extinct genus of trematosaurid temnospondyl amphibian found in Germany and Russia. It was first named by Hermann Burmeister in 1849 and the type species is ''Trematosaurus brauni''. History of study ''Trematosaurus'' w ...

''. Fastnacht therefore concluded that Huene's two specimens belonged to different species, with the first high-spined specimen referable to ''Sclerothorax''. However, three more specimens were uncovered in German museum collections with nearly complete skulls attached to vertebral columns. These specimens had broad heads, proving that Huene's second specimen also belongs to ''Sclerothorax''.

Classification

Huene originally classified ''Sclerothorax'' within the family

Actinodontidae Actinodontidae is an extinct family of temnospondyls Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') or temnospondyls is a diverse ancient order of small to giant tetrapods—of ...

as a close relative of '' Sclerocephalus'', another genus of temnospondyl from Germany that has a similarly broad skull. This classification put it as a close relative of a major temnospondyl group called

Stereospondyli The Stereospondyli are a group of extinct temnospondyl amphibians that existed primarily during the Mesozoic period. They are known from all seven continents and were common components of many Triassic ecosystems, likely filling a similar ecologi ...

, characterized by

centra Centra is a convenience shop chain that operates throughout Ireland. The chain operates as a symbol group owned by Musgrave Group, the food wholesaler, meaning the individual shops are all owned by individual franchisees. The chain has three ...

(central portions of vertebrae) that are primarily formed by bones called intercentra (non-stereospondyl temnospondyls have an additional bone called the pleurocentrum that forms a significant portion of each vertebra). The unusual vertebrae of ''Sclerothorax'' were thought to separate it from stereospondyls, but in 2000 it was reinterpreted as a tentative member of the group. A

phylogenetic analysis In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...

published in 2007 placed ''Sclerothorax'' as a derived stereospondyl in a clade called Capitosauria.

References

{{Taxonbar, from=Q2570648 Capitosauria Triassic temnospondyls of Europe Taxa named by Friedrich von Huene Fossil taxa described in 1932 Olenekian life