A haemal arch also known as a chevron, is a bony arch on the ventral side of a tail

vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

of a vertebrate. The canal formed by the space between the arch and the vertebral body is the haemal canal. A spinous ventral process emerging from the haemal arch is referred to as the haemal spine. Blood vessels to and from the tail run through the arch. In reptiles, the caudofemoralis longus muscle, one of the main muscles involved in locomotion, attaches to the lateral sides of the haemal arches. In 1956,

Alfred Sherwood Romer Alfred Sherwood Romer (December 28, 1894 – November 5, 1973) was an American paleontologist and biologist and a specialist in vertebrate evolution. Biography Alfred Romer was born in White Plains, New York, the son of Harry Houston Romer ...

hypothesized that the position of the first haemal arch was sexually dimorphic in crocodilians and dinosaurs. However, subsequent research established that the size and position of the first haemal arch was not sexually dimorphic in crocodilians and found no evidence of significant variation in tyrannosaurid dinosaurs, indicating that haemal arches could not be used to distinguish between sexes after all. Haemal arches play an important role in the taxonomy of sauropod

dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is ...

, as sauropods exhibit a wide range of morphologies of the haemal arches. In 1878,

Othniel Marsh Othniel Charles Marsh (October 29, 1831 – March 18, 1899) was an American professor of Paleontology in Yale College and President of the National Academy of Sciences. He was one of the preeminent scientists in the field of paleontology. Among ...

named the sauropod ''

Diplodocus ''Diplodocus'' (, , or ) was a genus of diplodocid sauropod dinosaurs, whose fossils were first discovered in 1877 by S. W. Williston. The generic name, coined by Othniel Charles Marsh in 1878, is a neo-Latin term derived from Greek διπ ...

'' after the distinctive shape of its haemal arches, which were forked to have both an anterior and posterior process. Though once thought to be a specialized characteristic of ''Diplodocus'' and its close relatives, forked chevrons are now known to have been widespread among sauropod dinosaurs, although titanosauriform sauropods returned to the unforked condition. File:Wintonotitan tail chevrons.jpg, Haemal arches of ''

Wintonotitan ''Wintonotitan'' (meaning " Winton titan") is a genus of titanosauriform dinosaur from late Albian (Early Cretaceous)-age Winton Formation of Australia. It is known from partial postcranial remains. Description and history Fossils that ar ...

''. File:Ichthyovenator laosensis tail vertebrae NMNS.jpg, Caudal vertebrae of ''

Ichthyovenator ''Ichthyovenator'' is a genus of spinosaurid dinosaur that lived in what is now Laos, sometime between 125 and 113 million years ago, during the Aptian stage of the Early Cretaceous period. It is known from fossils collected from the ...

'', showing haemal arches below tail. File:The Childrens Museum of Indianapolis - Chevron from Edmontosaurus - detail.jpg, Haemal arch of ''

Edmontosaurus ''Edmontosaurus'' ( ) (meaning "lizard from Edmonton") is a genus of hadrosaurid (duck-billed) dinosaur. It contains two known species: '' Edmontosaurus regalis'' and '' Edmontosaurus annectens''. Fossils of ''E. regalis'' have been found in roc ...

''. File:Oksoko & Rinchenia tail chevrons.jpg, Haemal arch series of ''

Oksoko The three-headed eagle (or triple-headed eagle) is a mythological or heraldic bird, as it were an augmented version of the double-headed eagle. A three-headed eagle is mentioned in the apocryphal Latin Ezra, featuring in a dream by the high p ...

'' and '' Rinchenia''. File:Ischioceratops caudal vertebrae.png, Caudal vertebrae of '' Ischioceratops'', showing haemal arches below tail.

References

* Vertebrate anatomy {{Vertebrate anatomy-stub