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 characteristi ...

of a

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

. 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 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 Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', ' lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their ...

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 the ...

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

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

''. 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 ...

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

References

* Vertebrate anatomy {{Vertebrate anatomy-stub