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

vertebra 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 spina ...

of a

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

. 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 b ...

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

, as sauropods exhibit a wide range of morphologies of the haemal arches. In 1878, Othniel Marsh named the sauropod '' Diplodocus'' 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''. File:Oksoko & Rinchenia tail chevrons.jpg, Haemal arch series of '' Oksoko'' and '' Rinchenia''. File:Ischioceratops caudal vertebrae.png, Caudal vertebrae of '' Ischioceratops'', showing haemal arches below tail.

References

* Vertebrate anatomy {{Vertebrate anatomy-stub