Ahaemal arch, also known as achevron, is a bony arch on the ventral side of a tailvertebra of avertebrate. The canal formed by the space between the arch and the vertebral body is thehaemal canal. A spinous ventral process emerging from the haemal arch is referred to as thehaemal 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.[1]
In 1956,Alfred Sherwood Romer hypothesized that the position of the first haemal arch was sexually dimorphic in crocodilians and dinosaurs.[2] 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.[3]
Haemal arches play an important role in the taxonomy ofsauropoddinosaurs, as sauropods exhibit a wide range of morphologies of the haemal arches.[1] In 1878,Othniel Marsh named the sauropodDiplodocus after the distinctive shape of its haemal arches, which were forked to have both an anterior and posterior process.[4] Though once thought to be a specialized characteristic ofDiplodocus and its close relatives, forked chevrons are now known to have been widespread among sauropod dinosaurs, although titanosauriform sauropods returned to the unforked condition.[5]
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