What distinctive anatomical feature defines the Order Xenacanthiformes?

A prominent spine projecting backward from the rear of the skull

The most defining characteristic separating the Order Xenacanthiformes, and thus the genus *Xenacanthus*, from most other extant and fossil sharks is the presence of a large, backward-pointing spine located at the rear of the skull, often described as serrated. This singular anatomical feature is so crucial to the identification of this extinct lineage that it is directly responsible for the names assigned to both the order (Xenacanthiformes) and the genus (*Xenacanthus*). This structure contrasts notably with the typical dorsal fin spines found on many modern sharks, marking it as a highly specialized evolutionary trait for the ancient inhabitants of Carboniferous and Permian waters.