Which combination of features is specifically noted as highly diagnostic for identifying Xenacanthus fossils?
Answer
Nuchal spine, caudal spine, and herringbone teeth
Paleontologists use a highly specific suite of traits to definitively classify a fossil specimen as *Xenacanthus*. The primary identifier is the backward-pointing spine emerging from the nuchal region (back of the head). This is strongly supported by the presence of a second caudal spine near the tail structure, and critically, the unique, paired herringbone pattern observed in the tooth structure. The combination of these three highly specialized skeletal and dental features separates this genus from contemporaneous fish inhabiting the same Carboniferous and Permian freshwater realms.
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