From which specific anatomical area does the defining nuchal spine of *Xenacanthus* extend backward?
Answer
Back of the head or nape region
The nuchal spine is identified as the morphological trait giving the genus its name, and its precise location is critical for identification. This long, sharp projection is situated at the rear of the head, specifically extending backward from the nape region. This positioning distinguishes it from a typical dorsal fin spine, which arises from the main body musculature along the back. The placement suggests a structure specifically evolved for protection of the head and neck area, providing a significant deterrent against predators attempting an engulfing strike from the front or above.
Related Questions
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