Anatomical exploration of Vipera ammodytes (family Viperidae) horn supports cephalic appendage sensory function in non-aquatic snakes

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Several snakes possess cephalic appendages over their eyes and snouts, but their exact function remains largely unknown. In one aquatic species, such structures have been associated with a mechanosensory role, and this suggests that other horned snakes may have evolved analogous appendages. To be considered sensory structures, cephalic appendages should be highly innervated by specific cranial nerves like the trigeminal, and perhaps carry specialised integumentary sense organs. The identification of trigeminal branches reaching the cephalic appendages would be consistent with sensory capability of the structures. Here we tested whether the rostral appendage of the nose-horned viper (Vipera ammodytes) could serve such a sensory function. We conducted an integrative anatomical exploration of the species' head and appendage using a combination of diffusible iodine-based contrast-enhanced micro computed tomography, classical histology and scanning electron microscopy and found support for our initial hypothesis. The rostral appendage of V. ammodytes is highly innervated by the ophthalmic branch of the trigeminal nerve, and we also found dermal sensory organs over the snake' snout and appendage scales. These findings suggest that a sensory function for cephalic appendages could also be possible in non-aquatic species, hinting that more analogous sensory structures may have evolved across horned snakes and other reptile groups.

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