Embryonic development of the neotropical pit viper Bothrops atrox (Serpentes: Viperidae: Crotalinae), with emphasis on pit organ morphogenesis and its evolution in snakes.

BACKGROUND: Bothrops atrox is a pit viper with a loreal pit organ, and its embryological development remains undescribed. Here, we provide a comprehensive description of the embryology of B. atrox, focusing on the loreal pit organ and cephalic scales. RESULTS: We characterized 13 developmental stages of B. atrox based on external features consistent with the embryogenesis of previously described snake species. The loreal pit organ originates from the circumorbital region and migrates to its final position. In Crotalinae, the pit organ first becomes visible at stage 28, whereas in Pythonidae labial, pit organs appear at Stage 35. Pit organs evolved independently three times in Serpentes, encompassing Boidae, Pythonidae, and Crotalinae. Boidae lacks embryological information for pit organs. Furthermore, we observed that head scalation onset occurs at Stage 33 in B. atrox, with fusion of scales surrounding the loreal pit organ. CONCLUSIONS: The embryology of pit organs in Pythonidae and Boidae species remains poorly understood. Our detailed embryological descriptions are critical for proposing developmental scenarios for pit organs and guiding future research on these structures.