Inception and organogenesis of the nasal glands in the embryonic Laysan albatross ( Phoebastria immutabilis ) and American alligator ( Alligator mississippiensis ), and a re‐evaluation of their homology

Although diverse manifestations of nasal glands occur within Tetrapoda, most studies have focused on those with ductal openings in the nasal vestibular ductal opening. Historically, comparative studies of adult animals often concentrated on specific clades, resulting in taxon-specific terminology (e.g., external nasal gland ENG in squamates and "Steno's gland" or "salt gland" in birds). The few embryological studies that exist are descriptions of individual unstaged embryonic specimens. In this study we examined two series of stage embryonic specimens of two archosaurs: Laysan Albatross (Phoebastria immutabilis: Aves) and the American alligator (Alligator mississippiensis: Crocodylia). Serial histological sections of mid to late embryonic staged specimens of both species were examined. A. mississippiensis possesses a single large gland, with a medial vestibular opening whereas P. immutabilis has two ducts (inner and outer in reference to their origin from the vestibulum). In comparison to other tetrapods, the alligator condition most closely resembles that of an ENG, which is typical of amphibians and many amniotes. Dual glands exist in many birds, echidna, turtles and Sphenodon. We argue that the inner one corresponds to the ENG and the outer one to Steno's gland; the entire complex can appropriately be called the dual nasal gland (dNG). Unique features of timing (earlier in birds and alligators, in comparison to rodents), nasal rotation (dual late embryonic nasal rotation in alligators) and the impact of ductal morphogenesis (and the putative role of signaling pathways in ductal development) reveal a hitherto underappreciated complexity of nasal gland evolutionary history.