The nasal salt gland of extant birds: anatomical structure and its relevance for inferring the behavior and habitat preferences of extinct birds

Abstract

The kidneys of extant sauropsids (birds, crocodylians, turtles, lepidosaurs) tend to be less efficient at removing excess salt from the blood than are those of mammals, and consequently, sites of extrarenal salt excretion are common in sauropsids. In birds, the nasal gland is involved with concentrating and excreting salt from the blood. In many marine birds (e.g., charadriiforms, procellariiforms), the nasal gland is hypertrophied and occupies a supraorbital position, but other positions are known in birds, such as within the orbit (e.g., suliforms). The basic physiology of avian nasal salt glands has been well‐understood for generations, but the detailed anatomy, particularly with regard to its osteology, is poorly documented. We present the results of our preliminary survey of nasal gland structure in birds to better resolve the osteological correlates of the gland, its ducts, and its vascular supply. The resulting osteological correlates (e.g., bony fossa, supraorbital process of the lacrimal, foramina/grooves for ducts) allow a more rigorous and informed assessment of the presence, position, and size of the nasal gland in extinct birds, potentially allowing rough assessments of salt loads and hence habitat preferences (e.g., marine vs. terrestrial). The extant studies emphasize marine and aquatic birds (e.g., albatrosses, shearwaters, gulls, gannets, cormorants, anseriforms), among others, such as, non‐marine outgroups (e.g., ostrich), drawing on the following anatomical approaches: (1) gross dissection, (2) diceCT (iodine enhanced microCT), and (3) radio‐opaque vascular injection followed by microCT. These studies allow not only detailed information about the relevant soft tissues and their osteological correlates, but also permit quantification of gland volumes and other metrics for comparative analysis. These extant studies shed new light on the interpretation of the nasal glands of extinct birds. Study of original fossil material and microCT scan data provide more refined information on the nasal glands of the Cretaceous marine toothed birds Hesperornis and Ichthyornis. The flightless diver Hesperornis had enormous nasal glands, suggesting that it was forced to consume seawater exclusively, whereas the volant and hence more vagile Ichthyornis had smaller salt glands suggesting that it perhaps also had access to fresh water sources. The Paleogene anseriform bird Presbyornis, typically regarded as inhabiting saline soda lakes, also was sampled and provides new evidence for a modest nasal salt gland.Support or Funding InformationUS National Science Foundation (IOB‐0517257, IOS‐1050154, IOS‐1456503), the Ohio University Honors Tutorial College, and the Ohio University Heritage College of Osteopathic Medicine