Evolution of vomerolfaction in squamate reptiles with a special emphasis on Lacerta agilis

The naso-palatal complex of squamates incorporates developmentally associated structures and contains elements of the main and vomeronasal olfactory systems. These structures include the nasal cavity, vomeronasal organ (VNO), choanal groove, lacrimal duct, and the associated parts of the superficial palate. One of the most distinctive features of the squamate naso-palatal complex is the separation of the VNO from the nasal cavity. The VNO establishes a direct connection with the oral cavity, enabling tongue-mediated vomerolfaction. In all squamates, the rostral part of the lacrimal duct reaches the VNO duct or is in close proximity to it, delivering a secretion from the Harderian gland, which acts as an additional solvent for odor molecules. More posteriorly, the lacrimal duct in most nonophidian squamates connects to the choanal groove, which is a ventral remnant of the embryonic nasal cavity. In lacertid lizards and Scincomorpha, the choanal groove and the lacrimal duct are so closely associated that they form a lacrimo-choanal gutter, making these two components difficult to distinguish. Here, we analyzed the embryonic development of the naso-palatal complex of the lacertid lizard Lacerta agilis (Squamata: Episquamata: Lacertidae) using histological serial sections and X-ray microtomography. We found that the general pattern of naso-palatal complex development in L. agilis is similar to that observed in other studied nonophidian squamates, with the exception of some noticeable differences related to the timing of developmental characters. Delayed development of the lacrimal duct and choanal groove (heterochrony), along with their initial contact extent (heterotopy), may influence lacrimo-choanal gutter formation. Our findings provide additional support for previous investigations that the anterior part of the lacrimo-choanal gutter is exclusively composed of the lacrimal duct. Therefore, the "complete" gekkotan-like choanal groove, confluent with the VNO duct, is only present in L. agilis embryos at the time of its formation and is likely recapitulated in all autarchoglossans. The embryonic VNO at stage 36 is well developed and contains a distinctive mushroom body as well as a dilated ventral channel. Scattered blood vessels were observed at the level of the vomeronasal sensory epithelium in late developmental stages. This suggests the presence of undulations in the basal lamina and the intrusion of connective tissue in the form of an irregular network, as was previously proposed to occur in lacertids, but also in varanid lizards. Additionally, we describe the formation of a distinctive vomerine raphe that separates two laterally located depressions called here palatine tongue tine recesses. We propose that the choanal groove, a structure seemingly overlooked in many recent studies on squamate chemoreception, is, on the one hand, important during embryonic development by allowing the lacrimal duct to establish its final destination on the medial aspect of the VNO duct. On the other hand, it is incorporated into the structure of the lacrimo-choanal gutter in adult anatomy and may be functionally linked to the delivery of chemicals to the VNO. We also discuss the potential link between this structure and the noteworthy morphology of the superficial palate to tropotaxis and the sensitivity of the vomeronasal sensory epithelium.