Cephalic salt gland evolution in Mesozoic pelagic crocodylomorphs

Abstract

Secondarily marine tetrapod lineages have independently evolved osmoregulatory adaptations for life in salt water but inferring physiological changes in extinct marine tetrapods is difficult. The Mesozoic crocodylomorph clade Thalattosuchia is unique in having both direct evidence from natural endocasts and several proposed osteological correlates for salt exocrine glands. Here, we investigate salt gland evolution in thalattosuchians by creating endocranial reconstructions from CT scans of eight taxa (one basal thalattosuchian, one teleosauroid, two basal metriorhynchoids and four metriorhynchids) and four outgroups (three extant crocodylians and the basal crocodyliform Protosuchus) to identify salt gland osteological correlates. All metriorhynchoids show dorsolateral nasal cavity expansions corresponding to the location of nasal salt glands in natural casts, but smaller expansions in teleosauroids correspond more with the cartilaginous nasal capsule. The different sizes of these expansions suggest the following evolutionary sequence: (1) plesiomorphically small glands present in semi-aquatic teleosauroids draining through the nasal vestibule; (2) moderately sized glands in the basalmost metriorhynchoid Pelagosaurus; and (3) hypertrophied glands in the clade comprising Eoneustes and metriorhynchids, with a pre-orbital fenestra providing a novel exit for salt drainage. The large gland size inferred from basal metriorhynchoids indicates advanced osmoregulation occurred while metriorhynchoids were semi-aquatic. This pattern does not precisely fit into current models of physiological evolution in marine tetrapods and suggests a unique sequence of changes as thalattosuchians transitioned from land to sea.

Orbital soft tissues, bones, and allometry: Implications for the size and position of crocodylian eyes

“…We found that these vascular traces diminished at the rostral tail of the nasal gland, usually about midway along the nasal bone. The nasal gland did not sit in a groove located on the ventral aspect of the nasomaxillary suture (Figure 8), as has been reported in some extinct, pelagic crocodyliforms (Cowgill et al, 2022;Herrera et al, 2013), but rather sat medial to it. This groove was instead filled by the nasolacrimal duct as it coursed toward the nasal cavity (Figure 8).…”

Neuroanatomy of the crocodylian Tomistoma dowsoni from the Miocene of North Africa provides insights into the evolutionary history of gavialoids

“…It is unclear whether this morphology is a preservational artefact of NHMUK PV R 4769, given that the entirety of the paranasal sinus protrudes dorsolaterally over the nasopharyngeal duct in Tomistoma schlegelii (Figure 4), whereas the paranasal sinus only protrudes dorsolaterally over the nasopharyngeal duct in its most posterior part in Gavialis gangeticus (Figure 3). This expansion results in depressions on the internal surface of the prefrontal and lacrimal in the two extant gavialoids, likely due to enlargement of the nasal salt glands (Cowgill et al, 2022; Pierce et al, 2017). In all three species, the dorsal surface of the olfactory region is characterised by a shallow midline groove.…”

“…The nasal cavity possesses nasal glands in all three species. These glands occupy small concavities located on the ventral surface of the nasals, on the nasomaxillary suture (Cowgill et al, 2022; Witmer, 1995). Whereas they are restricted to the posterior half of the rostrum in Tomistoma schlegelii (Cowgill et al, 2022), these concavities extend further anteriorly in both NHMUK PV R 4769 and Gavialis gangeticus .…”

Increasing the impact of vertebrate scientific collections through 3D imaging: The openVertebrate (oVert) Thematic Collections Network

“…Another obvious benefit is that a specimen may be used in different projects by multiple researchers at any given time. For example, during a 3-year period (2020–2022), more than 43 unique users downloaded the same data set of a gharial skull ( Gavialis gangeticus ; UF: Herp:118,998; https://doi.org/10.17602/M2/M39794 ), resulting in at least 10 publications from authors in seven countries (Hone et al 2020 , Bowman et al 2021 , Cowgill et al 2021 , 2022 , Pochat-Cottilloux et al 2021 , Serrano-Martínez et al 2021 , Iijima et al 2022 , Schwab et al 2022 a, 2022b, Tahara and Larsson 2022 ). These digital data grow the number of researchers able to interact with specimens, especially people from Low and Middle Income Countries lacking local collections or the resources to travel to other collections.…”