Active airflow of the paranasal sinuses in extinct crocodyliforms: Evidence from a natural cast of the thalattosuchianDakosaurus andiniensis

Abstract

The evolution of Thalattosuchia documents the unique shift among Crocodylomorpha from aquatic continental/coastal habitats to a fully pelagic lifestyle. This transition was coupled with deep modification of their skeletons, such as hydrofoil forelimbs, hypocercal tail, and loss of osteoderms. The natural snout casts of the rhacheosaurin Cricosaurus araucanensis showed that it also included changes in the internal anatomy of the snout like the enlargement of nasal glands (probably for salt excretion) and the rearrangement of the paranasal sinus system, including the internalization of the antorbital sinus. Here we described the snout natural cast of the geosaurin Dakosaurus andiniensis from the Late Jurassic of Patagonia. The information provided by it indicates that, despite having different external morphologies and ecology, D. andiniensis and C. araucanensis share the same facial anatomy. The new cast preserves a suborbital diverticulum of the antorbital sinus protruding into the orbit through the postnasal fenestra. Its location indicates that it was interleaved with jaw adductor muscles suggesting an active airflow in the paranasal sinus. We provide a putative functional interpretation of this peculiar arrangement where bellow pumps actions of musculature may help drain salt glands. The rearrangement of the paranasal sinuses predates the transition to a completely pelagic‐lifestyle. We proposed a stepwise evolutionary scenario of Thalattosuchia, implying changes in the preorbital region (and orbit orientation) where the internalized antorbital sinus via its subsidiary diverticulum was co‐opted for helping nasal glands drainage. Further scrutiny of facial anatomy of a larger sample of thalattosuchians will help to test this hypothesis.

Paranasal sinus system and upper respiratory tract evolution in Mesozoic pelagic crocodylomorphs

“…Recent work on a natural cast of another metriorhynchid, Dakosaurus andiniensis , also preserves compelling evidence for a suborbital diverticulum with a similar morphology to that of Cri. araucanensis (Fernández & Herrera, 2021). Our paranasal sinus reconstructions in three metriorhynchid species ( Cri.…”

“…If the suborbital diverticulum were present in metriorhynchids it would likely be a synapomorphy of the clade because compelling morphological evidence from natural casts is known in species from both metriorhynchine ( Cri. araucanensis ) and geosaurine ( Dakosaurus ) lineages (Fernández & Herrera, 2009, 2021). The diverticulum would not primarily be homologous with that of birds and other theropods because no positive evidence is known for its presence in other archosaur clades (Witmer, 1997).…”

Dispatches from the age of crocodiles: New discoveries from ancient lineages

“…They found thalattosuchians have reduced paranasal sinuses compared to other crocodyliforms but appear to evolve a more elaborate antorbital cavity, perhaps to help expel the derived nasolacrimal duct or ventilate possible suborbital air sacs. Following, Fernandez and Herrera (2022) describe the fossilized soft tissues of the faces of Cricosaurus and Dakosaurus . They detail the presence of salt glands, ducts, and suborbital diverticulae that may have been ventilated by the machinations of jaw muscle contractions.…”

El Estudio De Los Reptiles Marinos Mesozoicos en El Museo De La Plata

“…Fernández), en estrecha colaboración con colegas y discípulos del MLP y de otras instituciones de Argentina y del exterior. Dentro de esta última línea podemos destacar los trabajos en cocodrilos metriorrínquidos (e.g., Fernández y Herrera, 2009, 2021Herrera et al, 2013Herrera et al, , 2018 y en ictiosaurios (e.g., Campos et al, 2020;Fernández et al, 2020).…”