A time traveller in the late Jurassic and early Cretaceous, around 150 million years ago, would be wise to avoid the open sea, for there she would have encountered reptilian killing machines, thalattosuchians. These reptiles were 4-5 metres in length and belonged—just like modern crocodiles—to the Crocodylomorpha, a once diverse group of animals.
Palaeontologists have established the thalattosuchians had morphological features that can be linked to living in pelagic or oceanic environments. I will not dwell on those adaptations but concentrate on one which has been the subject of a recent paper from Argentina concerning a new specimen of Dakosaurus andiniensis.
When we wrote our monograph on salt glands in 1975, there had been no search for evidence of their presence in extinct reptiles. However, it seemed highly likely that they must have been there. As a result of what we wrote, in the late 1970s I had a very enjoyable day with the late Peter Whybrow (1942-2004) at the Natural History Museum in London. He wondered whether the obvious depressions in the skull of hadrosaurs—duck-billed dinosaurs—could have been salt glands. Although semi-aquatic herbivores, there was the possibility they could have been potassium secreting glands as in the Common Iguana of South America. We also took the opportunity to look at a range of extinct marine reptiles, including ones collected by Mary Anning, and I was able to see clear depressions in the skull indicative of the presence of nasal salt glands.
In recent years there has been considerable interest in the presence of salt glands in dinosaurs and their relation to the morphological adaptations associated with a presumed marine or brackish water habitat.
In Dakosaurus andiniensis the arrangement of the internal anatomy is such that the nasal glands drain through a ‘nostril’ on either side of the snout via the antorbital sinus. However, a new twist in the story was discovered—a rearward extension of the antorbital sinus to form a suborbital diverticulum. That diverticulum is interleaved between the muscles that operate the jaw. Therefore, it was argued, operation of those muscles could be used to compress the diverticulum and expel rapidly secretion from the nasal salt glands gathered there. An analogous clearing of the salt glands can be seen and heard in the Galapagos Marine Iguana which snorts to cast the concentrated salt solution from its nostrils to the four winds.
What seems to be unknown is their mode of reproduction. From the little I have read, crocodylomorphs were oviparous, with aquatic forms having to lay their eggs on land. If so then a mechanism to expel actively secretion from the salt glands makes sense in that it would prevent salt encrustation and potential blockage of the system caused by evaporation.
These and related studies on the thalattosuchians appear to show an evolutionary trend from forms that lived in fresh water to those completely at home in a fully marine environment. They are also interesting in that modern crocodilians, the only survivors of the crocodylomorphs, have no sign of a nasal salt gland. Instead, the Estuarine crocodile or Saltie to Australians, was found, thanks to the efforts of Gordon Grigg at the University of Queensland, to have salt glands in its tongue.
The authors of the paper on Dakosaurus andiniensis, from the Universidad Nacional de La Plata, make the point that the presence of what must be salt glands in their specimen suggests that the animals could have dealt with a variety of prey from fish (usually, I see, described as the prey), other reptiles, all of which would be relatively low in salt, to invertebrates like squid which have the osmotic concentration of salt water. A specimen of a species of thalattosuchian was discovered in Cambridgeshire with stomach contents that included cephalopod hooklets, fragment of belemnite rostrum and bones, possibly from a pterosaur, suggesting these animals eat anything they can get hold of. I wonder if somebody would like to sit down and calculate the likely weight of the salt glands of Dakosaurus or other thalattosuchians as a percentage of their body weight and then do some armchair physiology, taking known secretory rates of salt glands and food intake for a reptile of that size, in order to calculate whether the salt glands could have coped with a mainly vertebrate diet or one containing lots of osmoconforming invertebrates like squid and other cephalopods. The results would be interesting either way. In other words, we could indulge in physiological time travel.
|Young, Brusatte, De Andrade, Desojo, Beatty, Steel, Fernández,
Sakamoto, Ruiz-Omeñaca, Schoch. 2012.
PLoS ONE, 7, e44985, doi:10.1371/journal.pone.0044985
Fernandez MS, Herrera Y. 2021. Active airflow of the paranasal sinuses in extinct crocodyliforms: Evidence from a natural cast of the thalattosuchian Dakosaurus andiniensis. Anatomical Record, 1–16. https:// doi.org/10.1002/ar.24678