Caipirasuchus catanduvensis: A vocal sphagesaurid?
A new "crocodile" was just published and one with some fascinating implications.
The new taxon is Caipirasuchus catanduvensis, the sixth species of Caipirasuchus described so far. As a sort of broad overview, Caipirasuchus is a genus of small-bodied sphagesaurid notosuchian, reaching about 1 meter in length. As sphagesaurids, the genus is most closely related to the likes of Yacarerani, Adamantinasuchus as well as the robust Armadillosuchus.
Another interesting thing about Caipirasuchus, and one that's gonna come back later, is how close they are in terms of geography. Of the six species, all are from Brazil's Bauru Group. 5/6 are from the Adamantina Formation and 5/6 are Sao Paulo State. So unlike the also specious Araripesuchus, these are all comparably limited in their distribution.
left: Caipirasuchus escaping baurusuchids by Deverson da Silva right: Generalized Caipirasuchus distribution adapted from Iori et al. 2024
Now what is interesting about this new species is a certain feature referred to as the pterygoid chamber, a hollow pocket in the pterygoid bone, that is notably more complex and pronounced than in other Caipirasuchus species (thus also why it was given its own name). Now an older study using CT scans already showed that this chamber connects to the choanal septum, which functions as a sort of air duct that connects the pterygoid chamber with the airways of the animal. There is also a secondary chamber and evidence that suggest that these structures grew more complex as the animal aged.
Now, the interesting part is that this combination of large hollow bone pockets with the airways is seen in some other animals as well, notably the wildebeest Rusingoryx and hadrosaurs, in which the pockets serve as a resonating chamber. As was already suggested in 2020, this might mean that Caipirasuchus catanduvensis had a much more complex range of vocalisations, which in turn would have a large influence on social behavior.
Now remember how I brought up how all the species of Caipirasuchus lived in a fairly enclosed area relative to Araripesuchus, the other specious notosuchian? Well one hypothesis proposes that this rapid speciation was driven not by dietary preferences, but by them refining their vocalisation and social structure. This in turn might be tied to environmental factors (like needing to be louder in more crowded, forested environments).
One pointer to this being the case is how species of Caipirasuchus differ from each other. Rather than skull shape or even the teeth, the main differences seem to be found in the palate, i.e. a region of the skull that would be tied to vocalisation. Another, albeit more tenuous, piece of evidence stems from the senses of what was likely a major predator of Caipirasuchus. Baurusuchids, larger predator notosuchians growing up to 3 meters in length, appear to have a complex outer ear (meatal chamber specifically), which indicates that they had good hearing. We also know that they did feed on sphagesaurids, as evidenced by a specimen of Aplestosuchus being preserved with one (possibly Caipirasuchus) found in its stomach contents. So by extension, it would not be unreasonable to assume that baurusuchids used their good hearing to track down noisy Caipirasuchus. Though more research is required of course.
left: Aplestosuchus and its prey by Rodolfo Nogueira right: various images from Godoy et al. 2014 featuring the fossil material of Aplestosuchus
All of this is of course fascinating. Crocodiles and their extinct kin are rarely viewed as very vocal, despite the fact that modern crocodilians have a great range of sounds at their disposal. Of course we're all familiar with the sounds hatchlings make (often likened to video game ray gun sounds), but crocodiles are also able to reproduce a wide range of grows, bellows and other sounds one might even compare to roars as adults. Just these past two years we have found evidence of dwarf crocodiles in the Congo making "mooing" sounds while the extinct Hanyusuchus, a gharial that died out during China's bronze age, has both historical and anatomical evidence for producing loud sounds.
Examples of vocalisations in African Dwarf Crocodiles and a male Indian Gharial
And finally, to cap this off, I want to leave you with the paper's artwork of the new species, illustrated by Guilherme Gehr. An interesting thing I noticed, but can only speculate on, is the fact that the two individuals have different colours, which might be a cheeky reference to the alternate hypothesis that the resonance chamber is sexually dimorphic (although this theory is largely dismissed on account of C. catanduvensis having other distinguishing features beyond just the pterygoid chamber.