Kalthifrons and the Fall of Mekosuchines
The penultimate singular mekosuchine I'll talk about here is Kalthifrons, one of the more obscure members of this clade. After all, Kalthifrons was named only in 2017 and unlike some other animals like Ultrastenos, it does not stand out as much morphologically. But I still wanna take a moment to talk about this medium-sized Pliocene croc, its potential fate and the implications for its relatives.
Left: The skull of Kalthifrons (Ristevski et al. 2023)
Right: A reconstruction of how the skull may have looked like intact
In many respects Kalthifrons looks very typical for a crocodilian, with a triangular head, rounded snout and eyes that face upwards. There are some interesting factors to mention, at least imo, and one thing thats sorta unknown. Starting with the unknown, what we aren't sure about is how deep the skull is. Sure the skull of Kalthifrons looks very intact in top view, but when looking at it from the side its about as flat as a sheet of paper. So we can't exactly tell how deep the skull would be. Maybe it was not much different from a modern croc...or maybe it was comparable to Baru the cleaver-headed crocodile. We simply don't know. The other two interesting things about its anatomy are more subtle. For one, the tip of the snout is comparably narrow. Now generally, if you look at mekosuchines the premaxilla and external nares tend to be wider than long, even in Australosuchus, but in Kalthifrons they are longer than wide. And then there's the frontal. The name Kalthifrons derives from the Latin word for forehead and the Dieri word for spear. The reason why is that the frontal bone, which covers the forehead, has an exceptionally long spear-like process at the front. Sure such a process is also found in other crocs, but in Kalthifrons it makes up 64% of this bones entire length.
A hypothetical reconstruction showing the size of Kalthifrons, which may have been around 3.4 meters.
With a length of possibly up to 3.4 meters and a triangular skull, Kalthifrons was probably a fairly generalist hunter. Fossils of Kalthifrons are known from a single place, the Golden Fleece Locality along the shores of Lake Palankarinna in the Lake Eyre basin, South Australia. Now this locality primarily yields two things. Croc fossils and turtles. Based on this and certain minerals in the sands of this locality, it has been suggested that the Golden Fleece Locality once housed a waterhole that dried up, leading to the deaths of its inhabitants.
The death of Kalthifrons as illustrated by Joschua Knüppe.
Which leads us to a bit of discussion regarding the general state of mekosuchines during the time of Kalthifrons. Now if you have read my previous posts on mekosuchines, you'll notice one thing. Most species native to continental Australia were around during the Oligocene to Miocene. Australosuchus, Baru, Ultrastenos, "Baru" huberi, Trilophosuchus and the continental Mekosuchus species. After the Miocene, continental species become rarer. Really you got Paludirex, Kalthifrons and Quinkana. And even they wouldn't stick around for much longer, with the last mekosuchines of Australia going extinct at the end of the Pleistocene.
So obviously mekosuchines experienced a drastic fall in diversity towards the end of the Miocene, clinging on for a few more million years before disappearing forever. But there's another thing that happened around this time. Modern crocodiles showed up. Contrary to popular belief, today's crocodiles of Australia are relatively recent arrivals. Freshwater crocodiles only lived there since the Pleistocene and saltwater crocodiles were likely even more recent, with both of them having evolved from different ancestors and arrived in Australia independently. But there's at least one more species of Crocodylus. The as of yet unnamed Tirari Desert Crocodile is known from remains that date to strata slightly more recent than that of Kalthifrons, displaying similar generalized anatomy. Like freshies and salties, the Tirari Crocodile arrived on its own, with phylogenetics indicating that its closest relatives were the palaeoafrican crocodiles like Crocodylus thorbjarnarsoni (a giant that grew up to 8 meters long).
Crocodylus species of Australia
Top left: The freshwater crocodile (Ristevski et al. 2023)
Bottom left: The saltwater crocodile (Ristevski et al. 2023)
Right: Stratigraphy of the Lake Eyre Basin and successions of local crocodilians (Yates and Pledge 2017)
Did Crocodylus drive Kalthifrons to extinction? Well, maybe? Yates and Pledge discuss this possibility in the description of Kalthifrons, suggesting that theres some supporting pieces of information for this hypothesis. Not only does the arrival of the Tirari crocodile match up nicely with the disappearance of Kalthifrons, but they both share very similar anatomy. And conventional wisdom suggests that two crocs of the same morphotype don't really coexist in the same habitats.
But there's also points against that which aren't as openly discussed. Namely, the lack of material. Sure we have Kalthifrons in the older Mampuwordu Sand Member and the Tirari Crocodile in the younger Pompapillina Member, but remember. We only have that one Kalthifrons individual. We further know little about their habitat preferences. Sure conventional wisdom suggests crocs with similar morphology don't coexist, but that doesn't mean they don't overlap. Some species may very well be separated by the specific habitat they prefer. Slow open water, fast moving streams. Finally, is replacement necessarily the only answer? Not really. It has been deduced that Kalthifrons died in a drought and aridification is regarded as one of the main drivers of Australian megafauna extinction, as touched on in my summary of Paludirex. Maybe that's it? Kalthifrons was driven to extinction when environmental conditions grew too harsh, with Crocodylus entering the newly opened niche. This may parallel what happened in Miocene Africa, when rainforests disappeared osteolaemine crocodiles likely gave way to Crocodylus, which was better suited to the dryer climate.
Whatever the case, Kalthifrons is one of the most recent mainland mekosuchines and thus may be critical for us to determine what happened during this part of Australia's history. So sure it might not be flashy, but it's still an important little guy.
