#riversleigh

And so the Riversleigh #paleostream concluded! Lots of weird marsupials in this one. Since we chose to depict here a Miocene faunal zone you will not come across your typical Australian megafauna, like Diprotodon and Dromornis, that stuff came later.

We are looking at their ancestors, before the rainforest collapse.

Here some detail shots. Next time we will visit the Kimmeridge clay, a late Jurassic marine ecosystem.

So those that have been keeping up with my posts on mekosuchines might recall the name Ultrastenos, as I've talked about this genus back in August of last year. If you've read that post you might also remember how I highlight at multiple points that a lot of the info was tentative on the basis that Ultrastenos was highly incomplete and that close relatives awaited description.

You may also remember "Baru" huberi, a small mekosuchine that lived roughly around the same time, clearly distinct from Baru yet at that point still unnamed. Oh, how I wished for the former to get more material and for the latter to recieve a proper genus assignment.

My now outdated reconstructions for "Baru" huberi (the small one in the left image) and Ultrastenos (right image)

And then the monkey's paw curled.

As it turns out....they are the same damn animal. Now, back when "Baru" huberi was described, Willis also named a bunch of other mekosuchines from the same locality (the White Hunter Site of the Riversleigh WHA) and described even more material that remained unnamed, including the White Hunter Cranial Form 1. Now, when Ultrastenos was named in 2016, the type material was from the Low Lion Site (also Riversleigh), but importantly, the skull tables identified as WHCF1 were also assigned to the genus (and were the basis for my reconstruction).

Well, re-examination has shown that the WHCF1 and the holotype of "Baru" huberi aren't just a single species.....THEY ARE A SINGLE INDIVIDUAL.

Given the fact that the assignment of the skull table to the Low Lion Ultrastenos material still holds up, this means that Ultrastenos willisi and "Baru" huberi are a single taxon. Which consequently requires some reshuffling of the names.

"Baru" huberi was named first, so the species name takes priority and continues being used. However, since it was never given a genus name, Ultrastenos does stay valid. Except now it's called Ultrastenos huberi, not Ultrastenos willisi. A name that has aged like milk. Back in 2016 it was proposed that Ultrastenos had a very narrow snout (thus the name), so now that we know that the rostrum was flat and mesorostrine, the name really is just wrong.

So next up, lets examine what went wrong.

As I said before, Ultrastenos was fragmentary, so that certainly played a big part in it. But the team in charge of describing the animal still cited several lines of thinking to support their interpretation, most of which are now thoroughly debunked.

As an example, the lower jaw was rather shallow, however while this was initially taken as evidence for longirostry, the 2024 paper states that this only an argument against altirostry (a deep skull), not against a more generalized condition. The teeth were also initially used as evidence, citing their homodont condition (the teeth looked uniform), HOWEVER, the problem in that was that there were only a few teeth present, all of which notably do not bear any resemblance to the needle-like teeth seen in other long-snouted taxa. Another important clue initially taken to mean longirostry was the orientation of the quadrate area and the seemingly sudden constriction of the lower jaw. But the quadrate area was not found in articulation and would support a generalized skull form if simply rotated a little, while the constriction of the mandible appears to at least be partially exaggerated by preservation.

Of course, the fact that we now have proper material of the snout makes the interpretation of a generalized skull shape a lot more solid.

Image 1: The left and right halves of the mandible of Ultrastenos compared to that of Baru iylwenpeny (D), note how the right half is a lot more straight. Image 2: The initial reconstruction of the quadrate area of Ultrastenos compared to one that is slightly rotated Image 3: The revamped skull reconstruction by Yates and Stein

The size of the animal does stay relatively unaffected by these new discoveries. "Baru" huberi has been estimated at only around 1.5 meters in length and my own scaling of Ultrastenos got up to 2 meters, which seems to be in line with what is still assumed for this animal. So among aquatic mekosuchines, its still rather small.

There are however some interesting implications for mekosuchines at large. Now that we no longer have a longirostrine member of this family, one has to wonder, why is that? Well, there might be several reasons.

It could be that the types of environments that were present in Cenozoic Australia simply didn't support such animals. Even in the type description, its been noted that the Riversleigh isn't exactly known for its fish remains, leading to the idea that Ultrastenos might have gone for other small vertebrates like frogs. Hell, the ecology of Baru might suggest that the reason that this genus was so robust might tie to the fact that the local bodies of water just weren't deep enough to allow the typical crocodilian grab-and-drown tactic.

Competition might have been another factor. In environments that may have been more suitable for such morphology, mekosuchines might have been beaten to the punch by other types of crocodilians. Harpacochampsa for example, tho originally thought to be a mekosuchine, is now more often regarded as either an unrelated crocodile or a gharial and its very possible that it filling the nische of a longirostrine simply meant that mekosuchines didn't have the opportunity to expand into that space. Same goes for Gunggamarandu in the Pliocene and Pleistocene and Freshwater Crocodiles from the Pleistocene onwards. (Tho it should be noted that both Harpacochampsa and Gunggamarandu are so fragmentary that their snout shape is technically unknown).

Images: Gunggamarandu (Eleanor Pease), Harpacochampsa (ArtbyJRC) and Freshies (Antoni Camozzato) might have been key factors in why mekosuchines never evolved slender snouts.

Finally, its also possible that something in the growth of mekosuchines simply prevents them from evolving longirostrine skulls, which Yates and Stein liken to alligatoroids (notably the closest alligatoroids got to traditional longirostry as seen in gharials is the Rio Apaporis Caiman, and even that one is closer to some extant crocodiles in its morphology).

Whatever the case, I for one mourn the loss of our long-snouted Ultrastenos. Tho as a note for any paleoartists, there is not a single illustration of this new interpretation since nobody ever drew "Baru" huberi either. Wink wink nudge nudge

Links:

https://en.wikipedia.org/wiki/Ultrasteno

Fossil Novembirb: Day 19

So this one comes with a pretty big disclaimer. Ultrastenos, evenmoreso than any other mekosuchine, is incomplete and enigmatic. Whereas animals like Mekosuchus at least bolster a long research history or at least somewhat complete remains of specific body parts, Ultrastenos is both new and fragmentary. This, combined with ongoing research, means that this reconstruction is just a preliminary one based on current publications and should not be taken as gospel.

Ultrastenos is best known through assorted skull fragments and crania, notably part of the quadrate and quadratojugal, the occiput (back of the skull), an incomplete lower jaw, misc. postcrania like a tibia and vertebrae and some referred skull tables.

Already we enter some weirdness as the type material all stems from the Low Lion Site of the Riversleigh WHA, while the skull tables are from the White Hunter Site (also Riversleigh). Tho they are roughly of the same age, both being late Oligocene, but they are still different localities.

As is apparent both from my recon and the mandible, the lower jaw of Ultrastenos constricted weirdly rapidly, broad at the back and yet narrow towards the front (thus its name, which just means "extremly narrow").

A key issue here is that we don't know how long the rostrum was. We only have it up to the fourth-last tooth, so how long the snout would have continued we don't know. The idea that the snout was short has been entertained, but is deemed unlikely given that the mandible is very shallow. This would instead be more in line with long-snouted animals like gharials.

Size is another point of question. The type material is very small, only about two meters is what I recovered and the paper notes it's "only about as big as a freshwater crocodile", which matches fairly well I'd say. But it is not known whether or not the specimen was actually an adult, so we may find larger individuals in the future.

Which brings us to the disclaimer.

In 2023's "Migrations, diversifications and extinctions: the evolutionary history of crocodyliforms in Australasia", the confusion and uncertainty of Ultrastenos is once more brought up. Some of what is written is what you'd expect, which is that it's enigmatic and the interpretations of Stein et. al. need more research. Logical so far, its undeniably fragmentary and the interpretation is undeniably speculative. The fact that its phylogenetic placement among mekosuchines is a mess is likewise not a surprise. The original tree differs A LOT from more recent results, while the only other tree that included Ultrastenos is that of Rio and Mannion, which I generally avoid personally.

But the interesting nugget is that apparently, a close relative has been found since then which might have a big impact. But this research is still in prep courtesy of Adam Yates (who I've been told will publish the description of the Alcoota Baru soon enough). So will my recon become obsolete? Probably. But I'll update it accordingly once the time comes.

But lets briefly let go of safety and scepticism and consider the possibility that it really was a long-snouted mekosuchine. Aside from some of the anatomical clues like the shallow mandible, sameish teeth and narrow snout, a longirostrine croc would be expected, from the region, would it not? After all, long snouts evolved many times separately in crocs and despite having an incredibly diverse croc fauna, Australia is shockingly lacking in those. The only examples currently known are the Miocene Harpacochampsa, the Pliocene to Pleistocene Gunggamarandu and the current freshwater crocodile. The former two are gharials the later a crocodylid.

Furthermore, the White Hunter Site, if you remember prior posts, was positively teeming with mekosuchines of all different niches. Large semi-aquatic predators like Baru, small semi-aquatic ones like "Baru" huberi and terrestrial forms like Mekosuchus and Quinkana. So to fit in, Ultrastenos must have done something differently, and conveniently a longirostrine nische is still open. So why not.

Illustrations by Joschua Knüppe and Ashley Patch