Today Australia is the home of the Saltwater crocodile, the largest living reptile.
However, Mesozoic fossils of Australian crocodilians and their relatives, the crocodylomorphs,
are extremely rare.
Currently only one genus of crocodylomorph, Isisfordia, is known from Australia during
this time.
While small, Isisfordia has opened up many questions regarding the evolution of crocodilians.
Isisfordia was rather similar to modern crocodilians.
Its physical appearance has been compared to that of alligators with its broad snout.
However, it was still flatter, thinner, and longer than those of alligators.
However, it was just over a meter long, about the same size as Cuvier's dwarf caiman.
At such a small size it safe to say it wasn’t hunting adult dinosaurs like some of its larger
relatives.
Instead, it would have hunted much smaller prey, such as lizards, mammals, dinosaur hatchings,
fish, amphibians, and invertebrates.
Isisfordia consists of two to three species, all of which have been found in Australia.
The first one to be named, Isisfordia ducani, was found in the Winton Formation, which was
formed between the end of the Early Cretaceous and the beginning of the Late Cretaceous.
The Winton Formation was once a variety of different aquatic environments, including
swamps, rivers, and estuaries.
It was also home to the sauropod dinosaur Australotitan, the largest known dinosaur
from Australia, and the deadly predator Australovenator.
The holotype skeleton of Isisfordia ducani was almost complete, except for part of the
skull, which had been damaged due to exposure to the elements before it was found.
Another specimen, consisting of a larger, more complete skull, was found later, giving
science a complete picture of its skeleton.
Even more Isisfordia ducani have been found since then, some of which are even opalized.
The other two species of Isisfordia, Isisfordia molnari and Isisfordia selaslophensis, are
only known from much more fragmentary remains.
Indeed, due to the lack of overlapping material, they could turn out to be the same species.
They are both from the Griman Creek Formation.
Like the Winton Formation, the Griman Creek Formation was also formed during from the
Early/Late Cretaceous boundary in Queensland and was once a freshwater lagoon.
When Isisfordia was first found, it was thought to be a basal member of Eusuchia, the group
of crocodylomorphs that includes crocodilians and their closest relatives.
This classification was based on Isisfordia possessing both a ball-in-socket vertebral
joint and a crocodile style secondary palate, both hallmarks of Eusuchia.
At the time Isisfordia was discovered, only one other eusuchian as old was known, which
brought up the possibility that Eusuchia first evolved in southern, formerly Gondwanan continents
rather than in the northern Laurasia ones as has usually been thought.
However, a paper in 2015 found that the secondary palate in Isisfordia was more like those of
more primitive crocodilian relatives outside of Eusuchia.
Likewise, many of these comparatively more primitive relatives also had already evolved
crocodilian like vertebrate joints.
It is now agreed that Isisfordia was not an eusuchian, but still part of Neosuchia, the
larger clade of semi-aquatic crocodylomorphs Eusuchia is a part of.
The closest relative of Isisfordia is now thought to be Susisuchus.
Like Isisfordia, Susisuchus was small and lived in a formerly Gondwanan continent, South
American in its case.
Still, Isisfordia is the only known Mesozoic crocodylomorph in all of Australia.
All other Australian crocodylomorph fossils are too fragmentary to understand much about
them.
This is at least partly due to Australia being relatively geologically stable.
While this means rocks from older time periods are more common in Australia than elsewhere,
there were also fewer new sedimentary rocks formed by erosion during the Mesozoic era,
resulting in fewer fossils from this time.
Therefore, the lack of Australian crocodylomorph fossils from the time may just be due to the
low number of Mesozoic fossils from the continent in general.
However, there may be another reason for the rarity of crocodylomorphs in Mesozoic Australia,
one that adds to Isisfordia’s significance in crocodylomorph evolution.
During much of the Mesozoic era, Australia was much further south than it is today, with
parts of it even laying within the Antarctic circle.
While global temperatures were warmer during the Mesozoic era than today, Australia’s
southward position still resulted in temperatures in Australia being much cooler than what is
usually tolerable for crocodilians.
While early crocodylomorphs had higher metabolisms than today’s crocodilians, the metabolisms
of the amphibious neosuchians were already about as low as their modern descendants.
Instead, a branch of amphibians called brachyopoids, such as the bizarre Koolasuchus, filled the
niche of semi-aquatic predator.
Belonging to a larger group of amphibians called temnospondyls, their ancestors were
semi-aquatic predators during the Triassic period, long before the neosuchians evolved.
After the Triassic period, brachyopoids are only known from Australia.
This is thought to be because they were able to better withstand the cold there while neosuchian
crocodylomorphs took to the water elsewhere.
The appearance of Isisfordia 100 million years ago coincides with indicators that the region
had become warmer.
Unsurprisingly, no brachyopoid amphibians have been found from the same environments
as any species of Isisfordia or in any time afterward, suggesting they may have gone extinct.
Neosuchians like Isisfordia may have simply taken advantage of the ecological void left
in the wake of the extinction of their brachyopoid predecessors or their arrival may have been
what drove them to extinction.
Regardless while Isisfordia may not rewrite eusuchian evolution, it does provide important
information on the biodiversity of Australia and the likely extinction of the brachyopoid
temnospondyls.
Either way, as the sole named Mesozoic crocodylomorph from Australia, Isisfordia offers a glimpse
into an otherwise poorly understood part of crocodylomorph biodiversity.
Thank you for watching.
Also, a thank you to Flipped StuG for narrating this video.
This is a remake of an older video I did.
While that video in now unlisted, the link to it is in the description if you wish to
compare the two.
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