To most of the public, dinosaurs are considered almost synonymous with the Mesozoic Era, the
Age of Reptiles.
Despite this, dinosaurs were still rare and generally small during the first period of
the Mesozoic, the Triassic Period.
They were overshadowed by a number of creatures, most prominently the pseudosuchians.
Although they belonged to the same branch of reptiles as the dinosaurs, Archosauria,
the pseudosuchians were more closely related to crocodilians.
Among their number was the predatory Qianosuchus, a medium sized reptile which looked more like
a land drake than any dinosaur did.
Like the dinosaurs, most Triassic pseudosuchians, including the direct ancestors of the crocodilians,
were fully terrestrial animals.
However, Qianosuchus took to the water independently of them, yet curiously remained far more competent
on land than both crocodilians and most of the other contemporary amphibious reptiles.
Qianosuchus’s fossils are from the Guanling Formation in the Chinese Guizhou Province.
This site dates to the first half of the Anisian stage of the Middle Triassic, about 247 to
245 million years ago.
Despite the name, the Middle Triassic was actually near the beginning of the Triassic
period.
Two Qianosuchus skeletons have been found so far, and a third, isolated skull may also
belong to it.
The two skeletons are very well preserved, and only the lower half of Qianosuchus’s
forelimbs and the end of its tail are currently missing.
Qianosuchus was about three meters long.
While crocodilians only have a semi-erect posture, Qianosuchus’s limbs were placed
directly beneath its body.
This more dinosaur-like stance was actually ancestral to Pseudosuchia.
The scapulas of Qianosuchus’s forelimbs are thin and plate-like, a feature present
in many other marine reptiles, although its hindlimbs entirely lack any aquatic adaptions.
Indeed, its legs were notably much longer than those of crocodilians and most other
semi-aquatic reptiles.
While this would have allowed Qianosuchus to run faster while on land, its legs were
not its main source of propulsion in the water.
That was done by its tail.
Much like a crocodile’s, it was long, tall, compressed on the sides, and larger than the
tail of a terrestrial animal.
Qianosuchus’s tail was actually better optimized for swimming than the tails of the saltwater
crocodile and the fully marine reptile Hupehsuchus, although the latter also had flippers.
Like most pseudosuchians, Qianosuchus had bony osteoderms on its skin.
However, they were smaller and less extensive than of most other pseudosuchians.
This was likely an adaption to life in the ocean.
The modern saltwater crocodile has fewer osteoderms than its relatives, while the extinct metriorhynchids,
who were fully marine crocodylomorphs, lost them entirely.
Like crocodilians, Qianosuchus’s skull was elongated.
However, while most crocodilian skulls are flat, so as to create less resistance when
swimming, Qianosuchus’s skull instead deep, although the premaxilla at the end of the
snout was thin and upturned.
Qianosuchus’s teeth were ziphodont, meaning they were serrated, recurved, and laterally
compressed.
Ziphodont teeth are excellent as slicing through flesh and were typical of most predatory dinosaurs
and other predatory, terrestrial archosaurs.
However, the teeth of most aquatic and semi-aquatic archosaurs, including crocodilians, are conical.
In another departure from crocodilians, Qianosuchus’s neck was long, although nowhere near to the
extent of the contemporary the plesiosaurs and tanystropheids.
Its neck had long ribs which served as attachment points for extensive muscles, which would
have enabled it to make strikes that were both quick and powerful.
Therefore, Qianosuchus was equipped to catch both small, nimble prey like fish and larger
prey like other reptiles.
Although evolution has produced no shortage of semi-aquatic animals, Qianosuchus’s particular
combination of terrestrial and aquatic traits is unusual.
Its long legs would have produced excess drag when swimming, which is why most semi-aquatic
animals shorten their legs early in their evolution.
While they would have certainly been beneficial on land, its aquatic traits, like its heavy
tail, would have been major disadvantage when competing with dedicated terrestrial carnivores.
On the surface, Qianosuchus’s mixture of specialized aquatic and terrestrial traits
may seem like something that should have never evolved in a single animal.
These seemingly contradictory adaptions are explained by the environment it lived in.
When Qianosuchus was alive, southern China was not part of the mega-continent Pangea
and most of the Guizhou Province was underwater.
Therefore, Qianosuchus is thought to have lived on islands.
While most of the available land was too small to maintain a population of fully terrestrial
carnivores, Qianosuchus could viably fill their niche parttime.
Likewise, while it was not as well adapted to the water as the other marine reptiles,
it was still competent enough there to catch sufficient amounts of prey to make up for
the limited resources on the islands.
Besides fish, it would have also preyed upon these reptiles.
They included the long necked Dinocephalosaurus, early ichthyosaurs such as Mixosaurus, and
the unusual hammer-mouthed herbivore Atopodentatus.
Some of these reptiles, like the nothosaurs, would have still visited the shores, where
they would been the most vulnerable to Qianosuchus.
However, while it was their greatest threat on land, some contemporary species of Nothosaurus
grew to seven meters long, and were likely the greatest threats to Qianosuchus in the
water.
Qianosuchus belonged to a branch of Pseudosuchia named Poposauroidea.
As one can tell from this image, it was a very diverse clade.
It included sail-backed predators like Arizonasaurus and bipedal dinosaur mimics like the shuvosaurids
and Poposaurus itself.
However, none of them were very much like Qianosuchus.
Most studies of pseudosuchian evolution place Qianosuchus at the base of Poposauroidea.
Diandongosuchus, who also lived in the seas of Middle Triassic China, was once considered
potentially even less derived.
More recent research has found that Diandongosuchus was not a poposauroid at all but an early
phytosaur, an entirely different group of amphibious reptiles.
Therefore, in spite of its aquatic adaptations, Qianosuchus may be the closest science has
to what the first poposauroids looked like.
Qianosuchus may also be the oldest semi-aquatic archosaur.
Ironically, the only potential challenger to the title is once again Diandongosuchus,
since it is currently debated whether or not phytosaurs were early archosaurs or merely
close relatives of the archosaurs.
Either way, Qianosuchus did not give rise to the later water-loving archosaurs like
crocodilians.
While it still remains an evolutionary novelty among the poposauroids, Qianosuchus struck
a unique balance between land and water which set it apart not just from its surviving relatives,
but also from all other semi-aquatic animals.
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