The appearance of Velociraptor in Jurassic Park and its sequels thrust it into becoming
one of the most popular dinosaurs.
Of course, Velociraptor and other the “raptors”, properly called dromaeosaurids, looked very
different from their appearance in the movies.
However, there was another reptile who was reminiscent of them, in some ways more than
the dromaeosaurids themselves.
Named Poposaurus, it lived in what is now western North America during the Late Triassic
period.
While it was once mistaken for a dinosaur, Poposaurus was a pseudosuchian, meaning it
was more closely related to crocodilians than to dinosaurs.
When this crocodilian raptor was alive, true dinosaurs had just evolved and were still
the underdogs to Poposaurus and its fellow pseudosuchians.
While many other pseudosuchians looked similar to the later dinosaurs, Poposaurus was perhaps
the most extreme dinosaur look-alike.
Between several different skeletons, almost all of Poposaurus’s body is accounted for,
although the skull is still only known from a few fragments.
The Poposaurus genus consists of two species, Poposaurus gracilis and Poposaurus langstoni.
Poposaurus gracilis was about four and a half meters long.
Poposaurus langstoni was a bit larger, about five meters long, longer than the Jurassic
Park raptors.
However, most this length was taken up by its unusual tail.
Therefore, Poposaurus would have filled the niche of a medium sized predator while the
larger rauisuchids like Postosuchus were the apex predators.
Because Poposaurus’s legs were shorter than most theropods, it was only about one meter
tall, half the height of its Jurassic Park like-alikes.
Despite this, there is no doubt that it was a biped.
Examination of the muscle scars on Poposaurus’s leg bones has found that the muscles responsible
for back-and-forth movement were the strongest, a trait typical of bipedal animals.
Furthermore, its hindlimbs were still twice the length of the forelimbs.
While its feet lacked the famous sickle claws of the dromaeosaurids, they were still very
similar to the feet of theropods.
As long as Poposaurus’s small first digit didn’t touch the ground, it wouldn’t be
hard its footprints to be mistaken for those of dinosaurs.
The main difference between them is that Poposaurus would have had a shorter stride distance due
to its shorter legs.
Poposaurus’s neck was also long by pseudosuchian standards, closer to those of theropods.
On the other hand, the crocodilian raptor’s tail was odd compared to those of both theropods
and most other pseudosuchians.
Theropod tails gradually shrink as they extend from the body.
In Poposaurus, its tail simply keeps going long after becoming very thin.
Additionally, the spinous processes at the base of its tail don’t shrink along with
its vertebrae and at one point even grow taller.
Then the spinous processes shrink rather abruptly.
The exact purpose of this odd tail is unclear.
The base of the tail might have served as some sort of hump to store nutrients.
Alternatively, Poposaurus’s odd tail may have been the result of sexual selection and
may have been brightly colored or used in elaborate dances to attract mates.
While most of Poposaurus’s skull has yet to be found, a few jaw fragments have sharp,
recurved teeth, confirming it was a predator.
Combined with the skulls of its close relatives, it seems its skull would have looked very
much like what one would expect from a theropod dinosaur.
While Poposaurus is often called a “theropod mimic”, it lived not long after the theropods
had first evolved.
It is therefore entirely possible that Poposaurus’s theropod-like ancestors evolved before the
actual theropods.
Most of their similarities were the result of convergent evolution, while a few, such
as their skulls, were inherited from the first archosaurs.
Poposaurus is the namesake of the clade Poposauroidea.
The members of this clade are varied and include a few oddballs like Qianosuchus and Lotosaurus.
While Poposaurus is not really representative of any other poposauroid, it does bridge the
gap between two very different groups within it.
Most early poposauroids were ctenosauriscids.
Ctenosauriscids were quadrupedal predators with sailed backs.
It is possible that Poposaurus’s tall spinous processes at the base of its tail are remnants
of these sails.
The other major poposauroid clade is Shuvosauridae.
They lacked any remanents of sails and, like Poposaurus, were bipeds.
However, the shuvosaurids were herbivores with toothless beaks instead of teeth.
In phylogenetic analyses, Poposaurus is found to be between the two clades and it likely
represents how the transition from ctenosauriscid-like poposaurids to the shuvosaurids took place.
Poposaurus’s closest living relatives, crocodilians, grow very slowly due to their low metabolisms.
However, examination of Poposaurus’s internal bone histology has found signs of faster growth.
This would have required a high metabolism more typical of dinosaurs than crocodilians.
Unlike some of its other theropod-like characteristics, convergent evolution was not the culprit of
the crocodilian’s raptor’s high metabolism.
Archosaurs like Poposaurus and the dinosaurs originally had a high metabolism.
While the ancestors of the crocodilians eventually returned to a lower metabolism, more like
that of most other reptiles, Poposaurus evolved long before this happened.
Right now, only the growth rings of an immature Poposaurus specimen have been examined, so
it is not certain how long it took to reach its full size.
Assuming the growth rings were each laid out during a single year, the animal would have
been seven years old when it died.
Poposaurus was an amazing creature.
While modern crocodilians may be cold blooded and tied to the water, Poposaurus shows just
have active and dinosaur-like many of their distant relatives once were.
Its superficially dinosaur-like appearance is simply the cherry on top.
It also serves as one of the starkest of cases of convergent evolution and is an example
of how diverse and strange the world was during the Triassic Period.
Thank you for watching.
I hope you learned something interesting.
Also, a thank you to Domenic Pennetta for making art of Deinosuchus specifically for
this video.
If you would like to see more of his art, be sure to check out his website.
There is a link to it in the description.
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Finally, be sure to have a great day!