In 2019, the discovery of a new species of dinosaur, named Vespersaurus, was announced.
A small theropod, it lived 85 million years ago in southeastern Brazil.
While it is the first Brazilian dinosaur found from this time, Vespersaurus is not the first indication of their precedence there.
Dinosaur fossil footprints have also been found there, which likewise appear to have been made by a small theropod.
However, these footprints seem to have been made by one that was monodactyl, meaning it only had one toe on each foot.
Most other theropods instead supported their weight with three toes.
At first glance, Vespersaurus would not seem to be the track maker,
at it possessed the same number of toes typical of other theropods.
But a closer examination of Vespersaurus’s middle toe reveals it was much larger and longer than they rest,
strong enough to walk on without the support of the two outer toes.
Instead, these two toes seemed to be held off the ground.
While it wasn’t truly a one toed dinosaur, Vespersaurus was functionally monodactyl,
with feet matching those of the one toed fossil tracks.
But while they weren’t used for walking, Vespersaurus’s two outer toes were far from vestigial.
Their claws were similar to the sickle claws of the famous dromaeosaurids,
though Vespersaurus’s were instead both straighter and sharper.
They therefore seem to have been better at slicing than piercing.
It is still unclear why Vespersaurus only walked on one toe.
One possibility was too free up the outer toes to be better used as weapons,
as was the case for the less extreme adaptations in the dromaeosaurids.
Another was for the same reason as many other species: speed.
When the weight of an animal is concentrated in one area, like one toe instead of three, the foot is more resistant to stress.
This allows the animal to make run or hop faster without risk of hurting its feet.
Animals with reduced digits are usually found in open environments, like deserts or grasslands.
. If speed was why Vespersaurus evolved its odd feet, it would not have been an exception.
85 million years ago, southern Brazil was part of a vast desert.
It could be that both of these factors, or factors that have yet to be considered,
were behind the evolution of Vespersaurus’s unusual feet.
Since a biomechanical study of its feet and claws has yet to be performed, it is currently unknown what its exact
capabilities were.
Besides its feet, the details of the Vespersaurus’s appearance are not entirely certain either,
as only 40% of its skeleton was preserved.
It is not even clear if all of these other bones actually belong to Vespersaurus, as the skeleton was disarticulated
disarticulated and found along with pterosaur bone fragments.
The other bones from the site are assigned to Vespersaurus as they are the same size
and seem to come from the same group as Vespersaurus, though this assignment is only tentative.
From what can be gathered, Vespersaurus was very small, about a meter to a meter and a half long.
Besides its feet, Vespersaurus’s appearance seems to have been fairly typical of small carnivorous theropods.
Its arms, however, were rather short.
Despite dromaeosaurids also possessing toes held above the ground, Vespersaurus was not closely related to them.
Instead, it belonged to the much more obscure theropod clade Noasauridae. Noasauridae consisted of two major groups.
The first, the elaphrosaurines, were carnivorous when they hatched and shifted to a herbivorous diet as they became adults.
The branch Vespersaurus was a part of, Noasaurinae, instead consisted of small, lifelong predators.
While coelurosaurs like the dromaeosaurids were present in Gondwana,
they were less abundant there during the Late Cretaceous than they were in the northern continents.
Instead, the noasaurines were some of the most common small carnivorous dinosaurs in the south.
However, Vespersaurus and its relatives were still less common than this would imply.
During the Late Cretaceous, many of the niches held by small dinosaurs elsewhere in the world
were instead held by crocodylomorphs in the Gondwanan continents.
Unfortunately, the other noasaurines are of limited help in filling the gaps in Vespersaurus.
While Vespersaurus is known from less than half a skeleton, it is actually one of the most complete members of the clade.
Instead, it may help to fill in the gaps in other noasaurines.
The only noasaurine known from more material than Vespersaurus is Masiakasaurus,
with 65% of its skeleton known between different specimens.
Masiakasaurus is also the only noasaurine known from a mostly complete skull. Unusually, at the end of its mouth its teeth
protrude outwards,,though exactly what it used these teeth for remains debated. With the exception of
some isolated Masiakasaurus-like teeth found in Early Cretaceous Brazil, no other noasaurine fossils
could confirm or deny if Masiakasaurus’s bizarre teeth were ubiquitous to the clade or not.
Vespersaurus may help to answer this. Among the material assigned to it is a single tooth.
This tooth is from the same part of the skull as Masiakasaurus‘s odd teeth and it didn’t protrude outwards.
Therefore, while Vespersaurus may have had unusual feet, it, and at least some other noasaurines,
had teeth more typical of most theropods.
This is not the only impact it has had in deciphering noasaurine anatomy.
Besides the tracks left by Vespersaurus in Late Cretaceous Brazil,
similar one toed tracks have also been found in formations from the Early Cretaceous and Late Jurassic.
This suggests that some other closely related noasaurines shared Vespersaurus’s functionally monodactyl feet.
Different reconstructions of Vespersaurus in this video have portrayed it as either feathered or entirely scaly.
As of right now, no direct indications of the outer covering of Vespersaurus or any other noasaurid have been found.
Large scale impressions have been found in the abelisaurid Carnotaurus, the closest relative of Vespersaurus
with direct indications of integument. However, Carnotaurus was of similar to size to modern mammals
which have reduced fur to prevent overheating.
While discoveries over the past decade have found that feathers may have been ancestral to dinosaurs,
in general, they are usually found in smaller species. However, noasaurines may have been an exception.
Examination of Masiakasaurus’s growth rate found it to be 40% slower than most other similarly sized theropods
but still 40% faster than crocodilians. This suggests it had a lower metabolism than other theropods,
which would have reduced the need for the thermal insulation provided by feathers.
Whether this means Masiakasaurus actually lost its feathers
and whether Vespersaurus shared its apparently lower metabolism both remain unknown.
While not as ferocious as most of its larger relatives,
Vespersaurus has revealed an impressive new adaption in dinosaurs.
It has also proven very helpful in understanding other noasaurines.
Hopefully new Vespersaurus fossils will be found and a biomechanical study of it will be performed
so we can learn more about the dinosaur that ran with one toe.
Thank you for watching. I hope you learned something interesting.
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