Vespersaurus: Beast of the Week

Today we will be looking at a truly unique, recently discovered species of dinosaur.  Let's check out Vespersaurus paranaensis!

Vespersaurus was a theropod dinosaur that lived in what is now Brazil, during the Cretaceous Period, about 90 million years ago.  From snout to tail, Vespersaurus would have measured a little over six feet long.  The genus name translates from Latin to "Western Reptile" in honor to Cruzeiro do Oeste (which means Western Cross), the town where the bones were uncovered. 

Vespersaurus paranaensis life reconstruction in watercolors by Christopher DiPiazza.

Vespersaurus was a member of the noasaurid family.  Noasaurids were theropods that primarily flourished in the Southern Hemisphere during the Cretaceous period and are within the larger, more diverse ceratosaur group, which includes more famous dinosaurs, like Ceratosaurus and Carnotaurus.  Unlike these larger relatives, however, noasaurids tended to be much smaller, and more lightly built, with longer necks and proportionally smaller heads.  Another, recently more popular noasaurid, and therefore much closer relative to Vespersaurus, was Masiakasaurus, which was found in Madagascar and had a distinctive, down-turned lower jaw with teeth that jutted out forward.  Because so little of Vespersaurus' skull was found, it is unknown if it had a similar down-turned jaw or not.  If it did, Masiakasaurus may not be that unique, and the trait might just be a more widespread ancestral adaptation to the noasaurid family.

Articulated foot of Vespersaurus.

When it came to Vespersaurus' legs and feet, however, it truly was unique.  In fact, Vespersaurus' feet are unlike anything ever seen before in reptiles, let alone dinosaurs!  Vespersaurus had what are called functionally monodactyl feet.  This means that the animal was only using one toe on each foot to actually walk.  In the case of Vespersaurus, it had the first digit of each foot was higher on the leg and didn't touch the ground (typical for nonavian theropods), but the second and fourth digits, which normally would touch the ground in other dinosaurs, were unusually thin and would have been carried off the ground when walking.  All the weight was put on its third digit.  Amazingly enough, prior to the discovery of Vespersaurus' bones, paleontologists found very strange tracks from Argentina which show what appears to be a theropod dinosaur walking on its central toe.  These tracks are a bit older than Vespersaurus, and they possibly could have only simply appeared to have been only walking on one toe, and the side toes just weren't making as deep of an impression, but they are still important to note.  We know dromaeosaurids, the group that includes Velociraptor, carried one specialized toe off the ground and walked on two, and there are modern birds (also dinosaurs) that have only two toes on each foot, like ostriches, but only one walking toe in dinosaurs is completely unheard of until this.

One-toed theropod track, possibly an earlier relative of Vespersaurus.

Vespersaurus certainly had unique feet for a dinosaur, and even a reptile, but this sort of foot plan HAS evolved before...just in a different group of animals.  In fact, it happened in a very popular group of animals that everyone knows, horses!  Everyone knows horses only have one hoof per foot, which is just one big toe.  Fortunately, we have a very detailed fossil record of horse ancestors, which shows that millions of years ago prehistoric horses were walking on three toes, and over time the side toes grew shorter and eventually became useless, leaving everything to the middle toe, ultimately resulting hooves we recognize today.  It makes me wonder if Vespersaurus had not gone extinct if its descendants would have had only one toe per foot, too.  Since Vespersaurus lived 90 million years ago, tens of millions of years before the mass extinction event that wiped out the non-avian dinosaurs, maybe there was a later, more derived noasaurid that has this foot plan that paleontologists just haven't found?

Illustration of prehistoric horse foot bones.  Oldest to youngest from left to right.  Note how side toes become smaller and eventually disappear, leaving a singular, walking toe.(image from Outlines of Zoology, by J. Arthur Thomson.)

So why the unusual one toe on each foot?  Well, if we go back to horses, which have a convergently similar adaptation, we may get some clues.  We know horses evolved these feet to run faster.  When the weight of an animal's foot is on a more concentrated spot, it provides more resistance against stress when weight is applied on it, and therefore results in a stronger runner.  Looking at the rest of Vespersaurus' known bones, it makes sense that it would have been a very fast runner.  It also lived in an arid desert environment, with lots of open space, where being able to run long distances more easily would certainly be an advantage.  What environmental pressures would have caused Vespersaurus to have evolved such an extreme running adaptation?  Unfortunately the diversity of fossils from the site Vespersaurus was found in is still limited, Vespersaurus being the only known dinosaur so far, so we have no idea what kind of predators it would have had, if any.  On the other end, Vespersaurus had small teeth that were short, but serrated, so it was possibly at least eating some meat.  Perhaps Vespersaurus was really good at chasing down small prey? It's still a mystery!

Tooth of Vespersaurus.

That is all for this week! Comment below with your thoughts!

References

Glut, Donald F. (2003). "Appendix: Dinosaur Tracks and Eggs". Dinosaurs: The Encyclopedia. 3rd Supplement. Jefferson, North Carolina: McFarland & Company, Inc. pp. 613–652.

J. Arthur Thomson, M.A., LL.D. Outlines of Zoology (New York, NY: D. Appleton & Company, 1916)

Langer, Max Cardoso; de Oliveira Martins, Neurides; Manzig, Paulo César; de Souza Ferreira,, Gabriel; de Almeida Marsola, Júlio César; Fortes, Edison; Lima, Rosana; Sant’ana, Lucas Cesar Frediani; da Silva Vidal, Luciano; da Silva Lorençato, Rosangela Honório; Ezcurra, Martín Daniel Ezcurra (2019). "A new desert-dwelling dinosaur (Theropoda, Noasaurinae) from the Cretaceous of south Brazil". Scientific Reports. 9.

“Mechanics of Evolutionary Digit Reduction in Fossil Horses (Equidae).” Proceedings of the Royal Society B: Biological Sciences, royalsocietypublishing.org/doi/full/10.1098/rspb.2017.1174