Tuesday, March 13, 2018

Shringasaurus: Beast of the Week

Today we will be looking at a relatively recently discovered creature that was utilizing adaptations famous in different groups of dinosaurs...but wasn't a dinosaur.  Check out Shringasaurus!

Shringasaurus was a plant-eating archosauromorph reptile that lived in what is now India, during the Middle Triassic period, between 247 and 242 million years ago.  It measured a little over ten feet long from snout to tail and would have walked on four short, but robust limbs when alive.  The genus name translates to "Horned Reptile" because this animal possessed two, forward-curving horns on the top of its head.

Male Shringasaurus basking.  If I told you I wasn't a little inspired by Chuckwallas while painting this...I'd be lying.
 
Shringasaurus belonged to an interesting group of prehistoric reptiles, called allokotosauria.  These early archosauromorphs are characterized by having proportionally small skulls, semi-sprawling quadrupedal posture, and teeth and jaws that suggest they were herbivores in life.  During the middle Triassic, when many of these animals were alive, plant-eating animals are so far relatively rare on the fossil record.  So the more fossils that are discovered from these interesting creatures, the more we can paint an accurate picture of their environment!

Shringasaurus has a number of unique characteristics that set it apart as a beautifully unique animal.  Most notable are those horns.  Lots of reptiles have horns, but few groups of reptiles have forward-curved horns growing over the eyes.  Ceratopsians, the successful group of dinosaurs that includes Triceratops, are the most notable. Many species of modern chameleons also have horns like this.  Shringasaurus seems to have convergntly evolved similar horns, being only distantly related to both of these groups.  The real purpose for evolving these horns is somewhat of a mystery.  But there are clues!  Shringasaurus is known in the fossil record from several individuals.  Among these are smaller, younger individuals, that show smaller horns.  This means that the baby Shringasaurus started off with small, or perhaps no horns, and did not develop them until later in life, probably when they were sexually mature.  This supports the idea that the horns were some kind of an intraspecies display.  Further, there is one individual Shringasaurus skull that had no horns, but was otherwise the same size as other Shringasaurus skulls that had horns.  This strongly points to Shringasaurus having been sexually dimorphic, when the males and females are visibly different.  It is highly possible that Shringasaurus males used their horns to visually intimidate each other, maybe incorporating some kind of "dance" like many modern reptiles will do today, like bobbing their heads around to compete for mating rights.  They may have even used their horns to physically compete with each other, like shoving, or jousting, which is also observable in modern horned animals.

Graphic from Sengupta's 2017 paper describing Shringasaurus.  This image is showing a comparison between Shringasaurus' skull (top left) and the skull of the convergently evolved Arrhinoceratops (top right).  Below, letters d-g and h-k, show the  top and side views of the skulls from various Shringasaurus individuals.  Note how specimen g/k has no horns but is otherwise the same size as specimen f/j.

Another cool characteristic about Shringasaurus is its nostril holes.  They were connected, forming one hole, instead of two, in the front of the snout.  This kind of nose hole opening is not really known in reptiles until Shringasaurus, but is something seen in modern mammals.  Shringasaurus, for whatever reason convergently evolved an external nasal passage more similar to those of mammals from millions of years later!

Shringasaurus had small, leaf-shaped teeth in its mouth, probably for eating plants.  It also had a proportionally small head at the end of a very long neck.  This long neck is often compared to those of sauropod dinosaurs, but it reminds me a lot of Galapagos Tortoise necks too.  Shringasaurus probably used this neck for the same reasons sauropods and tortoises do, too, to help it reach more leaves to eat without moving its body.  Plants provide less energy to the consumer than meat, so plant-eating animals need to conserve as much energy as possible, being able to get as much food for the littlest amount of work is important.

References

Sengupta, S.; Ezcurra, M.D.; Bandyopadhyay, S. (2017). "A new horned and long-necked herbivorous stem-archosaur from the Middle Triassic of India". Scientific Reports. 7: 8366.

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