Silesaurus: Beast of the Week

 This week we will be checking out an interesting and a mysterious creature, Silesaurus opolensis!

Silesaurus was a reptile, closely related to dinosaurs, that lived in what is now Poland during the Triassic period, between 237 and 227 million years ago.  Footprints that could have been from Silesaurus, or at the least something very closely related to it, have also been found on the east coast of the United States, including Maryland.  When alive it would have been an omnivore, eating plants and also small animals, like insects.  Adult Silesaurus measured about 7.5 feet (2.3 meters) from beak to tail.  The genus name, translates to "Selesia Reptile" In reference to the region in Poland in which its bones were discovered. 

Life reconstruction of Silesaurus in watercolors by Christopher DiPiazza.

Silesaurus had a narrow snout with a beaklike mouth.  Its teeth were somewhat leaf-shaped and lacked prominent serrations.  Microwear studies on Silesaurus teeth suggest it ate lots of plants but fossilized dung associated with Silesaurus was also found to have beetle shells in it, showing it was more likely an omnivore, eating insects and maybe other small animals on occasion in addition to a mostly plant diet.

Silesaurus skeletal mount on display at Jurapark Science Park, in Poland.

Silesaurus would have walked on all fours most of the time but may have been able to rear up on its hind legs on occasion if it needed.  All four of its limbs were long and slender, making it look like a reptilian greyhound of sorts. (in my opinion at least) It had five very short fingers and claws on each of its hands and its feet each had three toes.  Thanks to tracks we know its thumb and pinky fingers were held off the ground when it walked. The neck was relatively slender and would have been held in a gentle S shape most of the time, a feature similar to dinosaurs. 

Hand (left) and foot (right) tracks from what may have been Silesaurus (or a very close relative) that were found in Maryland, United States, on display at the Natural History Society of Maryland in Baltimore.

Silesaurus is an interesting animal because its exact placement on the tree of life is still highly debated.  That by itself wouldn't be that unusual (loads of fossil animals we don't know the exact natural history of) but what makes Silesaurus' identity especially noteworthy is because the discourse is over whether or not it is a dinosaur. (dinosaurs are popular) The original placement of Silesaurus was that it was an extremely close relative of dinosaurs without quite being a true dinosaur, itself.  It shares a number of dinosaur-like characteristics, like having its limbs positioned directly under it's body, resulting in a fully erect posture while standing and walking.  It also has several diagnostic dinosaur characteristics in its leg and hip bones.  Because of these extreme similarities to dinosaurs, some experts are starting to propose that the silesaurid family should be reassigned as true dinosaurs, specifically as the ancestors of the first ornithiscian dinosaurs, the broad branch of dinosaurs which includes famous taxa like Triceratops, Stegosaurus, and Iguanodon.  This is partially due to the fact that there is very little, if any, ornithiscian dinosaur fossils known from the Triassic period. (unless they're the silesaurids). Other expert's, however, are still unconvinced, since Silesaurids still don't possess some other key dinosaur characteristics, like a specialized ridge on the anterior of the humerus for muscle attachments, called the deltopectoral crest, as well hip sockets that forms an open window completely through the pelvis. 

Images of the humerus of Silesaurus (left) from Piechowski's 2020 paper alongside the humerus of early ornithiscian dinosaurs, Heterodontosaurus (middle) and Abrictosaurus (right) from Galton's 2014 paper. Note the Silesaurus bone is lacking the deltopectoral crest at the top of the bone compared to the other two. 

Regardless if Silesaurus was a true dinosaur or not, it very well may have sported feathers of some kind when alive.  This is because both dinosaurs and pterosaurs are believed to have shared a common ancestor with some kind of feathers.  Since silesaurids were at least very closely related to dinosaurs (moreso than to pterosaurs) they likely would have had feathers as well.  This method of predicting features in fossil groups based on known features in surrounding groups is known as phylogenetic bracketing.  

References

Dzik J (2003). "A beaked herbivorous archosaur with dinosaur affinities from the early Late Triassic of Poland" (PDF). Journal of Vertebrate Paleontology. 23 (3): 556–574.

Galton, P.M. (2014). "Notes on the postcranial anatomy of the heterodontosaurid dinosaur Heterodontosaurus tucki, a basal ornithischian from the Lower Jurassic of South Africa" (PDF). Revue de Paléobiologie, Genève. 1. 33: 97–141.

Nesbitt, Sterling J. (2011). "The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades". Bulletin of the American Museum of Natural History. 352: 1–292.

Piechowski, Rafał; Tałanda, Mateusz (2020). "The locomotor musculature and posture of the early dinosauriform Silesaurus opolensis provides a new look into the evolution of Dinosauromorpha". Journal of Anatomy. 236 (6): 1044–1100.