Unenlagia: Beast of the Week

 This week we will be checking out an amazing little bird-like dinosaur.  Check out Unenlagia comahuensis!

Unenlagia was a bird-like theropod that lived in what is now Argentina during the late Cretaceous period, roughly 89 million years ago.  From snout to tail Unenlagia may have measured about seven feet (2.1m) long, but estimates vary since it's known from fragmentary remains.  When alive it likely ate meat, possibly specializing in smaller prey or fish.  The genus name translates to "Half-bird" due to the dinosaur's limbs which were particularly similar to those of modern birds.  In fact when it was first discovered in 1986, it was thought by paleontologists studying it to be a kind of prehistoric bird.  More recently Unenlagia was determined to be more closely related to dinosaurs like Velociraptor, being a member of, or very closely related to the family of dinosaurs called dromaeosaurids.

Watercolor reconstruction of Unenlagia comahuensis by Christopher DiPiazza.

Sadly Unenlagia is only known from several bones, so we aren't positive about how it looked in life exactly.  The bones we do have, however, have an interesting history to them regarding how paleontologists think about dinosaurs and their relationship to birds.  The upper arm bone and shoulder blade of Unenlagia was originally thought to be adapted for flapping by paleontologists studying it.  This is what led them to believe that Unenlagia may have been either a kind of prehistoric bird extremely closely related to birds.  Some even hypothesized that Unenlagia may have been capable of flight.  More recently, however, it was determined that Unenlagia's arms were positioned more similarly to those of dromaeosaurids, like Velociraptor and Deinonychus, and did not have the range of motion of modern bird wings, thus implying it was indeed flightless.  

Photograph of Unenlagia's bones from Gianechini's 2011 paper.

Since Unenlagia's discovery, dinosaurs that are more complete than, yet extremely similar to the parts of Unenlagia we do have, give us a better idea of how it looked in life.  These dinosaurs have been given their own subfamily, called unenlagiinae, named after the earliest discovered member.  Based on more complete members of this group, we can guess that Unenlagia may have had long, slender legs and a long, narrow snout, lined with cone-shaped teeth. Because of these features some hypothesize unenlagiines may have specialized in hunting small prey and/or fish in life.  Unenlagiines also tended to have proportionally smaller arms than those of other dromaeosaurids.  

That is all for this week!  As always feel free to comment below!

References

Agnolin, F.L.; Novas, F.E. (2011). "Unenlagiid theropods: are they members of the Dromaeosauridae (Theropoda, Maniraptora)?". Anais da Academia Brasileira de Ciências. 83 (1): 117–162.

Carpenter, K. 2002. "Forelimb biomechanics of nonavian theropod dinosaurs in predation". Senckenbergiana Lethaea 82: 59–76

Gianechini, F. A.; Apesteguía, S. (2011). "Unenlagiinae revisited: Dromaeosaurid theropods from South America". Anais da Academia Brasileira de Ciências. 83 (1): 163–95.

Hartman, S.; Mortimer, M.; Wahl, W. R.; Lomax, D. R.; Lippincott, J.; Lovelace, D. M. (2019). "A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight". PeerJ. 7: e7247.

Matías J. Motta; Federico L. Agnolín; Federico Brissón Egli; Fernando E. Novas (2020). "New theropod dinosaur from the Upper Cretaceous of Patagonia sheds light on the paravian radiation in Gondwana". The Science of Nature (journal). 107 (3): Article number 24.

Norell, M.A.; Makovicky, P.J. (1999). "Important features of the dromaeosaur skeleton II: information from newly collected specimens of Velociraptor mongoliensis". American Museum Novitates (3282): 1–45.

Novas, F.E.; Puerta, P.F. (1997). "New evidence concerning avian origins from the Late Cretaceous of Patagonia". Nature. 387 (6631): 390–2.

Novas, F. E.; Pol, D.; Canale, J. I.; Porfiri, J. D.; Calvo, J. O. (2008). "A bizarre Cretaceous theropod dinosaur from Patagonia and the evolution of Gondwanan dromaeosaurids". Proceedings of the Royal Society B: Biological Sciences. 276 (1659): 1101–7.

Stegouros: Beast of the Week

This week we will be checking out a recently described, and very unique little armored dinosaur.  Say hello to Stegouros elengassen!

Watercolor life reconstruction of Stegouros elengassen by Christopher DiPiazza.

Stegouros was a plant-eating dinosaur that lived in what is now Chile during the late Cretaceous period, between about 75 and 72 million years ago.  It was a member of the ankylosaur group of dinosaurs, known for sporting bony armor all over their bodies, but it was extremely tiny for that group, only measuring about six feet from beak to tail.  The genus name, Stegouros, translates to "roofed tail" in reference to the bony structures growing from the top of its tail (more about that in a bit) and the species name, elengassen, is the name of a mythical creature known for its armor from Aonik'enk folklore, the indigenous people of the area in which Stegouros' bones were unearthed. 

Photograph of the known skull parts of Stegouros. Image from the paper by Scoto-Acuna Et al.

Stegouros is known from an almost complete skeleton.  Almost the entire rear half of the body is known, plus armor plates, and even an almost complete skull. Stegouros had a slightly curved beak which could have been used for clipping vegetation to eat.  It also had small teeth in the back of its mouth with serrations, ideal for shredding plant material before being swallowed.  It had  relatively small osteoderms on most of the upper part of its body, but the real impressive armor was on its tail.  Unlike some of its relatives, like Ankylosaurus, which are famous for having bony clubs on the tips of their tails, Stegouros had a series of wide flat pieces of bone growing from the sides of the tail and eventually fusing together to form a solid flat piece of bone at the end of the tail, forming an almost plant-like shape.  The whole structure is best described as being similar to the Aztec weapon, called a macuahuitl.  Stegouros may have used this amazing tail to defend itself from potential predators, or perhaps even to fight members of its own species for dominance or over territory, mates, or resources.

Photograph of the tail of Stegouros.

Paleontologists have determined that Stegouros evolved separately from the two other major groups of ankylosaur dinosaurs, the narrow-snouted nodosaurids, and the club-tailed ankylosaurids, forming a third major branch of armored dinosaurs, called Parankylosauria that split off earlier from the other two.

References

Soto-Acuña, Sergio; Vargas, Alexander; Kaluza, Jonatan; Leppe, Marcelo; Botelho, Joao; Palma-Liberona, José; Gutstein, Carolina; Fernández, Roy; Ortiz, Hector; Milla, Verónica; Aravena, Bárbara; Manríquez, Leslie M. E.; Alarcón-Muñoz, Jhonatan; Pino, Juan Pablo; Trevisan, Cristine; Mansilla, Héctor; Hinojosa, Luis Felipe; Muñoz-Walther, Vicente; Rubilar-Rogers, David (2021). "Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile". Nature. 600 (7888): 259–263.