Avisaurus: Beast of the Week

 This week we will be looking at a true bird that lived alongside some of the most famous dinosaurs.  Check out Avisaurus darwini!

Avisaurus was a bird (theropod dinosaur) that lived in what is now Montana, United States, during the latest Cretaceous period, about 66 million years ago.  Based on the bones that are known it would have been roughly the same size as a modern hawk. Possibly weighing about 2lbs (.9kg) and having a wingspan of about 4 feet (1.2 meters).  (Again these numbers are very rough estimates based on literally some foot bones and are therefore subject to change/variation.). The genus name translates to "Bird-Lizard" because it was initially thought to be a different kind of theropod, and not a bird.  The species name is in honor of the famous scientist, Charles Darwin.  When alive, Avisaurus, was likely a meat-eater, which we will discuss more below. 

Avisaurus darwini life reconstruction in watercolors by Christopher DiPiazza.  Parallels between Avisaurus' foot bones and the same bones of modern birds imply it may have been a hunter of larger prey, including smaller dinosaurs. In this case it is shown with a captured baby Acheroraptor.

Unfortunately fossil birds are usually only known from very fragmentary remains because they tend to have hollow bones, which often decompose before they fossilize. Avisaurus, which is only known from some foot bones, is no exception. That being said, scientists were still able to learn a lot of interesting information about this prehistoric bird based on the little material they did find.  The parts where the foot bones would have met the toe bones are similar to the foot bones of certain modern birds, like hawks, eagles, owls, and falcons.  All these birds have extremely flexible and powerful toes, tipped with hook-like talons for capturing prey.  Here it is important to note that Avisaurus was not directly related to modern birds, but evolving this trait common to modern birds of prey isn't outlandish.  This is due to the fact that most modern birds of prey aren't related to one another and evolved these predatory feet independently of one another.  That's right, owls, falcons, and hawks/eagles are all from completely separate families of birds and do not share a common ancestor with grasping talons. (falcons in particular are much closer to parrots than they are to other birds of prey.) So it isn't outlandish for an extinct lineage of bird from the Cretaceous to have independently evolved this trait as well.

Foot bones from Avisaurus darwini. Photo from paper by Clark et. al., referenced below. 

We know nothing else about what Avisaurus looked like other than its feet, but based on more completely-known fossil birds that were related to it we can make a few educated guesses.  It likely had clawed fingers under its wings.  (A trait still present in certain modern birds, like ostriches and chickens, but was more widespread in more ancient birds) It also may have had some teeth in its mouth, unlike the toothless beaks of all living birds.  

When alive, Avisaurus would have lived in a seasonally wet, almost swampy environment.  It almost certainly could fly, and based on its feet, was likely hunting smaller animals, like insects, reptiles, small mammals, other birds, and likely even small/baby dinosaurs and pterosaurs.  It would have shared its world with some of the most famous dinosaurs, like Tyrannosaurus, Triceratops, Edmontosaurus, Ankylosaurus, Pachycephalosaurus, to name just a few.  

References

Chiappe, Luis M. (1992) "Enantiornithine (Aves) Tarsometatarsi and the Avian Affinities of the Late Cretaceous Avisauridae" "Journal of Vertebrate Paleontology" September 3, 1992, Volume 12 no. 3 pp. 344-350

Clark, Alexander D.; Atterholt, Jessie; Scannella, John B.; Carroll, Nathan; O’Connor, Jingmai K. (2024-10-09). "New enantiornithine diversity in the Hell Creek Formation and the functional morphology of the avisaurid tarsometatarsus". PLOS ONE. 19 (10): e0310686.

Mononykus: Beast of the Week

 This week we'll be going over an interesting little dinosaur with unique arms.  Check out Mononykus olecranus! 

Mononykus was a small, bird-like, likely feathered dinosaur that lived in what is now Mongolia, during the late Cretaceous period, about 70 million years ago.  It only measured a little over 1 meter (between 3 and 4 feet) long from nose to tail and was likely a meat-eater, specializing in insects, when alive. The name, Mononykus, translates to "one claw" because it only had one large functional claw on each hand.  

Life reconstruction of Monoykus in watercolor by Christopher DiPiazza.

As the name implies, Mononykus possessed only one claw on each hand, which is perhaps its most distinctive trait.  The claws are proportionally large and curved, and are backed up by extremely stubby but powerful arms.  In fact, despite their length, Mononykus' ulnas (outer forearm bone) extend out past the upper arm bones, implying they had large muscle attachments there in life.  We see similar features in living animals like aardvarks, which use those kinds of muscles and large claws to dig into termite mounds.  Even though aardvarks and dinosaurs are not closely related at all, this may provide insight as to how Mononykus was using its unusual anatomy.  

Arm bones of a modern Aardvark(top) and Mononykus (bottom).  Note the extended ulna.

Other parts of Mononykus' anatomy imply it may have been digging for insects other than the arms and hand claws.  Its vertebra would have been interlocked in a way that stiffened the spine in life, making it more sturdy, another trait that would benefit digging.  The inside of the skull suggests Mononykus had superb hearing, comparable to that of modern owls, who are specially adapted for hunting prey they cannot see.  Again, this is a useful trait to have for an animal that is hunting for insects underground or inside of dead logs.  Other, more completely known, relatives of Mononykus, imply it likely had a narrow, toothless, beaklike mouth, and large, forward-facing eyes.  

Mononykus may have used its superb hearing to detect insects underground or inside logs, then would have needed to press its body against that surface in order to dig them out with its short arms.

As stated before, Mononykus had extremely short arms, which seems to be the only trait inconsistent with animals that dig for their food.  The arms are so short, in fact, that the only way for Mononykus to have even reached a surface to dig is if it's torso was pressed up against it.  I can imagine Mononykus listening to the outside of a dead log, and then rapidly positioning itself to a diffing position with its body pressed against the log, vigorously tearing at the bark with its claws, and then hopping back up to a standing position to listen again or root out the now exposed insects with its beak-like snout.  

Mononykus skeletal mount on display at the American Museum of Natural History in New York.

Mononykus also had extremely long and slender legs.  This implies it was a fast runner when alive, which would have been a useful trait for avoiding predators, but it also has another function that may also be related to finding food.  Long legs means Mononykus could have more easily walked long distances without getting tired.  Since we know Mononykus lived in a desert, it would have needed to regularly travel across large expanses of land in order to find areas where insects actually were, like oasises, or sparsely occurring logs in an otherwise barren environment.  

When alive, Mononykus would have shared its environment with other dinosaurs, like Prenocephale, Gallimimus, Deinocheirus, Therizinosaurus, and the large tyrannosaur, Tarbosaurus, to name a few.  It likely avoided confrontation with the vast majority of these other animals by possibly being nocturnal.  

References

Choiniere, Jonah N.; Neenan, James M.; Schmitz, Lars; Ford, David P.; Chapelle, Kimberley E. J.; Balanoff, Amy M.; Sipla, Justin S.; Georgi, Justin A.; Walsh, Stig A.; Norell, Mark A.; Xu, Xing; Clark, James M.; Benson, Roger B. J. (2021-05-07). "Evolution of vision and hearing modalities in theropod dinosaurs". Science. 372 (6542): 610–613.

Perle, A.; Norell, M. A.; Chiappe, L. M.; Clark, J. M. (1993). "Flightless bird from the Cretaceous of Mongolia". Nature. 362: 623−626.

Perle, A.; Chiappe, L. M.; Rinchen, B.; Clark, J. M.; Norell, M. A. (1994). "Skeletal morphology of Mononykus olecranus (Theropoda, Avialae) from the late Cretaceous of Mongolia". American Museum Novitates (3105): 1−29.