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.