The most complete enantiornithine bird fossil from North America: “The stuff of legend to those in the paleo-ornithology community”

We recently published “The most complete enantiornithine from North America and a phylogenetic analysis of the Avisauridae“. In this study, researchers describe one of the largest birds known from North America from the Age of Dinosaurs.

Detailed analysis of the well-preserved fossil suggests the new species, Mirarce eatoni, was an advanced flyer. Author Jessie Atterholt from Western University of Health Sciences shares more on why this fossil is kind of a big deal and what it tells us about enantiornithine evolution.

Hello, world of PeerJ! I am a paleontologist and evolutionary biologist who works at Western University of Health Sciences, where I teach human gross anatomy to medical students and pursue my research on fossils and comparative anatomy. I’m also very passionate about science education and outreach, and maintain a “scicomm” Instagram account (@theladyanatomica).

My colleagues and I recently described and named an exciting new Late Cretaceous (~75Ma) fossil bird from Grand Staircase Escalante National Monument in Utah in PeerJ. In paleontological circles, this fossil is kind of a big deal. It was discovered by my co-author Howard Hutchison in 1992. Howard’s specialty is fossil turtles, but he recognized that this fossil was significant. Bird fossils from the Late Cretaceous of North America are relatively rare, and he had found parts of almost the entire skeleton (except the skull). Additionally, this thing was big, somewhere around the size of a turkey vulture. It was accessioned into the collections of the University of California Museum of Paleontology, where it became the stuff of legend to those in the paleo-ornithology community.

Two decades later, when I was a graduate student at UC Berkeley, I was doing research on fossil birds. I was particularly interested in a group called the Enantiornithes. They were super successful birds — we’ve so far found enantiornithine fossils from every continent except Antarctica, and they are known from rocks as old as 130 million years right up to the end of the Cretaceous 65 million years ago when they mysteriously go extinct with the rest of the non-avian dinosaurs. In many ways, they look quite similar to modern birds. They were fully-feathered and flew by flapping their wings like modern birds. In fact, this group is closely related to modern birds, but we know from the fossil record that they had significant developmental differences and that they evolved many of the structures we see in modern birds independently.

Part of my dissertation was focused on understanding the growth and evolution of this group. And it just so happened that my institution had one of the finest enantiornithine fossils ever discovered. I was lucky enough to have the chance to collaborate with Howard to describe the anatomy of this specimen and to name a new taxon. We also brought on board my friend and colleague Dr. Jingmai O’Connor, the world’s leading expert on enantiornithines.

We named the new bird Mirarce eatoni (Meer-ark-ee). Mirarce pays homage to the incredible, detailed, three-dimensional preservation of the fossil (“mirus” = “wonderful” in Latin), and anatomical evidence that it was an advanced flyer (Arce was the winged messenger of the Titans in Greek mythology). The species name honors Dr. Jeffrey Eaton for his decades of work as a paleontologist on fossils from the Kaiparowits Formation.

We conducted a thorough study of the anatomy of the preserved skeleton, comparing morphological features to those seen in other fossil birds and in modern birds. Luckily for us, it is preserved in three dimensions and exhibits exquisite detail. On many of the bones, we see tuburosities and scars where muscles attached to the bone, as well as foramina for nerves and blood vessels (for instance check out all of the bony landmarks we identified on the femur). This kind of preservation is unusual for Mesozoic bird fossils, which are very often squished flat between fine layers of rock.

Mirarce also is significant for what it tells us about enantiornithine evolution, and the interesting ways this clade is similar to and different from modern birds. For example, we documented adaptations for advanced flight capabilities much like what is seen in modern birds. We think the sternum (breastbone) had a deeper keel than other enantiornithines, for larger flight muscles. The furcula (wishbone) is more narrow. And, for the very first time in an enantiornithine bird, we found quill knobs on the ulna! These are large tuberosities where flight feathers actually attach to the bone. The large size in Mirarce indicates they functioned to improve the structural integrity of the feathers during flight. All of these features are seen in modern birds, but are unusual for enantiornithines.

On the other hand, even though Mirarce comes late in enantiornithine evolution and has many advance features, it also shows us that enantiornithines weren’t always doing exactly the same thing as modern birds. For instance, living birds have many compound elements—skeletal elements formed by fusing together several bones. The hand bone (carpometacarpus) and foot bone (tarsometatarsus) are examples of this. Throughout their evolution, modern birds increase the degree of fusion of these bones, to help make them more stable during flight. Enantiornithines also have compound elements, however, they exhibit a minimal degree of fusion; you can still clearly see many of the separate bones that comprise them. Even though it occurs late in enantiornithine evolution, Mirarce still has compound elements that are about as fused as early enantiornithines.

Right now we’ve just published a description of the anatomy of Mirarce, and a phylogenetic analysis of how it is related to other enantiornithines. But there is still so much to be learned from the specimen! Keep an eye out for studies about the growth of this animal (from bone histology), it’s behavioral habits (ecomorphology), and maybe even biomechanical reconstructions of its muscles!