The (fossil) eyes have it: Evidence that an ancient owl hunted in daylight

An extraordinarily well-preserved fossil owl was described in PNAS this past March. Owls are not new to the fossil record; evidence of their existence has been found in scattered limbs and fragments from the Pleistocene to the Paleocene (approximately 11,700 years to 65 million years ago). What makes this fossil unique is not only the rare preservation of its near-complete articulated skeleton but that it provides the first evidence of diurnal behavior millions of years earlier than previously thought. 

In other words, this ancient owl didn’t stalk its prey under the cloak of darkness. Instead, the bird was active under the rays of the Miocene sun.

Seeing the light

Its eye socket was key to making this determination. Dr. Zhiheng Li is the lead author on the paper and a vertebrate paleontologist who focuses on fossil birds at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in China. He explained in an email that the large bones around the eyes of birds (but not mammals) known as the scleral ossicles offer information about the size of the pupil they surround. In this case, the pupils of this fossil owl were small. And if the pupil is small, he wrote, it “means they can obtain good vision with a smaller eye opening.”

Co-author Dr. Thomas Stidham is an integrative biologist and avian paleontologist at the IVPP. He described the scleral ossicles as “roughly trapezoidal in outline (with the narrow part toward the center of the pupil)." They "overlap each other to make a ring with a smaller internal circular opening and a larger circumference on the outside of the ring. The internal opening emcompasses the iris and pupil," he said.

“Nocturnal birds,” he continued, “need a larger opening to let in more light than what is needed for eyes used during the day (where a smaller opening/pupil will let in enough light to see). We did statistical tests on the scleral ossicle rings of hundreds of species of birds and lizards that are active at night, dusk, and daytime.”

How and when owls evolved their day/night preference is exceedingly difficult to ascertain, as the owl fossil record in deep time is fragmentary. And one of the biggest clues to whether an owl is active at day or night lies within the skull, the fossils of which can be elusive.

A fragmentary history

Thus, having a well-preserved skull offers rare insight into at least one species during the Miocene. The researchers' analyses put this fossil owl within the Surniini clade ("clade" is a term that refers to a group with a common ancestor), which includes diurnal owls today, such as pygmy owls and the northern hawk owl. If this ancient owl was diurnal millions of years ago, it's likely that its subsequent close relatives were as well.

"From a parsimonious view of evolutionary history," Li clarified, "the explanation with the fewest evolutionary changes is the most likely."

He and his colleagues named this new mid-sized species Miosurnia diurna, a name that nods to its existence within the Miocene period, its resemblance to today’s northern hawk owl (Surnia ulula), and its diurnal behavior.

According to Li, the fossil was found some time ago by a farmer in Hezheng county and donated to the Shandong Tianyu Museum, where it remained among “thousands of feathered dinosaurs and a large number of much older birds fossils” until it caught the attention of Li and his team. Found within the Linshiu Formation in Tibet, it is approximately 6 million to 9.5 million years old.

This exquisite specimen is almost complete, missing only one forelimb and a few digits. Its preservation is so extraordinary that it even has stomach contents: small bone fragments of what the team thinks are equally small mammals based on bone residues. Li wrote that he and his team feel that, although it's still within the body, this stomach content is actually a gastric pellet “since the position of the residue is more likely in the upper part of [the] digestive tract.” He added that the “shape of the bone residue was quite pellet-like.”

Not alone

Dr. Denver Fowler, who was not involved in this research, thinks the discovery is interesting. “There are a few modern diurnal owls,” he told Ars, “and I suppose the question is whether these particular owls are secondarily diurnal (I suspect they are, as does the article), and if so, what ecological or evolutionary pressures might have prompted their switch.”

Fowler is the curator of the Badlands Dinosaur Museum in Dickinson, North Dakota. Remarkably, he and assistant professor of biology at Dickinson State University, Dr. Elizabeth Freedman Fowler, are working on another exceptionally well-preserved fossil owl. This one was found in Wyoming by Burke Museum’s John Alexander in 2007. They described the fossil owl at the 78th annual meeting of the Society of Vertebrate Paleontology in 2018 but haven’t yet published their work. 

Their specimen is only 45 percent complete, but even so, it is another extremely rare well-preserved fossil owl. Fowler described the specimen as “superb” and wrote, “It would have been 100 percent complete originally, but the weathering that originally exposed the skeleton destroyed most of the middle part of the animal… Fortunately, the most informative parts of the skeleton stayed intact and [are] mostly articulated: the 3D skull, distal wings, parts of the neck, and the lower legs and feet.” 

One particularly exciting aspect of the Wyoming fossil is a bony protuberance over its eye sockets, which is not a feature seen in nocturnal birds. The researchers expect to discuss this in more detail in their upcoming paper.

“The fossil record of owls (excluding 10,000-year-old skeletons from [the La Brea Tar Pits], which are of essentially living species) is mostly limited to occasional fragments, odd bones here and there providing limited but tantalizing glimpses of owl evolution,” Fowler said. “The new discovery is therefore very welcome, especially the 3D skull, of which there is only one other comparable specimen (the one we are working on). It's adding to the story of how [these] charismatic birds acquired their astonishing suite of predatory specializations. We shall certainly be citing this important paper in our own work!”

The discovery of Miosurnia diurna increases the number of fossil birds found in the Liushu Formation. It’s a fascinating ancient environment that we might recognize today, one that includes vultures, raptors, grouse, and ostriches. Li described how this fossil impacts our overall understanding of the ancient ecology of that previously arid savanna. And he hopes people will see that owls can be “quite flexible ecologically by morphological adaptation, and we can figure out their ancient behavior pattern in deep time through new fossil findings with rigorous hypothesis testing," he said.

Prior to this paper, many people thought that the few modern diurnal owls in existence were an evolutionary anomaly, added Stidham.

But, Stidham said, “our fossil and research show that for at least one of these diurnal owl taxa, it wasn't just the evolution of a single species or a recent change in behavior to the daytime but something that occurred millions of years ago and impacted a wider grouping of owls. Our discovery changes the context of how we should look at the evolution of diurnal behavior in owls.”