The first ғossιʟ of a daytime active owʟ found at the edge of the тιʙᴇтᴀɴ ᴘʟᴀтᴇᴀu

An amazingly well-preserved fossil skeleton of an extinct owl that lived more than six million years ago has been unearthed in China. The fossil was discovered nearly 7,000 feet (2,100 meters) up, in the Linxia Basin of China’s Gansu province, at the edge of the Tibetan Plateau.

Fossil skeleton of the daytime active owl Miosurnia diurna from China (below) with an expanded view of the skull (top left). The eye bones or scleral ossicles are false-coloured blue and set in comparison with an intact ring in the skull of a pygmy owl Glaucidium (top right).

It dates back to the late Miocene Epoch, around six million years ago.

Detailed analysis of the skeleton’s fossilised eye bones by researchers at the Chinese Academy of Sciences reveals that, unlike most modern owls, this species was active in the daytime, not the night.

The fossil comprises nearly the entire skeleton from the tip of the skull through the wings and legs to the tail bone, along with body parts that are rarely seen as fossils.

These include the bones of the tongue apparatus called the hyoid, the trachea, the kneecap, tendons for wing and leg muscles, and even the remnants of the last meal of a small mammal.

‘It is the amazing preservation of the bones of the eye in this fossil skull that allows us to see that this owl preferred the day and not the night,’ said Dr. LI, first author of the study.

Reconstruction of the extinct owl Miosurnia diurna perched in a tree with its last meal of a small rodent, overlooking extinct three-toed horses and rhinos with the rising Tibetan Plateau on the horizon.

The researchers named the species Miosurnia diurna in reference to its close living relative, the diurnal Northern Hawk Owl (Surnia ulula).

The features of the skull and skeleton, including a large bump on part of the cheekbone just behind the eye, show that Miosurnia is a part of the global owl group Surniini.

Their research shows that the Surniini, which includes Miosurnia, the Northern Hawk Owl, and pygmy owls, rejected the night millions of years ago.

This extinct species is the first record of an ancient owl being ‘diurnal’, or active during the day.  Scleral ossicles are small bones that form a ring around the pupil and iris in the outer region of the eye.  Nocturnal animals require overall larger eyes and bigger pupils to see in low-light conditions, but diurnal animals have smaller eyes and pupils.

In the Miosurnia diurna fossil, the soft parts of the eye had decayed long ago, leaving the small trapezoidal scleral ossicles randomly collapsed into the owl’s eye socket.

The palaeontologists, therefore, had to measure these individual small bones and do some basic geometry to rebuild the size and shape of the ring around the eye.

‘It was a bit like playing with Lego blocks, just digitally,’ said Dr. Stidham, describing how the 16 little similar bones overlap each other to form a ring around the iris and pupil.

He said that putting them back together correctly allowed the scientists to determine the overall diameter of the ring and the opening for light in the middle.

The IVPP scientists then compared the fossil owl’s scleral ossicles with the eyes of 55 species of reptiles and more than 360 species of birds including many owls.

Looking at the size and shape of the fossil’s eye and its relatively smaller opening for light, the scientists determined that it most resembles the eyes of living owls in the Surniini group. Furthermore, they studied behavioural data from over 360 species across a diversity of birds to determine which were likely nocturnal or diurnal.

Their results show that the ancestor of all living owls was almost certainly nocturnal, but the ancestor of the Surniini group was instead diurnal.

‘This fossil skeleton turns what we thought we knew about the evolution of owls on its head,’ said Dr. LI.

Dr. Stidham adds that Miosurnia diurnia is the first record of an evolutionary process spanning millions of years and stretching across the globe whereby owls evolved to ‘reject the night for some fun in the sun.’

The team’s findings were published in Proceedings of the National Academy of Sciences (PNAS) on March 28.

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