Houornis

Houornis is known from only one main fossil, which is not preserved as bones but as impressions left in the rock.1

Its tail was originally thought to be long and bony like Archaeopteryx, but scientists later realized it had a long pygostyle, a fused tail bone.1

The upper arm bone of Houornis is less than 30 millimeters (1.2 inches) long, making it a very small bird.1

Houornis was once classified as a species of Cathayornis, but in 2015 it was given its own genus because of unique features like its large pygostyle.1

Some scientists have considered Houornis a doubtful species because the fossil is poorly preserved, but a 2015 study found it was distinct enough to keep.1