Houornis

Houornis is a genus of enantiornithine birds from the Jiufotang Formation of Liaoning, People's Republic of China. It is known from a single species, Houornis caudatus, which had been once been classified as a species of Cathayornis, and has also been regarded as a nomen dubium.

Houornis caudatus was a small enantiornithine with a slightly elongated, toothy snout and perching feet. It is known only from a single specimen, the slab and counter-slab of a fossil catalogued as number IVPP V10917/2 in the collections of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. The specimen was found near the town of Boluochi in Liaoning Province, China. The actual bones of this specimen were not preserved, but rather can be seen by the impressions they left in the surrounding rock. It was originally thought to have a relatively long, bony tail intermediate in length between modern birds and long-tailed birds like Archaeopteryx, and so was given the name Cathayornis caudatus, meaning "long-tailed Cathay bird". This interpretation was later found to be in error, though it does appear to have an unusually long pygostyle (a component of the tail made of fused vertebrae).

While some researchers have considered the species dubious due to the poor preservation quality and incomplete nature of the fossil, a 2015 study by Wang Min and Liu Di was complete enough to be compared to similar species, including Cathayornis, and so they assigned it to its own genus, Houornis. Wang and Liu found that Houornis can be told apart from similar species by several anatomical details, including the large pygostyle that gave it its name. Two additional but very fragmentary specimens, IVPP V9936 and V10896, have been referred to C. yandica in the past, but cannot be directly compared with the type specimen because they do not preserve any of the same key parts of the skeleton.