Eopteranodon lii was named by paleontologists Lü Junchang and Zhang Baokun in 2005. The only known specimen was discovered in the Lower Cretaceous Yixian Formation of Liaoning, northeastern China. The name Eopteranodon means “dawn toothless wing,” a reference to it being one of the earliest known toothless pterosaurs.
The skeleton is disarticulated, incomplete, and preserved on two limestone slabs. It’s made up of most of the skull, some neck vertebrae, the sternum, shoulders, the majority of the wing bones, and one leg.
The skull is missing much of the area behind the eyes, but otherwise largely complete and would have been about 18 cm (7 inches) long when complete. The toothless beak is very long and triangular, and has a large fenestra that housed the nostrils and a large facial sinus. The upper margin of this fenestra, known as the nasoantorbital fenestra, seem to be higher than the level of the eye. At the rear of the skull, a flattened spike-shaped bone fragment appears to be part of a backwards-pointing crest. In profile the skull has a hump in front of and above the eyes, and the hump and rear spike might have been made taller by soft tissues.
Several neck vertebrae are present and they are notable for being unusually short for a toothless pterosaur. The sternum is large and the wings well muscled, showing that Eopteranodon was a strong flier. Its wingspan was 110 cm (44 inches), about the same as an average gull.
When first named, Lü and Zhang didn’t regard Eopteranodon as an ancestor of the Late Cretaceous Pteranodon, as the name might imply. Instead, they considered it to be a possible ancestor of all Late Cretaceous toothless pterosaurs including the pteranodontids, nyctosaurids, and azhdarchids. Most pterosaur family trees consider that to be an unlikely scenario however, with pteranodontids and nyctosaurids most closely related to the tooth-bearing ornithocheirids, and azhdarchids close to toothless tapejarids, thallasodromids, and chaoyangopterids as well as the tooth-bearing dsungaripterids.
The high upper margin of the nasoantorbital fenestra is a feature of the azhdarchids and their closest kin, a lineage known as the azhdarchoids. Within the azhdarchoids, the spike-like crest and short neck vertebrae are most similar to the chaoyangopterids, a poorly known family of pterosaurs known from Lower Cretaceous rocks considered to be the closest relatives of the azhdarchids.
The environment Eopteranodon lived in was temperate and forested with numerous rivers and lakes. Chaoyangopterids like Eopteranodon may have eaten fish, or hunted small animals in the forest.