Eosipterus yangi was the first pterosaur discovered from the Lower Cretaceous Yixian Formation in Liaoning, China, and was named by Ji Shu’an and Ji Qiang in 1997. The genus name translates to “dawn wing” in Greek. The species name honors the father of Chinese paleontology Yang Zhongjian, known in the West as C. C. Young for most of the 20th Century.
The species is known from two headless specimens, the original described by Ji and Ji in 1997, and a second described by Lü Junchang and colleagues in 2006. Both skeletons are articulated, but incompletely preserved, missing many details of the vertebrae and limb bones.
Eosipterus was a medium-sized short-tailed pterosaur with robust limbs. Both specimens were subadults, with wingspans of 1.2 m (4 feet). The proportions of the bones in its wings are distinctive and allow Eosipterus to be differentiated from other pterosaurs.
Ji and Ji did not assign Eosipterus to any family when they initially described it in 1997. In a 2006 followup paper, Lü Junchang and Ji Shu’an found it to be an archaeopterodactyloid close to both the ctenochasmatids and germanodactylids. Later in 2006 Lü and colleagues described the second specimen of Eosipterus and assigned it to the Ctenochasmatidae. In 2007 Wang Xiaolin and colleagues pointed out that many of the bones in the original specimen were modified by a fossil dealer prior to scientific study and could only confirm that it was a pterodactyloid, although probably an archaeopterodactyloid. In 2016, Lü and colleagues again considered it to be a ctenochasmatid.
Eosipterus lived in a lush temperate forest about 125 million years ago. Most archaeopterodactyloids are found at the margins of waterways, where they likely foraged. Ctenochasmatids were suspension feeders, using dozens of narrow outwardly pointing teeth to strain small animals from mud and water.