New insights on the osteology of istiodactyliforms from the Jehol Biota: new material of Hongshanopterus lacustris Wang et al., 2008 and restudy of Nurhachius ignaciobritoi Wang et al., 2005

The Istiodactyliformes, a group of short-toothed Cretaceous pterosaurs including istiodactylids and their relatives, have mostly been reported from the Jehol Biota of Northeastern China. Recently, a new specimen of Hongshanopterus lacustris, an istiodactyliform from the Lower Cretaceous Jiufotang Formation, has been unveiled. Here we present a detailed description of a new specimen, which comprises most of the skull, the mandible, several cervical vertebrae, and plenty of isolated teeth, providing additional anatomical information of this taxon. We also present the comprehensive redescription of the holotype of Nurhachius ignaciobritoi, the first reported istiodactylid from China. Most of the previously suggested variances between Nurhachius specimens are here found invalid to separate different taxa, but the holotype has a shorter rostrum compared with others. Additional morphological information revealed by the new specimen and redescription further supports the affinity between Hongshanopterus and istiodactylids, and between istiodactylids and other pteranodontoids, but istiodactylids possess narrower cervical vertebrae compared with anhanguerians.