Since their initial recovery and scientific description, most pterosaurs have generally come in two broad groups, the early long-tailed short-skulled "rhamphorhynchoids" like Rhamphorhynchus and Dimorphodon and the later short-tailed long-skulled pterodactyloids like Pterodactlus and Pteranodon. Until the early 21st century, there was a significant additional gulf separating these two groups. The discovery of the Wukongopterids, exemplified by the long-tailed long-skulled Darwinopterus, helped fill that gap.
A pterosaur named Allkaruen koi described in 2016 provides another example of a transitional anatomy between the more primitive "rhamphorhynchoids" and more derived pterodactyloids. Allkaruen was discovered in rocks of the Middle Jurassic Cañadón Asfalto Formation of Chubut Province, Argentina.
The type specimen of Allkaruen is made up of an extremely well-preserved braincase and some associated skull and neck bones. The specimen was found in a pterosaur bonebed deposited in an ancient lake bed. The name Allkaruen koi is derived from the Tehuelche language and means "ancient brain from the lake."
The braincase is so well preserved that the actual anatomy of the brain could be studied by CT scanning the fossils. The bone and brain anatomy was compared to other pterosaurs and Allkaruen was found to be in a transitional position between the long-tailed "rhamphorhynchoids" and the monofenstratans, the name given to the wukongopterids and pterodactyloids. The authors analyzed both the type specimen alone, as well as the type specimen plus the other pterosaur bones found in the bonebed and in both cases found Allkaruen to be in that phylogenetic position.
Allkaruen's skull sported two parallel crests on the top of skull and appeared to have teeth restricted to the middle and distal portion of its jaws. Based on its position within the pterosaur family tree, it is assumed that Allkaruen retained a long tail.