New data on the cranial anatomy of Pterogonaspis (Tridensaspidae, Galeaspida) from the lower Devonian of Yunnan, China and its evolutionary implications

New material of the tridensaspid Pterogonaspis yuhaii is described from the Xujiachong Formation in Qujing City, Yunnan Province, revealing the morphology of the ventral side of the headshield for the first time. Most significantly, the cranial anatomy of Pterogonaspis provides the first fossil evidence for the position of the esophagus in galeaspids. The esophagus and dorsal aortae display a central placement in the postbranchial region and do not share a common canal as in osteostracans. Moreover, the paired and symmetrically extending dorsal aortae in galeaspids strikingly resemble those of jawed vertebrates, probably representing a plesiomorphic condition for vertebrates because they are also present in amphioxus and hagfishes. In contrast, the unpaired and asymmetrically extending dorsal aorta in osteostracans is similar to that of lampreys, probably resulting from convergent evolution. In addition, our new data also exhibit some characters that galeaspids uniquely share with osteostracans, such as the complete endoskeletal postbranchial wall and the extensive lateral expansion of the headshield with the marginal arteries and veins.