The circulatory system of Galeaspida (Vertebrata; stem-Gnathostomata) revealed by synchrotron X-ray tomographic microscopy

Micro-CT provides a means of nondestructively investigating the internal structure of organisms with high spatial resolution and it has been applied to address a number of palaeontological problems that would be undesirable by destructive means. This approach has been applied successfully to characterize the cranial anatomy of Shuyu, a 428 million-year-old galeaspid (jawless stem-gnathostome) from the Silurian of Changxing, Zhejiang Province, China. Here, we use the synchrotron X-ray tomographic microscopy (SRXTM) to further describe the circulatory system of the head of Shuyu. Our results indicate that the circulatory system of galeaspids exhibits a mosaic of primitive vertebrate and derived gnathostome characters, including a number of derived gnathostome characters that are absent from osteostracans — the group conventionally interpreted as the sister lineage of jawed vertebrates. Our study provides a rich source of information that can be used to infer and reconstruct the early evolutionary history of the vertebrate cardiovascular system.