Article: The first Mesozoic chondrophorine (medusoid hydrozoan), from the Lower Cretaceous of Japan
Floating jellyfish-like hydrozoans belonging to the Suborder Chondrophorina are a little known but distinctive group extending back to the Precambrian with sporadic occurrences in the Palaeozoic. Their absence from post -Carboniferous rocks was puzzling, considering their abundance and wide distribution today. Palaelophacmaea annulata (Yokoyama), from the lower Cretaceous (Barremian) of Japan, is reinterpreted as the first known example of a chondrophorine fossil from the Mesozoic. The species was previously considered to be a patelliform gastropod. The remarkable similarity of chondrophorine pneumatophores to some univalved molluscs complicates precise indentification. Possible criteria to aid in distinguishing these little-known fossils are discussed.