Extinct craniids Most craniid genera are
extinct, known only from
fossils like other craniiforms. Craniids first appeared in the later part of the
Tremadocian, the first stage of the
Lower Ordovician. In the Lower Ordovician, they were mostly restricted to peri-
Gondwanan
terranes (modern central Europe) in the South Polar region. By the Middle Ordovician, they had spread northwards to
Baltica. In the Late Ordovician, their range expanded eastwards to
Avalonia before crossing the
Iapetus Ocean to
Laurentia. Craniid diversity and abundance was respectable but still fairly low during the Late Ordovician, and even lower through the rest of the
Paleozoic. The craniid fossil record is patchy, with fossils unknown from the
Upper Carboniferous,
Upper Permian, and the entire
Triassic. They reappear in the
Oxfordian stage of the
Late Jurassic and briefly regain their Ordovician level of diversity in the
Late Cretaceous.
Living craniids Craniids are remarkable for their slow rate of
evolution. Approximately 11 species of this 480-million-year-old lineage still
survive today, with minimal differences relative to their fossil counterparts. One species,
Valdiviathyris quenstedti, has remained essentially unchanged for the last 35 million years or so.
Craniscus has sometimes been cited as a fourth living craniid genus,
based on "Craniscus japonica
", a putative species from waters off Japan. Genetic evidence has clarified this misconception, revealing that "Craniscus japonica
" actually represents a misattributed species of Neoancistrocrania''. == List of genera ==