Two thin volcaniclastic sandstone beds in the Bryn-llin-fawr road section in North Wales
overlie an apparent sequence boundary within the uppermost Cambrian Acerocare Zone and are overlain
by lowest Ordovician (lower Tremadoc) Rhabdinopora faunas. U–Pb geochronology of zircons
from these sandstones yields a maximum Cambrian–Ordovician boundary age of 489±0.6 Ma. This
age indicates both that the Tremadoc Series (lowest Ordovician) may be shorter in duration than was
previously thought and that the duration of the Middle and Late Cambrian (c. 22 Ma) was much less
than that of the Early Cambrian (c. 33 Ma). Cambrian trilobite zones locally had an average duration
as short as 1 Ma.