1. Introduction In the 1990s, a precise geochronology based on U–Pb dating of volcanic zircons began to replace estimates on the lower and upper boundaries and on the higher- level chronostratigraphic divisions of the Cambrian that were based on glauconite and whole-rock analyses (compare Harland et al. 1990, pp. 134–8, with Bowring & Erwin, 1998, fig. 3). Among the key results of this more recent work are ages of the Proterozoic–Cambrian boundary (c. 543 Ma; Bowring et al. 1993; Grotzinger et al. 1995) and Lower–Middle Cambrian boundary interval (c. 511 Ma; Landing et al. 1998), and a date on the upper Upper Cambrian (491±1 Ma; Davidek et al. 1998). These U–Pb dates place the Cambrian evolutionary radiation into the context of a pre-trilobitic Early Cambrian, which with a duration of approximately 24 Ma, was approxi- mately half the length of the Cambrian. The rapid turnover of trilobite faunas in the later Early Cambrian to Late Cambrian allows a biostratigraphic resolution comparable to that of Ordovician grapto- lites or Mesozoic ammonites (Davidek et al. 1998). The low diversity ‘Cambrian fauna’ persisted into the Early Ordovician (e.g. Sepkoski, 1995; the Ibexian fauna of Adrain, Fortey & Westrop, 1998). Dates on the lower and upper boundaries of the Tremadoc, which is the lowest Ordovician series in the historic Anglo-Welsh type area of the Cambrian and Ordovician systems (Fortey et al. 1991; Fortey, 1995), are needed to determine the rates of biotic turnover in the later stages of the ‘Cambrian fauna’, as well as to provide a basis for comparison with subsequent Phanerozoic biotic turnover events (e.g. Bowring et al. 1999). U–Pb zircon ages of volcanic ashes from the Anglo-Welsh, Maritime Canadian, and eastern Newfoundland sequences of the late Proterozoic– Middle Paleozoic Avalon continent (Landing, 1996) have been particularly important in the determination of precise dates through the Cambrian–Ordovician boundary interval. The new datum documented here allows by far the best determination of the age of the Cambrian–Ordovician boundary. Davidek et al.’s (1998) U–Pb zircon age of 491±1 Ma on a volcaniclastic sandstone in the lower Peltura scarabaeoides Zone at Ogof-ddû, 1.25 km east of Criccieth, North Wales (Fig. 1), supported Shergold’s (1995, p. 4) estimate of a Cambrian–Ordovician boundary at an ‘arbitrarily selected date of 490 Ma … with the acknowledgement that it is likely to be substantially younger’. Previous proposals on this systemic boundary ranged from 505 Ma (Harland et al. 1990) to 495 Ma (Tucker & McKerrow, 1995). An upper bracket on the Cambrian–Ordovician boundary is the 483± 1 Ma age on an ash in the upper Tremadoc (Hunneberg Stage) from Cape Breton Island, Nova Scotia (Landing et al. 1997). The best section in Avalonia for determining a radiometric age on the Cambrian–Ordovician boundary Geol. Mag. 137 (5), 2000, pp. 485–494. Printed in the United Kingdom © 2000 Cambridge University Press 485 Cambrian–Ordovician boundary age and duration of the lowest Ordovician Tremadoc Series based on U–Pb zircon dates from Avalonian Wales E. LANDING*, S. A. BOWRING†, K. L. DAVIDEK†, A. W. A. RUSHTON‡§, R. A. FORTEY‡ & W. A. P. WIMBLEDON¶ *Center for Stratigraphy and Paleontology, New York State Museum, The State Education Department, Albany, NY 12230, USA †Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA ‡Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK ¶Countryside Council for Wales, 4 Castleton Court, Fortran Road, St. Mellons, Cardiff CF3 0LT, UK §Formerly British Geological Survey, Keyworth, Nottingham NG12 5GG, UK (Received 14 October 1999; accepted 10 May 2000) Abstract – 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 over- lain 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. * Author for correspondence: elanding@mail.nysed.gov