Didymograptus bifidus and the trans-Atlantic correlation of the Lower Ordovician
Version of Record online: 9 OCT 2007
Volume 6, Issue 4, pages 313–339, October 1973
How to Cite
BERGSTRÖM, S. M. and COOPER, R. A. (1973), Didymograptus bifidus and the trans-Atlantic correlation of the Lower Ordovician. Lethaia, 6: 313–339. doi: 10.1111/j.1502-3931.1973.tb01201.x
- Issue online: 9 OCT 2007
- Version of Record online: 9 OCT 2007
Bergström, S. M. & Cooper, R. A.: Didymograptus bi'us and the trans-Atlantic correlation of the Lower Ordovician.
The co-occurrence of Didymograptus bifidus (Hall) and Prioniodus mae Lindstrom, and other conodonts of the middle Arenigian P. mae Zone in the upper part of the Marathon Limestone, Texas, provides the first non-graptolitic evidence that at least part of the D. bifidus Zone, as recognized in the reference area for the standard graptolite zonal succession of the Ordovician of North America, is as old as the BritisK D. nitidus Zone. This has important implications for the correlation of Early Ordovician rocks within North America and across the Atlantic. A regional review of the available evidence from conodonts and shelly megafossils indicates that the D. bifidus—D. artus graptolite assemblage widely used for identification of the D. bifidus Zone in North America ranges in age from the middle Arenigian to, probably, the lower Llanvirnian, and hence is of relatively limited use for precise local and regional correlations. It is also noted that the base of the North American Middle Ordovician falls within hograptus ‘caduceus’ Zone strata that apparently correspond to an interval in the middle to upper part of the British D. hirundo Zone.