The Tiñu Formation of Oaxaca State is the only fossiliferous lower Palaeozoic unit between the Laurentian platform in northwest Mexico and Gondwanan successions in Andean South America. The Tiñu traditionally has been referred to the Lower Ordovician (Tremadoc) and regarded as having a provincially mixed fauna with Laurentian, Avalonian, and Gondwanan elements. Bio- and lithostratigraphic re-evaluation demonstrates that the Tiñu is a Gondwanan, passive margin succession. It includes a lower, thin (to 16 m), condensed, uppermost Cambrian Yudachica Member (new). The Yudachica nonconformably overlies middle Proterozoic basement as a result of very high late Late Cambrian eustatic levels. The Yudachica changes from storm-dominated, but slightly dysoxic, shelf facies (fossil hash limestone and shale) in the south to an upper slope facies with debris flows 50 km to the north. Three biostratigraphically distinct depositional sequences comprise the Yudachica. The Yudachica has Gondwanan-aspect trilobites with low-diversity conodonts characteristic of unrestricted marine/temperate facies. The upper Tiñu, or Río Salinas Member (new), is a Lower Ordovician (Tremadoc) depositional sequence that records strong early, but not earliest, Tremadoc eustatic rise marked by graptolite- and olenid-bearing dysoxic mudstones. Higher strata shoal upward into shell-hash limestones and proximal tempestite sandstones with upper lower Tremadocian unrestricted marine/temperate conodonts. New taxa includeOrminskia rexroadaeLanding gen. et sp. nov. from theCordylodus andresiZone; this euconodont is related to hyaline coniform genera best known from Ordovician tropical platform successions.Cornuodus?clarkeiLanding sp. nov. resembles the coeval, upper lower Tremadoc tropical speciesScalpellodus longipinnatus(Ji & Barnes).

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.

Ed Landing, Shanchi Peng, Loren E. Babcock, Gerd Geyer, and Malgorzata Moczydlowska-Vidal. Episodes 2007;30:287-9. https://doi.org/10.18814/epiiugs/2007/v30i4/004

New U–Pb radioisotopic ages on early Cambrian volcanic zircons condition a high-resolution Bayesian age model that constrains the first occurrences and zonations of West Gondwanan archaeocyaths and trilobites in southern Morocco. The oldest archaeocyaths in the Tiout Member of the Igoudine Formation (519.71 + 0.26/− 0.35 Ma) are c. 6 Ma younger than the oldest Siberian archaeocyaths. The oldest Moroccan trilobite fragments, from the lower member of the Igoudine, are constrained to 519.95 + 0.43/− 0.40 Ma. The succeeding Issendalenian Stage (i.e. Hupetina antique – Eofallotaspis tioutensis – Fallotaspis plana – Choubertella – Daguinaspis trilobite zones) spans c. 1.5 Ma (519.78 + 0.26/− 0.37 Ma to 518.43 + 0.25/− 0.69 Ma). Identifiable Moroccan fallotaspidids and bigotinids, among Earth’s oldest trilobites, occur above a positive δ¹³C excursion dated with our age model at 520.27 + 0.59/− 0.57 Ma, and correlated with the IV excursion peak within the lower range of Siberian Atdabanian Stage trilobites (Repinaella Zone). This excursion is the best standard for a Cambrian Series 2 base. The oldest West Gondwana trilobite fragments are c. 1 Ma younger than those in Siberia and c. 0.5 Ma older than the oldest Avalonian trilobites (Callavia Zone). This diachrony means a trilobite first appearance datum is an inappropriate chronostratigraphic base for Cambrian Series 2. Taxonomic differences in the oldest trilobites between Cambrian palaeocontinents are in accordance with trace fossil evidence for the group’s appearance possibly as late as c. 530 Ma in the Cambrian Evolutionary Radiation. Coeval 519–517 Ma dates from Avalonia (cool-water siliciclastic shelf) and West Gondwana (tropical carbonate platform) sections with distinct macrofaunas emphasize these successions were latitudinally separate by the late Ediacaran Period.