Precise early Cambrian U–Pb zircon dates bracket the oldest trilobites and archaeocyaths in Moroccan West Gondwana

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.

Archaeocyathan zonation of the Yangtze Platform: Implications for regional and global correlation of lower Cambrian stages

Detailed analysis of the distribution of archaeocyaths in five lower Cambrian sections in South China has resulted in the erection of four new archaeocyathan zones and one new archaeocyathan bed. Listed in order of ascending age, these are: theDailycyathus xiuqiensisZone; theDictyocyathus shaanxiensisZone; theSpirillicyathus duplexRange Zone; theSibirecyathus meitanensisRange Zone; and theArchaeocyathus yanjiaoensisbeds. These new subdivisions permit a correlation of lower Cambrian strata both within the area of the Yangtze Platform (South China) as well as between this region and Siberia, Australia, Western Europe and North America. Within the Yangtze Platform area, archaeocyaths of theDailycyathus xiuqiensisandDictyocyathus shaanxiensiszones co-occur with trilobites of the middle and upperEoredlichiaTrilobite Zone of the Qiongzhusian Stage. TheSpirillicyathus duplexand theSibirecyathus meitanensisRange Zone correspond to theMalungia, Yiliangella–YunnanaspisandDrepanuroidesTrilobite zones of the basal and middle Canglangpuan Stage, respectively. Finally, theArchaeocyathus yanjiaoensisbeds belong to thePalaeolenus fengyangensisTrilobite Zone of the upper Canglangpuan Stage. Global correlations based on archaeocyath assemblage zones suggest that: (1) the middle–upper Qiongzhusian of South China correlates with the middle Atdabanian Stage of the Siberian Platform, theWarriootacyathus wilkawillinensisArchaeocyath Zone in South Australia, the middle Issendalenian Stage of Morocco and the lower Ovetian Stage in Spain; (2) the lowermost Canglangpuan Stage of South China correlates with the uppermost Atdabanian of the Siberian Platform, theSpirillicyathus tenuis – Jugalicyathus tardusarchaeocyath zones in South Australia, the uppermost Issendalenian – lower Banian stages of Morocco, the middle Ovetian Stage of Spain and the middle Montezuman Stage of North America; (3) the middle Canglangpuan Stage approximates the lowermost Botoman Stage of the Siberian Platform, middle Banian Stage of Morocco, the uppermost Ovetian – lowermost Marianian stages of Spain and the upper Montezuman Stage of North America; (4) the uppermost Canglangpuan Stage is equivalent of the middle Toyonian Stage of the Siberian Platform,Archaeocyathus abacusbeds in Australia, the middle Bilbilian Stage in Spain and the middle Dyeran Stage (Bolbolenellus eurypariaTrilobite Zone) in North America.

Observations on the Early Cambrian helicoplacoid echinoderms

The Early Cambrian helicoplacoid echinoderms occur in the Cordilleran Geosyncline of western North America in strata correlated with the Atdabanian Stage of Siberia. Several higher taxa are recognized on the basis of inferred differences in the water vascular system, test organization, and external morphology. These are subclass Polyplacida Durham, with genus Polyplacus Durham; subclass Helicoplacida Durham and Caster, with n. family Helicoplacidae, type genus Helicoplacus Durham and Caster (with tubefeet emerging between two contemporaneous ambulacral plates); n. family Westgardellidae, with type n. genus Westgardella, type species H. curtisi (Durham and Caster) (with tubefeet emerging between two sequential ambulacral plates). The genus Waucobella Durham is also referred to Westgardellidae. Helicoplacus gilberti Durham and Caster, H. everndeni Durham, H. casteri n. sp., H. guthi n. sp., H. sp. a, and H. sp. b are assigned to Helicoplacidae. The genus Westgardella includes H. firbyi Durham, 1967, and W. blancoensis n. sp., in addition to the type species. No evidence of flooring plates separating the radial water vessel from the interior of the test is recognized. The mouth is at the top of the test in the interpretation adopted herein and not lateral as inferred by others; therefore, the ambulacral system is not triradiate. Illustration identified as Helicoplacus curtisi by Paul and Smith includes misidentified plates and should not be referred to this species.

Archaeocyathid fauna of the lowest fossiliferous levels of Tiout (Lower Cambrian, Southern Morocco)

SummaryThe oldest archaeocyathid fauna from Tiout (Anti-Atlas, Morocco) contains 15 species belonging to 11 genera. The following species: Aldanocyathus cribratus, Neoloculicyathus magnus, Coscinocyathus primus, C. secundus, C. fasciola, Tumulifungia marocana, Gordonifungia cowieiare new. The biostratigraphy of this archaeocyathid fauna demonstrates that the lower boundary of the Amouslekian stage should be moved downwards to within the ‘Calcaires Supérieurs’ of the Adoudounian (Assadassian or equivalent Ouneinian). One or perhaps more archaeocyathid and trilobite zones may yet be found beneath this boundary at Tiout. The uppermost ‘Calcaires Superiéurs’ may be dated, on the basis of Archaeocyatha, as equivalent to the Atdabanian stage, but not to the lowest part of it. The underlying fossiliferous horizons should be equivalent to all or part of the lower Atdabanian. The Precambrian–Cambrian boundary in Morocco can be looked for lower in the succession and defined on the basis of other fossil groups.