Deep-Water Ediacaran Fossils from Northwestern Canada: Taphonomy, Ecology, and Evolution

Abstract

Impressions of soft-bodied Ediacaran megafossils are common in deep-water slope deposits of the June beds at Sekwi Brook in the Mackenzie Mountains of NW Canada. Two taphonomic assemblages can be recognized. Soles of turbidite beds contain numerous impressions of simple (Aspidella) and tentaculate (Hiemalora, Eoporpita) discs. A specimen of the frond Primocandelabrum is attached to an Aspidella-like holdfast, but most holdfast discs lack any impressions of the leafy fronds to which they were attached, reflecting Fermeuse-style preservation of the basal level of the community. Epifaunal fronds (Beothukis, Charnia, Charniodiscus) and benthic recliners (Fractofusus) were most commonly preserved intrastratally on horizontal parting surfaces within turbidite and contourite beds, reflecting a deep-water example of Nama-style preservation of higher levels in the community. A well-preserved specimen of Namalia significantly extends the known age and environmental range of erniettomorphs into deep-water aphotic settings. Infaunal bilaterian burrows are absent from the June beds despite favorable beds for their preservation. The June beds assemblage is broadly similar in age and environment to deep-water Avalonian assemblages in Newfoundland and England, and like them contains mainly rangeomorph and arboreomorph fossils and apparently lacks dickinsoniomorphs and other clades typical of younger and shallower Ediacaran assemblages. Fossil data presently available imply that the classically deep- and shallow-water taxa of the Ediacara biota had different evolutionary origins and histories, with sessile rangeomorphs and arboreomorphs appearing in deep-water settings approximately 580 million years ago and spreading into shallow-water settings by 555 Ma but dickinsoniomorphs and other iconic clades restricted to shallow-water settings from their first known appearance at 555 Ma until their disappearance prior to the end of the Ediacaran.

Remarkable insights into the paleoecology of the Avalonian Ediacaran macrobiota

“…Clapham and Narbonne, 2002;Ghisalberti et al, 2014, though see Grazhdankin et al, 2008) Rangeomorphs have been described from several younger and shallower localities across the globe, extending the range of this lineage beyond the time interval and facies recorded in the Newfoundland and U.K. successions (Jenkins, 1985;Grazhdankin et al, 2008;Hofmann and Mountjoy, 2010;Gehling and Droser, 2013;Narbonne et al, 2014).…”

“…Several major Ediacaran morphotypes, such as the dickinsoniomorphs and erniettomorphs (cf. Laflamme et al, 2013), are not currently known from Newfoundland or the U.K., but the discovery of a Namalia specimen at Sekwi Brook (Narbonne et al, 2014) may demonstrate that members of the erniettomorphs at least were present in deep-marine environments prior to their appearance in shallower-water localities.…”

Rangeomorph classification schemes and intra-specific variation: are all characters created equal?

“…As such, no formal diagnosis of the branching pattern exists by which this genus or its constituent species may be definitively identified at other localities. The Bonavista specimens are often reconstructed as a two-dimensional fan or candelabrum in life (Hofmann et al 2008), while the single specimen assigned to Primocandelabrum from NW Canada suggests a more brush-like appearance (Narbonne et al 2014). In the specimens from Charnwood Forest that fit the broad diagnosis of Primocandelabrum, it is clear that the crown consists of three first order branches that split from the main stem at a single point (Fig.…”

New Ediacaran fossils from the uppermost Blueflower Formation, northwest Canada: disentangling biostratigraphy and paleoecology

“…Fedonkin (in Palij, Posti, and Fedonkin, 1979) and Sekwia excentrica Hofmann, 1981 were both reported along with the trace fossils Gordia Emmons, 1844, Gordia?, andTorrowangea Webby, 1970. Subsequent studies at Sekwi Brook have extended the range of Ediacara-type body fossils into the underlying 'June beds,' which were previously correlated with the uppermost strata of the Sheepbed Formation (sensu Macdonald et al, 2013) and have substantially increased the known diversity of Ediacaran body fossils and trace fossils in this succession (Narbonne and Aitken, 1990;Narbonne, 1994;Narbonne et al, 2014;Carbone and Narbonne, 2014). Most Ediacaran body fossils and trace fossils from the Mackenzie Mountains are from slope deposits (Dalrymple and Narbonne, 1996;MacNaughton et al, 2000;Macdonald et al, 2013;Narbonne et al, 2014;Carbone and Narbonne, 2014), although shallow-water deposits at the top of the Blueflower Formation have yielded single specimens of the probable dickinsonid Windermeria Narbonne, 1994 and an Ediacaria-morph of the holdfast disc Aspidella Billings, 1872 (Narbonne and Aitken, 1990), along with a low diversity assemblage of mainly simple, sub-horizontal burrows (Carbone and Narbonne, 2014).…”

“…Subsequent studies at Sekwi Brook have extended the range of Ediacara-type body fossils into the underlying 'June beds,' which were previously correlated with the uppermost strata of the Sheepbed Formation (sensu Macdonald et al, 2013) and have substantially increased the known diversity of Ediacaran body fossils and trace fossils in this succession (Narbonne and Aitken, 1990;Narbonne, 1994;Narbonne et al, 2014;Carbone and Narbonne, 2014). Most Ediacaran body fossils and trace fossils from the Mackenzie Mountains are from slope deposits (Dalrymple and Narbonne, 1996;MacNaughton et al, 2000;Macdonald et al, 2013;Narbonne et al, 2014;Carbone and Narbonne, 2014), although shallow-water deposits at the top of the Blueflower Formation have yielded single specimens of the probable dickinsonid Windermeria Narbonne, 1994 and an Ediacaria-morph of the holdfast disc Aspidella Billings, 1872 (Narbonne and Aitken, 1990), along with a low diversity assemblage of mainly simple, sub-horizontal burrows (Carbone and Narbonne, 2014). Shallow-water equivalents of the Blueflower Formation in the Wernecke Mountains, 250 km west-northwest of Sekwi Brook (Pyle et al, 2004), have yielded abundant Ediacara-type discoid body fossils and simple trace fossils along with a single specimen of the Ediacaran frond Charniodiscus Ford, 1958(Hofmann et al, 1983Narbonne and Hofmann, 1987;Pyle et al, 2004), and were previously cumulatively considered the only Ediacaran shallow-water assemblage from NW Canada.…”