Testing the protozoan hypothesis for Ediacaran fossils: a developmental analysis ofPalaeopascichnus

Abstract

The hypothesis that the Ediacara biota were giant protozoans is tested by considering the external morphology, internal organization, suggested fossil representatives and molecular phylogeny of the xenophyophores. From this analysis, we find no case to support a direct relationship. Rather, the xenophyophores are here regarded as a group of recently evolved Foraminifera and are hence unlikely to have a record from the Ediacaran Period. Further from the growth dynamics of Foraminifera, they are also unlikely to be related to the Palaeopascichnus organism. We also find significant distinctions in the growth dynamics of Palaeopascichnus and organisms usually referred to the Ediacara biota, such as Charnia and Dickinsonia. Developmental analysis of the Palaeopascichnus -central to the xenophyophore hypothesis -reveals unusual, protozoan features, including evidence for chaotic repair structures, for mergence of coeval forms, as well as complex bifurcations. These observations suggest that Palaeopascichnus is a body fossil of an unidentified protozoan but is unrepresentative of Ediacaran body construction, in general.Key words: Palaeopascichnus, Ediacara biota, Protozoa, xenophyophores, development, evolution.The fossil record contains a great conundrum; the modern animal phyla appear already distinguished as fossils within the Cambrian System (Brasier 1979). This system lies immediately above the recently named Ediacaran System (Knoll et al. 2004). From the time of Darwin to the discovery of the global Ediacara biota (Gürich 1933; Sprigg 1947; Ford 1958), the ancestry of animals has remained one of the most exciting mysteries in science. Indeed, Darwin (1859) stated that one of the greatest flaws in his hypothesis was the lack of demonstrable ancestry to the recognized phyla that appear distinct at the base of the known fossil record (then 'Silurian', now the base of the Cambrian). After the great antiquity of the Ediacara biota had been realized (Ford 1958), then the Ediacara biota was interpreted as primitive metazoans (Glaessner 1966(Glaessner , 1984 Glaessner and Daily 1959; Glaessner and Wade 1966; Wade 1972; Jenkins 1984 Jenkins , 1985 Jenkins , 1992 Jenkins , 1995 Jenkins and Gehling 1978; McMenamin 1986 McMenamin , 1998 Bengtson 2003), thereby providing putative solutions to the long posed problem regarding animal ancestry.This view of Ediacaran palaeobiology was refocused by Seilacher , 1985, 1989, 1992 also Buss and Seilacher 1994), who questioned the evidence for continuity between the Ediacara biota and succeeding Cambrian metazoans. Seilacher (1989) also challenged the taxonomy by stating that many Ediacaran organisms share a common construction. This was taken to imply that they are, for the most part, a single clade unrelated to modern groups. A profusion of taxonomic affinities has followed (e.g. Dzik 2003; Gehling 1991; Jenkins 1992; Valentine 1992; Seilacher 2003; Seilacher et al. 2003; Peterson et al. 2003; Narbonne 2005). These taxonomies were based upon simple morphologic...

Decoupling of body-plan diversification and ecological structuring during the Ediacaran–Cambrian transition: evolutionary and geobiological feedbacks

“…Structures previously interpreted as guided meandering grazing trails are no longer considered trace fossils, because careful re-analysis fails to reveal the presence of actual meanders, suggesting that these were body fossils instead [17,18,33,37]. Therefore, animals displaying sophisticated feeding strategies involving strophotaxis (i.e.…”

Ediacaran matground ecology persisted into the earliest Cambrian

“…Palaeopaschichnus, a softbodied fossil restricted to Ediacaran strata worldwide 39 , occurs in subtidal strata 0.2 m below the top of the Ediacaran at the GSSP 5 , but despite intensive search no Ediacara-type body fossils have been found in the overlying Cambrian succession. The disappearance of the Ediacara biota, and its replacement by a fauna of complex metazoans is still a puzzling issue, and three alternative hypotheses have been invoked: mass extinction of Ediacaran forms; biotic replacement, either as a result of predatory displacement or ecosystem engineering; and closing of the unique Ediacaran taphonomic window 40 .…”

The advent of animals: The view from the Ediacaran

“…The serial arcuate looping forms, Paleopascichnus and Yelovichnus, and chains hemispheres, referred to as Neonereites, are now considered encrusting body fossils, reinterpreted as xenophyophores by Seilacher and colleagues (63), but disputed by Antcliffe and colleagues (64). The oldest described trace fossils seem to be those of actinian-grade polyps that left simple straight or curved locomotive trails in ash-coated, deep-water substrates in the Mistaken Point Formation assemblage in southeastern Newfoundland (65).…”