HSC Earth and Environmental Science Practice Questions

Evidence. The oldest widely accepted large multicellular organisms are the Ediacaran fauna (about 575–541 Ma), first recognised in the Ediacara Hills of the Flinders Ranges, South Australia, and also preserved at Mistaken Point (Newfoundland) and the White Sea (Russia). Fossils such as Dickinsonia, Spriggina and the fronds of Charnia are impressions of soft-bodied organisms preserved when microbial mats and fine sediment covered them. Volcanic ash beds interbedded with some assemblages have been dated radiometrically (e.g. U–Pb ages near 565 Ma at Mistaken Point), giving absolute ages. Trace fossils show simple horizontal movement by the late Ediacaran. The Cambrian fauna (from about 541 Ma) is recorded in sites of exceptional preservation such as the Burgess Shale (Canada), Chengjiang (China) and Emu Bay Shale (Kangaroo Island), which preserve soft tissues as well as hard parts and show most modern animal phyla, predators and complex burrows. Biomarker molecules in rocks and molecular clock estimates suggest animal lineages existed earlier still.

Strengths. Several independent lines of evidence (body fossils, trace fossils, radiometric ages and chemical biomarkers) converge on large multicellular life appearing in the late Neoproterozoic and diversifying rapidly in the Cambrian. The fossils are found on several continents in the same time interval, which makes a global event likely rather than a local accident of preservation.

Limitations. Soft-bodied organisms fossilise only under unusual conditions, so the record is patchy and earlier multicellular organisms may not have been preserved. The relationships of many Ediacaran organisms are debated (animals, colonial organisms, lichens or an extinct group), so they show multicellularity but not necessarily the origin of animals. Molecular clocks place animal origins tens of millions of years before the oldest fossils, which suggests the fossil record underestimates the date. Older claims, such as about 2.1 Ga structures from Gabon, remain contested.

Judgement. The evidence is strong that large, complex multicellular organisms existed by about 575 Ma and diversified dramatically in the Cambrian, because fossils, trace fossils and radiometric dates agree. It is weaker on exactly when and how multicellularity first arose, because preservation bias and disputed interpretations leave the earliest stages poorly recorded.

Marking guidance (6 marks): Award 1–2 marks for describing relevant evidence (named fossils, sites, dates, trace fossils); 1–2 marks for strengths and limitations of the evidence; 1–2 marks for a clear, justified judgement about how convincing the evidence is. A description of the Ediacaran and Cambrian fauna without assessing the evidence is limited to 3 marks.