In the late twentieth century, paleontologist Adolf Seilacher challenged the prevailing consensus that the late Precambrian Ediacaran biota represented direct ancestral precursors to modern animal phyla. Observing that organisms such as *Dickinsonia* and *Charnia* lacked identifiable mouths, guts, or alimentary tracts, Seilacher proposed the 'Vendobionta' hypothesis, positing that these organisms were not metazoans, but rather a distinct, extinct kingdom of modular, fluid-filled quilted mattresses relying on direct nutrient absorption through epidermal surfaces. However, recent discoveries have complicated Seilacher's morphological paradigm. Geochemical analyses of pristine Ediacaran fossils from the White Sea region have detected cholesteroid lipid biomarkers—specifically 27-carbon steranes synthesized exclusively by animals—within the organic residue of *Dickinsonia*, while the surrounding benthic sediment contains only microbially derived cholestanes. Furthermore, preserved taphonomic traces known as 'death masks' reveal evidence of sequential sediment displacement beneath *Dickinsonia*, indicating active, directed locomotion rather than passive, sessile anchorage. While these findings largely refute Seilacher's claim of a separate kingdom, they do not resolve whether the Ediacarans were stem-group metazoans branching before the divergence of modern bilaterians or crown-group representatives of extant lineages.
Based on the passage, which of the following statements are logically supported by the text? (Select all that apply.)
- The presence of cholesteroid biomarkers in the organic residue of *Dickinsonia* distinguishes it biochemically from the microbial communities found in the surrounding sediment.Answer
- BThe demonstration of directed locomotion definitively establishes that *Dickinsonia* belonged to a crown-group lineage of modern bilaterian animals.
- The formulation of the Vendobionta hypothesis was motivated primarily by the absence of recognizable internal digestive structures in fossils such as *Dickinsonia* and *Charnia*.Answer
- DThe ultimate extinction of the Ediacaran biota was precipitated by severe fluctuations in marine oxygen concentrations during the transition to the Cambrian period.
- ETaphonomic death masks have demonstrated that *Charnia* exhibited the same active mobility across benthic substrates as *Dickinsonia*.