Oldest animal megafossils

The oldest confirmed animal megafossils (aka macrofossils), measuring up to 1.4 metres (4 feet 7 inches) long, are those of Dickinsonia, a genus of oval-shaped, soft-bodied organisms from the Ediacara biota that lived approximately 558 million years ago. There has long been controversy as to whether some Ediacaran life forms, older than other macrofossils currently known, were pseudofossils (inorganic objects, markings or impressions that resemble fossils), lichen, bacterial colonies or giant single-celled organisms, among other things, rather than animals, which has made their taxonomic classification extremely difficult. According to a study published in the journal Science on 21 September 2018, however, a biochemical study of organically preserved Dickinsonia fossils from the White Sea region of Russia extracted lipid biomarkers and revealed that they solely produced cholesteroids. This is a diagnostic feature of animals and, excluding all other interpretations (i.e., plants, fungi and any other plausible life form), thereby confirmed that, based on current knowledge, this particular Ediacaran was an animal.

Dickinsonia resembled a dorsoventrally flattened round or oval disc, almost bilaterally symmetrical, and divided into a series of rib-like segments. However, in total the Ediacara biota exhibited an extraordinary diversity and radiation of morphologies, and some researchers believe that certain of these life forms may have even belonged to taxonomic kingdoms that are now long-extinct, entirely unrepresented in the modern-day biota, having seemingly died out during the Cambrian explosion of new life forms approximately 541 million years ago without leaving any descendants.

As part of the same study, another genus of Ediacara biota – Andiva – was confirmed to be an animal; however it dates to approximately 555 million years old, so is slightly younger than Dickinsonia. Interestingly, also in 2018, using the same analysis technique, another Ediacaran life form – disc-shaped Beltanelliformis – was confirmed to be a colony of cyanobacteria. This just shows the diversity of fossils from this era and why their identification is so difficult.

The research behind this discovery was conducted by the Australian National University (Australia), the Russian Academy of Sciences (Russia), the Max Planck Institute for Biogeochemistry and Bremen University (both Germany), led by PhD student Ilya Bobrovskiy, Dr Janet Hope and Professor Jochen Brocks of the Australian National University. The results were published in the journal Science on 21 September 2018.

It must be noted that some members of the scientific community are not convinced that this evidence is enough to conclusively say that Dickinsonia was an animal, flagging that high levels of cholesterol molecules have also been associated with certain protists (mostly single-celled, microscopic organisms), fungi, "microbial mats" and even some plants.