Earliest invertebrates

The earliest invertebrate animals of which we have fossilized remains are the trace fossils of various soft-bodied and shelled animals (sometimes referred to generally as Ediacaran biota) dating to at least 560 million years ago (MYA) in the Precambrian Ediacaran Period. These include groups such as Arkarua (a possible ancestor of echinoderms), Eoandromeda (a possible ancestor of ctenophores), the sponge-like Palaeophragmodictya, oval-shaped Dickinsonia – which at up to 1.4 m (4 ft 7 in) long represent the oldest animal macrofossils, i.e., those visible with the naked eye) – and the primitive cnidarian (related to jellyfish and sea anemones) Auroralumina attenboroughii, found in the UK in 2007 and described in 2022. It's worth noting that many fossils from the Ediacaran, including some said to date back even earlier than 560 MYA, are greatly debated among scientists as to which represent true animals or other forms of life, such as fungi, plants and microbial colonies, or even inorganic matter in some cases.

For a long time, it was assumed that the earliest animals emerged in the Cambrian Period (538.8 MYA to 485.4 MYA), known as the "Cambrian Explosion" or "Biological Big Bang", but in the mid-19th century impressions of what appeared to be multicellular organisms or at least signs of their existence (e.g., imprints, trails, burrows, etc) began to be found in ichnofossils (trace fossils) within strata situated below Cambrian rock, indicating an earlier-than-expected emergence of animal life. The first of these reported were disc-shaped Aspidella found by the Scottish geologist Alexander Murray in Newfoundland, Canada, in 1868. As time went on, similar discoveries, seemingly representing a wide plethora of organisms, began to appear in various sites around the world including the UK, Namibia, Australia, Russia and China. As a group, these came to be called "Ediacaran biota" after their geological period.

Some of the earliest full-body fossils (as opposed to trace fossils) of invertebrates found to date are those of archaeocyathid sponges (unearthed in what is today Siberia, Russia), dating to the Tommotian Age of the Early Cambrian Period, circa 525 million years ago. Archaeocyathid sponges only appeared much later on in other regions of the world, during the Atdabanian (509–497 MYA). Moreover, they had completely died out by the end of the Cambrian (485 MYA).

A study published in the journal Nature in 2021 proposed a fossilized specimen of a sponge had been identified in rock from Canada dating back as far as 890 MYA, potentially pushing the age of animals back a staggering 330–350 million years or so. However, the findings drew a lot of scepticism from the scientific community, with some suggesting the mesh-like structures being likened to those of a sponge could be a similar-looking geological phenomenon, so more conclusive evidence is needed before this can be confirmed.

Thought to be the oldest animal predator, at around 560 million years old, Auroralumina attenboroughii is approximately 20 million years older than the next oldest-known examples (ancient jellyfishes). It is currently represented by just a single 20-cm-tall (8-in) specimen discovered in 2007 on a slab of quarry siltstone within the Bradgate Formation at Charnwood Forest in Leicestershire, UK. This specimen consists of two bifurcating polyps enclosed in a rigid, polyhedral, organic skeleton (also making it the oldest-known animal with a skeleton), with evidence of simple tentacles. The new species was first described in Nature Ecology & Evolution on 25 July 2022 and was named in honour of the British naturalist and TV presenter Sir David Attenborough.