Ikaria Was Called the Oldest Bilaterian. It Is an Oval

Chapters

1. 0:00 A dent no longer than seven millimetres
2. 0:22 Three questions
3. 0:35 The ancestor biologists predicted
4. 1:00 Ikara, the meeting place
5. 1:25 A furrow with no owner
6. 1:49 A correlation, not a date
7. 2:12 An animal of seven millimetres
8. 2:32 A front and a back
9. 2:51 An impression in the sand
10. 3:07 Flattened by burial
11. 3:22 108 on one bed
12. 3:43 The fossil does not carry the claim alone
13. 4:06 606 trails, 112 bodies
14. 4:22 What a P of 0.448 says
15. 4:43 One body beside one trail
16. 5:12 The authors' own verbs
17. 5:40 Oldest in South Australia
18. 6:10 A later chapter, with Cloudina
19. 6:33 The release and the paper
20. 6:53 Cylindrical, with muscles?
21. 7:11 Seen in the rock, read from the trail
22. 7:39 Measured, argued, open
23. 8:01 The easiest thing to overlook
24. 8:25 Next: a hole in a tube

Transcript

A dent no longer than seven millimetres 0:00

On the underside of a sandstone bed in South Australia, there is a dent no longer than seven millimetres. It is an oval. One end is wider than the other. In 2020, that dent was named one of the oldest bilaterians found in South Australia. The rock shows an oval. The rest of the claim has to be argued.

Three questions 0:22

This is Ikaria wariootia. By the end, you will know what the oval itself shows, what the name rests on besides the oval, and which parts of the headline never appeared in the paper.

The ancestor biologists predicted 0:35

A bilaterian is an animal with a left and a right, a front and a back. Insects, fish and humans are all bilaterians. Biologists working from living animals had predicted what the common ancestor of the group looked like: small, simple, able to move, with a front end unlike the back, and few senses. The question was whether the rock holds anything that fits.

Ikara, the meeting place 1:00

The rock is the Ediacara Member of the Rawnsley Quartzite, in South Australia. It is made of shallow-sea sandstone beds, fifty to five hundred metres below the base of the Cambrian. The fossils come from a site called Nilpena, where the landmark in view is Wilpena Pound. The Adnyamathanha people call it Ikara, the meeting place, and the animal was named for it.

A furrow with no owner 1:25

The clue was already in those beds. Across them runs a thin, unbranched furrow called Helminthoidichnites. Something small once moved through the sediment and left it. In these sections it appears more than a hundred metres below the first Kimberella, so it comes from low in the member. The furrow had a shape. No body fossil had been matched to it.

A correlation, not a date 1:49

The age is a correlation, not a date. The paper says plainly that there are no radiometric dates for the Ediacara Member. Its fossils overlap with those of the White Sea region of Russia, and that places the member between five hundred and sixty and five hundred and fifty-one million years ago. Ikaria first appears within that range, or possibly earlier.

An animal of seven millimetres 2:12

The animal itself is tiny. Along the long axis, the specimens run from one point nine to six point seven millimetres. The short axis runs from one point one to two point four. The authors think seven millimetres was about the limit, because nothing larger has the same shape.

A front and a back 2:32

Under a laser scanner, the shape is clear. It is an elongate oval. One end is wider and more broadly curved. The other is smaller and more tightly curved. The authors read that as a front and a back, with the left and right sides as mirror images.

An impression in the sand 2:51

What you see on the stone is not the animal. It is an impression pressed into the underside of a sandstone bed. Geologists call that negative hyporelief. The animal is gone. The sand kept its outline.

Flattened by burial 3:07

The depth of the impression is always less than its width. It runs from six tenths of a millimetre to one point six. So the body was flattened, and the shape that survives was moulded partly by burial, not only by the animal.

108 on one bed 3:22

There are plenty of them. The authors identified one hundred and eight on a single bed surface, and nineteen more from loose blocks at several places. In the same member lie bigger animals, like Dickinsonia. The paper treats Dickinsonia as probably not a crown-group bilaterian, though it moved and left traces.

The fossil does not carry the claim alone 3:43

Here is the problem. A flattened oval a few millimetres long, with one end wider than the other, could be many things. The authors say it directly. Given how simple Ikaria is, they could not confidently assign it to the bilaterians, or even to the animals, without its relationship to the trail. The fossil does not carry the claim alone.

606 trails, 112 bodies 4:06

So they measured. They took the widths of six hundred and six Helminthoidichnites, and of a hundred and twelve Ikaria. Every body width fell inside the range of the trail widths. And no body was wider than the widest trail.

What a P of 0.448 says 4:22

A statistical test found no significant difference between the two sets of sizes. The value was zero point four four eight. But failing to find a difference is not proof that there is none. And the comparison sets the average width of the trails against the maximum width of the bodies. It is a fit, not a proof.

One body beside one trail 4:43

Then comes the rarest piece of evidence. The paper shows one specimen found beside a trail. Not at its end: beside it. The authors explain the gap as the animal moving up off the bed between the end of the trail and where it came to rest. They add that even a body at the end of a trail would be hard to tell apart from it, because both are impressions in the same sand. Compare Yilingia, from our last episode, a body joined to its own trail.

The authors' own verbs 5:12

The authors are careful with their verbs. They write that they propose Ikaria as the maker of Helminthoidichnites. They add that it is not necessarily responsible for every example. And they note that moving and leaving a trace is not unique to bilaterians. Cnidarians, the group with jellyfish, and single-celled protists can do it too. They rule those out by size, and by the shape of the furrows.

Oldest in South Australia 5:40

Where does it sit? Older claims exist, and each one carries a caveat. Burrows from Uruguay, once read as older than five hundred and eighty-five million years, have uncertain age. Complex fossils from older deposits, like Mistaken Point, are read as cnidarians. Yilingia, from the last episode, is younger, larger and more complex. That is why the title says oldest in South Australia, not oldest anywhere.

A later chapter, with Cloudina 6:10

One more neighbour matters for the next episode. In Brazil, small traces lie thirty to forty metres above an ash dated to five hundred and fifty-five million years, in close association with Cloudina. The paper reads them as younger than Ikaria. So the tube we meet next time belongs to a slightly later chapter of the same story.

The release and the paper 6:33

Then came the university release. It called Ikaria the first ancestor on the family tree that holds most familiar animals, humans included. It called the animal the earliest bilaterian, and it dated the deposits at five hundred and fifty-five million years. The paper says something narrower.

Cylindrical, with muscles? 6:53

The release also described a cylindrical body with faintly grooved muscles. The paper says the fossils are compressed, and it describes no muscles. The age comes from fossils in Russia, like Kimberella, from the White Sea region, not from a date in Australia.

Seen in the rock, read from the trail 7:11

So what does the rock show? An oval whose ends differ. And in a few rare specimens, two to five divisions, which the authors call potential evidence of a modular body. A gut is not on that list. The mouth, the anus and the gut are inferred, from the way the sediment was moved, and the authors say they are unlikely to be preserved. The body is an oval. The anatomy is read from the trail.

Measured, argued, open 7:39

So here is what the rock allows. Measured: an oval no longer than seven millimetres, with one end unlike the other, and a hundred and eight of them on one bed. Argued: that it made the trails, and so belongs among the bilaterians. Open: whether it made all of them or only some, and what its relatives looked like.

The easiest thing to overlook 8:01

If the reading holds, the animal that biologists predicted was small, simple and mobile. And it was already crawling across a sea floor in South Australia, between five hundred and sixty and five hundred and fifty-one million years ago. It is a reminder that the first of anything may be the easiest to overlook: a seven-millimetre oval, beside a furrow nobody could explain.

Next: a hole in a tube 8:25

Next time, a hole. Cloudina is a tube of mineral, one of the first animals to build a skeleton. In 2003, researchers counted almost one in five of nearly a hundred tubes with a neat round boring through the wall. A second kind of tube, from the same rock, had none. The question is whether that is the oldest predator, and what a hole can honestly say about the thing that drilled it.

Description and sources

Ikaria wariootia (Evans, Hughes, Gehling & Droser, PNAS 117:7845, 2020), from the Ediacara Member of the Rawnsley Quartzite at Nilpena and Bathtub Creek, South Australia — undated itself; the paper places the member at 560–551 Ma by correlation with the White Sea, with no radiometric dates. A flattened oval 1.9–6.7 mm long, with one end wider and more broadly curved, preserved as an impression on the underside of sandstone beds: 108 on one bed surface and 19 loose. The authors say that, given how simple it is, it could not confidently be placed among the bilaterians, or even the animals, without its relationship to the trail Helminthoidichnites; they propose it as the trail's maker, from a size match (a test that fails to reject equality, P = 0.448) and the stratigraphy. Only one specimen is figured beside a trail, not at its end. Mouth, anus and gut are inferred, not preserved. A university release went further than the paper on ancestry, age and muscle.

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The Deep Time Record, Vol. 01 — the cores, the curves and the clocks

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PRIMARY SOURCES

• Evans, Hughes, Gehling & Droser 2020. PNAS 117(14):7845–7850, Discovery of the oldest bilaterian from the Ediacaran of South Australia. https://doi.org/10.1073/pnas.2001045117
• UCR Today, 23 March 2020: Ancestor of all animals identified in Australian fossils (Holly Ober). https://trilobyte.ucr.edu/news/2020/03/23/ancestor-all-animals-identified-australian-fossils
• Martin et al. 2000. Science 288:841, age of Neoproterozoic bilaterian body and trace fossils, White Sea (abstract read). https://doi.org/10.1126/science.288.5467.841
• Chen, Zhou, Yuan & Xiao 2019. Nature 573:412 (Yilingia). https://doi.org/10.1038/s41586-019-1522-7
• Hua, Pratt & Zhang 2003. Palaios 18(4–5):454–459, Borings in Cloudina shells (abstract read; next episode). https://doi.org/10.1669/0883-1351(2003)018<0454:BICSCP>2.0.CO;2

ARCHIVE IMAGES — author, date, archive and licence.

• Jesse Allen — 2010-05-16 | Wikimedia Commons | Public domain
• James St. John — 2016-04-14 01:13 | Wikimedia Commons | CC BY 2.0
• James St. John — 2006-08-16 21:15 | Wikimedia Commons | CC BY 2.0
• James St. John — 2006-08-18 19:53:21 | Wikimedia Commons | CC BY 2.0
• James St. John — 2016-10-04 22:50 | Wikimedia Commons | CC BY 2.0

The 2 generated plates were made for this channel with FLUX.1-schnell. No person is depicted in any of them.

Deep Time Archives rebuilds the pre-human Earth from the physical record — outcrops, drill cores, golden spikes, catalogued specimens. Every episode starts from an object you can go and check, and the sources are listed above.

About the images. The fossil itself is not shown in a photograph: the only images of Ikaria in open archives are artists' restorations under a share-alike licence, and the paper's figures are not openly licensed. What the paper measures (a flattened oval of 1.9–6.7 mm, one end wider, ends that differ) is drawn in code and kept schematic. Every fossil or rock shown is not Ikaria: Dickinsonia, a sample of Ediacara Member quartzite, rippled Rawnsley Quartzite and Kimberella from the White Sea (James St. John, CC BY 2.0), and Wilpena Pound seen from orbit (Jesse Allen, NASA Earth Observatory, public domain). The two sandy shore scenes are AI-generated reconstructions of places; no organism or trail is generated. This video is labelled as containing altered or synthetic content.

#DeepTime #EarthHistory #Ediacaran #Paleontology #Evolution