New Scientist - USA (2019-11-09)

40 | New Scientist | 9 November 2019

uncannily like modern creatures. “We used

to be obsessed with the Ediacaran-Cambrian

boundary,” says Simon Darroch at Vanderbilt

University in Tennessee. “Now it’s looking

smoother than previously thought.”

Perhaps the most compelling clues

come from rocks in Newfoundland, Canada,

that contain traces of the earliest Ediacaran

communities. Here, you don’t just stumble

on the occasional nicely preserved

specimen – you walk over bedrock exposures

120 square metres in area that each contain

thousands of fossils.

Each giant slab is a Pompeii-like snapshot

of the deep-sea floor community as it

was 570 million years ago. “It’s absolutely

astonishing in terms of preservation,” says

Emily Mitchell at the University of Cambridge.

It is what the rocks reveal about Ediacaran

organisms that really surprised her, though.

Mitchell and her colleagues mapped the size

and distribution of fossils of an oval-shaped

rangeomorph called Fractofusus. This

Ediacaran grew up to 40 centimetres in length

and was covered in peculiar, fractally repeating

pleats. The data, published in 2015, showed that

the largest individuals were scattered randomly

across the ancient sea floor, surrounded by

halos of smaller and smaller individuals.

This suggests that Fractofusus reproduced in

a sophisticated way. It generated waterborne

offspring that would drift and land on an

empty bit of sea floor. Then, as the offspring

developed into adults, each would form a

series of tentacle-like fingers, the ends of which

would then grow into a clone of the adult.

Surprisingly modern

That is a little like how certain modern

deep-sea animals such as sponges and corals

reproduce, says Mitchell. Fractofusus may

have had a fractal-like anatomy unlike that

of any modern animal, but it apparently

reproduced like some of today’s animals

do. That might hint that it was related to

those animals, although Mitchell says we

can’t rule out the possibility that Fractofusus

was instead related to fungi, which sometimes

also reproduce this way.

In any case, it isn’t just Fractofusus that

behaved surprisingly like a modern animal.

Another Ediacaran organism called Kimberella

left behind tracks that suggest it trundled

around, grazing on microbial mats on the

sea floor, which is a strikingly animal-like way

to behave. Darroch and his colleagues have

used computer models to show that another

Ediacaran, Tribrachidium, probably fed on

suspended particles, just as many modern

shellfish do. In a sense, it doesn’t even matter

whether these Ediacaran organisms were

animals or not: they were behaving and

reproducing like modern marine animals do,

which suggests that Ediacaran ecology was

more like today’s than we previously thought.

There might also be good reason to believe

that at least some strange Ediacaran organisms

really were animals. The strongest evidence

for this came last year. Ilya Bobrovskiy at the

Australian National University in Canberra

and his colleagues analysed the chemistry of

rocks containing an Ediacaran organism called

Dickinsonia. The rock around the fossils had

a chemical signature associated with algae,

which would make sense because the shallow

sea floor on which Dickinsonia lived was

probably coated in mats of algae. But the

molecules within the fossils themselves

included a particular type of steroid that

is produced only by animals, implying that

Dickinsonia was an animal.

These conclusions don’t necessarily lead

to a defusing of the Cambrian explosion.

Some people suspect that the Ediacaran

animals didn’t give rise to modern ones.

635 Million years ago 539 485

Kimberella

Yilingia

Pushing back the clock

We used to think that animals burst onto the scene in an abrupt “explosion” of complex life

during the Cambrian period. However, new discoveries are revealing animal-like creatures

in the earlier Ediacaran period

Fractofusus Marella

Anomalocaris

Hallucigenia

Reproduced much like

modern deep-sea

animals like sponges

Trundled around

the sea floor

grazing on

microbial mats

Segmented creature with

primitive legs, possibly an

early arthropod

Ediacaran Cambrian

“ There probably

wasn’t a Cambrian

explosion worthy of

the name after all”