Sсiеntifiс Аmеricаn (2019-06)

June 2019, ScientificAmerican.com 29

the communities of animals and plants that live on

open surfaces differ from those that occupy these

more hidden areas such as caves, crevices or under­

hangs. Our Ediacaran fossil discoveries indicate that

this distinction is as old as animal reefs themselves.

These observations are significant because reef

building represents an important ecological innova­

tion. By growing closely together and even cementing

to one another, individuals can become mechanically

stronger, rise above the seafloor away from competi­

tors, enhance feeding efficiency and gain protection

from predators. Like the earliest skeletons, then, the

appearance of reefs in the Ediacaran fossil record may

signal rising, complex ecological pressures. The Cam­

brian explosion, and indeed an arms race between

predator and prey, had already begun.

THE EDIACARAN WORLD

by the mid­ 2010 s it was becoming clear that the Cambri­

an did not mark the sudden, dramatic departure from

the Ediacaran that experts long envisioned. Not only

had researchers begun to amass evidence that animals

started evolving skeletons and building reefs earlier

than traditionally thought, but we had also developed

ecosystem models showing that Ediacaran animal

communities shared many ecological traits with Cam­

brian ones. The “explosion,” we were learning, had a far

longer fuse than was previously recognized.

Then, a few years ago, some key discoveries in Sibe­

ria and China blurred the Ediacaran and Cambrian

worlds even further. A group of researchers from China

and Germany found that Cloudina persisted into the

Cambrian. And my group, together with colleagues

from Russia and China, found fossils long thought to

be unique to the Cambrian in Ediacaran rocks. These

findings underscored to us that to solve the mystery of

the Cambrian explosion we had to figure out the

dynamics of the Ediacaran world in which these ani­

mals originated.

SOURCE: “INTEGRATED RECORDS OF ENVIRONMENTAL CHANGE AND EVOLUTION CHALLENGE THE CAMBRIAN EXPLOSION,” BY RACHEL WOOD ET AL., IN The possible role of shifting oxygen availability is

NATURE ECOLOGY & EVOLUTION

, VOL. 3; APRIL 2019

Illustration by Franz Anthony ( fossils)

Carbon Isotope Variations

12

8

4

0

-4

-8

-12

Ocean Chemistry

Over Space and Time

High

Shallow

Deep

Low

Mean

Millions of Years Ago: 660 640 620 600 580 560 540 520 500

Millions of Years Ago:

480

Cryogenian Ediacaran Fortunian

CAMBRIAN

PALEOZOIC

PROTEROZOIC

PRECAMBRIAN

Oxygen-rich

Iron-rich, oxygen-deficient water (tan) water (blue)

Regional variability (dots)

Widespread (solid)

Oxygen-rich water (blue)

Uncertain Uncertain Uncertain

Hydrogen sulfide–rich

water (red)

Lantianella laevis

Treptichnus pedum (trace fossil)

Charnia masoni

660 640 620 600 580 560 520 500

Cloudina

Ice ages

Fossil Evidence

Surprisingly sophisticated Ediacaran creatures include

Lantianella laevis, a possible forerunner of today’s

jellies; Charnia masoni, which appears to have grown

rooted to the seafloor and absorbed nutrients from

the surrounding water; Cloudina, one of the earliest

animals with a skeleton; and Treptichnus pedum, the

name given to the distinctive traces left behind by an

unknown burrowing animal.

B C D

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