Sсiеntifiс Аmеricаn (2019-06)

28 Scientific American, June 2019

discovered in Namibia in 1972 and was long assumed

to have grown attached to the seafloor. But in the past

few years researchers have identified many new speci­

mens of Cloudina from sites around the world that

have changed that view. My team’s work on specimens

from Namibia has shown that Cloudina had a variety

of growth styles. It could attach to mats made of mi ­

crobes that bound the soft sediment of the seafloor, or

it could anchor itself to layered mounds of cyanobacte­

ria. Most important of all, Cloudina individuals could

actually cement themselves to one another to form a

reef. This finding has established Cloudina as one of

the oldest reef­building animals, pushing back the

record of this way of life by some 20 million years.

Whether Cloudina was related to modern reef

builders such as corals remains uncertain. But we do

know that like reef­building corals, it lived in proximi­

ty to a number of other animals. Hints of this intimate

association have come from other skeletal fossils

found in rocks of the same age as those that contain

Cloudina fossils. A creature called Namacalathus,

known from fossil localities around the world, appears

to have been one of Cloudina’ s consorts. Its skeleton

was up to 50 millimeters long, composed of a delicate,

thin­walled stalk and a cup with a central opening at

the top and several openings around the sides. The ani­

mal’s soft tissue was probably mainly inside the cup,

although it is never preserved. Fossils of Namacala-

thus indicate that it grew rooted to microbial mats,

often near Cloudina.

Namapoikia, a creature known only from fossil

localities in Namibia, also fraternized with Cloudina.

This animal is remarkable for its large size—up to one

meter in diameter—and robust skeleton. On the basis

of its growth form, we think Namapoikia was a sponge

and so would have had an internal skeleton, in con­

trast to the external skeletons Cloudina and Namacal-

athus probably had. Intriguingly, Namapoikia grew

within the hidden places of reefs, encrusting the verti­

cal walls of open cracks and fissures. In modern reefs

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

Before the Cambrian Explosion

Many key innovations in animal evolution that were traditionally thought to have originated

in the Cambrian actually trace back much further in time to the Ediacaran. For example,

the first animals with skeletons debuted during this earlier period. Their ability to produce

mineralized tissue probably evolved as a means of protection from predators. Integrating

the fossil and geochemical records spanning the time between 670 million and 480 million

years ago reveals clues to the environmental factors driving this early evolutionary activity.

Geochemical Evidence

Animals need oxygen to survive.

The evolutionary diversification that

took place during the Ediacaran

occurred under wildly fluctuating

oxygen levels in the world’s oceans.

Carbon isotopes from Ediacaran

rocks show that the carbon cycle

was unstable and in a state of flux.

Analyses of the iron compounds in

these rocks, meanwhile, show that

dissolved oxygen in the oceans

probably reached a threshold or a

series of thresh olds in the Ediacaran

that allowed animals to diversify by

meet ing their increased metabolic

demands as they became more

active. Researchers now believe

the seas became progres sively

oxygenated not as one slow, gradual

increase but in a series of episodes

( A , B , C and D ) that appear

to coincide with the carbon isotope

varia tions. This trend continued

through out the Ediacaran and

probably well beyond.

A