Sсiеntifiс Аmеricаn (2019-06)

30 Scientific American, June 2019

one dynamic that researchers have been particularly

keen to pin down. Animals need oxygen, so a central

debate over the past few years has been to understand

whether at some point in the time spanning the Edia­

caran and Cambrian oxygen levels rose be yond a cer­

tain critical threshold, allowing animals to flourish. The

question is more complicated than it might seem

because animals do not all have the same oxygen re ­

quire ments. Simple, immobile creatures, such as spong­

es, may need less oxygen than mobile animals, and they

certainly require far less of the stuff than active, fast­

swimming predators do. We have borne this variation

in mind in the course of our investigations.

Fortunately for us, many new geochemical meth­

ods for estimating how much oxygen existed in these

ancient seas have been developed in recent years. One

especially powerful technique—Fe speciation—har­

nesses the characteristics of the various compounds of

iron, which behave differently depending on whether

oxygen is present or not. This method allows us to see RACHEL A. WOOD

WRITTEN IN STONE: Cloudina is one of the oldest organisms with an external
skeleton, known from fossils that preserve its delicate, tubular armor ( 1 ).
Cloudina individuals could cement themselves to one another, forming reefs.
Another early skeletal form is Namacalathus, whose cup-shaped hard parts
are typically preserved en masse ( 2 ). Cloudina and Namacalathus are often
found together ( 3 ). Namapoikia, an early sponge, was another of Cloudina’ s
associates, one that grew in the hidden crevices of the reef ( 4 ).

1 2

3 4