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The Latest Ediacaran Wormworld Fauna: Setting the
Ecological Stage for the Cambrian Explosion
GSA TODAY | NOVEMBER 2016 James D. Schiffbauer*, John Warren Huntley, Gretchen R.
O’Neil**, Dept. of Geological Sciences, University of Missouri,
Columbia, Missouri 65211, USA; Simon A.F. Darroch, Dept. of Earth
and Environmental Sciences, Vanderbilt University, Nashville,
Tennessee 37235, USA; Marc Laflamme, Dept. of Chemical and
Physical Sciences, University of Toronto Mississauga,
Mississauga, Ontario L5L 1C6, Canada; Yaoping Cai, State Key
Laboratory of Continental Dynamics and Dept. of Geology,
Northwest University, Xi’an, 710069, China
ABSTRACT
As signposted by the fossil record, the early Cambrian period
chronicles the appearance and evolutionary diversification of
most animal phyla in a geologically rapid event, traditionally
termed the Cambrian Explosion. The uniqueness of this event
pleads for a cause, and over the years, numerous biotic and abiotic origin of animals temporally removed from their morphological
factors have been offered as possible triggers. Many such explana-
tions, however, either fail to correspond in time or do not provide 2000; Xiao, 2014). In this case, the significant lag between the
a functional mechanism to explain the evolutionary pattern of
animal diversification. We support the notion that a series of
requisite biotic and abiotic events ushered in the Cambrian
Explosion, wherein each event was necessary for the implementa-
tion of later events but did not guarantee their occurrence. The
evolution of the terminal Ediacaran vermiform fauna was integral
in the construction of the Eltonian pyramid, fostered an escala-
tion of ecosystem engineering and macropredation, and repre-
sented a turning point in benthic ecosystems from those governed
primarily by competition for space and resources to those also
shaped by these novel pressures.
INTRODUCTION
Relative to its mass, the biosphere disproportionately impacts
other components of the Earth system. It oxygenates the oceans
and atmosphere, regulates global geochemical cycles, and influ-
ences rates and patterns of global change. Life leaves distinctive
signals in the rock record, and few are more striking than those at Competition and Nutrient Acquisition
the onset of the Phanerozoic—one of the most intensely studied
intervals in the geologic record. The Cambrian Explosion—as
famously revealed in the Burgess and Maotianshan shales—
represents the geologically abrupt (~25 million years [m.y.])
appearance and diversification of nearly all major metazoan
phyla. This proliferation of the roots of the animal phylogenetic
tree and rapid expansion of morphological complexity was one of
the more significant macroevolutionary events in life history and
GSA Today, v. 26, no. 11, doi: 10.1130/GSATG265A.1.
*Email: schiffbauerj@missouri.edu
**Now at North Dakota State University, Dept. of Geosciences, Stevens Hall, 1340 Bolley Drive #201, Fargo, North Dakota 58102, USA.
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coincided with a variety of global-scale biotic and abiotic changes
(Fig. 1; Briggs et al., 1992; Erwin, 2007)—some of which were
brought about by metazoan activities, while others elicited a
response by metazoans.
Molecular divergence time estimates (e.g., Erwin et al., 2011;
Peterson et al., 2008) suggest that the last common ancestor of all
animals evolved in the Cryogenian (ca. 800 Ma; although see dos
Reis et al., 2015, for caveats). The earliest interpreted stem-group
animals, however, are the ca. 600 Ma Doushantuo embryo-like
microfossils (Chen et al., 2014a; Yin et al., 2016), leaving a
200-m.y. interlude between the fossil and molecular records.
This hiatus between the estimated origin of Metazoa and their
first appearance in the fossil record highlights the growing real-
ization that the earliest stages of animal diversification were
neither truly Cambrian nor explosive—with the phylogenetic
and ecological diversification by a long fuse (e.g., Conway Morris,
establishment of the developmental toolkits necessary for the
origin of novelty and their later implementation and ecological
success can perhaps be attributed to the uniqueness of newly
developing animal ecosystems. Between the ignition of the fuse
and the subsequent evolutionary boom, three major eco-environ-
mental feedbacks (see Erwin et al., 2011) arose that helped to pave
the way for the Cambrian Explosion: (1) linkages between the
pelagic and benthic ecosystems; (2) expansion of ecosystem engi-
neering; and (3) metazoan macropredation. These feedbacks are
explored herein in the context of the terminal Ediacaran fossil
record of vermiform organisms. This “wormworld” biota—
comprised of various tubicolous body fossils (Figs. 2A–2C), such as
the cloudinids, and increasingly complex vermiform ichnofossils
(Figs. 2D–2F)—critically occupied a fundamental phase shift from
competition- to predation-governed marine benthic ecosystems.
BUILDING THE ELTONIAN PYRAMID
The classic Ediacara biota were unheralded in life history,
emerging ca. 578 Ma with new and complex multicellular
morphologies unlike anything seen before (Narbonne, 2005).
While some of these sessile and epibenthic curiosities have been
posited as stem-group animals (e.g., Budd and Jensen, 2015), for
the most part they lack convincing metazoan synapomorphies,
leaving their positioning within the tree of life unresolved. These
fossils, however, do provide clues as to how Ediacaran ecosystems