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GSA TODAY | NOVEMBER 2016 Stump, 1977). In addition, the advent of predation added selective
pressure to drive infaunalization (Dzik, 2007), expanding biotur-
bation vertically, reducing the availability of matground substrate
upon which many Ediacara organisms grew, and advancing the
ecosystem engineering feedback. The gregarious habit of some
vermiform taxa may have additionally served as an antipredatory
strategy and propagated ecosystem engineering via sediment
baffling. It is important to note that the suggested mass extinction
of the Ediacara biota in the context of our wormworld model is an
ecologically driven event rather than an environmentally driven
cataclysm akin to more recent (Phanerozoic) mass extinctions,
and thus may have been comparatively protracted—as evidenced
by Ediacara holdovers in the early Cambrian (Conway Morris,
1993; Hagadorn et al., 2000; Jensen et al., 1998). Nonetheless,
whereas the static synecology and comparatively passive feeding
modes of the classic Ediacarans had once emplaced a boundary on
evolutionary possibility, the successful expansion of innovative
traits of herbivory and carnivory, and their causal ties to infaunal- permitted the establishment of more oxygen-demanding motile
ization, reef-building, and biomineralization, permitted a new
scaling of this bounding “right wall” (sensu Knoll and Bambach,
2000) as realized by the organisms of the wormworld fauna. Over
time, the evolutionary breakthroughs conveyed by these neoteric
organisms, including novel strategies, behaviors, and physiologies,
increased the heterogeneity of benthic ecosystems, allowed for
enhanced exploitation of resources, and established insurmount-
able increases in ecospace that ultimately signaled the curtain call
for the Ediacara-type guilds.
THE SEARCH FOR A TRIGGER
The stark pattern of the Cambrian Explosion has steered many
to identify a “trigger” (see reviews by Conway Morris, 2000;
Erwin et al., 2011; Marshall, 2006; Xiao, 2014; Zhang et al., 2014).
Previously proposed triggers can be categorized into three broad
types (Erwin, 2015b): genetic, ecological, and environmental. For
instance, a few examples include [genetic] the origin of the genetic
toolkit for animal body plans; [ecological] bioturbation, preda-
tion, roughening of fitness landscapes, and adaptive radiation
following an end-Ediacaran extinction; and [environmental] the
Snowball Earth glaciations, increasing ocean oxygenation, and
other dramatic seawater chemistry changes. While this is not the
appropriate forum for an exhaustive discussion of triggers, we will
briefly review three recent but distinct iterations of trigger
hypotheses, centered on changing ocean chemistry, ties between
oxygenation and carnivory, and nutritional incentive.
The Great Unconformity
Emphasizing the role of global environmental change, Peters and
Gaines (2012) suggest that the Sauk transgression over the Great
Unconformity flooded the continents and delivered excess ions to
the ocean, necessitating a physiological response to intracellular
calcium toxicity in the form of metazoan biomineralization. This
evolutionary milestone, in conjunction with the expansion of
shallow marine environments, promoted the explosive radiation of
marine animals. The dominance of non-biomineralizing taxa in
Burgess Shale–type biotas (Chen and Zhou, 1997; Conway Morris,
1986), however, implies that the Cambrian Explosion would have
occurred with or without biomineralization (Butterfield, 2003). The
temporal linkage of metazoan biomineralization and diversification
8
begs the question as to whether they can be explained by the same
trigger or are instead compounded causes and consequences of the
immediately preceding and supervening events. Comparably, the
initiation of the Mesozoic plankton radiation and contemporaneous
expansion in planktic biomineralization (Knoll, 2003) proceeded
without the presence of a global unconformity, echoing a disconnect
between the oceanic influx of ions and evolutionary radiation.
The Hypoxia Hypothesis
Exploring the relationship between oxygen minimum zones
and polychaete worm-feeding ecology in modern oceans as an
analogue for terminal Ediacaran benthic communities, Sperling
et al. (2013) propose an eco-environmental trigger. Their work
emphasizes a series of requirements. First, an increase from
suboxia to hypoxia (and stabilization of minimum oxygen
content) removed a key limiting factor upon animal body size and
life modes. This resulted in increasing trophic complexity from
expanding diversity and abundance of carnivorous taxa—and
ultimately drove the evolutionary arms race resulting in the
Cambrian Explosion.
The Savannah Hypothesis
While the former two hypotheses note the importance of
predation and antipredatory (as presumed from the importance
of biomineralization) strategies, the Savannah hypothesis of Budd
and Jensen (2015) posits that the diversification of metazoan
tracemakers was driven by resource heterogeneity from patches of
post-burial Ediacara biota. Shallow burrowing behavior was the
key evolutionary innovation that allowed exploitation of this
resource. In conjunction with environmental heterogeneity and
patch dynamics, such burrowing would have spurred the radia-
tion of the bilateria—previously also tied to the advent of hard
parts (e.g., Bengtson, 2004).
While there are broad similarities between the Savannah model
and our proposed wormworld model, there are some key differ-
ences, which offer two testable hypotheses: (1) the majority of
classic Ediacara-type organisms are interpreted by Budd and
Jensen (2015) as stem metazoans and, thus, there would have
been no biotic crisis among classic Ediacarans prior to the
Cambrian (contra Laflamme et al., 2013); and (2) there should be
clear positive spatial and temporal associations between Ediacara
biota and trace fossils supporting the proposition that early bilat-
erians exploited decaying Ediacarans as a food source. Regarding
the first prediction, on local-, regional-, and global-scales, latest
Ediacaran fossil communities have been shown to be depauperate
with respect to many iconic and readily preserved forms (Boag et
al., 2016; Darroch et al., 2015; Xiao and Laflamme, 2009),
supporting an extinction/biotic replacement scenario. With
respect to the second prediction, available data suggest that direct
associations between Ediacara-type organisms and bilaterian
trace fossils are rare. Metazoan traces occur most frequently as
isolated monospecific assemblages lacking Ediacara biota,
suggestive of niche partitioning (Darroch et al., 2016). In China,
vermiform trace fossils are found in the same stratigraphic
sections as Ediacara biota, but only sometimes on the same beds,
suggesting that their co-occurrence within communities was
limited or that the classic Ediacara-type forms were relatively