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(Amthor et al., 2003); the Gaojiashan Lagerstätte, China (Cai et
al., 2010; Cortijo et al., 2015a); Estena River, Spain (Cortijo et
al., 2010; Cortijo et al., 2015b); and elsewhere (Hagadorn and
Waggoner, 2000; Hofmann and Mountjoy, 2001; Zhuravlev et
al., 2012). The best known from this group is the terminal
Ediacaran index fossil and one of the first biomineralizers,
Cloudina (Fig. 2A). The cloudinids and similar taxa (Figs. 2B
and 2C) remain phylogenetically enigmatic because of a lack of
preserved soft-tissues (Schiffbauer et al., 2014); although, based
on tube morphologies and growth patterns, these organisms
have drawn comparisons with modern worms or anthozoan
corals (Cai et al., 2014; Hua et al., 2005; Penny et al., 2014).
While the presence of worm-like animals was previously estab-
lished from the ichnofossil record, the cloudinids and similar
taxa provide the first vermiform body fossils. Perhaps more
importantly, the addition of 2° consumers of the third trophic
tier is marked by site-/taxon-specific and size-selective preda-
tory drillholes in the biocalcified tubes of Cloudina (Bengtson
and Yue, 1992; Hua et al., 2003). This novel feeding strategy
would have presented a severe ecological pressure; indeed, ~20%
of individuals in some populations were drilled by predators
(Hua et al., 2003). In addition to the added trophic level, the
presence of these latest Ediacaran drillholes signifies three major
evolutionary themes: (1) prey selectivity indicates the neural
sophistication of the predator; (2) failed attempts (incomplete
drillholes) demonstrate that mineralized exoskeletons impeded
predators; and (3) predation pressure may have played a signifi-
cant role in the proliferation of mineralized skeletons. With such
a sophisticated predatory mechanism recorded in the tubes of
Cloudina, it is likely that the origin of predation preceded this
first occurrence. Given the similarities between drillholes in
Cloudina and modern shelly prey, the organism responsible for
Ediacaran drillholes was mechanically, if not phylogenetically,
comparable to Phanerozoic drilling gastropods using a radula-
like structure to rasp the prey skeleton. With Kimberella inter-
preted as a radula-bearing stem-group mollusk (Gehling et al.,
2014), comparable physiological machinery for drilling preda-
tion may have already been in place ~10 m.y. before the first
drillholes appear in the fossil record. However, from this fossil
first occurrence and with improved preservation as a conse-
quence of taphonomically robust biominerals, the record of
macropredation is observed to increase in both frequency and
predator/prey diversity well into the Phanerozoic (Huntley and
Kowalewski, 2007).
THE IMPORTANCE OF THE WORMWORLD
While local facies and environmental controls have been shown
to be of importance to understanding and appreciating the
ecological contexts of classic Ediacaran communities (Gehling
and Droser, 2013), currently available diversity data imply that the
terminal Ediacaran is characterized by a considerably reduced
Ediacara biota (Boag et al., 2016; Darroch et al., 2015; Shen et al.,
2008; Xiao and Laflamme, 2009). Concurrently, eumetazoan
vermiform body fossils and bilaterian trace fossils show an
increase in diversity (Cai et al., 2014; Cai et al., 2011; Carbone and Ediacarans show an apparent decline approaching the Ediacaran
Narbonne, 2014; Wood and Curtis, 2015), offering glimpses into
more modern ecosystem dynamics. The coupling of these
records—the expansion of the wormworld fauna and diversity
loss of classic Ediacarans (Laflamme et al., 2013; Xiao and
Laflamme, 2009)—suggests that vermiform metazoans may have GSA TODAY | www.geosociety.org/gsatoday
played a role in displacing preexisting biotic components in the
terminal Ediacaran marine ecosystems (Darroch et al., 2015;
Darroch et al., 2016). However, much work remains to (1) estab-
lish what (if any) biotic interactions occurred between these two
broad groups; and (2) determine how many Ediacaran morpho-
clades actually represent metazoan lineages, such that further
discussion does not become mired in artificial and polyphyletic
groupings (MacGabhann, 2014).
Contrary to the influence of competition in shaping
Ediacaran communities, vermiform organisms witnessed an
expansion in ecological strategies through the onset of three new
life modes: (1) macropredation (Bengtson and Yue, 1992; Hua et
al., 2003); (2) reef-building (Penny et al., 2014); and (3) motile
grazing (Carbone and Narbonne, 2014). While it can be difficult
to quantify competition in fossil communities, evidence for
niche partitioning can be indicated by over-dispersion of body
size (e.g., Huntley et al., 2008) and spatial arrangement
(Clapham and Narbonne, 2002; Ghisalberti et al., 2014). In a
sessile epibenthic ecosystem relatively free of predation and with
few motile organisms, the availability of space and nutrients on
and above the matground must have been limiting factors
shaping community structure. In contrast, nutrient acquisition
was presumably non-limiting for those few mobile organisms
feeding on the microbial substrate (Tarhan et al., 2015), at least
early in the development of motility and herbivory. Thus, it is
likely that the evolutionary importance of competition for
resources was not equal across environments and trophic tiers,
and that factors such as predation and disturbance were more
influential in shaping wormworld communities. Escalation,
organisms responding evolutionarily to their enemies (Vermeij,
1987), places an evolutionary premium on predators rather than
competitors. Consequently, counter to the largely sessile guilds
of the Ediacaran, the evolution of motility and bioturbation,
grazing by 1° consumers, and predation by 2° consumers, marks
the most significant difference between earlier Ediacaran and
Cambrian ecosystems (Butterfield, 2007). The wormworld fauna
thus captures an explicit tipping point where predation and
disturbance became dominant ecological factors.
We suggest that the wormworld fauna and the ecological
complexities that they ushered in led to the displacement and
eventual biotic replacement (Darroch et al., 2015) of the classic
Ediacara-type communities. These vermiform organisms were
equipped with innovative adaptations of active feeding modes and
sediment restructuring capabilities, biomineralized armament
against predators, generalist and opportunist adaptability to
varying substrates (Cai et al., 2014), sexual and asexual reproduc-
tion for enhanced dispersal (Cortijo et al., 2015a), resilience to
environmental disturbance (Cai et al., 2010), and presumably
high fecundity and rapid achievement of sexual maturity. The
wormworld organisms were likely more adept at attaining
ecological success over the comparatively ineffectual occupation
of niches by the classic Ediacara biota. Indeed, while classic
-Cambrian transition (Darroch et al., 2015), several reports
indicate that at least some terminal Ediacaran tubicolous organ-
isms (including Cambrotubulus, Platysolenites, Cloudina, and
Sinotubulites) may traverse this boundary (e.g., Kontorovich et al.,
2008; McMenamin, 1985; Rogov et al., 2015; Yochelson and
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