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intolerant to bioturbation (Chen et al., 2014b). This may indicate
that direct interactions between Ediacara organisms and motile
vermiform bilaterians became common only when forced by the
diversification of motile animals and expansion into new
ecospace. Possibly capturing a snapshot of this changeover, the
Wood Canyon Formation in the southwestern Great Basin of the
United States preserves traces, a diverse tubicolous fauna, and
classic Ediacarans in the same stratigraphic section (Hagadorn
and Waggoner, 2000).
SUMMARY—A SERIES OF SWITCHES?
These three trigger hypotheses, among many others, illustrate
the complexity of the Cambrian Explosion and the many factors
that must be considered in its explanation—exemplifying why the
identification of a lone trigger has been a thorny process.
Revisiting the three categories of triggers, the genetic toolkit for Antcliffe, J.B., Callow, R.H.T., and Brasier, M.D., 2014, Giving the early fossil
animal body plans is a requirement for the Cambrian Explosion,
but its likely Cryogenian origin demonstrates that it was not an
immediate trigger (Erwin, 2015a; Tweedt and Erwin, 2015). Purely
environmental triggers correlate with the timing of the Explosion
but cannot explain the evolutionary pattern, which is outwardly
better explained by ecological triggers that create novel evolu-
tionary opportunity (Butterfield, 2009; Sperling et al., 2013). With Bengtson, S., and Yue, Z., 1992, Predatorial borings in late Precambrian
regard to the discussed hypotheses, it is important to note their
overlap. Both the Great Unconformity and Hypoxia hypotheses
involve predatory and antipredatory strategies, and both the
Hypoxia and the Savannah hypotheses emphasize the role of
vermiform organisms (or more broadly, bilaterians) in increasing
predation, burrowing, and scavenging behaviors (with implica-
tions for neurological development; Budd and Jackson, 2016). The
wormworld model underscores the role of the terminal Ediacaran
vermiform fauna in significantly expanding the second trophic
tier, intensifying ecosystem engineering behaviors, and estab-
lishing antipredatory mechanisms. In conjunction, these develop-
ments commenced a suite of antagonistic coevolutionary
feedbacks between 1° and 2° consumers (i.e., the Red Queen
hypothesis; Van Valen, 1973) amid the emerging Phanerozoic-
style ecological landscape, ultimately setting the stage for the
Cambrian Explosion.
In sum, the Cambrian Explosion appears to be but a part of an
extended series of molecular, ecological, and environmental revo-
lutions spanning the late Neoproterozoic to early Paleozoic.
Therefore, the search for a lone trigger, by nature, does not
account for this complexity; instead, integrated constructs
accounting for taxonomic, morphological, and ecological expan-
sion facilitated by environmental changes and genetic toolkit
exaptations should provide more inclusive explanations for the
Cambrian Explosion. As such, we view the events of this interval
as a series of switches, wherein each switch served to fundamen-
tally alter and increase the complexity and functionality of ecosys-
tems. We view the step-wise building of the Eltonian pyramid, in
large part owing to the ecological and evolutionary novelties
conveyed by the rise of the wormworld fauna, as prime examples
of these switches—but only a part of a series of interconnected
switches. These other requisite steps must have included the
genetic toolkits for bilaterianism, the linkage of the pelagic and
benthic ecosystems, the increase of oxygen to support animal
metabolisms, and the creation of new habitats through both biotic
processes and those related to contemporaneous supercontinent
breakup—which collectively enabled, but did not guarantee, the GSA TODAY | www.geosociety.org/gsatoday
rise of animals yet to come.
ACKNOWLEDGMENTS
For D.L.S. We thank G. Dickens and three anonymous reviewers for
constructive feedback, and we are grateful for the conversations with
numerous colleagues that helped shape our thoughts on this topic.
REFERENCES CITED
Amthor, J.E., Grotzinger, J.P., Schröder, S., Bowring, S.A., Ramezani, J., Martin,
M.W., and Matter, A., 2003, Extinction of Cloudina and Namacalathus at
the Precambrian-Cambrian boundary in Oman: Geology, v. 31, no. 5,
p. 431–434, doi: 10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2.
record of sponges a squeeze: Biological Reviews of the Cambridge
Philosophical Society, v. 89, p. 972–1004, doi: 10.1111/brv.12090.
Bengtson, S., 2004, Early skeletal fossils: Paleontological Society Papers, v. 10,
p. 67.
mineralized exoskeletons: Science, v. 257, 5068, p. 367–369, doi: 10.1126/
science.257.5068.367.
Boag, T.H., Darroch, S.A.F., and Laflamme, M., 2016, Ediacaran distributions in
space and time: Testing assemblage concepts of earliest macroscopic body
fossils: Paleobiology, doi: 10.1017/pab.2016.20.
Bottjer, D.J., and Ausich, W.I., 1986, Phanerozoic development of tiering in soft
substrata suspension-feeding communities: Paleobiology, v. 12, no. 4,
p. 400–420, doi: 10.1017/S0094837300003134.
Bottjer, D.J., Hagadorn, J.W., and Dornbos, S.Q., 2000, The Cambrian substrate
revolution: GSA Today, v. 10, no. 9, p. 1–7.
Briggs, D.E., Fortey, R.A., and Wills, M.A., 1992, Morphological disparity in the
Cambrian: Science, v. 256, 5064, p. 1670–1673, doi: 10.1126/
science.256.5064.1670.
Buatois, L.A., Narbonne, G.M., Mángano, M.G., Carmona, N.B., and Myrow,
P., 2014, Ediacaran matground ecology persisted into the earliest
Cambrian: Nature Communications, v. 5, 3544, doi: 10.1038/
ncomms4544.
Budd, G.E., and Jackson, I.S.C., 2016, Ecological innovations in the Cambrian
and the origins of the crown group phyla: Philosophical Transactions of
the Royal Society of London, Series B, Biological Sciences, v. 371, doi:
10.1098/rstb.2015.0287.
Budd, G.E., and Jensen, S., 2015, The origin of the animals and a ‘Savannah’
hypothesis for early bilaterian evolution: Biological Reviews of the
Cambridge Philosophical Society, doi: 10.1111/brv.12239.
Butterfield, N.J., 1997, Plankton ecology and the Proterozoic-Phanerozoic
transition: Paleobiology, v. 23, p. 247–262, doi: 10.1017/
S009483730001681X.
Butterfield, N.J., 2003, Exceptional fossil preservation and the Cambrian
explosion: Integrative and Comparative Biology, v. 43, p. 166–177, doi:
10.1093/icb/43.1.166.
Butterfield, N.J., 2007, Macroevolution and macroecology through deep time:
Palaeontology, v. 50, p. 41–55, doi: 10.1111/j.1475-4983.2006.00613.x.
Butterfield, N.J., 2009, Oxygen, animals and oceanic ventilation: An alternative
view: Geobiology, v. 7, p. 1–7, doi: 10.1111/j.1472-4669.2009.00188.x.
Cai, Y., Hua, H., Xiao, S., Schiffbauer, J.D., and Li, P., 2010, Biostratinomy of
the late Ediacaran pyritized Gaojiashan Lagerstätte from southern
Shaanxi, South China: Importance of event deposits: Palaios, v. 25,
p. 487–506, doi: 10.2110/palo.2009.p09-133r.
Cai, Y., Schiffbauer, J.D., Hua, H., and Xiao, S., 2011, Morphology and
paleoecology of the late Ediacaran tubular fossil Conotubus hemiannulatus
from the Gaojiashan Lagerstätte of southern Shaanxi Province, South
China: Precambrian Research, v. 191, p. 46–57, doi: 10.1016/j
.precamres.2011.09.002.
Cai, Y., Hua, H., Schiffbauer, J.D., Sun, B., and Yuan, X., 2014, Tube growth
patterns and microbial mat-related lifestyles in the Ediacaran fossil
Cloudina, Gaojiashan Lagerstätte, South China: Gondwana Research,
v. 25, p. 1008–1018, doi: 10.1016/j.gr.2012.12.027.
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