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Figure 2. Representative wormworld body (A–C) and trace (D–F) fossils. (A–C) Tubular representatives from the Gaojiashan Lagerstätte, China; (D) surficial
traces with micropustular elephant skin-like textures, Blueflower Formation, Canada (courtesy of C. Carbone); (E) surficial traces, Ediacara Member, Rawnsley
Quartzite, South Australia (courtesy of L. Buatois); (F) complex undermat traces, Dengying Formation, China (courtesy of M. Meyer).

evolution in these sessile epibenthic ecosystems would have been different shifts in biotic interactions. Notably, the expansion of
driven by competition as reflected by niche partitioning and func- the pelagic realm to accommodate larger mesozooplankton

tional morphology in sessile communities (Bottjer and Ausich, (Butterfield, 2009) would favor filter-feeding strategies over the
1986; Clapham and Narbonne, 2002). passive diffusion of organics in osmotrophy. Furthermore, the

GSA TODAY | NOVEMBER 2016 Benthos Modification innovation of metazoans with one-way guts (i.e., bilaterians)
would have packaged nutrients in the form of fecal pellets, thus
Set against this competitive landscape, the first motile members efficiently transporting nutrients from the water column to the
of the second trophic tier (1° consumers) began to exploit wide- substrate (Sperling et al., 2011) and benefiting detritivores at the
spread microbial mats, sedimentary organic carbon, and possibly cost of those reliant on dissolved nutrients. Nonetheless, there
the decaying material of fallen Ediacarans (Budd and Jensen, remains a delay between the emergence of surficial grazing behav-
2015). Metazoan trace fossils from the last ~25 m.y. of the iors and significant sediment mixing from vertical bioturbation
Ediacaran period (ca. 565–541 Ma): (1) display behavioral evolu- (Tarhan et al., 2015). With an ostensibly limitless food source of
tion of the second trophic tier (Carbone and Narbonne, 2014; microbial substrates (which persist into the Cambrian; Buatois et
Chen et al., 2013; Meyer et al., 2014); (2) signify the development al., 2014) and the lack of macroscopic predation, there may not
of sensory-muscular activity (Gehling et al., 2014; Jensen et al., have been sufficient ecological stressors to drive metazoans into
2005); (3) provide tangible evidence for both grazing and deposit less hospitable or more physiologically challenging infaunal life
feeding (Carbone and Narbonne, 2014); and (4) confirm a modes. Nonetheless, the introduction of infaunalization and
burgeoning and sophisticated motile component to benthic increasing intricacy of horizontal burrow networks at the
ecosystems, marking an important expansion of ecosystem engi- Ediacaran-Cambrian transition (Hagadorn and Bottjer, 1999;
neering behaviors. Even with increasing species richness and Jensen, 2003) signaled a keystone development in ecosystem engi-
ecosystem complexity through early Ediacaran assemblages (Shen neering, and began to propagate a shift—albeit protracted
et al., 2008; Xiao and Laflamme, 2009), the diversity of nascent (Tarhan et al., 2015)—in the physical and chemical properties of
bioturbating behaviors was restricted, possibly by benthic oxygen the substrate.
levels (Fike et al., 2006; Sperling et al., 2015) or by sharp sediment
redox gradients maintained by the, at the time, still pervasive micro- Ecological Antagonism

bial blanketing of the shallow seafloor (Hagadorn and Bottjer, 1999). The first occurrence of metazoan predation appears in the
The complexity of traces in the ichnofossil record grew (Carbone terminal Ediacaran (ca. 550–541 Ma), in concert with several
and Narbonne, 2014), however, perhaps foreshadowing the coming other firsts, including metazoan biomineralization and the occu-
revolution in the abiotic and biotic structure of the benthos pation of biohermal ecological niches (Cai et al., 2014; Penny et
(Bottjer et al., 2000). The redistribution of nutrients between the al., 2014; Wood and Curtis, 2015). The earliest mineralizing taxa
water column and the substrate in the latest Ediacaran, a significant appear in several contemporaneous units, such as the Nama
consequence of bioturbation, could have resulted from many Group, Namibia (Grotzinger et al., 2000); the Ara Group, Oman

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