2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 69-2
Presentation Time: 1:55 PM


PORTER, Susannah M., Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106, porter@geol.ucsb.edu

Predators play a key role in the evolution of their prey, driving both evolutionary innovation and diversification. Most studies have focused on the role of predation during the Phanerozoic, but the wide distribution of predators across both bacterial and eukaryotic clades suggests a much deeper history. Preliminary molecular clock analyses suggest that eukaryophagy evolved independently in several eukaryotic clades ca. 800 Ma, consistent with other indirect evidence for predation on eukaryotes at this time, including the first appearance of mineralized skeletons and organic tests in protists, and body fossil evidence for testate amoebae, whose modern relatives consume both bacteria and eukaryotes. Here I report the presence of circular holes in cyst and cell walls of eukaryotic microfossils from the ~780–740 Ma Chuar Group, Grand Canyon, Arizona. The holes are regular in shape and occur in specimens that may otherwise show no signs of pitting or degradation. They range in size from 0.2–2.9 µm, but show much narrower ranges within individual specimens (vs. among specimens in the same sample). They are irregularly distributed on the fossil wall and number from 1 to >30 (median=5 per specimen). Some holes are beveled, with hole diameters narrowing toward the interior of the fossil. Their irregular number and distribution within specimens and occurrence in a variety of species suggest they are not biological characters of the fossils, and their regular shape and narrow size range within specimens suggest they are not formed by mineral puncture. Their shape and distribution are also inconsistent with post-mortem biodegradation. Instead, the holes are interpreted to be the work of predators that perforated the walls of their prey to consume the cell contents inside. Similar behavior is known today in some opisthokonts, amoebozoans, and rhizarians, as well as in Myxobacteria; the Chuar holes are most closely comparable to perforations formed by members of the Vampyrellidae (Cercozoa: Rhizaria), a diverse group of predatory amoebae found in marine, freshwater, and terrestrial environments. These holes provide the earliest direct evidence for predation on eukaryotes, and support the view that, by ~740 Ma, predation was an important agent shaping eukaryote ecology and evolution.