Proterozoic Biotic Innovations – Biomineralization Begins

Eukaryotes Appear

            •          Middle Proterozoic, Beck Springs Dolomite (finely laminated carbonates), 1.0 BY; unicells > 60 μm

            •          Ichnofossil radiation ~1200 Ma

            •          Branched filaments & vase-shaped microfossils (testate amoeba; Porter & Knoll 2000), Coexist with prokaryotes (e.g., Gunflint Chert)

Martin & Miller Hypothesis

            •          Primitive mitochondria-free eukaryotes never existed (1998)

            •          Methanogen archaea needing H₂ and CO₂

            •          Proteobacterium respires aerobically & anaerobically (fermentation producing H₂, CO₂, & acetate waste)

            •          Microscosmic Carbon cycle with nucleation

            •          Eukaryotes in anoxic environments have no mitochondria, but hydrogenosomes (DNA-less) control anaerobic metabolism

Acritarchs

            •          Unicellular, spherical microfossils, Resistant, single-layered walls, 20 to 120 μm in diameter

            •          Planktonic algae with resistant walls developed during "resting phase" resembling modern dinoflagellates

            •          First major group to undergo significant extinctions in response to late Proterozoic glaciation.

Other Unicellular Eukaryotes

            •          Abundant protozoans ~800 Ma, resistant shell and skeletal structures

            •          Bitter Springs Formation in Australia (> 850 Ma)

            •          Chert beds yield diverse Proterozoic fossils including cyanobacteria, chlorophycean algae, assortment of ?fungi, & ?heterotrophic bacteria

Multicellular Vendobionts

            •          Late Proterozoic (Belt Supergroup) first evidence of ribbon-shaped algae.

            •          Metazoans first appear in a cosmopolitan fauna about 630 MY BP, fauna survived ~ 50 MY

            •          Soft-bodied organism impressions in quartzites with familiar forms but not systematic affinities = Seilacher's Vendoza (vendobionts)

DISCOID FORMS

            •           Cyclomedusa & Tribrachidium are most common and widespread, ranging from few mm to 1m diameter.                                

            •          Interpreted planktonic (floating) jellyfish, now considered a benthic (bottom- dwelling) polyp (e.g., sea anemone)

FRONDOSE FORMS

            •          Charnia, Rangea

            •          Resembles living sea pen

            •          Basal attachment point is a ringed disk functioning as a holdfast

            •          Complex, often leaflike forms

            •          Composed of tubular units

OVATE-ELONGATE FORMS

            •          Dickinsonia may be ?annelid worm b/c apparent similarity to extant polychaete – Spinther.

            •          May be a cnidarian polyp, e.g., soft- bodied version of the"banana coral," Fungia.

SEGMENTED FORMS

            •          Bilateral symmetry

            •          Parvancorina shield-like molds (1-1.5 cm), “Arthropoid” Spriggina from Ediacara Hills

                        •          Crescent-shaped head with numerous segments and posterior tapering

                        •          About 3 cm long

CALCIUM PHOSPHATE SHELL-BEARERS

            •          Cloudina – Tubular to conical, lightly mineralized calcium carbonate shells, few cm in length; Systematic affinity not known

            •          Namacalathus found as framework builders in Namibia microbial reefs, 60 m relief; Systematic affinity not known

Doushantuo Fossils

            •          Doushantuo Formation 590-600 Ma, phosphate Mine near Guizhou

            •          Fossils in shales & chert include multicellular red algae, ornamented eukaryote microfossils, multicellular animals

            •          Evidence for Embryogenesis

                        •          400-500 μm spheres

                        •          Single, paired, quartets, octads and higher numbers

                        •          Many metazoan groups have similar embryogenesis

Other Metazoan Evidence

            •          Simple forms of trace fossils (ichnofossils) first at ~1000 Ma

            •          Increasing complexity in structure and, hence, behavior towards Paleozoic

            •          Cambrian substrate Revolution