Large spinose microfossils in Ediacaran rocks as resting stages of early animals

Abstract

Large (>100 microm), profusely ornamented microfossils comprise a distinctive paleontological component of sedimentary rocks deposited during the Ediacaran Period (635-542 million years ago). Smaller spinose fossils in Paleozoic rocks have commonly been interpreted as algal cysts or phycomata, but the Ediacaran populations differ from modern algal analogs in size, shape, ultrastructure, and internal contents. In contrast, cysts formed during the diapause egg-resting stages of many metazoans share features of size, ornamentation, and internal contents with large ornamented Ediacaran microfossils (LOEMs). Moreover, transmission electron microscopic observations of animal-resting cysts reveal a 3-layer wall ultrastructure comparable to that of LOEM taxa. Interpretation of these distinctive Ediacaran microfossils as resting stages in early metazoan life cycles offers additional perspectives on their functional morphology and stratigraphic distribution. Based on comparisons with modern marine invertebrates, the recalcitrant life stage represented by LOEMs is interpreted as an evolutionary response to prolonged episodes of bottom water anoxia in Ediacaran shelf and platform environments. As predicted by this hypothesis, the later Ediacaran disappearance of LOEM taxa coincides with geochemical evidence for a marked decline in the extent of oxygen-depleted waters impinging on continental shelves and platforms. Thus, the form, diversity, and stratigraphic range of LOEMs illuminate life cycle evolution in early animals as influenced by the evolving redox state of the oceans.

Fig. 1.

Size frequency distributions of Ediacaran and Cambrian acritarch species, with modern analog ranges. Data are from refs. –.

Fig. 2.

Stratigraphic relationships of major events in Ediacaran oceans. Dates are indicated by: # (28), * (27), ^ (29), and & (30). Carbon isotope data are from refs. , , and . Geochemical data suggesting oxygenation of the oxygen miniumum zone are bracketed by the beginning and end of the so-called Shuram event, the large negative C isotopic anomaly within the Ediacaran Period (2, 76, 77, 81).

Fig. 3.

Morphologies of LOEM taxa and a modern analog. (A) SEM of resting cyst, modern arthropod Brachinella longirostris. (B) Alicesphaeridium sp., Vychegda Formation, northern Russia. (C) LOEM microfossil, Doushantuo Formation, China. (D) LOEM microfossil, Officer Basin, Australia. (E) LOEM microfossil, Kursovsky Formation, Siberia. (Scale bar: 100 μm for A and D; 200 μm for B and C; 150 μm for D and E.)

Fig. 4.

Comparison of a modern algal analog and Ediacaran smooth-walled microfossil. (A) Halosphaera sp. phycoma from Puget Sound. (B) TEM of Halosphaera sp. phycoma. I indicates inside of phycoma. (C) Ediacaran Leiosphaera sp. (D) TEM Ediacaran Leiosphaera sp. (Scale bars: A and C, 50 μm; B and D, 500 nm.)

Fig. 5.

Comparison of a LOEM fossil and a modern crustacean analog. (A–C), Gyalosphaeridium sp. (A) Light micrograph. (B and C) TEM. (D–F) Branchinella longirostris. (D) SEM. (E) TEM. (Inset) Hollow process. (F) TEM of outer wall. (Scale bars: A and D, 100 μm; B and C, 500 nm; E, 4 μm; F, 200 nm.)