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Windows into the Marine Ecosystems of 600 Million Years Ago

Shuhai XiaoDepartment of Geosciences

Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

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Tiksi, Arctic Siberia

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The best mosquitoes are dead mosquitoes…

Namibia

(Dunn et al., 2008)

South China

(Dunn et al., 2008)

“Consequently, if my theory be true, it is indisputable that before the lowest Cambrian stratum was deposited, … the world swarmed with living creatures”.

“I look at the natural geological record, as a history of the world imperfectly kept … only here and there a short chapter has been preserved …”

(Darwin, 1859)

The Cambrian Explosion

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Billion years (Ga = 1000 Ma)

4.5

4.0

2.5

0.5

Hadean

Archean

Proterozoic

PhanerozoicCambrian

Neoproterozoic

Mesoproterozoic

Paleoproterozoic

1.0

1.6

CryogenianEdiacaran

TonianNeoproterozoic

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Doushantuo Formation: Localities and Windows

(Zhu, 2004)

Yangtze Gorges(C, Si)

Weng’an (P)

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• Subtidal, early diagenetic chert nodules are abundant in Doushantuo Fm;• Three dimensional preservation of microfossils in nodules;

Lower Doushantuo Chert Window

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Chert window: Acanthomorphic (spiny) acritarchs100 µm 50 µm

50 µm 50 µm

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What are Ediacaran Acanthomorphs?

• Some Ediacaran acanthomorphs may represent animal embryos;

• They extend to the base of the Doushantuo Formation, ~632 Ma;

(Yin et al., 2007)

Doushantuo Shale Window

Carbonaceous compression fossils in uppermost Doushantuo

551 Ma ash

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Shale window: Seaweeds or Algae

• Millimetric to decimetric;• Holdfast; Dichotomous branching;• Apical meristematic growth;

1 cm

0.5 cm

15Scales: 5 mm

Shale window: Seaweeds & Animals

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Shale window: Animals?

• Doushantuo annulated tubes; some were interpreted as animals;• Holdfast, -- benthic; Possibly siphonous, coenocytic algae;

Shale window: Animals?

• Large flanged tubes;• Cnidarian or pogonophoran tubes?

1 cm 1 cm

Doushantuo tubular fossil: Calyptrina striata Modern pogonophoran tubes

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• Eoandromeda: Each consists of eight clockwise spiral arms;• Similar fossils from South Australia;• Early animals?

(Tang et al., 2008; Zhu et al., 2008)

Shale Window: Eight-armed animals?

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Upper Doushantuo Phosphorite Window

• 40 m thick;• Two phosphorite horizons separated by dolostone and karstification surface;• Fossils in upper phosphorite;

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• Similar acanthomorphs are found in Australia, South China, northern India, Svalbard, southern Norway, Siberia, East European Platform;

Acanthomorphs in different taphonomic windows

phosphatization silicification carbonaceous compression

100 µm 100 µm 50 µm

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Phosphorite window: Algae

• Cellularly preserved multicellular algae;• Found in both chert and phosphorite windows;

50 µm

Phosphatized Silicified

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Phosphorite window: Algae

• Cell division events were preserved, indicating rigid cell wall;

20 µm

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Phosphorite window: Algae

• Cellular, tissue, and reproductive differentiation (e.g., conceptacles);

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Phosphorite window: Animal Embryos

• Animal stereoblastulae from the Doushantuo Formation;• Cellular and sub-cellular preservation;

(Hagadorn et al., 2006)

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Comparison between vesicles in fossil embryo and modern sponge embryo (lower right)

Phosphorite window: Sub-cellular Structures

500

µm

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Phosphorite window: Animal Embryos

Embryo ~600 µm in diameter

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• Three clockwise coils, with no exception;• Earliest known evidence for biological chirality;

(Courtesy of Phil Donoghue)

Phosphorite window: Spiral Embryos?

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Modern and Fossil Resting Eggs

200

µmFossil

Modern

Fossil

Modern

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White Sea, Russia: Yorgia

• Passive intermittent movement;• Alternate bilateral symmetry;• Bilaterally symmetric animals?

(Ivantsov and Malakhovskaya, 2002; Fedonkin, 2003)

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White Sea, Russia: Kimberella

• Dorsal-ventral and anterior-posterior differentiation;• Grazing activities and active self-powered locomotion;• Likely bilaterian animals;

(Fedonkin, 2003; Ivantsov, 2009)

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1 cm

Animal Burrows

• The presence of Ediacaran bilaterian animals is also supported by trace fossils;

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Nama Group, Namibia:Animal Biomineralization and Predation

• Cloudina, the earliest biomineralizing animals;• Drill holes suggest inter-species ecological interactions;

0.5 mm

(Bengtson and Yue, 1992; Hua et al., 2003)

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Conclusions

• Multiple windows (or “volumes in different dialects”): Ediacaran Period represents a fuse to the Cambrian explosion;

• But what ignited the fuse?

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Acknowledgments

• NSF Sedimentary Geology & Paleobiology;• Virginia Tech X-ray CT laboratory;• Phil Donoghue, Jim Schiffbauer, Xunlai

Yuan, Chuanming Zhou;

Nama assemblage

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Marinoan snowball

An Ediacaran Fuse of the Cambrian Explosion?

• Doushantuo (635-551 Ma), Avalon (575-560 Ma), White Sea (560-550 Ma), Nama (550-541 Ma);

Cryogenian

Cambrian

635 Ma

545 Ma

555 Ma

565 Ma

575 Ma

585 Ma

595 Ma

605 Ma

615 Ma

625 Ma

White Sea assemblageE

diac

aran

Per

iod

Avalon assemblage

Doushantuo biota

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Temporal Distribution of Avalon-White Sea-Nama Fossils

• Many have unusual bodyplans (triradial, tetraradial, pentaradial, octaradial);• Diploblastic grade animals with a greater diversity of bodyplans in Ediacaran?

(Xiao and Laflamme, 2009)

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• Tubular fossils with perforated crosswalls: possible stem group cnidarians?• Ontogenetically related to the spheroidal microfossils?

100

µmPhosphorite window: Adult animals?

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Phosphorite window: Embryonic development?

Are blastula stages and spiral fossils ontogenetically or phylogenetically related?

Blastulas

100 µmSpiral embryos

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Phosphorite window: Algae

50 µm

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Phosphorite window: Animal Embryos

• Animal stereoblastulae from the Doushantuo Formation;• Cellular and sub-cellular preservation;

(Xiao et al., 2007)(Hagadorn et al., 2006)