The Cambrian-Precambrian Boundary

Treptichnus pedum

Key Faunas Before/After the Cambrian Explosion:

• Burgess Shale (505 Ma) • Chengjiang (520 Ma) • Small Shellies (Manykaian Stage) (544-530 Ma) • Ediacara (575-545 Ma) • Doushantuo embryos (580-570 Ma)

1 Small Shellies

Small Shellies

2 Small Shellies

The Tommotian fauna is a small shelly fauna found first on the Siberian Platform. The scale bars are all 1 millimetre.

Photo: S. Bengston, in Evolution of the Earth, 1994 edition.

3 What are small shellies?

Halkieria, Sirius Passet, Greenland

Lapworthella www.palaeos.com/Invertebrates/ Procoelomates/Lapworthella.gi

4 Key Faunas Before/After the Cambrian Explosion:

• Burgess Shale (505 Ma) • Chengjiang (520 Ma) • Small Shellies (Manykaian Stage) (544-530 Ma) • Ediacara (575-545 Ma) • Doushantuo embryos (580-570 Ma)

The Chengjiang Fauna

5 The Chengjiang fossil lagerstätte: Major outcrop in the Qiongzhusi Formation near Chengjiang, Yunnan Province. (c) 1997 by E. Landing

Microdictyon – an onychophoran

6 Waptia – an arthropod

Canadaspis – an arthropod

7 Hallucigenia – an onychophoran

Anomalocaris – a (very large) arthropod

8 Naraoia – a trilobite

Cindarella

9 Saperion

Fuxianhuia – an arthropod

10 Yunnanozoon - the oldest known chordate

11 Haikouella – The oldest known vertebrate??

Science, February 2003

12 Key Faunas Before/After the Cambrian Explosion:

• Burgess Shale (505 Ma) • Chengjiang (520 Ma) • Small Shellies (Manykaian Stage) (544-530 Ma) • Ediacara (575-545 Ma) • Doushantuo embryos (580-570 Ma)

Factoids about the Burgess Shale

• Discovered in 1909 by C.D. Walcott • Middle Cambrian age • Walcott did not think the Burgess creatures were that remarkable • Restudy in the 1980s showed that many Burgess taxa were very unusual • Best-selling 1989 book by S.J. Gould made Burgess famous and encouraged more study • Burgess and Chengjiang share many taxa

13 (1989)

The Burgess Shale

Charles Walcott

14 15 Wiwaxia - a mollusk?

Aysheaia - an onychophoran

16 A modern onychophoran

New reconstruction ⇑

⇐ Old reconstruction

Hallucigenia – an onychophoran

17 Thamnauptilon – a penatulacean (sea pen)

Pikaia - a chordate

18 Opabinia – affinities unknown

Reconstructions of Opabinia

19 Marella – an arthropod

Anomalocaris – an arthropod

20 Anomalocaris

Anomalocaris

21 22 Reconstruction of the Burgess Shale fauna www.emc.maricopa.edu/faculty/ farabee/BIOBK/1ord04b.gif

Evidence of predation in the Burgess Shale

23 Temporal distribution of Burgess-type faunas

How might all this fit together?

• The fossils • Snowball Earth • Molecular clocks • Developmental genetics

24 How might all this fit together?

• The fossils • The environment: Snowball Earth • Molecular clocks • Developmental genetics

How might all these faunas fit together? The fossils. • If there were animals before Ediacara, they were small • Ediacara mostly soft-bodied; many did not leave decendants; some did • Skeletons appeared abruptly at the boundary • Many early skeletonized animals had multi- element skeletons • The “explosion” was truly sudden (a few million years) and produced almost all the phyla

25 Halkieria, Sirius Passet, Greenland

26 Similarities between Wiwaxia and Halkieria Halkieria

Wiwaxia

27 How might all this fit together?

• The fossils • The environment: Snowball Earth • Molecular clocks • Developmental genetics

28 Late Proterozoic Environmental Changes

• Major glaciations • Major carbon burial events

29 Late Proterozoic Glaciations

Duoshantuo? • ≈ 580 Ma - Gaskiers Glaciation Duoshantuo? • ≈ 680 Ma - Marinoan Glaciation

• ≈ 710 Ma - Stirtian Glaciation

Snowball Earth

30 Stage 1: Prologue Stage 3: Snowball as it Thaws

Stage 2: Snowball Earth at Its Coldest Stage 4: Hothouse Aftermath

How might all this fit together?

• The fossils • The environment: Snowball Earth • Molecular clocks • Developmental genetics

31 (from Bromham & Hendy, Proc. R. Soc. Lond., 2000, 267:1041)

32 How might all this fit together?

• The fossils • The environment: Snowball Earth • Molecular clocks • Developmental genetics

The role of development

Recent discoveries in developmental biology tell us that it is possible for major evolutionary changes in morphology to occur abruptly through relatively small genetic changes.

33 Phylogeny of the Burgess/Chengjiang arthropods

From Bioessays 19, pg 431. Fortey R.A., et al, 1997.

The Cambrian Explosion: An attempt at synthesis • One Possible Scenario: The phyla originated sometime in the late Proterozoic (1000-600 Ma), but remained small during environmental changes associated with major late Proterozoic glaciations (which may have reached the level of “snowball earth”). These environmental changes -- combined with one or more major genetic/developmental changes -- led to the Cambrian Explosion.

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