The Cambrian Explosion - 550-544 Ma ?
Milestones in the first 3 billion years of life
• Origin of life - before 3.8 Ga • Origin of eukaryotes - before 1.4 Ga; before 2.7 Ga ? • Origin of animals (multicellularity) - 600-800 Ma ? • Origin of skeletons - 550 Ma ? • The Cambrian Explosion - 550-544 Ma ?
The relatively sudden appearance and diversification of almost all of the phyla (all but Bryozoa) in the early Cambrian. This event began around 550 million years ago and lasted no more than 20-30 million years.
1 Key Faunas Before/After the Cambrian Explosion:
• Burgess Shale (505 Ma) • Chengjiang (520 Ma) • Small Shellies (Manykaian Stage) (544-530 Ma) • Doushantuo embryos (580-570 Ma) • Ediacara (575-545 Ma)
The Ediacara Biota
2 The Ediacara Biota
Parvancorina Dickensonia Cyclomedusa
Tribrachidium
Mawsonite s
The Ediacara Biota
Spriggina
Charnia
3 Spaniard's Bay in eastern Newfoundland.
4 The “traditional” interpretation of Ediacara (from Glaessner (1984)
Ediacara fronds: Comparison with living Pennatulacean Cnidarians
5 “Traditional” reconstructions of the Ediacara Biota
A radical alternative interpretation of the Ediacara Biota: Vendobionta (From Seilacher, 1989)
6 A Chordate in the Ediacara?? (2003)
Length ca. 5 cm)
The latest on Ediacara… Nov. 2003 • There may be actual bilaterians among the Ediacaran biota • There are at least 3 biostratigraphically recognizeable assemblages: – Avalon 575-565 (e.g., Mistaken Point, Newfoundland) – Nama 565-550 – White Sea 550-545
7 The Ediacara Biota at Mistaken Point, Newfoundland
http://geol.queensu.ca/museum/exhibits/ediac/mistaken_point/mistaken_pt.html
Key Faunas Before/After the Cambrian Explosion:
• Burgess Shale (505 Ma) • Chengjiang (520 Ma) • Small Shellies (Manykaian Stage) (544-530 Ma) • Doushantuo embryos (580-570 Ma) • Ediacara (575-545 Ma)
8 Doushantuo embryos
http://more.abcnews.go.com/sections/science/DailyNews/fossils0204.html Bengtson & Zhao 1997, a SEM image depicting a suggested metazoan embryo – possibly Olivooides multisulcatus – at approximately the 256-cell stage.
The Cambrian-Precambrian Boundary
Treptichnus pedum
9 Key Faunas Before/After the Cambrian Explosion:
• Burgess Shale (505 Ma) • Chengjiang (520 Ma) • Small Shellies (Manykaian Stage) (544-530 Ma) • Doushantuo embryos (580-570 Ma) • Ediacara (610-530+? Ma)
Small Shellies
10 Small Shellies
Small Shellies
11 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.
What are small shellies?
Halkieria, Sirius Passet, Greenland
12 Lapworthella www.palaeos.com/Invertebrates/ Procoelomates/Lapworthella.gi
Key Faunas Before/After the Cambrian Explosion:
• Burgess Shale (505 Ma) • Chengjiang (520 Ma) • Small Shellies (Manykaian Stage) (544-530 Ma) • Doushantuo embryos (580-570 Ma) • Ediacara (575-545 Ma)
13 The Chengjiang Fauna
The Chengjiang fossil lagerstätte: Major outcrop in the Qiongzhusi Formation near Chengjiang, Yunnan Province. (c) 1997 by E. Landing
14 Microdictyon – an onychophoran
Waptia – an arthropod
15 Canadaspis – an arthropod
Hallucigenia – an onychophoran
16 Anomalocaris – a (very large) arthropod
Naraoia – a trilobite
17 Cindarella
Saperion
18 Fuxianhuia – an arthropod
Yunnanozoon - the oldest known chordate
19 Haikouella – The oldest known vertebrate??
20 Science, February 2003
Key Faunas Before/After the Cambrian Explosion:
• Burgess Shale (505 Ma) • Chengjiang (520 Ma) • Small Shellies (Manykaian Stage) (544-530 Ma) • Doushantuo embryos (580-570 Ma) • Ediacara (610-530+? Ma)
21 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
(1989)
22 The Burgess Shale
Charles Walcott
23 Wiwaxia - a mollusk?
24 Aysheaia - an onychophoran
A modern onychophoran
25 New reconstruction ⇑
⇐ Old reconstruction
Hallucigenia – an onychophoran
Thamnauptilon – a penatulacean (sea pen)
26 Pikaia - a chordate
Opabinia – affinities unknown
27 Reconstructions of Opabinia
Marella – an arthropod
28 Anomalocaris – an arthropod
Anomalocaris
29 Anomalocaris
30 Reconstruction of the Burgess Shale fauna www.emc.maricopa.edu/faculty/ farabee/BIOBK/1ord04b.gif
31 Evidence of predation in the Burgess Shale
Temporal distribution of Burgess-type faunas
32 How might all this fit together?
• The fossils • Snowball Earth • Molecular clocks • Developmental genetics
How might all this fit together?
• The fossils • The environment: Snowball Earth • Molecular clocks • Developmental genetics
33 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
34 Halkieria, Sirius Passet, Greenland
Similarities between Wiwaxia and Halkieria Halkieria
Wiwaxia
35 36 How might all this fit together?
• The fossils • The environment: Snowball Earth • Molecular clocks • Developmental genetics
Late Proterozoic Environmental Changes
• Major glaciations • Major carbon burial events
37 Late Proterozoic Glaciations
Duoshantuo? • ≈ 580 Ma - Gaskiers Glaciation Duoshantuo? • ≈ 680 Ma - Marinoan Glaciation
• ≈ 710 Ma - Stirtian Glaciation
38 Snowball Earth
Stage 1: Prologue Stage 3: Snowball as it Thaws
Stage 2: Snowball Earth at Its Coldest Stage 4: Hothouse Aftermath
39 How might all this fit together?
• The fossils • The environment: Snowball Earth • Molecular clocks • Developmental genetics
40 (from Bromham & Hendy, Proc. R. Soc. Lond., 2000, 267:1041)
How might all this fit together?
• The fossils • The environment: Snowball Earth • Molecular clocks • Developmental genetics
41 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.
Phylogeny of the Burgess/Chengjiang arthropods
From Bioessays 19, pg 431. Fortey R.A., et al, 1997.
42 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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