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Cambrian Explosion

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Cambrian Explosion 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. 34 35 36 37.
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