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The Cambrian Explosion, Fossil Record, and Origin of Vertebrates

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The Cambrian Explosion, Fossil Evolution Biology 4974/5974 Record, and Vertebrate Origins D F Tomback The Cambrian Explosion, Fossil Record, and Origin of Vertebrates Biology 4974/5974 Evolution Science, May 2002, AAAS--cover Hall and Hallgrimson 2014, Strickberger’s Evolution. The fossil record gives us a remarkable picture of the evolution of multi- cellular life during the last 600 million years. Well, maybe we don’t know everything….. Learning goals Also read Carroll, chapt. 6, end boxes p. 349-357 Know and understand: •How fossils are formed, and why the fossil record is incomplete. •Possible explanations for the Cambrian “explosion” of body plans. •The Ediacaran metazoans (animals) in the Pre-Cambrian. •The Cambrian “explosion” Burgess Shale fauna. •Major events in the fossil record. •The origin of Chordates from Echinoderms (star fish/sea urchins). •The origin of Vertebrates from Cephalochordates (lancelets) and Urochordates (tunicates). 1 The Cambrian Explosion, Fossil Evolution Biology 4974/5974 Record, and Vertebrate Origins D F Tomback Limitations to the fossil record Many intermediate forms are known. But, the formation and discovery of fossils are chance events. Requirements: 1. Organisms must have hard parts to become fossilized. 2. The catastrophic anoxic events that preserve soft parts are rare. 3. Fossilization requires that organisms are trapped in wetlands, so buried in sediment. 4. Sediments must be accessible to be discovered. 5. The longer a species lives, the greater Archaefructus, possible early angiosperm from upper Jurassic. the chance for fossilization. Sun, G. et al. 1998. Science 282: 1692. The first metazoans Gene sequences and molecular clock techniques suggest that many metazoan lineages had evolved as early as 700 million to 1 billion years ago. • Early metazoans were soft-bodied. • Followed an increase in deep sea oxygen levels. • Earliest records from the Ediacaran strata, a Pre- Cambrian formation discovered in the mid-1940s in So. Australia, but later found in many regions. • As early as 650 to 545 million years old. • These soft-bodied organisms were directly or indirectly related to organisms that appeared in the early Cambrian. The Ediacaran fauna • Ranged in size from a few millimeters to a meter. • Lack familiar body plans: no heads or tails, teeth, eyes, muscles, mouths, or discernible internal organs. • Shaped like fern fronds or feathers, or lumpy coins; many show folded surfaces. • One view (Seilacher): they absorbed nutrients and exchanged gases by diffusion (thus surface area). • One species (Kimberella) now considered to be an ancestors of several invertebrate groups. Most Ediacaran species were evolutionary dead ends, but a few probably transitioned to the Cambrian. Fig. EB 17.3 2 The Cambrian Explosion, Fossil Evolution Biology 4974/5974 Record, and Vertebrate Origins D F Tomback The Ediacaran fauna “It’s the most dramatic moment in the history of life, and we can’t even name the cast of characters.” (M. McMenamin) Dickinsonia Tribrachidium Spriggina Charniodiscus Wright, K. 1997. When life was odd. Discover, March, pp.52-61. Illustrated by Jack Unruth. The “Cambrian explosion” By about 545 million years ago, the beginning of the era of multi-cellular life, many different forms with hard parts simultaneously appear in the fossil record—thus the “explosion” of metazoan life. Why then? • Sufficient levels of oxygen for mineralization producing hard skeletons? • Sufficient oxygen for aerobic metabolism, supporting larger bodies? • Sea level changes and continental drift? • Warming trend after global glaciation (Snowball Earth hypothesis). • Evolution of Homeobox gene systems and other regulatory genes? (Carroll, Fig. 6.8. Fig. 17.10 in text.) The Burgess Shale fauna Shallow water marine organisms with hard parts, discovered by Charles Walcott above the town of Field, British Columbia, in the Stephen formation, Yoho National Park (1909). At least 12 other sites with similar fossils since discovered around the world. • Preserved by a sudden mudslide, covering the reef, producing anoxic conditions. • Analysis by Walcott followed by Conway Morris indicates about 124 genera, 140 species, in 12 major groups representing living phyla • Predominance of arthropods and Hemichordates. • “Incertae sedis” comprises 19 very different body plans— failed “experiments”? • Includes some Ediacaran forms plus Pikaia, a chordate-like organism. 3 The Cambrian Explosion, Fossil Evolution Biology 4974/5974 Record, and Vertebrate Origins D F Tomback A. Number of body plans Wiwaxia Fig. EB 17.1 Relative number of organisms represented Morris, C. 1989. Science 246:339-346. Major events of the fossil record Geological time Event (see Table 1.2) Precambrian Formation of earth through early metazoans 4.5 bya Phanerozoic Paleozoic______545 mya Cambrian Explosion of invertebrate forms, Burgess Shale Ordovician First vertebrates (agnathans) Silurian First land plants and invertebrates Devonian Diversification of fishes; first amphibians Carboniferous Giant forests; early reptiles; radiation of insects Permian Diversification of reptiles Mesozoic______248 mya Triassic Spread of conifers; first mammals Jurassic First birds; diversification of dinosaurs Cretaceous Diversification of angiosperms and insects Cenozoic______65 mya Tertiary Modern genera: vertebrates, invertebrates, plants Quaternary Rise of genus Homo Phylum Chordata: From invertebrates to vertebrates Phylum Chordata is composed of the following subphyla: • Subphylum Vertebrata (vertebrates) • Subphylum Cephalochordata (lancelets) • Subphylum Urochordata (tunicates) • Subphylum Hemichordata (acorn worms) Traits in common • Gill slits • Notochord • Dorsal, hollow nerve cord 4 The Cambrian Explosion, Fossil Evolution Biology 4974/5974 Record, and Vertebrate Origins D F Tomback “Echinoderm” hypothesis Chordates from Echinoderms Pluteus larva Chordates and echinoderms are deuterostomes and share several traits: • Blastopore produces adult anus. • Radial cleavage of early zygotic cells. Oceanexplorer.noaa.gov • Larvae in echinoderms are “pluteus-type.” • These are morphologically similar to auricularia larvae in acorn worms. This provides a connection from non-chordates to chordates Auricularia larva Genetic support for the “echinoderm hypothesis” Sea Urchin Genome Sequencing Consortium. 2006. Science 314:941-952. Echinoderms (star fish) closely related to Hemichordates and both to Urochordates. Urochordates (tunicates) found to be the closest relatives of Cephalochordates (lancelets) and Vertebrates. Cephalochordates from Urochordates: Garstang’s hypothesis • Urochordate (Tunicate) larvae have notochord, dorsal nerve cord, gill slits, and swim. Metamorphose into sessile adults. • Paedomorphosis, or delay of metamorphosis, could result in a sexually mature “lancelet-like” larval form (Garstang’s hypothesis, 1920’s); so, no sessile adult needed. • Stage now resembles the Cephalochordates, or lancelets, Fig. 17-11: Strickberger’s Evolution, 4th edition. which are small, free-swimming, fishlike animals. • The first vertebrates (jawless fish) evolved from these. 5 The Cambrian Explosion, Fossil Evolution Biology 4974/5974 Record, and Vertebrate Origins D F Tomback Vertebrate evolution Mammalia (Triassic) Aves (Jurassic) Reptilia (Carboniferous) Amphibia (Devonian) Osteichthyes (Silurian) Agnatha (Ordovician) www.healthsciences.okstate.edu Study questions • How are most fossils formed? Why, most likely, is the fossil record incomplete? • What are the Ediacarans? Are they space aliens ? Did these leave many descendants? • What explanations have been proposed for the “Cambrian explosion” of multi-cellular animals? • What are the Burgess Shale fauna? Were these connected to today’s living taxa? • What traits are shared by all chordates? What invertebrate phyla are most closely related to the Subphylum Vertebrata? • What features of the Echinoderms connect them to Phylum Chordata? • What is Garstang’s hypothesis, and how does that explain the evolution of Cephalochordates (lancelets) from Urochordates (tunicates)? 6 .
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