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Home , Charniodiscus, Chordate, Dickinsonia, Spriggina, Stephen Formation

The Explosion, 4974/5974 Record, and Origins D F Tomback

The , Fossil Record, and Origin of

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 during the last 600 million .

Well, maybe we don’t know everything…..

Learning goals

Also read Carroll, chapt. 6, end boxes p. 349-357 Know and understand: •How are formed, and why the fossil record is incomplete. •Possible explanations for the Cambrian “explosion” of body plans. •The metazoans () in the Pre-Cambrian. •The Cambrian “explosion” fauna. •Major events in the fossil record. •The origin of from (star /sea urchins). •The origin of Vertebrates from () and Urochordates ().

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 , the greater Archaefructus, possible early angiosperm from upper . the chance for fossilization. Sun, G. et al. 1998. Science 282: 1692.

The first metazoans Gene sequences and 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 or tails, teeth, , 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 () now considered to be an ancestors of several 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 , and we can’t even name the cast of characters.” (M. McMenamin)

Dickinsonia Tribrachidium

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 ? • Sufficient oxygen for aerobic metabolism, supporting larger bodies? • Sea level changes and continental drift? • Warming trend after global glaciation ( 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, , 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 and . • “” comprises 19 very different body plans— failed “experiments”? • Includes some Ediacaran forms plus , a -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) Formation of earth through early metazoans 4.5 bya Phanerozoic Paleozoic______545 mya Cambrian Explosion of invertebrate forms, Burgess Shale First vertebrates (agnathans) First land and Diversification of ; first Giant forests; early ; radiation of insects Diversification of reptiles Mesozoic______248 mya Spread of ; first Jurassic First ; diversification of Diversification of angiosperms and insects Cenozoic______65 mya Tertiary Modern genera: vertebrates, invertebrates, plants Quaternary Rise of

Phylum Chordata: From invertebrates to vertebrates

Phylum Chordata is composed of the following subphyla: • Vertebrata (vertebrates) • Subphylum Cephalochordata (lancelets) • Subphylum Urochordata (tunicates) • Subphylum Hemichordata (acorn )

Traits in common • slits • • Dorsal, hollow cord

4 The Cambrian Explosion, Fossil Evolution Biology 4974/5974 Record, and Vertebrate Origins D F Tomback

“” hypothesis Chordates from Echinoderms Pluteus Chordates and echinoderms are and share several traits: • Blastopore produces adult . • 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 () larvae have notochord, , gill slits, and swim. Metamorphose into sessile adults. • Paedomorphosis, or delay of , could result in a sexually mature “-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)

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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)?

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