bild 3 lecture 14

bild 3 lecture 14 - The fossil record changed dramatically...

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The fossil record changed dramatically in the early Cambrian, however, when organisms resembling most of the 35 phyla of present-day animals appear for the first time. These complex multicellular animals are known collectively as the metazoan (“middle animals”) phyla, to distinguish them from the single- celled protozoa (“first animals”). Reconstructions of how some of these organisms might have appeared in life are shown below. The reconstructions are based on well-preserved metazoan fossils of animals from the middle of the Cambrian, 520 million years ago, that lived in ancient mudflats of western Canada. The mud has been transformed by pressure into a black rock called shale. T h a u m a p t i l o n w a l c o t t i , a sea pen S i d n e y i a i n e x p e c t a n s , an arthropod Pi kai a grac i l ens, a primitive chordate Marrel l a spl endens, a primitive arthropod O l enoi des serratus, a trilobite O pabi ni a regal i s, an unusual arthropod W i w axi a c orrugata, close to the ancestor of the annelid (segmented) worms H al l uc i geni a sparsa, an onychophoran ("velvet worm") There must have been a long “fuse” period of animal diversification that led up to the explosion. We know this because three animal lineages that were genetically very divergent from each other at the start of the Cambrian became large and complex simultaneously during the explosion. This could only have occurred if these three lineages had a common ancestor long before the Cambrian, and if they had diverged since that time without leaving a fossil record. It seems that the selective pressures that led to the Cambrian explosion were able to affect all these lineages simultaneously in similar ways, making them larger and with harder and more easily fossilizable body parts, even though they were already genetically very different. The molecular evidence for a fuse period was gathered by phylogenetic analysis of ribosomal RNA sequences from many different groups of organisms. A phylogenetic tree of these organisms, based in part on the fossil record, is shown below. The molecular tree allows the relationships among these major
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groups to be determined unambiguously, and the amount of molecular change allows the branch points to be dated to times hundreds of millions of years before the Cambrian explosion, even after accounting for possible errors. Deuterostomia Blastopore forms anus Integument is molted Hollow feeding tentacles or a trochophore larva Representative phyla (not a complete list) Time = molecular evidence = fossil evidence Radiation of Radiation of Radiation of Radiata (radial symmetry) Bilateria (bilateral symmetry) Protostomia Blastopore forms mouth Annelids (segmented worms) Molluscs (clams, squid, octopus) Platyhelminthes (flatworms) Bryozoa ("moss" animals) Arthropods (insects, crustaceans) Nematodes (roundworms) Priapulid worms Chordates (including vertebrates) Hemichordates such as Amphioxus Echinoderms Causes of the Cambrian Explosion.
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