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Chapter 18

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BIOL 359
Jonathan Witt

Chapter 18 – The Cambrian Explosion and Beyond - First animal appeared in the fossil record 565 million years ago - Cambrian period: First appearance of most of the animals today “large & complex”, represent <1% of Earth’s history o There’s total of 5 episodes of cataclysmic extinction o Phanerozoic: Interval between the start of Cambrian & the present 18.1. The Nature of the Fossil Record - How Organic Remains Fossilize o Fossil: Any trace left by an organism that lived in the past o 4 category distinguished by the method of formation  Compression fossils: When organic material is buried in water/wind-borne sediment before it decomposes. An impression is left in the sediment  Casts & molds: When remains decay after being buried in sediments, preserve information about external & internal surfaces  Molds: unfilled spaces  Casts: When new material infiltrates the space, fills it, and hardens into rock  Permineralized fossils: When structures are buried in sediments & dissolved minerals precipitate in the cells, preserves details about internal structures  Unaltered remains: Preserved in environments that discourage loss from weathering, consumption by animals & decomposition by bacteria & fungi, EXTREMELY RARE o All fossilization processes requires durability, burial, and lack of oxygen - Strengths and Weaknesses of the Fossil Record o 3 types of sampling bias in the fossil record  Geographic: Majority of fossils come from lowland & marine habitats  Taxonomic: Marine organisms dominate the fossil record (~90%) but make up only 10% of extant species today  The full two-third of animal phyla living today are underrepresented in the fossil record because of their lack of mineralized hard parts for fossilization (e.g. plants)  Temporal: Earth’s crust is constantly being recycled “tectonic plates subduction, older rocks are rarer & out ability to sample life forms decline with time  Found that older rocks still contain enough fossils to accurately record the order of branching event, so the taxonomic bias is not a big deal o The fossil record has characteristics that limit the types of information that can be retrieved & how broadly the data can be interpreted, the goal of paleontologists is to recognize the constraints and work creatively within them - Life through Time: An Overview o Geologic time scale: Divided into eons, eras, periods, epochs, and stages o Absolute time was assigned after the discovery of radioisotopes & the development of accurate dating techniques  Eras (3): Paleozoic (ancient life)  Mesozoic (middle life)  Cenozoic (recent life) o Fossils are separated by the appearance of new traits 18.2. The Cambrian Explosion - Almost all animal phyla currently recognized make their first appearance in the fossil record during the Cambrian period (~40 million years span) - The Ediacaran Fauna of South Australia - Precambrian o Contains the first unequivocal evidence for animals o Ediacarna animals are small, morphologically simple, are either asymmetrical or have radial symmetry o Bilaterally symmetric organisms first appeared in the late Precambrian “prior to the Cambrian Explosion”, they are small in size  Fossils of embryos, trace fossils etc. - The Burgess Shale Fauna of British Columbia – Cambrian o Contains animals that are large, complex & bilaterally symmetric  Hard exoskeletons, complex body parts, diversification in basic body shapes & organization o E.g. Opabinia regalis, elongated with serially repeated lateral plates, five dorsal eyes & an elongated nozzle on the anterior end o These animals were capable of performing a varieties of movements o Most of the animal phyla living today make their 1 appearance in the fossil record during the Cambrian - Phylogeny and Morphology o The Porifera, Cnidaria and Ctenophora predominate in the Ediacaran fauna o The Arthropoda and annelids appeared in the Burgess Shale fauna o Diploblasts & Triploblasts:  Cnidaria & Ctenophora are diploblastic  2 embryonic tissue types  Ectoderm – skin & nervous system & Endoderm – gut & associated organs  Radial symmetrical or asymmetrical  Remaining animals are triploblastic  3 embryonic tissue types  Ectoderm, endoderm & mesoderm – gonads, heart, muscle, CT & blood o Mesoderm is important for the muscle-lined, fluid-filled cavities to evolve  Hydrostatic skeleton, directed movement became more efficient  Bilateral symmetrical o Protostomes & Deuterostomes:  Gastrulation: Mass movement of cells that rearranges the embryonic cells after cleavage & defines the ectoderm, endoderm & mesoderm  Protostomes “Mouth first” & Deuterostomes “Anus first”  Both found in the Burgess Shale o Lophotrochozoans & Ecdysozoans:  Lophotrochozoans: Animals with a feeding structure called a lophophore  Ecdysozoa: Molting animals  Both found in the Burgess Shale – Evolutionary innovations of Cambrian explosion o Hence: Most basal groups of animals populate the Ediacaran fauna & the vast majority of more derived groups first appeared in the Cambrian - Was the Cambrian Explosion Really Explosive? o Molecular clock is used to estimate when the earliest branches on the animal phylogeny occurred o Molecular clock is calibrated in terms of the amount of change expected per million years  Differences among hemoglobin AA sequence is used to date origin of the bilaterians  Prediction of molecular clock: Divergence between groups occurred hundreds of millions of years before their first appearance in the fossil record  Hence, the Cambrian explosion is an explosion of morphological diversity “long fuse”, but not a necessarily an explosion of lineages, which occurred much earlier - What Caused the Cambrian Explosion? o Radiation of bilaterally symmetric animals was driven by how the organisms made a living o Multicellularity & large body size is due to rising oxygen concentration in seawater, leading to higher metabolic rates for powered movement  Beginning of both traits are recorded in the Ediacaran faunas  Massive extinction @ end of Proterozoic  opportunity for tiny deuterostomes & protostomes to evolve in response to the changed conditions o Ediacaran animals: Sessile filter feeders or floating predators o Burgess Shale animals: Benthic & pelagic predators, filter feeders, grazer, scavengers & detritivores, most actively chased their prey 18.3. Macroevolutionary Patterns - The rapid diversification of species in response to a morphological innovation or ecological opportunity - Adaptive Radiations: When single or small group of ancestral species rapidly diversifies into a large number of descendant species that occupy a wide variety of ecological niches o Predominant pattern, as if the tree of life suddenly sprouts a large number of highly diverse branches  Ecological Opportunity as a Trigger  When a small number of individuals/species is suddenly presented with a wide & abundant array of resources to exploit o Dispersal & colonization: Few competitors & wide variety of resources & habitats  Rapid diversification & speciation  E.g. Galapagos finches & Hawaiian Drosophila o Opportunity can also be present after a mass extinction  Lack of competition  Morphological Innovation as a Trigger  Important new adaptations o E.g. Diversification of arthropods, variety of ecological niches occupied by insects, crustaceans, and spiders, remarkable numbers  Modifications & elaborations of jointed limbs  Move efficiently & find food o E.g. Adaptive Radiations in Land Plants  1 : Radiation of terrestrial plants from aquatic ancestors in the early Devonian (~400mya)
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