Chapter 15

1. Chapter 15 The Cambrian Explosion and Beyond

2. Multicelluarity • First multicellular organisms began approximately 600 Mya • First animals about 565 Mya • In about 40 My nearly every major animal phylum appeared in fossil record • The Cambrian Explosion • The Phanerozoic Eon • Visible life

3. Nature of the Fossil Record • Fossil = any trace left by an organism that lived in the past • Fossils originally defined boundaries of eons, eras, periods, and epochs • Which part of the organism is preserved and available for study? • What kinds of habitats produce fossils?

4. Nature of the Fossil Record • Four categories of fossils • Compression and Impression • Organic matter is buried in water- or wind-borne sediment before decomposition • Weight of sediment causes impression in material below • Permineralized fossils • Structures buried in sediment and dissolved minerals precipitate in cells

6. Nature of the Fossil Record • Four categories of fossils • Casts and Molds • Remains decay or shells dissolve after being buried in sediment • Molds are unfilled spaces • Casts form when new material infiltrates space, fills it, and hardens to rock

8. Nature of the Fossil Record • Four categories of fossils • Unaltered remains • Preserved in environments that discourage loss from weathering, scavenging, and decomposition • Humans in peat bogs, mammoths in ice, dried dung from giant ground sloths, animals trapped in amber • Rarest type of fossil

10. Nature of the Fossil Record • Key factors for fossilization • Durability • Burial • Lack of oxygen • Fossils are primarily hard structures deposited in river deltas, beaches, floodplains, marshes, and sea floors

11. Nature of the Fossil Record • Biases in fossil record • Geographic • Lowland and marine habitats • Taxonomic • Marine organisms dominate fossil record but make up only 10% of extant species • Soft parts such as flowers almost never preserved • Temporal • Most recent species dominate • Not destroyed by tectonic activity yet

12. Nature of the Fossil Record • Geologic time scale divided into eons, eras, periods, epochs, and stages • Determined by diagnostic fossils • Absolute dated after discovery of radioactive isotopes • Before relative dating used • Lengths of time units not equal • Three eras of Phanerozoic • Paleozoic, Mesozoic, Cenozoic

18. The Cambrian Explosion • Nearly all animal phyla that ever existed first appear in fossil record during Cambrian • Very short geologic time • Animal body plans (bauplans) diversified incredibly during this time • Three key divisions of body plans determined by embryology

19. The Cambrian Explosion • Diploblasts and Triploblasts • Two or three embryonic layers • Ectoderm, Endoderm, and Mesoderm • Endoderm forms gut • Ectoderm forms skin • Mesoderm forms muscles, and most organs • Radially symmetrical animals are diploblastic • Bilateria are triploblastic

21. The Cambrian Explosion • Coelomates, Pseudocoelomates, and Acoelomates • Coelom = fluid filled cavity derived from mesoderm • Increases mobility • Acoelomates include flatworms • Pseudocoelomates include roundworms • Cavity not completely lined by peritoneum

23. The Cambrian Explosion • Protostomes and Deuterostomes • All have true coeloms • Difference in gastrulation • Protostomes form mouth first • Flatworms, roundworms, arthropods, annelids, mollusks • Deuterostomes form anus first • Echinoderms and Chordates

25. The Cambrian Explosion • All of these bauplans arose during Cambrian Explosion • Other major morphological innovations too • Segmented bodies • Shells • Appendages • Notochords

26. The Cambrian Explosion • Two major fossil faunas give evidence of emergence of new bauplans • Ediacaran faunas • 565—544 Mya • Burgess Shale • 520—515 Mya

27. The Cambrian Explosion • Ediacaran faunas • First discovered in Australia in 1940s • Now found at 20 sites around world • Late Precambrian • Compression and impression soft-bodied fossils • Sponges and jellyfish • Maybe annelids, arthropods, and mollusks

29. The Cambrian Explosion • Burgess Shale • Found in British Columbia • Chengjian Biota found in Yunnan, China • 525—520 Mya • Impression and compression fossils • Arthropods, annelids, mollusks, chordates • Agnathans • Little overlap with Ediacaran faunas

31. The Cambrian Explosion • Earliest members of most animal lineages appeared suddenly in fossil record at same time around globe • No obvious precursor older than Ediacaran fauna • Was the Cambrian Explosion really explosive?

32. The Cambrian Explosion • Estimated phylogeny of all animals using DNA sequence data • Did the branching events take place 565—525 Mya? • Molecular clock estimated earliest branches to be 900 Mya • Way before Cambrian Period • Chordates and echinoderms 1 Bya • Protostomes and deuterostomes 1.2 Bya

34. The Cambrian Explosion • Results indicate much earlier radiation and no Cambrian “explosion” • If true, older fossils should be found • Earliest chordate is from Chengjiang fauna 530 Mya • Therefore, chordates must be older than this time

35. The Cambrian Explosion • If molecular clock is correct, animal lineages are much older • But morphological explosion did occur in Cambrian • Maybe ecological explosion • New benthic and pelagic predators filling open niches • Rising oxygen in sea water made larger size possible • New ways of feeding and locomoting

36. Macroevolutionary Patterns • Adaptive radiation = single species quickly diversifies into many species • Hawaiian Drosophila • Galápagos finches • New ecological niches available to species • Can examine a phylogeny and see long later branches and short internodes

38. Macroevolutionary Patterns • Stasis • Long periods of little morphological change followed by rapid periods of speciation • Darwin proposed speciation was gradual • Eldredge and Gould (1972) proposed punctuated equilibrium • Stasis and rapid diversification • “Fits and starts” • Part of argument is problem of scale • Slow for biologists is rapid for paleontologists

40. Macroevolutionary Patterns • Punctuated equilibrium • Includes anagenesis = change in a species without diverging into other species • Phyletic transformation • Opposite is cladogenesis = one species splitting into two • Some species appear to demonstrate stasis • Living fossils • Horseshoe crabs, ginkos

42. Extinction • Rate of global extinctions has not been constant • Raup broke last 543 My into 1 My increments to examine constancy of extinction rate • Found several mass extinctions • Over 60% of living species went extinct

44. Extinction • Benton quantified how many extinctions occurred over last 510 My • Found the Big Five • Terminal Ordovician 440 Mya • Late Devonian 365 Mya • End Permian 250 Mya • End Triassic 215 Mya • Cretaceous-Tertiary (K-T) 65 Mya

46. Extinction • Over 96% of all extinctions did not occur during mass extinction events • Background extinctions • Likelihood of any clade becoming extinct is constant and independent of how long taxa have been in existence • Survivorship curves indicate this • Geographic range of species correlates negatively with extinction probability

48. Extinction • K-T High Impact Extinction • It is now well-known that an asteroid at least 10 km in diameter struck Chicxulub, Mexico causing the K-T extinction • Evidence from irridium deposits, shocked quartz, and microtekites • Glass spheres created by high impact and temperature