Why an “explosion”?

1. Notes: This nautilus lives in waters at a depth of 600 to 800 meters. Often their habitat is at the bottom of a steep slope under coral reefs. At night they use their natural jet propulsion to maneuver up over 325 meters to feed.

2. Why an “explosion”? • What were conditions like at the end of the Proterozoic?Supercontinent was breaking up climate warming after glacial period Increased oxygen in atmosphere More coastal habitat – shelf environments Eukaryotic cells added to diversity Stage was set for larger cells, specialized cells, and multi-celled organisms Sexual reproduction allowed for shuffling of alleles increasing variation within species Result? Rapid diversification of life forms and numbers of families and species occurred during the early Paleozoic. No “explosion” – it took millions of years.

3. Burgess Shale • Diorama of the environment and biota • of the Phyllopod bed of the Burgess Shale, • British Columbia, Canada

4. Burgess Shale Soft-Bodied Fossils • On August 30 and 31, 1909, • Charles D. Walcott, • geologist and head of the Smithsonian Institution, • discovered the first soft-bodied fossils • from the Burgess Shale, • a discovery of immense importance in deciphering the early history of life • beautifully preserved on bedding planes

5. More Complete Picture of a Middle Cambrian Community • a rare glimpse into a world previously almost unknown • that of the soft-bodied animals that lived some 530 million years ago • a much more complete picture • of a Middle Cambrian community • than deposits containing only fossils of the hard parts of organisms (the bias of the fossil record favors hard parts to be preserved)

6. Sixty Percent Soft-Bodied • 60% of the total Burgess Shale assemblage • of more than 100 genera is composed of soft-bodied animals, • a percentage comparable to present-day marine communities • What conditions led to the remarkable preservation of the Burgess Shale fauna? • The site of deposition of the Burgess Shale • was located at the base of a steep submarine escarpment

7. Reason for the Preservation • The animals lived in and on mud banks • that formed along the top of this escarpment • Periodically, this unstable area • would slump and slide down the escarpment as a turbidity current • At the base, the mud and animals carried with it were deposited in a deep-water anaerobic environment devoid of life, predators, and O2.

8. Carbonaceous Impressions • In such an environment, • bacterial degradation did not destroy the buried animals • and they were compressed by the weight of the overlying sediments • and eventually preserved as carbonaceous impressions

9. Study of Paleozoic Life • Evolution and plate tectonics forcing: Early Paleozoic characterized by • The opening and closing of ocean basins, (Wilson Cycles) • transgressions and regressions of epeiric seas, • as evidenced by Cratonic Sequences • the formation of mountain ranges, • Orogenies that formed Mobile Belts • and the changing positions of the continents • had a profound effect on the evolution • of the marine and terrestrial communities

10. Tremendous Biologic Change • appearance of skeletonized animals • near the Precambrian-Cambrian boundary • marine invertebrates • began a period of relatively rapid evolution of body types • Paleozoic marine invertebrate community greatly diversified • Actually the “explosion” had taken place over millions of years during the Early Cambrian Period

11. The Cambrian Explosion • At the beginning of the Paleozoic Era, • animals with skeletons appeared rather abruptly in the fossil record Microscopic, then larger, visible shelly fauna

12. Lower Cambrian Shelly Fossil • A conical sclerite* of Lapworthella from Australia * a piece of the armor covering • This specimen is several millimeters in size

13. Lower Cambrian Shelly Fossil • Archaeooides, an enigmatic spherical fossil from the Mackenzie Mountains, Northwest Territories, Canada • This specimen is several millimeters in size

14. What are advantages of an Exoskeleton • : (1) It provides protection against ultraviolet radiation, allowing animals to move into shallower waters; (2) it helps prevent drying out in an intertidal environment; (3) it provides protection against predators • Recent evidence of actual fossils of predators • and specimens of damaged prey, • as well as antipredatory adaptations in some animals, • indicates that the impact of predation during the Cambrian was great

15. Cambrian Predator • Reconstruction of Anamalocaris • a predator from the Early and Middle Cambrian • It was about 45 cm long and probably fed on trilobites • Its gripping appendages presumably carried food to its mouth

16. Wounded Trilobite • Wounds to the body of the trilobite Olenellus robsonensis • The wounds have healed, demonstrating that they occurred when the animal was alive and were not inflicted on an empty shell

17. Advantages of an Exoskeleton • With predators playing an important role • in the Cambrian marine ecosystem, • any mechanism or feature • that protected an animal • would certainly be advantageous • and confer an adaptive advantage to the organism (4) A fourth advantage is that • a supporting skeleton, whether an exo- or endoskeleton, • allows animals to increase their size • and provides attachment sites for muscles

18. Planktonic organisms • Plankton are mostly passive and go where currents carry them • Plant plankton • such as diatoms, dinoflagellates, and various algae, • are called phytoplankton and are mostly microscopic • Many are photosynthesizers • Animal plankton are called zooplankton and are also mostly microscopic • Examples of zooplankton include foraminifera, radiolarians, and jellyfish

19. Nektonic organisms • The nekton are swimmers • and are mainly vertebrates • such as fish • the invertebrate nekton • include cephalopods -- chambered, coiled shellfish

20. Benthic organisms • Organisms that live • on or in the seafloor make up the benthos • They can be characterized • as epifauna (animals) or epiflora (plants), • for those that live on the seafloor, • or as infauna, • which are animals living in and moving through the sediments

21. Sessile and Mobile • The benthos can be further divided • into those organisms that stay in one place, • called sessile, • and those that move around on or in the seafloor, • called mobile

22. Marine Ecosystem Infauna: worm, bivalve Mobile epifauna: gastropod, starfish

23. Marine Ecosystem • Where and how animals and plants live in the marine ecosystem Plankton: Jelly fish Sessile epiflora: seaweed Nekton: fish cephalopod Sessile epifauna: bivalve Benthos: d-k crinoid coral

24. Cambrian Skeletonized Life • Although almost all the major invertebrate phyla • evolved during the Cambrian Period • many were represented by only a few species • While trace fossils are common • and echinoderms diverse, • the organisms that comprised the majority of Cambrian skeletonized life were • trilobites, • inarticulate brachiopods, • and archaeocyathids

25. Cambrian Marine Community • Floating jellyfish, swimming arthropods, benthonic sponges, and scavenging trilobites Reconstruction

26. Trilobites • Trilobites were • by far the most conspicuous element • of the Cambrian marine invertebrate community • and made up about half of the total fauna • Trilobites were • benthonic • mobile • sediment-deposit feeders • that crawled or swam along the seafloor

27. Trilobites • They first appeared in the Early Cambrian, • rapidly diversified, • reached their maximum diversity • in the Late Cambrian, • and then suffered mass extinctions • near the end of the Cambrian • from which they never fully recovered • As yet no consensus exists on what caused the trilobite extinctions

28. Trilobite Extinctions • possible reduction of shelf space, • increased competition, • rise in predators • cooling of the seas may have played a role, at the end of the Ordovician Period

29. Cambrian Brachiopods • Cambrian brachiopods • were mostly primitive types called inarticulates • They secreted a chitinophosphate shell, • composed of the organic compound chitin • combined with calcium phosphate (like teeth) • Inarticulate brachiopods • also lacked a tooth-and-socket-arrangement • along the hinge line of their shells

30. Articulate Brachiopods • The articulate brachiopods, • which have a tooth-and-socket arrangement, • were also present • but did not become abundant • until the Ordovician Period

31. Archaeocyathids • The third major group of Cambrian organisms • were the archaeocyathids • These organisms • were benthonic sessile suspension feeders • that constructed reeflikestructures

32. Cambrian Reeflike Structure • Restoration of a Cambrian reeflike structure built by archeocyathids

33. Back to the Burgess Shale…. • The diversity of organisms preserved in the mud that is now the Burgess Shale shows • Most modern phyla are represented • Additional organisms cannot be placed in the evolutionary tree • A primitive chordate may have existed – “Pikaia”

34. Pikaia, chordateancestor of vertebrates??

35. Rare Preservation: Burgess Shale • Ottoia, a carnivorous worm

36. Rare Preservation: Burgess Shale • Wiwaxia, a scaly armored sluglike creature whose affinities remain controversial

37. Rare Preservation: Burgess Shale • Hallucigenia, a velvet worm

38. Rare Preservation: Burgess Shale • Waptia, an anthropod

39. Cambrian Phyla • Thus, the phyla of the Cambrian world • were viewed as being essentially the same in number • as the phyla of the present-day world, • but with fewer species in each phylum • According to this view, the history of life • has been simply a gradual increase in the diversity of species • within each phylum through time • The number of basic body plans • has therefore remained more or less constant • since the initial radiation of multicelled organisms

40. Striking Changes in Ordovician • The Cambrian invertebrate community • was dominated by trilobites, inarticulate brachiopods, and archaeocyathids, • the Ordovician was characterized • by the adaptive radiation of many other animal phyla, • such as articulate brachiopods, bryozoans, and corals

41. Middle Ordovician Seafloor Fauna • Recreation of a Middle Ordovician seafloor fauna with cephalopods, crinoids, colonial corals, trilobites, and brachiopods

42. Mass Extinctions • The end of the Ordovician • was a time of mass extinctions in the marine realm • More than 100 families of marine invertebrates became extinct, • and in North America alone, • approximately one-half of the brachiopods and bryozoans died out • What caused such an event? • Many geologists think these extinctions • were the result of the extensive glaciation • that occurred in Gondwana • at the end of the Ordovician Period

43. Mass Extinctions • Mass extinctions, • those geologically rapid events • in which an unusually high percentage • of the fauna and/or flora becomes extinct, • have occurred throughout geologic time • for instance, at or near the end of the • Ordovician, • Devonian, • Permian, • and Cretaceous periods • and are the focus of much research and debate

44. Silurian and Devonian Reefs • The Silurian and Devonian reefs • were dominated by • tabulate and colonial rugose corals and stromatoporoids • While the fauna of these Silurian and Devonian reefs • was somewhat different • from that of earlier reefs and reeflike structures, • the general composition and structure • are the same as in present-day reefs

45. Middle Devonian Reef • Reconstruction of a Middle Devonian reef from the Great Lakes area • with corals, cephalopods, trilobites, crinoids, and brachiopods

46. Silurian Brackish-Marine Scene • Restoration of a Silurian brackish-marine bottom scene • near Buffalo New York • with algae, eurypterids, gastropods, worms, and shrimp

47. Ammonoids • Ammonoids are excellent guide fossils • for the Devonian through Cretaceous periods • with their distinctive suture patterns, • short stratigraphic ranges, • and widespread distribution

48. Ammonoid Cephalopod • A late Devonian ammonoid cephalopod • from Erfoud, Morocco • The distinctive suture pattern, short stratigraphic range, and wide geographic distribution make ammonoids excellent guide fossils

49. Another Mass Extinction • Another mass extinction • occurred near the end of the Devonian • and resulted in a worldwide near-total collapse • of the massive reef communities • On land, however, the seedless vascular plants • were seemingly unaffected, • Thus, extinctions at this time • were most extensive in the marine realm, • particularly in the reef and pelagic communities

50. Permian Period • Paleogeography of North America during the Permian Period