Evolution and the Fossil Record

1. Evolution and the Fossil Record The Cambrian and Beyond

2. The nature of the fossil record How organic remains fossilize

3. Four categories of fossils • defined by method of formation • Compression and impression fossils • Permineralized fossils • Casts and Molds • Unaltered Remains

4. Compression and Impression fossils • Made when organic material is buried in water or wind-borne sediment before it decomposes • The weight of the sediment causes the structure to leave an impression in the material it is resting on • Analogous to footprints in mud or leaves in wet concrete • Fig 17.1

5. Permineralized fossils • Form when structures are buried in sediments and dissolved minerals precipitate in the cells • Can preserve details of internal structure • Fig 17.2

6. Casts and molds • Molds are unfilled spaces left behind as organic material decays or dissolves away • Casts are made when the molds are filled in with new material which then hardens into rock • Provide information about external and internal surfaces. • Fig 17.3

7. Unaltered remains • mummified remains that are protected from weathering, animals and decomposition by bacteria and fungi • Found in peat bogs, permafrost very dry desiccating environments (dessert caves). Preserved in plant resins (amber) Fig 17.4 • Saturated tar sands

8. Trace Fossils • Basically these are signs left behind by living organisms rather than parts of the organisms themselves • Includes tracks, burrows, fecal material • Can be used to get a general idea of the type of life in some areas

9. Features of Objects Which Fossilize • Durable • Buried before or shortly after death (usually in water-saturated sediment) • Located in areas devoid of oxygen • Therefore…… Most fossils are of hard materials left in areas of deposition such as river deltas, flood plains, marshes, beaches, ocean bottoms and river beds • There is an abundant fossil record of organisms that normally burrow in sediments, such as bivalves

10. Strengths and Weaknesses of the fossil record • Bias - a potential weakness • 3 types of sampling bias • GEOGRAPHIC BIAS • TAXONOMIC BIAS • TEMPORAL BIAS

11. GEOGRAPHIC BIAS • Most fossils come from lowland and marine habitats where the conditions for fossilization are most prevalent

12. TAXONOMIC BIAS • Marine fossils dominate the fossil record but only 10% of extant species are marine • 2/3 of extant animal species have no hard parts which would lend themselves to being easily fossilized • Critical parts of plants, like flowers, are seldom fossilized

13. TEMPORAL BIAS • Old rocks are more rare than new rocks because when tectonic plates subduct or mountains erode they take their fossils with them • Therefore our sampling of ancient life forms is poor

14. Biases must be accounted for • Therefore…. • Paleontologists need to be aware of limitations in what the fossil record can tell us • We need to remember that bias is not, however, unique to paleontology • There are many other areas of research which are biased

15. DEVELOPMENTAL GENETICS • can work with only a few model systems which by no means represent all living groups • Examples are roundworms, fruit flies, and zebra fish for animals • E. coli and Saccharomyce cerevisieae are models that are used for molecular and cell biology • Ecology focuses on the upland havitats in North America and Europe.

16. The Geologic time scale a look at life through time

17. Geologic time scale • Is divided into Eons, Eras, Periods, Epochs, and Stages • First formulated as a relative time scale in the early 1800’s • Absolute times were added later as more accurate dating techniques were developed • The time scale is constantly being refined as more rocks are sampled and dating techniques get more sophisticated

18. Please become familiar with the Phanerozoic Eras periods as shown below. Cenozoic Era(65 mya to today) Quaternary (1.8 mya to today)Holocene (11,000 years to today)Pleistocene (1.8 mya to 11,000 yrs)Tertiary (65 to 1.8 mya)Pliocene (5 to 1.8 mya)Miocene (23 to 5 mya)Oligocene (38 to 23 mya)Eocene (54 to 38 mya)Paleocene (65 to 54 mya) Mesozoic Era(245 to 65 mya) Cretaceous (146 to 65 mya)Jurassic (208 to 146 mya)Triassic (245 to 208 mya) Paleozoic Era(544 to 245 mya) Permian (286 to 245 mya)Carboniferous (360 to 286 mya)       Pennsylvanian (325 to 286 mya)        Mississippian (360 to 325 mya) Devonian (410 to 360 mya)Silurian (440 to 410 mya)Ordovician (505 to 440 mya)Cambrian (544 to 505 mya)Tommotian (530 to 527 mya) Phanerozoic Eon(544 mya to present) Entire timeline

19. The Cambrian “Explosion” • Called such because almost all of the currently recognized animal phyla first make their appearance in the fossil record in the Cambrian • The Cambrian spanned “just” 40 million years • When the fossil record is scrutinized closely, it turns out that the fastest growth in the number of major new animal groups took place during the Tommotian

20. Important fossil records • EDIACARAN SHALE • BURGESS SHALE • CHENGJIANG BIOTA

21. EDIACARAN Fauna • South Australia • first fossil evidence of multicellular animals • Pre-Cambrian - 565 mya, late Proterozoic (Vendian) • mostly compression and impression • entirely soft-bodied examples, sponges, jellyfish etc • many are trace fossils

22. BURGESS SHALE • Slightly younger than Ediacaran shale, 520-515 mya • British Columbia • Primarily impression and compression • Have extraordinary detail • Wide variety of arthropods (including trilobites), segmented worms, molluscs, several chordates, including jawless vertebrates

23. Not much overlap between the two except for a few Cnidarians (sea pens) • Therefore it appears from these important fossil records that there was an “explosion” of animals in the Cambrian.

24. CHENGJIANG BIOTA • From Yunnan Province in China • veryimportant area • recently made accessible again • very rich in fossils • Found Zygotes and blastocyst that indicate bilateral symmetry

25. Was there really a Cambrian “Explosion” ? • EVIDENCE FROM MOLECULAR CLOCKS • Using molecular clock data from DNA and protein sequences estimates have been made on the order of branching in the animal phylogeny Fig 17.12

26. Cambrian Explosion • Estimates show that the earliest branches occurred somewhere between 1200 and 900 mya • This is hundreds of millions of years before they are found in any fossil record • This is a highly controversial area and implies a long history of animal evolution for which we have no fossil record

27. Evidence From Proterozoic Rock • If these projections are correct we should eventually find fossils of these animals in the Proterozoic rock • Some jawless fishes (vertebrates) have been found in China in the Chengjiang fauna that are 530 million years old • This would be indirect evidence that chordates arose much earlier than this

28. TAKE HOME MESSAGE? • The Cambrian explosion is an explosion of morphological forms but not necessarily of lineages • The evolution of these lineages may have been occurring gradually during the Proterozoic but existed as small and larva-like organisms which left no fossils • However, there is still no explanation for the dramatic changes in body size in the brief period of the Cambrian where these fossils are found

29. What caused the Cambrian “Explosion”? • Changes in the ecology of the earth most likely led to these changes.

30. ECOLOGICAL CHANGES • Organisms were filling new niches due to changes in • FEEDING BEHAVIORS • FROM: predominantly either sessile (attached) filter feeding organisms or those floating high in the water column living off of plankton • TO: to a huge variety of feeding mechanisms • LOCOMOTION. • FROM:Sessile or free floating organisms • TO: swimmers, walking, burrowing, both benthic and pelagic predators, scavengers and on and on

31. What Factors Led to These Changes • Locomotion changes? • Rising O2 levels • allowed larger body size • allows evolution of tissues and higher metabolic rates needed for powered movement • Shells formation? • Probably as a result of predator selection pressure • Have found shells that have holes drilled by predators • Evidence from the types of holes drilled that predators were selecting their prey by size.

32. What other ecological interactions may have led to selection pressures? • New types of food such as diversification in the plankton, may have favored novel feeding mechanisms • Anatomy that favors swimming or grasping (for example) may have been favored as a way to obtain prey

33. All of these changes require • genetic variation to be present • Would require changes in the genes that control embryonic development

34. Macroevolutionary patterns • An important part of evolutionary research is looking for broad patterns in the fossil record • Can give insight into how macroevolution may occur • A common pattern seen in the fossil record is Adaptive Radiation

35. Adaptive Radiation • A single ancestral species diversifies into a large number of species which occupy a wide variety of ecological niches • Where have we seen and talked about examples of adaptive radiation? • Darwin’s Finches • Hawaiian drosophilids

36. Factors That Trigger Radiation of Species • What factors were responsible for the radiation in the finches? • ecological opportunity • Colonized a habitat that had few competitors and wide variety of resourcese • Leads to morphological innovations like the beak types

37. Ecological Opportunities • Ecological opportunity is not created solely through colonization events. • Mass extinction • Mammals diversified rapidly after the dinosaurs became extinct.

38. Adaptive radiation • Morphological innovations lead to radiation. • Example: arthropods ( insects, spiders, crustaceans). • inhabit a wide variety of niches based on modification of their jointed limbs • swimming, flying, running, jumping, grasping, walking

39. Examples from plants • as plants moved from aquatic to terrestrial habitat in the early Devonian ( 400mya) • developed leaves and vascular tissue

40. Explosion of flowering plants in the Cretaceous • The flower structure allowed a rapid expansion into new niches • Pollination strategies • including co-evolution with insects • dispersal mechanisms for seeds

41. Stasis vs. Gradualism

42. GRADUALISM • The Darwinian approach • In this pattern organisms are continually changing gradually form one from to another. • Occurs by a progressive accumulation of micromutations which leads to the formation of a new species.

43. STASIS • New morphologies appear in the fossil record and then remain unchanged for millions of years • Often, evolutionary innovations appear at the same time as new species • This results in morphological evolution that consists of long periods of no change (stasis) occasionally punctuated by speciation events that appear instantly in the geologic record

44. Gradual changes are rarely seen in the geologic record • What do you suppose Darwin would say when confronted with today’s fossil record? • He predicted that gaps in the fossil record would be filled in over time, with gradual transitions

45. WHY STASIS • Possibly a lack of genetic variation to work on • There is strong evidence that lack of genetic variation is not the cause of stasis • One living fossil, the horseshoe crab does not have any less genetic variation than groups that have evolved significantly • May be in dynamic stasis. Think of the finches and how they change with drought vs. flood years. there is an oscillation back and forth, fluctuating about a mean, but in the fossil record we perceive it as stasis

46. Theory of Punctuated Equilibrium • Proposed in 1972 by Eldredge and Gould (we will be reading this paper later.) • Led to some very heated debates for over 20 years • Debate revolved around differences in observing patterns of speciation and change on a biological time scale of years or decades vs. a geological time scale of millions of years • On a biological time scale gradual change and natural selection are important (and observable) • On a geological scale “instantaneous” seeming changes could actually be taking millions of years

47. EXTINCTION • The ultimate fate for all species • Several clear patterns of extinction • Global extinction rates are not constant • Two basic categories of extinctions • Mass extinctions • Background extinctions

48. To be a mass extinction requires • A broad range of organisms being affected • Global extinction • Rapid relative to the expected life span of the taxa that are lost • A mass extinction leads to the loss of over 60% of the species in a period of a million years

49. “ The Big Five” • During the Phanerozoic there have been 5 mass extinctions • Together these account for 4% of all extinctions • At the terminal Ordovician 440 mya • Late Devonian 365 mya • End-Permian 250 mya • end Triassic 215 mya • Cretaceous-Tertiary (K-T) 65 mya

50. Background Extinctions • occurred at constant rates • make up 96% of all extinctions • The likelihood of subclades becoming extinct is constant and independent on how long the taxa have been in existence • The probability of a subgroup becoming extinct is constant over the lifespan of the larger clade • Rates of extinction are constant within clades but highly variable across clades • The extinction rate of marine organisms vary depending on how far the larvae disperse after the egg is fertilized • Greater distance leads to greater colonizing ability which might reduce extinction rate