Lecture 3 Flashcards
Fossils-What are they, what do they tell us, examples
- remains of former life
- give a view of extinct morphologies, species, ecosystems, and evolutionary transitions between now clearly separate forms
- include bacteria, algae, plants, invertebrates, and vertebrates
- also include tracks, traces, burrows, that give us a view of the activities of extinct life
Modes of preservation
- actual remains, such as frozen mammoth, wood, shells, bone
- mineral replacement “petrified”
- impressions
- casts
- special modes like amber
- chemical fossils-event ancient DNA
- trails and other trace fossils
- coprolites
- eggs, nests, fossil fetuses
- environmental traces, including fossil raindrops, ripples, dunes, mud cracks, glacial scrape marks
Pre-modern interpretation of fossils
-generally seen as items created in place and mimicking shells, teeth, or bones
Leonardo da Vinci and Nicklaus Steno
- realized that fossils were remains of former life
- da Vinci speculated that shells on mountains showed that they were once covered by the sea
- most famous demonstration was of fossil tongue stones to teeth of living sharks
John Woodward
-by 1680 fossils are seen as remains from the Deluge (flood of Noah)
George Curvier
- introduces reality of extinction, long history of life, repeated catastrophes and re-population by new organisms
- use of biostratigraphy and index fossils
- invented archeology
Modes of preservation
- sediments-almost all fossils in sedimentary rocks
- biases: these affect what we see as fossils
conglomerate
-rocks cemented
sandstone
-sand cemented
shale
-mud cemented
limestone
-shell bits
chert
-silica
Environmental Bias
- fossilization requires sedimentation
- best sites: shallow marine basins, reefs, lakes, river flood plains
Preservational bias
-hard parts of large creatures best; small creatures with soft bodies least likely to leave fossils
Depositional environmental bias
-fossilization unlikely if scavengers are present, soil is acidic, there are high O2 levels which encourage decah, strong water currents or bioturbation moves things around
How do fossils come to be?
- have to know how rocks are made
- sediment and these things get stuck under layers and layers of sediment over thousands of years make a fossil
- most of the time what dies rots away so no fossil is made
- so the fossil record is kind of sparse, not a complete record of everything that ever lived
Sampling biases and gaps
- over and under representation by environment (shallow water marine–>good, mountains–>poor)
- by taxon: mollusks, brachiopods, corals, echinoderms, all well represented while insects, spiders are poorly represented and flatworms, nematodes, rotifers, and ctenophores are almost not at all represented
- by geological time period
- by continent: Australia is a poor record of invertebrates
taphonomy
-do experiments to determine how dead things get preserved
Nematodes
- most diverse species
- only have fossils of 5 kinds
gaps
- missing rock sequenes common in geological record
- due to non-deposition, non-preservation, destruction by tectonic forces
- destruction by erosion
- the higher you go in the taxonomic category the more representation you get so you can see the big picture
Importance of fossil record
-without it we would know that whales are mammals but we wouldn’t know how they got there
special and unusual preservation in fossil record
- most valuable kinds of fossils
- unusual deposition, fast, fine sediment, no O2 microbial action to replace and mineralize soft tissue
- entire small soft bodied animals, hair, feathers, dinosaur eggs with embryos, body tissue of frogs and mammals, wings/internal ears of bats
Cabrian
-Burgess shale, Chengjiang
Pennsylvanian
-Mason Creek
Jurassic
-Solnhoffen
Eocene
-Messel
Mesozoic/Cenozoic
-Ambers
What we learn from fossil record
- sampling-how complete is our view of life history in the fossil record?
- extinct diversity
- times of origination and extinction-confidence intervals and data to use for determining evolutionary rates
- observe evolutionary transitions
- rates and patterns of evolution in the past
- paleobiogeography
- paleoclimate
- paleoecology
Sampling
- if we look at living marine animals, only 20% would leave fossils under normal circumstances
- gaps in deposition/erosion
- if only a tiny minority of extinct species is known, how good can our record of the major patterns of evolution be?
- ex: for animal fossil record should recover nearly all phyla, most classes and proportionally fewer lower taxonomic categories
Extinct diversity
- reveals former life on earth. Trilobites, dinosaurs, strange Cambrian animals all known only via the fossil record
- patterns change through time
Kinds of questions
- was live always as divers as now?
- does the record reveal trends or underlying regularities? (past extinctions only known this way)
- is there progress?
- can we find transitional forms in evolutionary series?
Evolutionary transitions
- to observe speciation
- to observe macroevolution, transitions between major groups: fish to amphibian, reptile to mammal, reptile to bird, land mammal to whale
- to test theories of evolution: gradual vs. punctuational
Vertebrates in the fossil record
- origins and new structural features (jaws, lungs, legs)
- rise and fall in diversity
- transition to land life
- meaning of extinctions-competition vs. convergence
Species origins
-difficult to pick up in fossil record because of the sampling problem. However some species to species transitions are known from the fossil record
Transitional forms between distinct groups
- several
- fish to amphibian, dinosaur to bird, land mammal to whale, lizard to snake, hominid to human
