BIOL 4160


Phil Ganter

301 Harned Hall



The Fossil Record

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Geological Review


  • are classified by many different schemes (mineral composition, etc.)
  • Classification by Origin
    • Igneous - rock formed by cooling of fluids coming from the mantle (basalts, granites)
    • Sedimentary - rock formed from layers of sediment or precipitate that accumulate at the bottom of bodies of water or the particles that settle out of air (Limestone, Shale, Conglomerate, and Chalk, dear to faculty)
      • The particles can be of biotic origin (mostly skeletons of CaCO3 and SiO2)
      • The rock forms at the bottom of the accumulations due to pressure from the sediments that overlie the deep layers
      • Three types based on the origin of the materials (Clastic - particles, Organic - biological origins, Chemical - from layers that precipitated out of supersaturated water)
    • Metamorphic - rocks that are the product of extreme pressure and heat acting on pre-existing rocks (sedimentary, igneous or even metamorphic) (gneiss, slate, marble, schist, or quartzite)

Fossils are traces of pre-existing life

can be formed from:

  • the entire organism (insects in amber)
  • the hard (usually mineral) parts of organisms (including external shells)
  • structures built by organisms (like the tubes that tube worms live in or coral formations)
  • rock that has replaced the original material
  • impressions of the original organism preserved as the rock formed
  • impressions left by the activities of the organisms (tracks in sediment, burrow holes, etc.)
  • Fossils are never found in igneous rock and rarely in metamorphic but are common in sedimentary rock


Not time enough to review Plate Tectonics here but know:

  • Plate
  • Subduction Zone
  • Spreading Zone
  • Continental Drift
  • Mid-Oceanic Ridge
  • Rift Lake
  • Volcano and Hot Spot
  • Oceanic Trench
  • Fault
  • Earthquake
  • Magnetic Striping


Relative Dating

  • Relative dates only tell which organisms precede others in the record and these are based on observations of their occurrence in strata and two assumptions from geology:
    • Strata - distinct layers in sedimentary rock (note that folding of the rock may mean that strata are occasionally not horizontal but vertical and even upside down)
    • Principle of Superposition - the oldest layer is at the bottom and each layer above is younger until the youngest layer, which is on the top of the strata
    • Principle of Uniformitarianism - the geologic process that are observable today are the same that operated long ago
  • Thus, we can date organism origins and extinctions depending on the strata in which their fossils are found

    Geological Time Scale

    • This is the relative scale developed before the ability to absolutely date rock formation
    • Phanerozoic (Gr. "visible animals" - the period from now until the point in the past when animal fossils become very rare) is divided into three eras, each of which is divided into a different number of Periods, each of which is divided into Epochs (we will only know the epochs of the most recent Period, the Cenozoic

Absolute Ages

  • Radiometric Dating
    • Dating by comparing the amount of a radioisotope with the amount of its decay product in a rock (often adjusted for the amount of decay product expected in a newly formed rock)
    • Unstable nuclei decay by emitting energy and particles (both are "radiation" from the nucleus, which changes the number of protons in the nucleus and, thus, the element to which the nucleus belongs
    • This process will reduce the amount of the parent element in a rock and increase the amount of the decay product over time
  • Half Life
    • Although it is impossible to predict when an individual unstable nucleus decays, the behavior of a large number of nuclei is predictable and each type of unstable nucleus has a characteristic probability of decay during a given period of time and therefore a large number of nuclei have a characteristic rate of decay
    • We use the idea of a half life to describe this rate
    • A half life is the time -in the appropriate scale (from millions of years to millionths of seconds) - it takes for half of the radioactive nuclei in a sample to decay
  • The type of rock affects the possibility of dating
    • Igneous rocks can often be dated from when the solidified (an subsequently cooled to below the closure temperature - see below)
    • Metamorphic rock can be dated from when it cooled below its Closure Temperature (the temperature where parent an decay products are trapped in the rock) when its clock was set to zero
    • Sedimentary rocks are made of rocks that are older than the formation of the sedimentary rock
    • If either metamorphic or igneous rocks are heated to the closure temperature of the particular elements being used for dating (different elements have different closure temperatures), then the clock is reset to 0
  • The actual dating of rocks is done by choosing the radioactive species suitable for the expected age of the rock (14C would not do for even moderately old rocks) and often more than one isotope is chosen to cross-check the dating
    • accuracy of some clocks is very good (2 million years out of 2 billion for some uranium clocks)
    • agreement of different clocks measured from the same sample can also be good (to within less than 1 %)

The Record

Below is the Geological Record - you should know the Eons, Eras, Periods, and Epochs listed and the starting dates of Eons and Eras

  • Also know the smallest division of the record during which the following "First" fossils appeared and these significant periods of expansions/radiations:
    • First Prokaryotes, Invertebrates, Land Plants, Jawed Fish, Land Plants, Land Vertebrates, Mammals, Flowering Plants
    • Expansions/Radiation of: Oxygen in the Atmosphere, Jawed Fish, Forests; Reptiles, Flowering Plants, Mammals

Quality of the Fossil Record

  • Fossils are at risk of destruction by geologic forces (from subduction to metamorphosis to weathering of exposed fossils) once they are made
    • Therefore the fossil record of any time deteriorates as time goes on and we should expect to see what we do see: more complete records of more recent events
  • Only some environments lead to fossilization
    • Even where fossils might form, the opportunity to form fossils might be very episodic
    • Some areas only accumulate sediment under extreme conditions
  • The anatomy of the organisms affects their likelihood of fossilization
    • Hard parts (cell walls, endo- and exoskeletons, etc.) are most likely to become fossilized
    • Examples: some benthic organism like echinoderms (endoskeletons), invertebrates with exoskeletons (trilobites) or shells (snails or cephalopods with shells like ammonites), and protists with mineral shells (silica shells of diatoms and radiolarians and the calcareous shells of foraminifera)
  • This means that the fossil record is very incomplete and will vary greatly for different groups of organisms
    • The record of most important evolutionary events was never in the fossil record
    • Many events that were recorded in fossils have since been lost
    • Many fossils remain as of yet undiscovered and uncatalogued by science
      • Now is an exciting time as parts of the world that have received less attention are now being more thoroughly searched (South America and Central Asia, to name a couple)
  • Thus, we must view the fossil record as one that can provide "positive" evidence far more reliable than it can provide "negative" evidence (another instance of the saw "the absence of evidence is not the same as the evidence of absence")
    • Given the flaws in the record, there are a remarkable number of important events well documented in the record
    • Most are macroevolutionary but, in some instances, microevolutionary events (within species) can be documented in the record

Some Macroevolutionary records illustrating Intermediates in the origins of modern groups

  • We have two independent sources of evolutionary history: sequence data (DNA, protein) and the fossil record
    • To a high degree, the two records agree and often the earliest branches of the tree have the oldest fossil record (book uses bristletail - earliest fossil insect and earliest branch - as an example)
      • One common discrepancy between fossil and sequence records is that the age of groups is often older when estimated from sequence data than the oldest known fossil
        • This is not unexpected, as fossilization may not commence with the origin of the clade
        • largest discrepancy is in the origin of animal phyla (was the Cambrian Explosion an explosion in new phyla or in new hard parts to fossilize?)
  • Because the fossil record is incomplete, it is reasonable to ask if there are lineages with a record complete enough to document long-term macroevolution
    • Many cases of well-documented macroevolution exist and we illustrate the point with a brief discussion of four cases here

Amphibia -

  • Amphibia consist of the salamanders and the frogs/toads - they are the oldest (according to the fossil record) of the four-legged land vertebrates (called the Tetrapoda for their limbs)
  • Closest relatives are the Sarcopterygii ("fleshy fin"), the lobe-finned fish (the famous Coelacanth and lungfish), based on molecular and anatomic evidence
    • Lobe-finned fish have fins attached to short limbs that protrudes from the side of the fish's body (like a primitive tetrapod limb) and often have lungs
    • Rhipidistia - extinct group of lobe-finned fish with both lungs and gills
  • Earliest Amphibian fossils (Ichthyostega) fossil has both Rhipidistian and Tetrapod features
    • Bones of the braincase, dermal skull bones, lateral line canals, teeth were all Rhipidistian
    • Pelvic and pectoral girdles were large, as in Tetrapods, and their limbs were fully walking limbs (like Tetrapods) except that the digits are variable in number and not as strong as in Tetrapods (which all have definite digit numbers)
      • Rhipidistians have variable, small digits


  • Bird origins were, until recently, a hot topic for argument - are birds a separate lineage or are they a kind of dinosaur,  the only remaining linage of that once earth-shaking and dominant group?
    • Fossils from China have provided so many transitional forms that the origins of the birds from within the dinosaur lineage is no longer disputed
    • Dinosaurs have been found with "bird" features such as: feathers, hollow long bones, long "flight" feathers on limbs (probably not for actual powered flight), fused tail vertebrae, keeled breastbone, loss of digits


  • Mammal skeletons fossilize well and the record reflects the change in lineages
    • Many changes are those in the skull due mostly to adaptations to different diets (structure of the jaw, teeth, muscle attachments and size)
  • Mammals belong to the Synapsid lineage of the Amniota
    • Synapsids have a fenestra (window) posterior to the orbit
  • Changes from early Synapsids to Mammals
    • Jaw Structure changes from a lower jaw composed of several bones to the Mammalian condition of a lower jaw composed of a single bone
    • Articulation changes from between Quadrate (skull) and Articular (lower jaw) to an intermediate form with both the primitive condition and the derived, to the final Mammalian condition of Squamosal (skull) and Dentary (lower jaw)
    • Early Synapsids had only a single bone for sound transmission but Mammals have three: the original Stapes (stirrup) and two jaw bones become modified to transmit sound as the Incus ("anvil"- once the Quadrate) and malleus ("hammer" - once the Articular)
    • Teeth in early Synapsids had mostly single cusps (points) but Mammals teeth range from a single cusp (canines) to four or five (molars)


  • Hominid evolution is important to us but is not a particularly rich fossil record and so there are limits on our knowledge
    • Better record than Pan fossils (none) - probably due to habitat change (decomposition on forest's moist floor is complete) - makes you wonder if there are not hominid lineages in the forests!
  • The order Primates is composed of two suborders split into four infraorders
    • One suborder is the Haplorhini (also called the Similiformes - us, monkeys and apes) which has three infraorders, of which one is the Catarrhini (the name means down-turned nostrils)
      • Catarrhini is composed of two superfamilies Cercopithecoidea ("tailed apes" the old world monkeys - monkeys without prehensile tails [dentition is also different from apes]) and the Hominoidea
        • Hominoidea is composed of three families Hylobatidae (gibbons - old world), Pongidae (great apes - Gorilla, Pongo [orangs], and Pan [chimps]- old world), and Hominidae (Homo is the only living genus)
  • Paucity of specimens for many extinct Hominds makes it hard to define separate species
    • Normally done by looking at variation among fossils - species boundaries are set by large jumps in variation (within species variation is small but variation much larger if two species are included in the calculations)
    • Leads to disagreement over naming of fossils
  • Earliest fossil hominid is from late Miocene (7 MYA)
    • Hominid vs. Pongid characters
      • Ancestral characters - lower face projects out beyond upper face, large canine teeth, long arms (relative to leg length), curved bones in fingers, small brain case
      • Derived hominid characters - flattened face, bipedalism, large brain case
    • Several lineages are extinct (represented by different genera - Australopithecus, Sahelanthropus, Orrorin, Ardipithecus, Kenyathropus)
  • Genus Homo - brain case volumes increase, more erect - may be one species and what we are seeing is anagenesis or different clades
    • habilis - 1.9 MYA - tool makers (may have originated before Homo)
    • erectus - 1.6 MYA - more sophisticate tools, used fire
    • sapiens - 0.2 MYA - controversy over status of neanderthals (different species, same species?) - no doubt that the more slender "sapiens" form moved out of Africa 0.12 MYA and overlapped with populations of neanderthals in Europe and Asia and replaced the earlier forms about 40,000 years ago in Europe
      • Note - earliest evidence of Australian culture dates from at least 60,000 years ago
      • No evidence that the overlap of "sapiens" and "neanderthals" lead to interbreeding

Evolutionary Trends

  • Within-lineage trends
    • example - horse and whale lineages get larger through time
      • There are a host of these trends, often named for the biologist who first described them
      • Cope's rule (larger body size over time, Bergman's rule, etc.)
    • Reversals linked to loss of adaptive value of traits
    • Dollo's Law - once lost, complex characters are never regained
    • Development often contains evidence of primitive conditions (used to be known as the "ontogeny recapitulates phylogeny" rule)
      • Expected because adaptations are modifications of pre-existing phenotypes, not "re-designs"
      • Phylogenetic Burden - Some primitive structures may be found in development because they are important in the embryo, although they are not retained in the adult
        • Notochord is primitive, vertebra are derived but vertebrates all have a notochord in their embryos
        • Notochord is important in induction of the neural tube and formation of the CNS
  • Punctuated Equilibrium
    • Describes lineages in which morphology within species seems to be in "stasis" (state of no change) for long periods of time
      • Stasis is occasionally broken ("punctuated") for short periods during which large changes appear in the record
    • Punctuated equilibrium is an alternative to Phyletic gradualism (long periods of gradual change)
    • Cases of both patterns are known
      • Reason for punctuated equilibria are hard to discern from contemporary dynamics of populations
      • Stasis may be due to stabilizing selection or (as the originators of p.e. proposed) to developmental constraints on phenotypes
      • Sudden appearance of new forms harder to explain (founder effect? some undocumented genetic event such as genome duplication (hybrid polyploidy?)
Last updated January 25, 2010