Notes towards Biodiversity Chapter 6
Introductory/Title Slide (1)
Hello. My name is Gwen Raitt. I will be presenting this chapter on extinction over time.
What is Extinction?
Extinction is the process through which a species or higher taxonomic category ceases to
exist (Fiedler & Jain 1992, Wikipedia Contributors 2006a). This definition may be
applied within selected geographical boundaries (Fiedler & Jain 1992). Under this
definition, extinct species may or may not have descendants (Wikipedia Contributors
2006a). Extinction may also be defined as the disappearance of any evolutionary lineage
(from populations to species to higher taxonomic categories) because of death or the
genetic modification of every individual (Cox 1997, Broswimmer 2002).
Where a lineage has changed such that a new (daughter) species is recognised, the
extinction of the original (parent) species may also be called pseudoextinction (Futuyma
1998, Wikipedia Contributors 2006a). Pseudoextinction is hard to show (Wikipedia
Contributors 2006a). The new and original species are known as chronospecies
(Futuyma 1998). Extinction may be regarded as the result of failing to adapt to
environmental changes (Futuyma 1998). Extinction is a natural process. Every species
will become extinct (Cox 1997, Freeman & Herron 1998, Lévêque & Mounolou 2001).
Calculations from the fossil record show that most genera have a relatively short lifespan
of less than 50 million years. The average lifespan of a genus is 5—10 million years
(Gaston & Spicer 1998) while a species has an average lifespan of 4 million years
(Groombridge 1992, Barbault & Sastrapradja 1995). The picture shows a representation
of the extinct Steller’s sea cow (Hydrodamalis gigas). Steller’s sea cow was native to the
Bering Sea. The last individual was killed in 1767 (Caughley & Gunn 1996).
The Fossil Record – Key to the Past
The Occurrence of Fossil-Bearing Rocks
Fossils are usually found in sedimentary rocks (Futuyma 1998, 2005). The picture shows
a piece of limestone containing fossils. Fossils in metamorphic rocks are normally
changed beyond recognition. Fossils never occur in igneous rocks (Futuyma 1998,
2005). Sedimentary deposits are most likely in low-lying areas such as aquatic
environments. Marine sediments are most common (Freeman & Herron 1998, Futuyma
1998). Each site may have fossils representing a limited fraction of geological time
because sediment deposition was not continuous and sedimentary rocks erode (Futuyma
1998). The further back in time, the fewer the sedimentary deposits that are available
because there has been more time for erosion to act or metamorphosis to occur (Futuyma
1998).
The Fossil Record – Key to the Past
An Incomplete Record
The fossil record is known to be incomplete because some time periods are poorly
represented by sedimentary rock formations (Futuyma 1998, 2005) (The picture shows a
sedimentary rock formation in the Grand Canyon, USA.), Lazarus taxa show up at widely
separated time intervals (Raup 1988, Futuyma 1998, 2005), many large extinct species
are only represented by a few specimens and the rate of description of new fossil species
is steady (Futuyma 1998, 2005)
Fossil formation depends on the durability of the specimen, burial and lack of oxygen
(Freeman & Herron 1998). Most organisms do not form fossils because they do not have
hard skeletal parts that will endure long enough for fossilization to occur, they get eaten
so nothing is left to become fossilized, they occur in places where decay is rapid (e.g.
tropical forests) or deposition does not occur (higher elevations) or they did not live/die
during a period of sedimentation (Freeman & Herron 1998, Futuyma 1998, 2005).
The Fossil Record – Key to the Past
Problems with Interpretation and Classification
Determining fossil’s age is difficult because radiometric methods cannot be used directly
on the fossil, fossils deposited over a brief time interval are often mixed before the
sediment becomes rock (Futuyma 1998). Identifying fossils may be difficult because the
nature of the fossil may hide the diagnostic traits because it has been crushed (e.g. insect
imprint fossils) or fragmented (e.g. different plant parts that cannot be matched or bone
fragments) (Futuyma 1998). The picture shows an example of an imprint fossil. For
palaeontology, a species is a morphologically identifiable form. Some living species
cannot be morphologically separated by skeletal features so a single fossil “species” may
consist of more than one biological species. For some groups, living species can be
differentiated by skeletal features so the fossil species are probably also skeletally unique
(Futuyma 1998). Species representation in the fossil record is poor so palaeontologists
tend to consider genera and higher taxa (Futuyma 1998).
The Geologic Time Scale
The geologic time scale illustrates the magnitude of time period that fossils occur in. The
oldest fossil dates from 3.5 billion year ago. Older deposits from 3.8 billion years ago
containing lots of carbon are regarded as the earliest evidence of life on earth (Wikipedia
Contributors 2006b). The five recognized mass extinction events in prehistoric time and
the present mass extinction event are marked with red arrows.
Background Extinction and Extinction Events
The normal extinction rate is known as background extinction or the background
extinction rate (Futuyma 1998).
Background extinction rates are constant within clades but vary greatly between clades
(Freeman & Herron 1998).
Extinction events were used to demarcate the geological time periods (Leakey & Levin
1995). The principle subdivisions are identified by distinctive fossils and major faunal
breaks are used as the boundaries (Raup & Sepkoski 1982, Raup 1998, Futuyma 1998).
Raup & Sepkoski (1984) suggest that mass extinction events occur periodically at about
26 million year intervals.
Some Quantified Effects of Mass Extinctions
Table 6.1: The Effects on Skeletonized Marine Invertebrates of the ‘Big Five’ Mass
Extinctions (modified from p713, Futuyma 1998). The modifications come from
Anderson (1999), Lévêque & Mounolou (2001), Broswimmer (2002), Futuyma (2005)
and Wikipedia Contributors (2006c). Time periods are given for the older mass
extinctions because the literature gives variable dates. A median time value has been
selected (with references) for the slides on each mass extinction event. The species
percentages are estimated from statistical analyses of the numbers of species per genus
(Futuyma 1998).
Causes of Mass Extinctions
Most of the extinction events are likely to be caused by a combination of factors (Leakey
& Lewin 1995, Broswimmer 2002), for example, dropping sea levels were recorded
between extinction events as well as for most extinction events (including all five mass
extinction events) so other factors were involved in the extinction events. A drop in sea
level may be caused by several factors including changes in continental configuration due
to plate tectonics and extensive polar glaciation (Leakey & Lewin 1995). A supernova
explosion is considered very unlikely (Leakey & Lewin 1995) and recent research shows
that a gamma ray burst is impossible because our galaxy is metal rich (Wikipedia
Contributors 2006c). The consequences of the asteroid/comet impact at the end of the
Cretaceous Period are debated (Freeman & Herron 1998). It has been suggested that the
asteroid impact could cause increased volcanic activity elsewhere in the world (Leakey &
Lewin 1995).
End Ordovician Mass Extinction
This is the earliest of the five mass extinctions (Raup & Sepkoski 1982). Of the five, it
had the second largest impact (Futuyma 1998). It may have occurred as three peaks
spread over 500 000 years (Groombridge 1992) or two peaks separated by one to two
million years (Caughley & Gunn 1996, Wikipedia Contributors 2006c). This extinction
event took place about 439 million years ago coinciding with glaciation (Groombridge
1992, Futuyma 1998). Plants, insects and tetrapods had not yet developed so they were
not affected (Anderson 1999). The marine organisms affected included brachiopods,
cephalopods, echinoderms, graptolites, solitary corals and trilobites (Dobson 1996,
Primack 1998, Lévêque & Mounolou 2001). The picture shows an Ordovician
brachiopod fossil assemblage including bryozoan, coral, annelid and gastropod fossils.
Suggested causes include climate change (a drop in temperature), a drop in sea level
(Futuyma 1998, Anderson 1999, Wikipedia Contributors 2006c), asteroid or comet
impacts and a gamma ray burst (Wikipedia Contributors 2006c).
Late Devonian Mass Extinction
This is the second of the five mass extinctions. It may have occurred in two stages over
about 15 million years (Raup & Sepkoski 1982) or as two or more peaks over 3 million
years (Caughley & Gunn 1996) or as a series of peaks over 20 million years
(Groombridge 1992, Wikipedia Contributors 2006c). This extinction event took place
about 365 million years ago (Leakey & Lewin 1995, Lévêque & Mounolou 2001).
Insects and tetrapods had yet to develop but of the plants, the rhyniophytes decreased
(Anderson 1999). The marine organisms affected include ammonoids, brachiopods,
corals, agnathan fish, placoderm fish, ostracods and trilobites (Caughley & Gunn 1996,
Dobson 1996, Primack 1998, Lévêque & Mounolou 2001). The picture shows a
Devonian example of a trilobite. Suggested causes include climate change (a drop in
temperature) (Caughley & Gunn 1996, Wikipedia Contributors 2006c) and multiple
asteroid impacts (Anderson 1999).
End Permian Mass Extinction
This is the third and biggest of the five mass extinctions (Groombridge 1992, Barbault &
Sastrapradja 1995, Futuyma 1998). It may have taken place over 5—8 million years
(Groombridge 1992, Caughley & Gunn 1996, Dobson 1996) or been a single event that
took less than 10 000 years or occurred as two peaks over 5 million years (Barbault &
Sastrapradja 1995). This extinction event took place about 245 million years ago
(Barbault & Sastrapradja 1995, Gaston & Spicer 1998, Lévêque & Mounolou 2001).
Plants were severely affected (Barbault & Sastrapradja 1995). The previously dominant
Ottokariales (glossopterids) became extinct (Futuyma 1998, Anderson 1999). About two
thirds of the insect families became extinct and six insect orders disappeared (Barbault &
Sastrapradja 1995, Anderson 1999, Lévêque & Mounolou 2001). About 70% of the
vertebrate families were lost (Lévêque & Mounolou 2001). The tetrapods that suffered
losses were the amphibians and the mammal-like reptiles (therapsids) (Dobson 1996,
Futuyma 1998, Anderson 1999). The picture shows a Permian amphibian
Branchiosaurus. Land extinctions were less severe than the marine extinctions (Futuyma
1998). The marine organism losses include benthic foraminifera, brachiopods,
bryozoans, echinoderms, 44% of fish families (possibly including freshwater fish), all
graptolites, solitary corals, all trilobites (Groombridge 1992, Barbault & Sastrapradja
1995, Dobson 1996, Lévêque & Mounolou 2001). Suggested causes include a variable
climate (Futuyma 1998) or climate change (cooling (Dobson 1996)) (Groombridge 1992,
Gaston & Spicer 1998, Broswimmer 2002, Wikipedia Contributors 2006c), a drop in sea
level (Barbault & Sastrapradja 1995, Gaston & Spicer 1998, Futuyma 1998, Broswimmer
2002), massive carbon dioxide (CO2) poisoning (Anderson 1999), oceanic anoxia, the
explosion of a supernova (Barbault & Sastrapradja 1995, Gaston & Spicer 1998), asteroid
or comet impacts (Primack 1998, Wikipedia Contributors 2006c), plate tectonics during
the formation of Pangea (Groombridge 1992, Broswimmer 2002, Wikipedia Contributors
2006c) and high volcanic activity (Groombridge 1992, Futuyma 1998, Primack 1998,
Wikipedia Contributors 2006c).
End Triassic Mass Extinction
This is the fourth mass extinction (Raup & Sepkoski 1982). It may have been a
protracted event taking place over 15 million years (Groombridge 1992, Caughley &
Gunn 1996, Lévêque & Mounolou 2001) or it may have been caused by a series of
catastrophes within 100 000 years (Broswimmer 2002). This extinction event took place
about 210 million years ago (Leakey & Lewin 1995, Caughley & Gunn 1996). From the
plants, several orders of gymnosperms were lost and the Umkomasiales (Dicroidium)
became extinct. Insects were apparently not severely affected by this extinction event
(Anderson 1999). Some reptile lineages were affected – the mammal-like reptiles
(therapsids) especially (Caughley & Gunn 1996, Dobson 1996, Primack 1998, Anderson
1999, Broswimmer 2002). The picture shows Phytosaur teeth. Phytosaurs became
extinct during this extinction event (Humboldt State University, Natural History Museum
2005). The marine organisms affected included ammonites, ammonoids, bivalves
(Molluscs), brachiopods, corals, gastropods and sponges (Caughley & Gunn 1996,
Dobson 1996, Futuyma 1998, Primack 1998, Lévêque & Mounolou 2001, Broswimmer
2002). Suggested causes include one or more asteroid/comet impacts (Anderson 1999,
Broswimmer 2002), climate change (Broswimmer 2002) and volcanic activity
(Broswimmer 2002, Wikipedia Contributors 2006c).
End Cretaceous Mass Extinction
This is the final and best known of the five mass extinctions (Groombridge 1992,
Barbault & Sastrapradja 1995). It may have been a protracted event (Barbault &
Sastrapradja 1995) or it may have occurred as a series of peaks over a relatively short
period of time (100 000—1 million years) (Groombridge 1992, Barbault & Sastrapradja
1995). This extinction event took place about 65 million years ago (Barbault &
Sastrapradja 1995, Caughley & Gunn 1996, Freeman & Herron 1998, Lévêque &
Mounolou 2001, Wikipedia Contributors 2006c). Anderson (1999) indicates that plants
were not seriously affected but Caughley & Gunn (1996) indicate that up to 75% of plant
species disappeared. The impact on insects was minor (Anderson 1999). Tetrapods lost
thirty-six families from three groups (dinosaurs (all non-avian), plesiosaurs and
pterosaurs (Groombridge 1992, Freeman & Herron 1998, Anderson 1999, Lévêque &
Mounolou 2001, Broswimmer 2002, Wikipedia Contributors 2006c). The marine
organisms affected include ammonites, ammonoids, cephalopods, bivalves, foraminifera,
icthyosaurs, mosasaurs, plackton and rudists (Barbault & Sastrapradja 1995, Freeman &
Herron 1998, Futuyma 1998, Lévêque & Mounolou 2001). The picture shows a juvenile
Tyrannosaurus rex skeleton. Suggested causes include asteroid/comet impact
(Groombridge 1992, Barbault & Sastrapradja 1995, Leakey & Lewin 1995, Freeman &
Herron 1998, Anderson 1999, Broswimmer 2002, Wikipedia Contributors 2006c),
climate change (Broswimmer 2002) and volcanic activity (Wikipedia Contributors
2006c). The occurrence of an impact event has been verified (Leakey & Lewin 1995,
Freeman & Herron 1998).
Present Mass Extinction
There is evidence that the extinctions on New Zealand and the Pacific Islands after
human colonization were ultimately caused by humans (Caughley & Gunn 1996).
Human Extinction?
Conclusions – the Future?