BIOL10005 Lecture Notes - Lecture 20: Cambrian Explosion, Trilobite, Paleozoic
LECTURE 20
ANIMAL DIVERSITY - THE BIG PICTURE
CHAPTER 32 - EVOLVING EARTH
GEOLOGICAL TIME SCALES MARK MAJOR EVENTS IN
EVOLUTION
•Geological time is split into a number of defined eras and periods.
•Division into periods is based on differences between rocks.
•The Precambrian era comprises two time periods/eons: the Archean eon and
the Proterozoic eon. 580-545 million years ago (mya)
•We know about Precambrian animals from fossils in rocks (bacteria,
cyanobacteria, stromatolites)
•Precambrian organisms were prokaryotes, some were photosynthetic.
•Oldest eukaryotes in 1.4 billion year old rocks in North America, China and
India.
•Precambrian complex metazoans MAY have been Cnidarians as they resemble
jellyfish, corals, and anemones, but there are many specimens that defy
categorisation.
EDIACARAN FAUNA: EVIDENCE OF MULTICELLULAR
ORGANISMS
•Major phyla of invertebrates are present as fossils in the Cambrian, the beginning of
the Palaeozoic era.
•The ediacaran fauna is remarkable due to the preservation of soft bodied animals in
sandstone.
•They are the earliest known complex animals.
•Previously thought such animals arose in the ‘Cambrian Explosion’.
MARINE LIFE
•Trilobites (sub phylum: Trilobita)
were the most common marine
multicellular animal. They are extinct
arthropods with hard skeletal parts,
large eyes, segmentation, jointed legs,
long antennae and biramous
appendages for swimming, walking
and feeding.
•More than 17,000 species known (from fossil record!)
•They exhibited a huge diversity of forms.
•Existed from early Cambrian (530 Mya) until end of the Permian (250
Mya) but declined after end of Devonian (360 Mya).
•Many Cambrians had compound eyes.
Lecture 20 - Wednesday 7 September 2016
BIOL10005 - GENETICS & THE EVOLUTION OF LIFE
THE CAMBRIAN EXPLOSION
THE BURGESS SHALE
•Much of our knowledge of the
Cambrian explosion comes from one
site in Canadian Rocky Mountains.
•Enormous number of fossilised
animals found in the Burgess Shale
(510 million years old).
•Fossil bed was once the base of an
underwater cliff where anoxic mud
(and dead animals) collected, which
would have covered animals and they
wouldn’t have been eaten by bacteria
or other animals and thus there are
good fossil records.
•The Burgess Shale shows the appearance of
nearly all modern phyla, plus some
unrecognisable ones, which some (including
Walcott) claim these are just unusual forms
of the modern phyla.
•Others (Eg.Stephen Jay Gould) say that the
Cambrian explosion produced many
hundreds of phyla, only some of which
continued.
•Four main reasons lead to have caused the
Cambrian explosion.
•1. Body plans
•The appearance of the triploblastic body form allowed more diversity
•2. Genetics
•The origin of homeobox (Hox) genes
•3. Environment:
•Oxygen levels reached optimum (known to increase at the end of the Pre- Cambrian)
•Break-up of land masses (more shallow seas) more movement
•4. Ecological communities
•Greater interactions between organisms led to evolution of many forms (including more
advanced sensory structures)
EXTINCTION
•A species (or class/phylum) can either evolve or go extinct
•Average lifespan for a species is surprisingly short;
•Mammals’ average species lifespan = 1 million
years.
•No mammals currently living older than 10 million
years.
•On average, the fossil record indicates that between
10 and 100 species go extinct every year.
•But there is enormous variation in extinction rates.
•There have been periods of relatively sudden major
declines in biodiversity, known as extinction events or
mass extinctions (right).
Lecture 20 - Wednesday 7 September 2016
BIOL10005 - GENETICS & THE EVOLUTION OF LIFE
Document Summary
Evolution: geological time is split into a number of defined eras and periods, division into periods is based on differences between rocks, the precambrian era comprises two time periods/eons: the archean eon and the proterozoic eon. 5 0-545 million years ago (mya: we know about precambrian animals from fossils in rocks (bacteria, cyanobacteria, stromatolites, precambrian organisms were prokaryotes, some were photosynthetic, oldest eukaryotes in 1. 4 billion year old rocks in north america, china and. India: precambrian complex metazoans may have been cnidarians as they resemble jellyfish, corals, and anemones, but there are many specimens that defy categorisation. Marine life: trilobites (sub phylum: trilobita) were the most common marine multicellular animal. Mya) but declined after end of devonian (360 mya): many cambrians had compound eyes. The burgess shale: much of our knowledge of the. Walcott) claim these are just unusual forms of the modern phyla: others (eg. stephen jay gould) say that the.
