Lecture 6: The Cambrian explosion of animal life Flashcards
Metazoan:
Multicellular animals
Metazoans share a number of characteristics:
1) multicellular body frond from different kind of cells
2) The ability to manufacture the protein collagen
3) A reproductive cycle with gametes produced by meiosis
4) Nervous system composed of neutrons (except in sponges )
Recent advances in several fields have been important in addressing the problem of the origin of metazoans:-
1/ New fossil finds
2/ Phylogenetic analysis of anatomical and molecular data
3/ Molecular clock studies
4/ Molecular genetics of animal development
Diploblastic:
having a body derived from only two embryonic cell layers (ectoderm and endoderm, but no mesoderm), as in sponges and coelenterates.
Triploblastic:
having a body derived from three embryonic cell layers (ectoderm, mesoderm, and endoderm), as in all multicellular animals except sponges and coelenterates.
E.g of diploblastic animals
- Sponges and Archaeocytathids
- Jellyfish & corals
E.g. of triploblastic animals
Other animal (BILATERIA)
BILATERIA:
a major group of animals, including the majority of phyla but not sponges, cnidarians, placozoans and ctenophores.
Late Precambrian [Vendian] dates:
635-542Ma
Late precambrian organism e.g.
- The Ediacara biota
- Trace fossils of triploblastic organisms
- Egg cases containing embryos
Ediacaran organisms have no mouth, gut or anus! How did they feed?:-
- The compartments contained unicellular photosynthetic algae (unlikely due to water depth and light penetration)
- They took in substance through the body wall (particulate food; dissolved organic matter; photons)
- Chemosymbiosis (utilize sulphide oxidising bacteria)–cf. deep sea vents
Ediacara organisms have been interpreted as:-
- Simple ancestors of several modern phyla (sponges, jellyfish, sea pens etc.)
- Diploblastic animals showing a range in variation not seen in living examples
- An entirely separate attempt at multicellular life that ultimately failed
Late Precambrian [Ediacaran] at 630Ma
Large acritarchs (?organic-walled egg cases) similar to those in younger deposits that contain phophatized embryos
Late Precambrian [Ediacaran] at 580Ma
Phosphatized embryos (including forms preserved within organic-walled egg cases.
In phosphate rich environments …
small organisms (<1mm) even those composed entirely of soft tissues that normally rot very quickly, can be perfectly preserved
acritarchs:
organic microfossils, present from approximately 1,400 to 3,200 million years ago to the present
we know that some ‘acritarchs’ are..
the cases of animal embryos
Early Cambrian [Manakayan] dates and type of fossil
542-530Ma
-small shelly fossils
small shelly fossils are probably:
disarticulated elements of a skeletal covering that had yet to evolve into a large discrete “shell” covering the entire organism. They are almost certainly an adaptation against predation. These organisms probably represent either:-
(i) an extinct phylum close to the mollusca
(ii) some form of annelid
Middle Cambrian [Tommotian & Atdabanian] Date and what happened:
530-520 Ma
-The Cambrian explosion (fossilised hard parts appear, including representatives of
all modern phyla). Very rarely we also find soft parts preserved in exceptionally
preserved biotas such as the Chenjiang biota (China), the Sirius Passet biota
(Greenland) and the Burgess Shale biota (Canada).
When did animals begin to diversify?
During Ediacaran times with a slow fuse or explosively during the Middle Cambrian
What caused the Cambrian Explosion?
1) Environment
2) ecology
ENVIRONMENTAL EVENTS AT THE PRECAMBRIAN-CAMBRIAN TRANSITION
- long lived supercontinent begins to break apart
- series of glaciations
- boulder clays
- C-isotopes conc. in carbonate above and below
- BIFs
- There is a mysterious final, short, isotope excursion (very low C-isotope values) in the latest Precambrian. It coincides with the extinction of skeletal fossils from microbial reefs.
- Atmospheric O2 levels begin to rise dramatically at the Precambrian/Cambrian boundary (?did this trigger the evolution of large animals?).
Why have no new phyla appeared since the Cambrian Explosion and what happened to all of the phyla we don’t see today?
???????
HIGHLY SPECULATIVE EVOLUTIONARY PROGRESSION:Late Precambrian
Ediacaran animals inhabit the sea floor. They are unprotected but have no predators. Triploblastic animals [cf. trace fossils + embryos and their egg cases (some large acritarchs)] are present but they are very small and their ecology unknown.
HIGHLY SPECULATIVE EVOLUTIONARY PROGRESSION:Early-Middle Cambrian
Triploblastic predators with teeth evolve. Most Ediacara organisms become extinct. Other multicellular animals protect themselves by evolving armour (spines, sclerites etc.) or burrowing in the sea floor.
HIGHLY SPECULATIVE EVOLUTIONARY PROGRESSION:
Late Cambrian
Predators become more efficient (+eyes) and multicellular animal evolve better armour (continuous shells) and exploit more burrowing niches.
