The Cambrian explosion Flashcards
(20 cards)
what characteristics do metazoans share
1) Multicellular body formed from different kinds of cells
2) The ability to manufacture the protein collagen- act as a glue
3) A reproductive cycle with gametes produced by meiosis
4) A nervous system composed of neurons (except in sponges- very simple organisms)
what are the recent advances and why are they important to addressing the problem of the origin of metazoans
1) New fossil finds- have to go back to the fossil record
2) Phylogenetic analysis of anatomical and molecular data
3)Molecular clock studies- god for recent diversity (20-30 mil years)- modifying clocks
4) Molecular genetics of animal development (Evo-Devo studies- knockout genes, put into new organisms)
what are the different classification categories of animals
-dipoblastic: sponges and archaeocyathids
-triploblastic: other animals (bilateria)
what does the Late Precambrian [Ediacaran] at 630 Ma consist of
-Large acritarchs (?organic-walled egg cases) similar to those in younger deposits that contain phosphatized embryos
what does the Late Precambrian [Ediacaran] 630-542 Ma consist of
-The Ediacara biota
-Trace fossils of triploblastic organisms
-Egg cases containing embryos
what does the late Precambrian [Ediacaran] at 580 Ma consist of
-Phosphatized embryos (including forms preserved within organic-walled egg cases.
what does the Early Cambrian [Manakayan] 542-530 Ma consist of
-Small shelly fossils
what does the Middle Cambrian [Tommotian & Atdabanian] 530-520 Ma consist of
-The Cambrian explosion (fossilized 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).
what the ediacaran biota
-consists of lots of compressing fossils
-up to a meter in size
-all have similar characteristics- the way they’re preserved and symmetry
-2 types, both lay on the sea floor
what do ediacaran organisms not have
-no mouth, gut or anus
-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)
=Chemosymbiosis (utilize sulphide oxidising bacteria)–cf. deep sea vents
what have. Ediacara organisms 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
-‘Hot-off-the-press’ geochemical evidence suggesting some are indeed animals
what occurs in phosphate rich environments with small organisms
-even those composed entirely of soft tissues that normally rot very quickly, can be perfectly preserved
-organisms preserved perfectly
-We now know that some large “acritarchs” are the cases of animal embryos and similar acritarchs occur in the late Precambrian
small shelly fossils: early Cambrian
-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
what are Cambrian fossil with hard preserved parts like
-During the Middle Cambrian fossils appear, at the same time and worldwide, of organisms with preserveable hard parts. Interestingly these include essentially representatives of all modern phyla with hard parts
-they find the fossil and find the legs and soft body has been perceived as well as the chitinous body
Does the Cambrian Explosion represent the appearance of fossilizable parts
-Here we have a long Precambrian history of animals. However, we only get fossils once they developed armour thus increasing their fossilization potential. Evidence for a long Precambrian history comes from molecular clocks, fossilized embryos etc.
Does the Cambrian Explosion represent a true evolutionary burst to large size and greatly increased anatomical variety
-Here animal diversity exploded rapidly, with little subsequent change (did some biological or environmental event trigger the radiation).
Why have no new phyla appeared since the Cambrian Explosion and what happened to any phyla we don’t see today
-The exceptional preservation of the Burgess Shale, Sirius Passat, Emu Bay and Chenjiang biotas tells us that the Cambrian explosion resulted in hugely diverse biotas of animals (with and without hard parts). These represent numerous animal phyla (all living phyla and possibly others that are now extinct).
If there was a ‘Cambrian Explosion’ what caused it?
1) Environment
2) Ecology
what are the environmental events at the precambrian-cumbrian transition
-The long lived and large supercontinent that had dominated the Precambrian begins to break apart.
-There two extensive glaciations extending into equatorial latitudes that produce snowball/slushball Earth scenarios:- Cryogenian (635-720 Ma) Marinoan Ice Age (635-650 Ma), Sturtian Ice Age (660-720 Ma)
-There is a mysterious 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?).
what is the highly speculative evolutionary progression
-Late Precambrian:
Ediacaran animals inhabit the sea floor. They are unprotected but have no predators. Triploblastic animals are present [cf. trace fossils + embryos and their egg cases (some large acritarchs)] but they are very small and their ecology unknown.
-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.
-Late Cambrian:
Predators become more efficient (+eyes) and multicellular animal evolve better armour (continuous shells) and exploit more burrowing niches.
