The Cambrian Explosion of Life (Wellman) Flashcards
Metazoans
- Multicellular body formed from different kinds of cells.
- The ability to manufacture the protein, collagen (no cell wall so they require for structure)
- A reproductive cycle with gametes produced by meiosis (sexually reproducing)
- A nervous system composed of neurons (except in sponges)
Advances addressing the problems of the origin of metazoans
- New fossil finds
- Phylogenetic analysis of anatomical and molecular data
- Molecular clock studies
- Molecular genetics of animal development
Timeline
Late Precambrian [Ediacaran] (635-542Ma)
- The Ediacara biota
- Trace fossils of triploblastic organisms
- Egg cases containing embryos.
Late Precambian [Ediacran] (630 and 580Ma)
-Large acritarchs that contain phosphatized embryos
Early Cambrian [Manakayan] 542-530Ma
-Small shelly fossils
Middle Cambrian
-Fossilised hard parts.
Ediacaran Biota - Late Precambrian (635-542Ma)
- Quilted compartments
- Large, can be metres in size
- Very symmetrical
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 (utilise sulphide oxidising bacteria) - deep sea vents.
How have Ediacaran organisms been interpreted?
- Simple ancestors of several modern day 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.
Trace fossils of triploblastic animals - Late Precambrian (635-542 Ma)
Organism eating sediment
-Need a gut and needs to be triploblastic
Egg cases containing embryos (phosphatised eggs) - Late Precambrian (630-580Ma)
In phosphate rich environments, small organisms (<1mm), even those composed entirely of soft tissues that normally rot very quickly can be perfectly conserved.
- Embryos of early animals are preserved well because of phosphate
- Shows that the life cycle is similar to other modern animals.
- We now know that some acritarchs are the cases of animal embryos.
- E.g. Cambrian water bears - trapped by phosphate
Small shelly fossils - Early Cambrian (425-540 Ma)
Small shelly fossils are probably disarticulated elements of a skeletal covering that had yet to evolve into large discrete “shell” covering the entire organism. They are almost certainly an adaption against predation.
These organisms probably represent either:
- An extinct phylum close to the mollusca
- Some form of annelid
- Animals evolved skeletons at Cambrian boundary (adaptation against predation).
Middle Cambrian (542-530Ma)
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)
Cambrian fossils with hard preservable parts
During the Middle Cambrian fossils appear, at the same time and worldwide, of organisms with preservable hard parts.
-Interestingly, these include essentially representative of all modern phyla with hard parts.
Cambrian fossils wit hard and soft parts
Biota has been extensively conserved
- Clay mineral
- No organisms eating sediment
- Arthropods
When did animals begin to diversify?
During Ediacaran times with a slow fuse or explosively during the Middle Cambrian.
- The appearance of fossilizable parts
- 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
- A true evolutionary burst to large size and greatly increased anatomical variety
- Here, animal diversity exploded rapidly, with little subsequence change (did some biological or environmental event trigger the radiation).
What could have caused the Cambrian Explosion?
- Environment
- Ecology
How could the environment have caused the Cambrian Explosion?
The long lived and large supercontinent that had dominated the Precambrian begins to break apart.
- There are a series of extensive glaciations extending into equatorial latitudes that produce snowball/slushball Earth scenarios: (765 (?confined to Africa), 710 (global), 600 (global), 542 Ma (relatively small))
- The boulder clays representing the glaciations are bounded below by carbonates and above by cap carbonates.
- The carbonates below have exceptionally high C-isotope values. (Due to continental break up producing narrow seaways with enhance C burial. These reduced atmospheric CO2 leading to the glaciations.)
- Banded Iron Formations occur with the boulder clays–for the first time in 1.8 billion years iron accumulates in the oceans.
- The cap carbonates have exceptionally low C-isotope values. (Due to rapid accumulation and/or absence of organisms and/or a burst of methane that ended the glaciations.)
- 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?). O2 have been high ever since.
