Lecture 6 - the Cambrian explosion of animal life Flashcards
What is another word for animals?
metazoans
What characteristics do metazoans share?
- Multicellular body formed from different kinds of cells
- The ability to manufacture the protein collagen
- A reproductive cycle with gametes produced by meiosis
- A nervous system composed of neurones (except in sponges)
What have recent advancements allowed in addressing the problem of the origin of metazoans?
- new fossils
- phylogenetic analysis of anatomical & molecular data
- molecular clock studies - assumed molecular evolution is consistent in terms of time (controversial)
- molecular genetics of animal development
How do you classify animals?
- Diploblastic (sponges & archaeocyathids, jellyfish & corals)
- Triploblastic (other animals - bilateria)
How many germ layers do Diploblastic animals have?
2 germ layers (ectoderm & endoderm)
How many germ layers do Triploblastic animals have?
3 germ layers (layersectoderm, endoderm & mesoderm)
`What occurred during the Late Precambrian (Ediacaran) 630-542 Ma?
- the ediacara biota - contain strange markings (fossils found all over the world - e.g. Namibia)
- trace fossils of triplobastic organisms
- egg cases containing embryos
- possible traces of triploblastic animal - they’ve got a gut so triploblastic animal
What occurs during the late Precambrian 630Ma?
Large acritarchs (organic-walled egg cases) similar to those in young deposits that contain phosphstized embryo?
What occurred during late Precambrian 580Ma?
Phosphatized embryos
What occurred during Early Cambrian (Manakayan) 542-530Ma?
Small Shelly fossils
What occurred during Middle Cambrian (Tommotian & Atdabanain) 530 - 520 Ma?
The Cambrian explosion (fossilised hard parts appear, including representatives of all modern phyla).
What are Small shelly fossils (SSF)?
Small shelly fossils are probably disarticulated elements of a skeletal covering that had yet to evolve into a large discrete ‘shell’ covering the entire organisms.
They are almost certain an adaption against predation
What do small shelly fossils (SSFs) represent?
- an extinct phylum close to the mollusca
- some form of annelid
What occurred during the Cambrian exceptional preservation of hard & soft parts?
- eyes evolved to evade predation
Does the Cambrian Explosion represent?
- The appearance of fossilisable parts
- A true evolutionary bust to large size & greatly increased anatomical variety
What environmental events at the precambrian-Cambrian transition?
- the long-lived & large supercontinent that had dominated the Pre-Cambrian begins to break apart.
- there are 2 extensive glaciation extending into equatorial latitudes that produce snowball/slushball Earth scenarios.
- 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 - may have triggered the evolution of large animals
What could’ve caused the ‘Cambrian Explosion’?
- Environment
- Ecology - eyes and/or predation
What highly speculative evolutionary progression was seen in the Late Precambrian?
Ediacaran animals inhabit the sea floor. They are unprotected but have no predators. Triploblastic animals are present, but they are very small & their ecology is unknown
What highly speculative evolutionary progression was seen in the Early-Middle Cambrian?
Tripoblastic predators with teeth evolve. Most Ediacara organisms become extinct. Other multicellular animals protect themselves by evolving armour (spines, scleritis etc.) or burrowing in the sea floor
What highly speculative evolutionary progression was seen in the Late Cambrian?
Predators become more efficient (+ eyes) & multicellular animal evolve better armour (continuous shells & exploit more burrowing niches
