Lecture 6: The Cambrian explosion of animal life

Question 1

Metazoan:

Answer 1

Multicellular animals

Question 2

Metazoans share a number of characteristics:

Answer 2

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 )

Question 3

Recent advances in several fields have been important in addressing the problem of the origin of metazoans:-

Answer 3

1/ New fossil finds
2/ Phylogenetic analysis of anatomical and molecular data
3/ Molecular clock studies
4/ Molecular genetics of animal development

Question 4

Diploblastic:

Answer 4

having a body derived from only two embryonic cell layers (ectoderm and endoderm, but no mesoderm), as in sponges and coelenterates.

Question 5

Triploblastic:

Answer 5

having a body derived from three embryonic cell layers (ectoderm, mesoderm, and endoderm), as in all multicellular animals except sponges and coelenterates.

Question 6

E.g of diploblastic animals

Answer 6

• Sponges and Archaeocytathids

- Jellyfish & corals

Question 7

E.g. of triploblastic animals

Answer 7

Other animal (BILATERIA)

Question 8

BILATERIA:

Answer 8

a major group of animals, including the majority of phyla but not sponges, cnidarians, placozoans and ctenophores.

Question 9

Late Precambrian [Vendian] dates:

Answer 9

635-542Ma

Question 10

Late precambrian organism e.g.

Answer 10

• The Ediacara biota
• Trace fossils of triploblastic organisms
• Egg cases containing embryos

Question 11

Ediacaran organisms have no mouth, gut or anus! How did they feed?:-

Answer 11

• 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

Question 12

Ediacara organisms have been interpreted as:-

Answer 12

• 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

Question 13

Late Precambrian [Ediacaran] at 630Ma

Answer 13

Large acritarchs (?organic-walled egg cases) similar to those in younger deposits that contain phophatized embryos

Question 14

Late Precambrian [Ediacaran] at 580Ma

Answer 14

Phosphatized embryos (including forms preserved within organic-walled egg cases.

Question 15

In phosphate rich environments …

Answer 15

small organisms (<1mm) even those composed entirely of soft tissues that normally rot very quickly, can be perfectly preserved

Question 16

acritarchs:

Answer 16

organic microfossils, present from approximately 1,400 to 3,200 million years ago to the present

Question 17

we know that some ‘acritarchs’ are..

Answer 17

the cases of animal embryos

Question 18

Early Cambrian [Manakayan] dates and type of fossil

Answer 18

542-530Ma

-small shelly fossils

Question 19

small shelly fossils are probably:

Answer 19

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

Question 20

Middle Cambrian [Tommotian & Atdabanian] Date and what happened:

Answer 20

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).

Question 21

When did animals begin to diversify?

Answer 21

During Ediacaran times with a slow fuse or explosively during the Middle Cambrian

Question 22

What caused the Cambrian Explosion?

Answer 22

1) Environment

2) ecology

Question 23

ENVIRONMENTAL EVENTS AT THE PRECAMBRIAN-CAMBRIAN TRANSITION

Answer 23

• 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?).

Question 24

Why have no new phyla appeared since the Cambrian Explosion and what happened to all of the phyla we don’t see today?

Answer 24

???????

Question 25

HIGHLY SPECULATIVE EVOLUTIONARY PROGRESSION:Late Precambrian

Answer 25

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.

Question 26

HIGHLY SPECULATIVE EVOLUTIONARY PROGRESSION:Early-Middle Cambrian

Answer 26

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.

Question 27

HIGHLY SPECULATIVE EVOLUTIONARY PROGRESSION:

Late Cambrian

Answer 27

Predators become more efficient (+eyes) and multicellular animal evolve better armour (continuous shells) and exploit more burrowing niches.