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Introductory Lecture F:  What does invertebrate phylogeny reveal about evolution and ecology?

1. Origins of multicellular animals

Cambrian Explosion - the rapid (~10 million years) appearance of major metazoan (animal) taxa in the early Cambrian.


Several important fossils discoveries have extended the length of time over which this diversification occurred.

The Early Metazoan Fossil Record -In reverse order of their appearance:

First appearance of mineralized skeletons of such phyla as:

  • Mollusca,
  • Brachiopoda
  • Arthropoda
  • Echinodermata
  • Cnidarans
Furthermore, soft-bodied phyla, also make their appearance such as:
  • Annelida - Polychaeta
  • Priapulida
  • Onycophora
  • Ctenophora
  • Porifera
  • Chordata
As well as other taxa that can not be confidently placed into modern phyla (is this a problem?).
So were the organisms that produced these trace fossils fairly sophisticated?

Some studies using "molecular clocks" to estimate the origin of multicellular animals are in agreement with the appearence of trace fossils.
  What modern organisms have a similar body plan?
There is some debate on whether the ediacara fauna represent a single monophyletic group.


40 million years now considered by many as the length of time from first multicellular animals to appearance of most major phyla.  This is still relatively short in geological terms.


Some hypotheses on factors initiating the rapid diversification of the Cambrian explosion:

  • Increase in oxygen that allowed for production of collagen.

    • So what"s the problem with this hypothesis?
  • Co-evolutionary feedback with the advent of predators.

  • Origin of hox genes - regulatory pathways can be repeated and modified among various lineages.

The bottom line is that the evolution of multicellular life may be older than previously believed but was still relatively rapid, and when compared with life today produced most of the major animal body plans.     Given that diversification appears to have occurred over a scant 40 million years (over 500 million years ago), reconstruction of the evolutionary relationships of modern phyla is quite a challenge!

2. The subsequent history of multicellular animals 

Here are some questions to consider over the course of the semester as we survey the modern phyla present today (and consider these questions in light of what you know about evolution and natural selection):

Some ecological terms:
* planktonic versus benthic versus nekton
* sessile versus mobile
* infuana versus epifauna
* suspension-feeder versus deposition-feeder
* parasite versus predator

(from SC Morris' Crucible of Creation)


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