|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:
- Cambrian macrofauna
First appearance of mineralized skeletons of such phyla as:
Furthermore, soft-bodied phyla, also make their appearance such as:
- Annelida - Polychaeta
As well as other taxa that can not be confidently placed into modern phyla (is this a problem?).
Additional images of Burgess and other soft-bodied Cambrian fauna: http://park.org/Canada/Museum/burgessshale/faunaandflora.html
- The first shelly faunas
The first evidence of animals with hard parts are fossils that are small (generally 1 to 5 mm) and only parts of organisms that appear in the middle part of the Early Cambrian.
- Later small shelly fossils - sclerites of worm-like animals or as early representatives of the major fossil groups.
- Early small shelly fossils - tiny tubes, spines, cones and plates that are not clearly allied with modern groups.
- The earliest "trace" fossils
What kind of traces can animals leave in soft sediment?
There is considerable debate on when trace fossils first appear in the fossil record, though these trace fossils clearly predate the assemblage above.
What do trace fossils imply about animal body plans that predate later fossils of the Cambrian Explosion:
- What does the lack of fossilized body parts imply about the outer covering of these early animals?
- What do long unidirectional trails suggest about the symmetry of these early animals?
- Complex trails that clearly indicate sophisticated locomotion appeared several million years prior to fossilized hard parts. What do these trail patterns imply about neural integration of sensory organs?
- Latter trace fossils indicate vertical movement through sediment associated with burrowing. What does burrowing suggest about the musculature and body form of these early animals?
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.
- Ediacara fauna (Vendobionta)
Quilt-like body plans forming feathery fronds, pouches, disk and worm-shapes, without heads or obvious circulatory, nervous or digestive systems (Late Proterozoic to the Middle Cambrian).
Additional images of Ediacaran fauna: http://geol.queensu.ca/museum/exhibits/ediac/ediac.html
There is controversy over what these Edicaran organisms represent:
- Ancestors of the Cambrian (and hence modern) metazoa - animals before the evolution of Hox genes?
sea pen ?
Is this quilt-like body plan common in modern animal phyla?
- "Failed experiments" in early animal evolution that left no modern day representatives,
What may have caused extinction?
- Not multicellular animals at allWhat 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):
- Has the rate of evolution of new body plans been constant since the origin of animal life, or has the last 500 million years has been mostly "tinkering" ?
- Has the rate of speciation and extinction been constant among all body plans (i.e. Do species fall into distinct clusters of organism types or along a continuum of form across the entire animal Kingdom. Do some phyla seem "constrained" by their body plan while others have greater success in diversifying)?
- Should evolution result as a sequence of linear progress where less complex living phyla represent perfect representatives of past life?
- Taxonomically, could life today have been very different if past circumstances had been slightly different? Does the bizarre diversity of the Burgess shale and the subsequent branching of major taxa suggests that life on earth as it is today may have been very contingent on past extinctions (Stephen Jay Gould's Wonderful Life)?
arise early in multicellular evolution and survive to today? If so what does this suggest about constraints on evolution (Simon Conway Morris's The Crucible of Creation)?
- Ecologically, are the general types of niches in ecological systems limited? In otherwords, is there a similarity of fauna ecologically but not taxonomically over history of animals? For example, are the following niches of Burgess shale organisms occupied by modern ecologic equivalent taxa?:
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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