Evolutionary paths and relationships

Although it is impossible to work out the exact relationship between creatures only known from the fossil record because we will never have access to their DNA, we can work out the general relationships by comparing their morphology. As we look at these relationships the evolutionary paths start to become clear and the fossils and the body shapes of the creatures we see on the planet today begin to make more sense.

For example, the comparison of the sclerites of the early Cambrian creature halkieria and the later wiwaxia show a remarkable resemblance to each other. Not only that, the sclerites also show a relationship with the chaeta of annelid worms. Therefore wiwaxia can be considered as a transitional form between halkieria and annelida (Ref 1.)

The shells of halkieria also show a close resemblance to brachiopoda shells and it is hypothesised that over time the creatures shortened and folded over joining their two shells together for increased protection. Note that although they look like bivalve mollusc their shells are formed differently. Bivalves molluscs have two symmetrical shells which grow joined at a hinge whereas brachiopods have two separate, differently shaped shells which form at the ends of the embryo and come together only because of folding (Ref 2.)

The lobopods appear to have evolved from predatory priapulid worms while changing from a burrowing to a crawling lifestyle (Ref 3). These are thought to have gone on to evolve into the incredibly diverse arthropods, including arachnids, crustaceans, insects and the extinct trilobites. Reinforcing this hypothesis is the recent discovery of the fossil remains of a creature called Miraluolishania which appears to have features of both lobopods and arthropods (Ref 4.)

Although not complete nor beyond controversy, these proposed relationships allow us to sketch out a general picture of connections between the groups of creatures. Note that it's hard to separate ancestors from sister groups purely from fossils. For example, the anomalocarids may actually be a sister group to the ancestors of arthropods rather than being direct ancestors themselves.
Brachiopods Annelids Arthropods Wiwaxia Halkieria Kimberella Priapulid worms Hallucigenia Hallucigenia Lobopods Aysheaia Aysheaia Anomalacaris Opabinia Mollusc Ottoia Evolutionary paths

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References:
  • Ref 1 : Simon Conway Morris, the Crucible of Creation, ISBN 0-19-286202-2: 188-195
  • Ref 2 : Bernard L. Cohen et.al, 'The brachiopod fold: a neglected body plan hypothesis', Palaeontology 46(1): 59-65.
  • Ref 3 : Jianni Liu et al., 'Origin and diversification of Cambrian lobopods', Gondwana Research vol 14 (2008): 277283
  • Ref 4 : Jianni Liu et al., 'Comparative study of Cambrian lobopods Miraluolishania and Luolishania',
    Chinese Science Bulletin, vol 53 (2008): 87-93