Wonderful Life The Burgess Shale and the Nature of History

The Burgess Shale During a survey of the Canadian Rockies in 1909, Charles Walcott of the Smithsonian Institute discovered a Cambrian fossil assemblage known as the Burgess Shale. The fossils he discovered represent a strange fauna seemingly unlike the animals alive today.

Contemporary Interpretation New insights from the field of Evolutionary Development (Evo Devo), have caused a reevaluation of the 140 species and 10 phyla of animals represented by the Burgess Shale. Two main groups of the Burgess Shale are the Lobopodians, that have simple un-jointed appendages, and the Arthropods, that have jointed appendages.

Arthropods Arthropods are the largest phyla of animals on Earth today, and 1/3 of the species in the Burgess Shale were arthropods, including species like: 1. Trilobites 2. Marella 3. Waptia

Marella

Waptia

Lobopodians Today’s onychophora are likely living descendants of the Lobopodians, the extinct species of which may represent the transition to the arthropods. 1. Aysheaia 2. Opabinia 3. Anomalocaris

Aysheaia Aysheaia lacks hallmark arthropod characteristics

Opabinia Opabinia is segmented like arthropods but lacks biramous appendages

Anomalocaris Anomalocaris is segmented, has biramous appendages, but lacks a fully developed exoskeleton.

Evolutionary Development Genetic analysis of the living Onychophora shows that the common ancestor of both Lobopodia and Arthropoda had the 10 Hox genes shared by these two groups. Interesting enough, the limb building gene distal-less is expressed in the development of the limbs of both groups suggesting that the birmous limb evolved from an intermediate with two unbranched lobopods.

The Uniramous Chelicerata? Gene expression patterns support the idea that the following structures spider structures evolved from an ancestral arthropod gill. 1. Book lungs 2. Tracheae 3. Spinnerets

The Uniramous Insects? The expression pattern of the genes Apterous and Nubbins in wings of terrestrial flyers and the gills of aquatic insects supports the idea that wings evolved from the gills of aquatic insects like the stoneflies, mayflies, or dragonflies. More interesting, if the gene Ultrabiothorax, which is expressed in the diversification of the hind wings of more specialized insects, is inhibited their hind wings develop like their forewings.

Segment Diversity Appendages now function in: 1. Feeding 2. Locomotion 3. Respiration 4. Burrowing 5. Sensation 6. Copulation 7. Brooding young 8. Defense

Bibliography Gould, Stephen J. (1989) Wonderful Life, W.W. Norton, New York. Sean Carroll (2005) Endless Forms Most Beautiful, W.W. Norton, New York. Meet the Cambrian Critters at Berkeley’s Evolution Website Burgess Shale Fossil Sampler at the Smithsonian Institute

Sydneyia

Hallucigenia

Wiwaxia