Biology : Semester II

Sections:

IntroductionSection 1 | Section 2 | Section 3 | Section 4 | Section 5

  Section Three:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 | Part 11 | Part 12 | Part 13 | Part 14

Biology : The Time of Ancient Life : Part Three

The Burgess Shale Fauna:
Snapshot of the Cambrian World


presentation View this presentation to learn about the Cambrian Explosion. Be sure your volume is set at a reasonable level.


The Canadian rocks known as the Burgess Shale form a deposit of rare and exquisitely preserved fossils has been much touted in numerous publications. The deposit was discovered by American geologist Charles D. Walcott (1850-1927), who stopped to look at his horse's hoof and spotted an odd fossil by the trail. Walcott returned the next field season (and four others) and excavated over 50,000 fossils, which he later formally named and described in scientific publications. Modern studies suggest the presence of approximately 125 different genera from the Burgess Shale, making it the most complete and famous known Paleozoic assemblage. The organisms range from the prokaryotic cyanobacteria to eukaryotic green and red algae, to sponges, brachiopods, priapulids, annelids, and many different arthropod groups, as well as echinoderms and possibly one of the first chordates.

Charles D. Walcott, the American paleontologist who discovered and made the first collections from the Burgess Shale in British Columbia, Canada. Diorama of the Burgess Shale seafloor.
Used with permission from
http://www.mnh.si.edu/museum/VirtualTour/images/vt43b.jpg.

One of the most common Burgess animals is the arthropod Marella. These animals were small, and exhibited the jointed appendages characterizing the arthropods. These delicate looking arthropods are referred to as "lace crabs". The quality of preservation of these fossils is quite extraordinary. On many specimens dark stains are interpreted as body fluids that have leaked from the specimen after burial.

Fossil impression, right reconstruction of the animal in life position. Marrella, a Burgess arthropod
Used with permission from
http://www.carleton.ca/~tpatters/teaching/intro/cambrian/cambrianex27.html

Trilobites, a dominant Cambrian animal, also occur in the Burgess Shale in abundance. Trilobites are an extinct group of arthropods that were most dominant in the Cambrian before finally becoming extinct at the close of the Paleozoic. Like Marella and other arthropods, trilobites have a hard exoskeleton (that enhanced the chances of fossilization) and numerous jointed appendages. In the trilobite shown, details of the appendages are not clear.

The Burgess deposits also yielded the oldest representative of the phylum Chordata, a small fossil known as Pikaia. Most specimens of Pikaia are between one and one and a half inches long. Chordates, the phylum to which humans belong, has three distinguishing characteristics, all of which are present in Pikaia. The extant animal known as a lancet bears at least a superficial resemblance to Pikaia.

http://tabla.geo.ucalgary.ca/~macrae/Burgess_Shale/Olenoides.gif, photo © Andrew MacCrae, used by permission.


Pikaia, the oldest known chordate. Note the head region of the animals shown above is on the left side of each image.

While many of the Burgess animals can be ascribed to modern groups, a startling number of them do not belong in such a group. Opabinia shows a segment body, but has five eyes and a feeding appendage. No living animal is similar.


Among the animals of uncertain taxonomic affinity that characterize the Burgess fauna, Anomalocaris must rank as surely the biggest and strangest of the lot. Ranging up to about one half meter in length, this animal was the largest predator to range the Cambrian seas. Trilobite fossils have been recovered with what appears to be bite marks from the complex mouth structures of Anomalocaris. Usually Anomalocaris is found disassociated; in fact the mouthparts, feeding appendages, and body were all described under different genus names. Rare fossils that show the articulation of these body parts have allowed scientists to reconstruct this animal.

 

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