ANATOMY OF THE BURGESS SHALE

Name: ______

Background

As you know, zoology is the study of animals. But as J.B.S. Haldane acknowledges in the quote, “If one could conclude as to the nature of the Creator from a study of his creation it would appear that God has an inordinate fondness for beetles”, most animals don’t come in forms that familiar to ourselves. In fact, eighty percent of all animals are arthropods of some sort. Arthropod animals are named for their “jointed foot”, and members of this group are categorized by the differentiation (and fusion) of their segments, and the specializations of their appendages. Similarly, all arthropods have an external or exoskeleton.

Through analysis of currently living and extant species of arthropods it is believed that each ancestral arthropod segment had a single pair of appendages, and that each appendage branched into two parts or rami. Thus, the appendages were biramous (two branches). In many arthropods, the inner and outer rami are used in respiration/swimming and locomotion, respectively and are thus called the gill branch and the leg branch as shown below (although the leg branch may be modified and no longer used in locomotion).

As arthropods have evolved some have lost one of the rami as they specialized, focusing either respiration or locomotion. Similarly, in many arthropod lineages the ancestral segments, having a single pair of appendages, have fused forming segments with more than one pair of appendages.

The Burgess Shale fauna of British Columbia, discovered by the paleontologist Charles Walcott, represents the great diversity of animals existing in the Cambrian period of the Paleozoic era 600 million years ago. In the following activity, you will become familiar with the evolutionary differences between the four major subphyla within the phyla Arthropoda (Trilobitiomorpha, Chelicerata, Crustacea, and Uniramia) so that you can analyze fossils from the Burgess Shale in order to better understand the evolutionary history of our the Kingdom Animalia.

  • The concept for this activity was inspired by reading Stephen J. Gould (1989) Wonderful Life: The Burgess Shale and the Nature of History, W.W. Norton & Company, New York. Drawing in the packet are by Marianne Collins.

Materials

Spider and scorpion as a representative chelicerates

Crayfish as a representative crustacean

Insect and millipede as a representative uniramia

Procedure & Questions

1. Skim through Chapters 23 (Trilobitomorpha, Chelicerata, Crustacea) and 24 (Uniramia) in your textbook to find information on the four arthropod subphyla so that you can accurately label the segments and appendages of your sketches for each of the arthropod subphyla (also see the list of important terminology).

Trilobitomorpha

/

Chelicerata

Crustacea

/

Uniramia

2. Based on the label drawings that you have completed, note the number and names of the general body segments, the type (ramous or biramous), names, and locations of the appendages, as well as list representative organisms in each group.

Characteristics

/

Examples Organisms

Number of General
Body Segments /

Appendages

Trilobitomorpha
Chelicerata
Crustacea
Uniramia

3. Draw the two possible evolutionary trees that your textbook shows including labels of terminology you now know.

4. Given the sketches of what some of the Burgess Shale animals are believed to have looked, based on the fossil record, 1) analyze their segmentation and appendages by making specific notes on what you see in the drawings, and then 2) propose the current lineage that you think each is most related to by listing its name in the space provided below each specimen.


Nararia /
Aysheaia

Canadaspis /
Sanctacaris

Wiwaxia /
Anamalocaris

Opabina

5. Based on the background writing, sketch an ancestral arthropod.

6. Does your analysis of the Burgess Shale fauna support evolutionary tree show below? Explain. What does this suggest about the importance of contingency in evolution? (If you have to look up the word do so).

ANATOMICAL TERMINOLOGY

abdomenantennaebiramousbook lungs

cephaloncheliceraeheadmandible

maxillamaxillapedopisthosomapedipalps

post-oral appendages pre-oral appendagesprosomapygidium

swimmeretsthoraxuniramouswalking legs

Characteristics / Examples
Number of General Body Segments /

Appendages

Uniramia / 3 Head, Thorax, Abdomen / Uniramous
3 pairs for walking
antennae, mouth parts
Chelicerata / 2 Prosoma, Opisthosoma / 6 pairs uniramous on first segment, 1 chelicerae, 1 pedipalps, 4 legs,
uniramous makeup book lungs in opithosoma
Crustacea / 3 Head, Thorax, Abdomen / 5 pairs on the head (2 antennae uniramous pre-oral, 3 feeding uniramous post-oral), Trunk pairs per segment biramous
Trilobita / 3 Cephalon, Thorax, Pygidium / 4 pairs on the head (1 antennae pre-oral, 3 feeding post-oral), trunk pairs biramous

Horseshoe Crab Image

Spider