Lab #4: Arthropods

At the end of this lab, you should be able to:

1. Identify a fossil as a trilobite, ostracod, barnacle or eurypterid
2. Be able to suggest a possible life habit for a trilobite based on its morphology
3. Know the skeletal structure and material of each of these animals.
4. Know the ecological characteristics of each of these animals.
5. Know the geologic range of each of these groups.
6. Know a few important genera (mentioned by name in this handout) for each group.

Display

A. Trilobites:

I. Hard part morphology - Using fig. 14.3 in your book , locate these morphological features: cephalon (head), thorax, pygidium (tail), appendages, the three lobes, eyes. Note lines on face where the exoskeleton split open during molting (sutures).

1. plastic replica (#8)
   
2. plaster reproduction (#62)
3. Compare the trilobite with the horseshoe crab in the resin block

II. Molting - Like all arthropods, trilobite had to periodically shed their skeleton to grow.

1. #1
2. unnumbered specimen

The unnumbered specimen nicely shows how the skeleton split (the cheeks have broken off and been lost). This is one of several molting patterns. In the #1 sample, the cephalon has separated from the body to allow the organism to wiggle out. These trilobites are often found as a "hash" of separate cephalon, body segments, and pygidium.

III. Eyes - Trilobites had compound eyes like insects.

1. #16 Phacops
2. #1625 Phacops
3. # 608 Perinopsis
   

The overlapping fields of each of the hundreds of lenses in the eye of Phacops (#16, 1625) gave it 360 degrees of stereo vision. Other trilobites, such as the tiny agnostids (#608), had no eyes. This suggests very different modes of life for these two groups.

IV. Life Habits - Trilobites were largely epifaunal deposit feeders, with some exceptions. Early trilobites have few anti-predatory devices

1. moderately spiny forms - (#27)  fed on the surface of the sediment, using spines to keep on top of mud.  Perhaps deposit feeders.
2. very spiny forms - (unnumbered) could be swimmers, with spines supporting the body in the water
3. long spines on cephalon - possibly stabilizing animal on soft sediment (epifaunal)
4. large smooth forms with protruding eyes - (#187, #114, no #) shallow burrowers, keeping eyes above the mud
5. tiny blind smooth forms - (#2) pelagic plankton (these are widely distributed over the globe, supporting the idea that they were floating in the open ocean.
6. enrollment - (#655 Flexicalymene rolled, #1889 Flexicalymene unrolled, two unnumbered specimens) anti-predatory device, suggesting surface or shallow burrowing.
7. many legs with feathery gills (plaster replica of Triarthus: deep water epifauna in oxygen-poor environments (using all those gills to absorb available oxygen)

B. Crustaceans

I. Ostracods: (#1415, 93) tiny bean-shaped bivalved arthropods. They use their feathery legs for filter feeding, are found in marine and fresh water, and are immensely abundant in the modern world. As fossils, they are useful as biostratigraphic index fossils. Compare these samples to fig. 14.23 in your book.

II. Barnacles: (#1594, 1510, 1060-1061, unnumbered) These animals start life like other crustaceans, with swimming larvae. The larvae develop a bivalved shell, then settle to the bottom and replace that shell with the plates of the barnacle. Preferred habitat is rocky intertidal.

III. Other Crustaceans: (#1555) Lobsters, crabs, shrimp, copepods and amphipods are all very abundant in the modern world, but fossilize poorly due to their chitinous skeleton (except a few crabs with calcite skeletons).

C. Insects: Although insects make up half the known species on earth, they have a dreadful fossil record due to their fragile bodies and largely terrestrial habits.

1. #123 fly in volcanic ash
   
2. #1874  wings in lake sediments
   

D. Eurypterids: The "sea scorpions" are rare but important fossils in the Paleozoic, representing some of the only large predators of the time. These fossils are rarely found together with other organisms of the normal marine environment, and seem to have specialized in brackish water habitats of the coastline. They were apparently swimmers, but did not survive the Paleozoic.

1. Unnumbered plastic model
2. Unnumbered plaster model
3. #149 fragments in mud
4. Unnumbered slab
   

Questions

NOTE: Not all the arthropod groups you are responsible for are covered in the questions below (we are short of specimens of ostracods, eurypterids and most crustaceans - pretty much everything but trilobites.  Make sure you review all the groups in the display, as you are responsible for learning all the groups out here today except the crab/lobster/shrimp crustaceans.

1. #713, 709, 609, 25, 925, unnumbered. Compare these samples of Elrathia kingii. Which represent molts of trilobites and which are the trilobites themselves? Suppose you were doing a population study on Elrathia. How would this observation affect your results?

2. no number - Is this a collection of organisms that died catastrophically, or is this an accumulation of molts or dead trilobites? What kind of preservation is this?

3. #1068, 1644, 1231, 4 - None of these trilobites have any legs. Why not?

4. Sort the pictures of trilobites into these life habit categories: swimmers, shallow burrowers, benthic surface feeders, pelagic floaters.

5. No #- Is this a trilobite or a molt? How could you tell?

6. # 167- How is this crab preserved?  Why do crabs have a poor fossil record?

7. # 1075- how is this barnacle adapted to life in its preferred environment?

8. no #- From the lithology and the preservation, what can you deduce about the environment of deposition?

9. Three trilobites, three different modes of preservation. What are they?
    #1066:

    #4:

    unnumbered:

10.  Here are various bits and pieces of trilobites.  For each one, figure out if you have the entire fossil (or at least the entire top side - dorsal side - of the critter).  If not, what piece of the trilobite do you have?

    #1895:

    #1574:

    #211:

    #6:

    #15:

    #1070:

    #1894: