BODY FORM AND RELATIVE DRAG IN TRILOBITES


        Trilobites were an early and extremely successful group of arthropods, that dominated marine faunas in the Early Paleozoic. Trilobites have relatively stereotyped body plans consisting of three body regions; the cephalon (or head), the trunk, and the pygidium. The cephalon and pygidium were composed of fused segments, so all body flexibility was restricted to unfused trunk segments. All body segments bore similar appendages that lacked the specializations seen in many later arthropods. Each appendage was biramous with a dorsal gill and a ventral walking leg. Trilobites lacked specialized feeding appendages and, instead, used clusters of stout spines on the bases of the legs, called gnathobases.

        Trilobites probably had several different feeding modes. Many species apparently were detritivores, which plowed through soft sediments feeding on detritus that may have been resuspended with leg movements and filtered out by the gills. Other were carnivorous on soft-bodied, or poorly defended, animals. Still other species are believed to have been true suspension-feeders that used their gills to collect particulate food from the water.

        Trilobites are generally considered to have been capable of crawling, burrowing and swimming locomotion. Despite the overall conservatism within trilobites, they evolved into a number of different body plans that probably varied greatly in locomotory ability. Spiny forms seem unlikely to have regularly engaged in swimming or burrowing. On the other hand, highly streamlined forms were undoubtedly capable of relatively rapid swimming, but rarely burrowed.

Body Size and Reynolds Number

        Unfortunately, not all trilobites are of the same size. In order to produce data of comparable quality between species of different sizes, we will need to adjust water velocity to body size. The appropriate scaling factor is Reynolds number (Re), which is used for scaling models that are suspended within a fluid. Reynolds number is calculated as:

The length of a model is any homologous measurement. Since both the density and viscosity of water are approximately 1 and are constant during a set of experiments, this equation reduces to:

        In testing models of different sizes, we want to be able to compare them under similar conditions, which means they must be tested at the same Reynolds number. Therefore, the relationship between velocity (V) and length (L) of the two models is important:

Once the drag has been determined for one model of known length (L1) and at a known velocity (V1), the correct velocity (V2) for a second model of different length (L2) can then be determined:


        The purpose of this exercise is to experimentally evaluate the swimming and resting capabilities of different trilobite species. Swimming capability will be approximated by determining the relative drag and inherent stability of freely-suspended body forms in a water current. Models of six trilobite species will be suspended in a flow tank at predetermined water velocities and their relative drags estimated from the displacement of the model in the water current.

        The problem is how to estimate relative drag (F) for a trilobite model (Figure 1) of immersed weight W.

Figure 1.  Displacement of a trilobite model in a water current.

When suspended in a current on threads of length L the model has both a linear displacement (x) and an angular displacement (A). Relative drag can be estimated either from the linear displacement:

or from the angular displacement:
While either the linear or angular displacement can be used for determining F, the angular displacement is somewhat easier to measure, and will be used in this exercise. As long as measurements are obtained at equivalent Reynolds numbers, the estimated relative drags of different species will be directly comparable.
 


Procedures for Freely-Suspended Models

There are life size, epoxy resin models of six trilobites available in the laboratory. The genera to be studied (listed in size from shortest to longest) are Flexicalymene, Metacanthina, Ceraurus, Phacops, Paralejurus and Isotelus.

To suspend the models in the flow tanks you will need to attach a pair of fluorescent orange threads. One thread should be attached at the junction of the cephalon and trunk and the other at the junction of the trunk and pygidium. The free ends of the threads should extend dorsally over the midline of the body at least 20 cm. For each model perform the following steps:

1. Measure the body length (BL) and record.

2. Suspend the model from the undercarriage hook of an elevated electronic scale. Completely immerse the model in a beaker of water. Record the weight (W).

3. Suspend the model from the horizontal support rods of the flow tank with the orange threads. Do not tie the threads to the support rods. Instead, wrap each thread several times around its support rod and hold in place with a small piece of modeling clay. This means of attachment permits slight adjustments in thread length to be made quickly and easily. The model should be suspended equidistant from the bottom of the flow tank and the surface of the water. The cephalon of the model must be upstream (i.e., away from the motor). The model also must be perfectly horizontal. Make certain that the lengths of both threads are equal, and that the distance between the support rods is equal to the distance between the attachment points of the threads to the model. Finally, the midline of the body must be parallel with the sides of the flow tank. Once the model has been correctly aligned, anchor the support rods to the top of the flow tank with small pieces of modeling clay.

4. Mount a protractor onto the near side of the tank by sliding the reference hole of the protractor onto the anterior support rod. The baseline of the protractor MUST be aligned with the anterior thread. Hold the protractor in place with a small piece of modeling clay.

5. Set the velocity control rheostat to zero. Set the master switch to the ON position. Slowly turn the velocity control to the selected velocity.

6. Measure the angular displacement (A) by aligning a straight edge with the reference hole of the protractor and the anterior thread. Record this value.

7. Repeat these procedures with the remaining five models.

8. Calculate values for tan A and relative drag (Fs).

9. Plot a graph of body length vs. relative drag. Print a copy of this graph.

Make your own trilobite:

  • Using the modeling clay and wire provided, modify one of the trilobite models to simulate some other trilobite taxon, preferably one with some unusual morphology (spines, eye-stalks, etc.)

  • Measure relative drag for your new trilobite

    Questions

    Did any of the models exhibit unstable behavior (i.e., changed attitude unexpectedly)?
     

    Are there any structural characteristics of trilobites that appear to be associated with either stability or instability while swimming?
     

    Is there any relationship between body length and relative drag?

    Is there any relationship between body morphology and relative drag?