Do Science: Morphometrics

Now's your chance to use morphometrics to better understand fossils. The fossils we'll look at are of the trilobite Phacops rana. We believe that trilobites are members a group we call the Phylum Arthropoda, for many reasons. If they are arthropods, they periodically shed their shells as they grow. Therefore, we expect trilobites of the same species, living in the same area, to not be of all different sizes, but to fall into size groups. All trilobites of about the same biological age (say, 1 year old), from the same area and from the same time will be about the same size! Trilobites of different biological ages will be a different size, but still be about the same size within the age group.

Using morphometrics, we can test this prediction. We'll measure a group of Phacops fossils and plot their length and width. If the measurements don't fall into distinct groups, then our prediction is proven false. Either the fossils weren't from the same area and time, or trilobites aren't arthropods.

[ pygidium of Phacops ]

We're going to measure one part of the trilobite, and look at how it's size varies between fossils. The part we'll look at is the tail-end of the trilobite, called the pygidium (pij-ID-ee-um), not because there's anything special about the pygidium, but because it's easy to find and easy to measure. The pygidium looks like a small, semi-circular shield, as you see on the right.

We want to measure the width, the distance between the left and right sides, and the height, the distance between the top and bottom. There are a couple points to remember to help you find these points

  1. The bottom edge of the pygidium of Phacops is smooth; the parts of the trilobite beyond the top have a jagged edge.
  2. The pygidium has lines on it that look like ribs, and there's a gap between the topmost rib on the pygidium and the rib right below it.

In many cases, the pygidium is broken. Use what you know about the shape of a whole pygidium to estimate (guess) where the edge would be if it weren't broken.

We're going to use a Java applet to take measurements from pictures of trilobites. If you don't have a Java-enabled browser (Netscape or Microsoft Internet Explorer before version 3), you'll need to upgrade to a more recent version before you can make the measurements. If you have a Java-enabled browser (Netscape or Microsoft Internet Explorer version 3 or later), you must make make sure that the ability to use Java is turned on in your preferences.

NOTE: Some version's of Microsoft's JDK won't run this applet. The applet runs perfectly under other releases from Microsoft, and always work using Sun's JDK. If you experience problems, get the Sun JDK.