A basic trilobite morphometric exercise

John F. Taylor
Indiana University of Pennsylvania
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A lab exercise and follow-up classroom activity in which students measure some trilobite specimens, evaluate the statistical significance of differences documented in data acquired for two different populations, and then use a similar data set and graphic plots to discriminate three different species of an Upper Cambrian trilobite genus

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Upper level (junior-senior) Geology majors and Earth and Science Education majors.

Skills and concepts that students must have mastered

The exercise requires some preparation (review of pertinent chapters in Hammer and Harper (2006) and/or previous lectures) to familiarize students with a few basic statistical techniques (e.g., Shapiro-Wilk Test, linear regression, etc.) and concepts. Rudimentary familiarity with the Geologic Time Scale also is required.

How the activity is situated in the course

It can be done at any point in the semester, as one of the early labs in the first few weeks or as a later lab toward the end. It simple requires a week or two in advance to allow for the required preparation/lectures.


Content/concepts goals for this activity

That the students develop

  • the ability to collect basic biometric data from fossil specimens and apply the standard biometric techniques to characterize a species and compare morphologies of different populations/taxa
  • some familiarity with the basic anatomy of trilobite exoskeletons

Higher order thinking skills goals for this activity

That the students develop the ability to

  • confidently evaluate the statistical significance of results of a biometric analysis
  • correctly interpret biometric data and draw accurate conclusions regarding morphospecies discrimination
  • accurately interpret bivariate graphs and the lines produced by linear regression
  • assess the type of growth (isometric vs. non-isometric) reflected in the morphology of invertebrate fossils
  • give appropriate consideration to ontogenetic morphologic variation in characterization and comparison of fossil species

Other skills goals for this activity

That the students develop

  • a familiarity with the stratigraphic importance of trilobites as tools for correlation of Early Paleozoic strata
  • an understanding of the biomere concept, its relationship to the chronostratigraphic units (series and stages) into which Cambrian and Ordovician strata have been divided in Laurentian North America

Description of the activity/assignment

This activity entails a basic morphometrics lab, followed up by an in-class exercise to reinforce some of the same key concepts. The lab exercise familiarizes the student with basic methods of quantitative characterization and statistical comparison through measurement of pygidia (tails) of two species of the Ordovician trilobite Bellefontia – one from New York and one from Pennsylvania. Actual specimens, while nice, are not required; data acquired by measurement from photo collages will suffice. The exercise culminates in a statistical test of significance (using the Z-statistic) of the difference in slopes of the lines acquired for data from the two species. The data also serve to pose questions and prompt consideration of growth trajectories and discrimination of isometric from anisometric growth. The in-class activity builds on the knowledge base built in the lab but applies it to species discrimination based on the cranidia (central part of the head) of three species of the Upper Cambrian genus Bartonaspis, known to be of identical age from their occurrences within the very thin (everywhere 2m or less) Irvingella major Zone of the Elvinia trilobite Zone. The importance of that subzone, which is the "critical interval" at the top of the Pterocephaliid Biomere the basal unit of the Sunwaptan Stage traceable throughout Laurentian North America, also contributes to the significance of the exercise. With the insight developed from the lab, students are able to confidently distinguish the three species of Bartonaspis (from three photo collages), but must thoughtfully evaluate the data presented in bivariate plots of cranidial morphologic data to do so. The exercise gives the students a good sense of the level of familiarity and morphologic characterization necessary to do species-level identification, and also some worthwhile practice in basic quantitative methods.

Determining whether students have met the goals

A lab hand-in that includes the bivariate plots created for trilobite pygidial (tail) data for New York and Pennsylvania species, and the conclusions that they have drawn regarding statistical significance of the results produced by linear regression, provides an objective assessment of their success in mastering the basic skills.

The success in reaching higher level goals is reflected in the answers students provide to the questions in the exercise and on lecture exams.

More information about assessment tools and techniques.

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Supporting references/URLs

Hammer and Harper (2006)
Westrop and Adrain (2007)

(full references provided in the Word document)