Ontogeny-Anisometric Growth

Judy Massare

SUNY Brockport

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Initial Publication Date: June 4, 2009 | Reviewed: June 24, 2014 (see revision history: 2 events)

Summary

Randomly select growth rates for different parts of a trilobite skeleton, and follow the animal through two molt stages to see how morphology can change with ontogeny.

Development/Growth and Form/Morphometrics | College Lower (13-14), College Upper (15-16)

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Context

Audience

Paleontology course taken by geology and earth science majors; students are sophomore or junior majors.

Skills and concepts that students must have mastered

Background reading on ontogeny that covers the major terms: isometric, anisometric; incremental growth, accretionary growth;

How the activity is situated in the course

Can be a stand-alone exercise on ontogeny and the difficulties in recognizing adults and juveniles of the same species in the fossil record.
I use the morphologies generated in a later exercise on cladistics and phenetics.

Goals

Content/concepts goals for this activity

Illustrate incremental and accretionary growth.
Ilustrate isometric and anisometric growth.

Higher order thinking skills goals for this activity

Problems in recognizing juveniles and adults of the same species in the fossil record.
Importance of differential growth in generating diversity.

Other skills goals for this activity

Develop technical drawing skills

Description of the activity/assignment

Each student receives the same simple drawing (on graph paper) of a trilobite. By flipping coins, each student determines a growth rate for different parts of the trilobite exoskeleton. They then draw (to scale) a first molt stages of the trilobite by applying the growth rates to the 'juvenile' diagram. They then apply the same rates to the 'first molt' to produce an 'adult' morphology. The adult morphologies for each ontogenetic series, as well as an adult produced by purely isometric growth, are displayed for the entire class to see. These diagrams serve as the basis for class discussion.

Later in the semester, the 'adult' trilobites are used to generate a data matrix of morphologic features of related species. Students work in groups so that there are a few different data matrices for the same set of species. A cladistic analysis and a cluster analysis (phenetics) produce possible phylogenies for the data matrices. We compare the results in a class discussion.