Page 387 - Introduction to Paleobiology and The Fossil Record

374 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Permian

Carboniferous Illaenina Harpina Calymenina Phacopina Cheirurina

Devonian Trinucleina

Silurian Agnostina Asaphina Ptychopariina

Ordovician Eodiscina Redlichiina

Cambrian Olenellina

Orders Agnostida Redlichiida Corynex- Ptychopariida Proetida Phacopida Lichida Odontopleurida
ochida

Figure 14.11 Stratigraphic distributon of the main trilobite groups. (From Clarkson 1998.)

Box 14.4 Landmarks: the Silurian trilobite Aulacopleura

Landmarks, as the name suggests, are recognizable geographic features. Such features can also be

defined on fossil organisms and they form the basis for geometric morphometrics. The aim of these
statistical techniques is to define precisely how shapes differ from each other, and the landmarks are

the fi xed points of comparison. Each landmark can be recorded as a set of coordinates or the dis-
tances between points, and they can be recorded from digital photographs or image analysis systems

and stored in spreadsheets. For example, 22 landmarks were necessary to define shape variations in
the exoskeletons of well-preserved Aulacopleura from the Silurian rocks of Bohemia (Fig. 14.12).
The data can be used in a variety of ways. For example it is relatively easy to see, visually, how the
trilobite actually grew; the most substantial growth took place in the thoracic region during ontogeny.
In some studies it is necessary to translate this into quantitative terms, and landmark analysis is the
key.
A large dataset is available at http://www.blackwellpublishing.com/paleobiology/. These data may
be analyzed and manipulated using a range of morphometric techniques such as principal component
analysis (see also Hammer & Harper 2005).