The Morphometric Ontogeny of the Trilobite Calyptaulax

Jennifer McDanielMentor: Dr. Jesse Carlucci

The Morphometric Ontogeny of the Trilobite Calyptaulax

Trilobites are extinct arthropod that make up the class Trilobita. They are the most diverse fossil group of the Paleozoic.

Trilobites could enroll for protection.

Armadillidium vulgare

Trilobites provide us with the earliest record of an advanced invertebrate visual system.

In the Cambrian and Lower Ordovician, trilobites are abundant and their remains are often the primary components of limestones

Our specimens have been silicified, so that only the limestone is dissolved when exposed to Hydrochloric Acid.

CaCO3(s) + 2H+(aq) + [2Cl-

(aq)] --> Ca2+(aq) + CO2(g) + H2O(l) + [2Cl-

(aq)]

The trilobite exoskeleton

What are trilobites and how do they grow? (Ontogeny)

• Protaspid: Disk or globular shape, no true segmentation.

• Meraspid: Adds thoracic segments with each molt.

• Holaspid: Gets larger with each molt, adult number of thoracic segments.

Hypotheses to be tested

• Shape change during the development of the trilobite Calyptaulax is allometric (Adults are not simply scaled up versions of larger ones).

• The transition from meraspid to holaspid stage can be seen in the head and trail and is not just dependent on an adult number of thoracic segments.

Landmark Selection

• Landmarks are a biological coordinate.• Must provide adequate coverage of the form• Can be found reliably across specimens in different

states of preservation.• Consistency of relative position• Coplanarity of landmarks (accounting for

orientation)

Type 1 landmarks are the most useful for defining shape they occur at intersections.Type 2 landmarks are at the end of a line or other feature.Type 3 landmarks are the most unreliable and are only used when they can not be avoided and occur on a curved line.

These landmarks, A,B, and C work as a axis of symmetry allowing to use partial samples as we can flip the

image over the base.

AB

C

These landmarks, A,B, and C also act as a baseline to show the direction of

shape change vectors.

AB

C

Type 2

• The next three land marks are at the inside edge of furrows S1, S2 and S3

Type 1

• The next landmarks are where the same sections (S1 S2 and S3) meet the Axial furrow.

Type 1 • Where the occipital and the axial furrows intersect. Where the axial and the palpebral furrow intersect. Where the palpebral and posterior furrow intersect. At the outside most point of the palpebral furrow.

Scientific Awesome

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Landmarks x/y coordinates

Semi-landmark coordinates along a curve.

Scaling Factor

Centroid Size

Centroid size is the square root of the sum of squared distances of a set of landmarks from thier centroid. The centroid is determined by the average of the x,y coordinates.

Future Work

• We are still collecting data, when we are done we need to apply multivariate statistics to the data.

Future Work (Procrustes Superimposition)

These are trilobite landmark coordinates after they have been superimposed: rotated, scaled by centroid size, and moved so that only shape differences remain. The denominator of the formula for the Procrustes distance between two sets of landmark configurations is the product of their Centroid Sizes.

Kim et al. 2002

Testing the hypothesis of merapsid-holaspid shape change

• Procustes Distance = shortest distance along an arc between two landmarks

Kim et al. 2002

Mer

aspid

Holaspid

PCA

• Principle Components Analysis to find out which aspects of shape change explain the most variance as the trilobite grows.

• Thin Plate Spline analysis to show how coordinates deform during different portions of ontogeny and the movement of which coordinates best explains the shape differences.

Webster et al. 2011