GRAPTOLITE PLASTIC SURGERY: A CASE STUDY IN SHAPE RECONSTRUCTION OF DEFORMED GRAPTOLITE SPECIMENS

One of the major impediments to the accurate description and reconstruction of fossils is the tectonic deformation they have undergone. In the past, paleontologists generally solved these alteration problems by concentrating on rare, well-preserved fossils that allowed them to eliminate the deformation enigma all together. Since the majority of fossils in deformed rocks nonetheless have undergone some degree of alteration in the features used for species discrimination, is not a satisfactory solution. Fortunately, taxonomic and strain information can be recovered from these altered fossils. In particular, those fossils with bilateral symmetry or known angles and dimensions may be reconstructed by analyzing the strain and restoring (retrodeforming) the fossils to their original shape.

The concept of shape then becomes paramount to the accurate description and statistical analysis of fossil organisms. Geometric morphometrics employs landmarks (discrete anatomical points that can be recognized as homologous in all specimens of the intended study set), which document aspects of shape and size that are not derivable from traditional morphometric approaches. The shape of a geometrical figure can be understood as those geometrical attributes that remain unchanged when the figure is translated, rotated, or scaled. The primary goal of this research is to construct a reliable and complete step-by-step method of applying a geometric morphometric landmark-based computer program as a reasonable solution to the deformation problem.

Graptolite fossils with known original shape have been used to considerable advantage in determining the finite strain of the enclosing rock mass. With this in mind, we analyzed as a test set three samples of graptolites with isograptid symmetry constituting initial arbitrary alterations of deformed, slightly deformed, and non-deformed specimens. The analysis statistically compared and contrasted the differences between initial deformation and retrodeformed graptolites within each sample set to gauge the reliability of the new retrodeformation method. The resulting retrodeformed landmark sets appear suitable for use in the powerful geometric morphometric formalism and provide a basis upon which further shape comparisons may be reliably conducted.