Principal component and discriminant analyses as powerful tools to support taxonomic identification and their use for functional and phylogenetic signal detection of isolated fossil shark teeth

Abstract

Identifying isolated teeth of fossil selachians only based on qualitative characters is sometimes hindered by similarity in their morphology, resulting often in heated taxonomic debates. On the other hand, the use of quantitative characters (i.e. measurements) has been often neglected or underestimated in characterization and identification of fossil teeth of selachians. Here we show that, employing a robust methodological protocol based on principal component and discriminant analyses on a sample of 175 isolated fossil teeth of lamniform sharks, the traditional morphometrics can be useful to support and complement the classic taxonomic identification made on qualitative features. Furthermore, we show that discriminant analysis can be successfully useful to assign indeterminate isolated shark teeth to a certain taxon. Finally, the degree of separation of the clusters might be used to predict functional and probably also phylogenetic signals in lamniform shark teeth. However, this needs to be tested in the future employing teeth of more extant and extinct lamniform sharks and it must be pointed out that this approach does not replace in any way the qualitative analysis, but it is intended to complement and support it.

Fig 1. Morphometric tooth terminology.

Abbreviations: BCW, basal crown width; CH, crown height; DCL, distal crown edge length; DS, degree of slant; IDCL, internal distal cutting edge length (cusp only); IMCL, internal mesial cutting edge length (cusp only); LCH, height of lateral cusplets; MCL, mesial crown edge length; PCH, height of principle cusp; PCW, width of principle cusp; RA, angle between root lobes; RH, root height; RW, root width; TH, total height of tooth.

Fig 2. PCA.

Results of PCA performed on the entire sample of standardized and log-transformed measurements for Brachycarcharias, Lamna, Carcharias and Carcharomodus, represented as convex hulls; (A) Morphospace plotted on PC1 and PC2; (B) Morphospace plotted on PC1 and PC3; (C) Loading values showing the variables associated with the first three PC axes.

Fig 3. CVA.

Results of CVA performed on the entire sample of standardized and log-transformed measurements. All genera are significantly separated (Hotelling’s p < 0.05) but the test failed to assign indeterminate teeth to a certain taxon (p > 0.05).

Fig 4. PCA.

Results of the PCA performed on the standardized and log-transformed measurements for Brachycarcharias and Lamna. (A) Morphospace plotted on PC1 and PC2; (B) Discriminant analysis results; (C) Loading values showing the variables associated with the two PC axes.

Fig 5. PCA.

Results of the PCA performed on the standardized and log-transformed measurements for Brachycarcharias and Carcharias. (A) Morphospace plotted on PC1 and PC2; (B) Discriminant analysis results; (C) Loading values showing the variables associated with the two PC axes.

Fig 6. PCA.

Results of the PCA performed on the standardized and log-transformed measurements for Brachycarcharias and Carcharomodus. (A) Morphospace plotted on PC1 and PC2; (B) Discriminant analysis results; (C) Loading values showing the variables associated with the two PC axes.

Fig 7. DA.

Results of the discriminant analyses comparing indeterminate teeth with those of Brachycarcharias, Lamna, Carcharomodus and Carcharias.

Fig 8. Hierarchical classification tree.

Hypothetical relationships between Carcharomodus, Brachycarcharias, Carcharias and Lamna based on the percentage of teeth correctly classified by discriminant analysis performed as pairwise comparisons. All the other possible combinations (Carcharomodus/Lamna; Carcharomodus/Carcharias) are related to a 100% of teeth correctly classified.