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. 2024 Nov 12;7(1):1490.
doi: 10.1038/s42003-024-07221-2.

Distinct causes underlie double-peaked trilobite morphological disparity in cephalic shape

Harriet B Drage  1 , Stephen Pates  2   3
Affiliations

Distinct causes underlie double-peaked trilobite morphological disparity in cephalic shape

Harriet B Drage et al. Commun Biol. .

Abstract

Trilobite cephalic shape disparity varied through geological time and was integral to their ecological niche diversity, and so is widely used for taxonomic assignments. To fully appreciate trilobite cephalic evolution, we must understand how this disparity varies and the factors responsible. We explore trilobite cephalic disparity using a dataset of 983 cephalon outlines of c. 520 species, analysing the associations between cephalic morphometry and taxonomic assignment and geological Period. Elliptical Fourier transformation visualised as a Principal Components Analysis suggests significant differences in morphospace occupation and in disparity measures between the groups. Cephalic shape disparity peaks in the Ordovician and Devonian. The Cambrian-Ordovician expansion of morphospace occupation reflects radiations to new niches, with all trilobite orders established by the late Ordovician. In comparison, the Silurian-Devonian expansion seems solely a result of within-niche diversification. Linear Discriminant Analyses cross-validation, average cephalon shapes, and centroid distances demonstrate that, except for Harpida and the Cambrian and Ordovician Periods, order and geological Period cannot be robustly predicted for an unknown trilobite. Further, k-means clustering analyses suggest the total dataset naturally subdivides into only seven groups that do not correspond with taxonomy, though k-means clusters do decrease in number through the Palaeozoic, aligning with findings of decreasing disparity.

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Conflict of interest statement

Competing interests The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Examples of trilobite specimens included within the dataset.
A Bohemoharpes naumanni Senckenberg Museum X286a; B snapshot of 3D model, Acaste downingiae NHMUK 44409; C Modocia typicalis Senckenberg Museum unnumbered Westphal collection; D Pterocephalia norfordi Senckenberg Museum unnumbered Westphal collection; E Orygmaspis contracta Senckenberg Museum unnumbered Westphal collection; F snapshot of 3D model, Bumastus barriensis NHMUK I1029; G snapshot of 3D model, Calymene blumenbachii NHMUK 44215; H Bumastus iorus Senckenberg Museum unnumbered Westphal collection; I Megalaspides sp. SMNKPAL 3887. Scale bars = 1 cm.
Fig. 2
Fig. 2. Principal components analysis (PCA) of trilobite cephalic outline morphometry, grouped by taxonomic order.
A PCA total plot; B PCA facet wrap plot; C linear discriminants analysis (LDA) plot. Principal components scree plot (D) is applicable to the entire dataset and contains no grouping. A, B visualise the same data, but (B) has the overlapping convex hull polygons separated out onto individual morphospaces for ease of visualisation. Group labels in (A, C) display the centroid positions in morphospace. N = 983 independent biological samples.
Fig. 3
Fig. 3. Linear discriminants analysis (LDA) cross-validation table for data grouped by taxonomic order.
Y-axis is the true order of a new data entry, and the x-axis is the order that this new entry would be predicted as being under this dataset, with a given probability in the relevant cell. Empty cells mean the probability is effectively zero. N = 983 independent biological samples.
Fig. 4
Fig. 4. Mean cephalic shape for each taxonomic order compared pairwise, and pairwise centroid distances for the same.
Bottom-right half of matrix: mean cephalic shape for data grouped within each taxonomic order, compared in a pairwise fashion with the colour of the outline reflecting the colour of its axis. Top-left half of matrix: pairwise centroid distances for taxonomic orders, showing their relative positions in PCA morphospace; blue figures indicate particularly close pairwise distances (<0.100), and red figures indicate particularly far pairwise distances (>0.399). N = 983 independent biological samples.
Fig. 5
Fig. 5. Box and whisker plots showing disparity measures calculated for each taxonomic order grouping.
A sum of variances; B sum of ranges. The black lines show the median value, the hinges correspond to the first and third quartiles, and the error bars represent 1.5 × the interquartile range. Black plotted points represent potential outliers (outside of the error bars), and grey plotted points show the total underlying data. N = 983 independent biological samples.
Fig. 6
Fig. 6. Principal components analysis (PCA) of trilobite cephalic outline morphometry, grouped by geological Period.
A PCA total plot; B PCA facet wrap plot; C linear discriminants analysis (LDA) plot; D LDA cross-validation table. A, B visualise the same data, but (B) has the overlapping convex hull polygons separated out onto individual morphospaces for ease of visualisation. For (D), the y-axis is the true occupied Period of a new data entry, and the x-axis is the Period that this new entry would be predicted as being found in under this dataset, with a given probability in the relevant cell. Empty cells mean the probability is effectively zero. The scree plot for the PCA (A, B) is the same as that presented in Fig. 2D. Group labels in (A, C) display the centroid positions in morphospace. N = 983 independent biological samples.
Fig. 7
Fig. 7. Mean cephalic shape for each geological Period compared pairwise, and pairwise centroid distances for the same.
Bottom-right half of matrix: mean cephalic shape for data grouped within each geological Period, compared in a pairwise fashion with the colour of the outline reflecting the colour of its axis. Top-left half of matrix: pairwise centroid distances for geological Periods, showing their relative positions in PCA morphospace; blue figures indicate particularly close pairwise distances (<0.100), and red figures indicate particularly far pairwise distances (>0.200). N = 983 independent biological samples.
Fig. 8
Fig. 8. Box and whisker plots showing disparity measures calculated for each geological Period grouping.
A sum of variances; B sum of ranges. The black lines show the median value, the hinges correspond to the first and third quartiles, and the error bars represent 1.5 × the interquartile range. Black plotted points represent potential outliers (outside of the error bars), and grey plotted points show the total underlying data. N = 983 independent biological samples.
Fig. 9
Fig. 9. Separate principal components analysis (PCA) morphospace plots of trilobite cephalic outlines for only specimens present in each geological Period.
The convex hulls represent the taxonomic orders present in each Period (A–F, labelled with the relevant Period; order denoted by colour and point shape, see legend for each of A–F). N = 983 independent biological samples.
Fig. 10
Fig. 10. K-means analysis displaying the natural clustering inherent in the trilobite cephalic outlines dataset, separated for each geological Period.
Both the elbow plot (displaying the most suitable number of clusters) and the clustering hypothesis are presented for each Period (A–F, labelled with the relevant Period). The arrows display the number of clusters at which there is an observable change (downwards trend) in y-axis steepness. N = 983 independent biological samples.
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References

    1. Foote, M. Discordance and concordance between morphological and taxonomic diversity. Paleobiology19, 185–204 (1993a).
    1. Fortey, R. A., Briggs, D. E. G. & Wills, M. A. The Cambrian evolutionary ‘explosion’: decoupling cladogenesis from morphological disparity. Biol. J. Linn. Soc.57, 13–33 (1996).
    1. Bapst, D. W., Bullock, P. C., Melchin, M. J., Sheets, H. D. & Mitchell, C. E. Graptoloid diversity and disparity became decoupled during the Ordovician mass extinction. P. Natl Acad. Sci.109, 3428–3433 (2012). - PMC - PubMed
    1. Hopkins, M. J. Decoupling of taxonomic diversity and morphological disparity during decline of the Cambrian trilobite family Pterocephaliidae. J. Evo. Biol.26, 1665–1676 (2013). - PubMed
    1. Puttick, M. N., Guillerme, T. & Wills, M. A. The complex effects of mass extinctions on morphological disparity. Evolution74, 2207–2220 (2020). - PubMed

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