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Ecol Evol
2022 Aug 26;128:e9247. doi: 10.1002/ece3.9247.
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A new index for quantifying the ornamentational complexity of animals with shells.
Miao L
,
Dai X
,
Song H
,
Backes AR
,
Song H
.
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Morphological complexity reflects the biological structure of an organism and is closely linked to its associated functions and phylogenetics. In animals with shells, ornamentation is an important characteristic of morphological complexity, and it has various functions. However, because of the variations in type, shape, density, and strength of ornamentation, a universal quantitative measure of morphological complexity for shelled animals is lacking. We propose an ornamentation index (OI) derived from 3D scanning technology and a virtual model for quantifying ornamentation complexity. This index is designed to measure the extent of folding associated with ornamentation, regardless of shape and size. Ornamentation indices were measured for 15 ammonite specimens from the Permian to Cretaceous, 2 modern bivalves, 2 gastropods from the Pliocene to the present, and a modern echinoid. Compared with other measurements, such as the fractal dimension, rugosity, and surface-volume ratio, the OI displayed superiority in quantifying ornamentational complexity. The present study demonstrates that the OI is suitable for accurately characterizing and quantifying ornamentation complexity, regardless of shape and size. Therefore, the OI is potentially useful for comparing the ornamentational complexity of various organisms and can be exploited to provide further insight into the evolution of conchs. Ultimately, the OI can enhance our understanding of morphological evolution of shelled organisms, for example, whether shell ornaments simplify under ocean acidification or extinction, and how predation pressure is reflected in ornamentation complexity.
FIGURE 1. Photographs showing organisms with shells involving varying strength and types of ornamentation.
FIGURE 2. Image of the procedure used to create a model for Pseudotirolites acuticostatus highlighting (a) cutting horizontally along the plane of symmetry to obtain the side exhibiting better preservation, (b) cutting the adoral part to restore the state of an ammonite shell, (c) capturing the curve of the smooth part of shells using 3ds Max to construct an idealized 3D digital model, which preserves the original geometric ratio of the shell conch without ornamentation, and (d) the TurboSmooth process to enhance meshes of the idealized 3D digital model (scale bar = 1 cm).
FIGURE 3. Illustration of the quantification of the morphological complexity for ammonite species including Douvilleiceras mammillatum, Pseudotirolites acuticostatus, Pseudoceltites multiplicatus, Cleoniceras madagascariense, Flemingites rursiradiatus, Nyalamites angusticostatus, Nyalamites angusticostatus, Mullericeras gujiaoense, Mesohedenstroemia kwangsiana, Dactylioceras commune, Teloceras sp., Perisphinctes sp., and polished ammonite fossil (specimens marked with an asterisk were not fully developed and showed intraspecies differences). Results are presented for the OI, S/V ratio (mm2/mm3) and rugosity (scale bars = 1 cm), and based on the OI, the ornamentation complexity varies from high (Douvilleiceras mammillatum) to low (polished ammonite fossil).
FIGURE 4. Image highlighting the diversity of the ornamentation index for shelled organisms. (a) a modern sea snail shell (Murex pecten), OI = 61.81%, (b) modern sea urchin shell (Cidaris cidaris), OI = 16.09%, (c) a modern sea bivalve shell (Trachycardium enode), OI = 12.96%, (d) polished bivalves (Codakia tigerina), OI = 0.42%, (e) a gastropod fossil specimen (Naticarius plicatella) obtained from the Digital Atlas of Ancient Life database (https://www.digitalatlasofancientlife.org), OI = 0.74%. Scale bar = 1 cm.
FIGURE 5. Plot showing the variation of the surfaceâvolume (S/V) ratio (mm2/mm3), ornamentation index (OI), and the rugosity of a model as the size and shape change. (a) the 3D component of the model is magnified by two, (b) the height of the ideal model is doubled.
FIGURE 6. Plot showing the relationships between shape and complexity metrics including the associated Pearson's correlations coefficient. The strengths of the correlations are differentiated based on the colors and sizes of the circles (*p < .05, **p < .01, ***p < .001). The parameters U/D = umbilical diameter/diameter, W/H = width/height, W/D = width/diameter, S/V = surfaceâvolume ratio (mm2/mm3) and OI = ornamentation index.
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