The quantification of shape in 3D for marine extinct arthropods remains rarely documente Based on both heads and tails of some trilobites, we compare the overall shape and explore th ontogenetic patterns and the phylogenetic signal for the first time in 3D versus 2D. We dem onstrate that there are rather congruent results between 3D and 2D to discriminate taxa; 2 and 3D landmarks capture different levels of detail, and the third dimension in 3D is ver important for making taxonomic distinctions at the genus level; there is congruity betwee 2D and 3D datasets for ontogenetic patterns; the phylogenetic morphospaces show tre branches that do not intersect, suggesting possible phylogenetic constraints on morphospac occupation for each species; and the morphological descriptors in morphometric analyses i 2D and 3D throughout trilobite evolution are effective.

Phacopid trilobites are well documented during the Paleozoic. Nevertheless, while 2D quan titative analyses have advanced our understanding of the morphological relationships amon trilobites, the quantification of their morphological traits in 3D remains rarely documented Based on two sets of morphological data (head and tail), 2D versus 3D shape quantificatio approaches were used to explore shape allometries as well as to explore how the shape vari ations can be explained by the phylogenetic relationships among phacopid trilobite species fo the first time. We demonstrate that (1) there are similar patterns of morphological variabilit across taxa in 3D and 2D; (2) there are rather congruent results between 3D and 2D to dis criminate taxa; (3) 2D and 3D landmarks capture different levels of detail, and the thir dimension in 3D is very important for making taxonomic distinctions at the genus level (4) there is congruity between 2D and 3D datasets for allometric patterns with results showin similar allometric slopes among species exhibiting a glabellar length decrease during growt leading to wider cephala; (5) the phylomorphospaces show tree branches that do not intersect suggesting possible phylogenetic constraints on morphospace occupation for each species an supporting the idea that the Austerops and Morocops groups are sister clades that experience different modes of morphological evolution; and (6) the morphological descriptors in mor phometric analyses in 2D and 3D throughout phacopid evolution are effective.