Documents the ontogeny and growth dynamics of Kaotaia globosa, suggesting implications for early arthropod evolution.
The exceptional fossil record of trilobite ontogeny allows the elucidation of the developmental evolution of early arthropods. However, the paucity of detailed segmental growth data limits the understanding of growth dynamics during post-embryonic development. Rich trilobite material is preserved in the Cambrian Kaili Biota (Miaolingian Series, Wuliuan Stage), and many articulated specimens of the “ptychopariid” Kaotaia globosa Chang & Zhou in Lu et al., 1974, from a relatively narrow stratigraphic interval provide valuable opportunities for a detailed ontogenetic study. This study documents the ontogenetic sequence and phenotypic changes of K. globosa from meraspid degree 3 to the holaspid period, integrating traditional morphological descriptions and geometric morphometric analyses. Analyses revealed that the primary changes in the K. globosa cranidium during ontogeny occurred in the glabella, facial suture, and palpebral lobes. The trunk of K. globosa follows a hemianamorphic pattern with a protomeric developmental mode. Growth model fitting suggests that the trunk-segment growth rate of K. globosa is controlled by a growth field (TGmold), which exhibits a continuously increasing growth gradient from front to back. A comprehensive analysis of existing data revealed significant variation in holaspid exoskeletal length, suggesting that an increasing growth gradient may have enabled these trilobites to undergo shorter anamorphic development followed by prolonged epimorphic development for body-size increase.
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Chen et al. (2026) studied this question.