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| Primicaris larvaformis Zhang, Han, Zhang, Liu & Shu, 2003 in Liu, Zeng, Zhao, Y. Zhu, Li, Yin et M. Zhu, 2025. |
Abstract
The mandibulate euarthropods are the most speciose animal group, but the evolutionary gaps in origin of mandibulate body plan remain unresolved. Marrellomorphs, a common Paleozoic euarthropod group, had a long evolutionary history from Cambrian to Devonian. With computed microtomography, here we report the fine-scale soft-bodied morphoanatomy of the oldest marrellomorph Primicaris larvaformis, a millimeters-sized euarthropod from the ~ 518-million-year-old Chengjiang biota, China. Primicaris possesses a body plan featuring morphologically similar post-antennular biramous appendages, but also mandibulate diagnostic features including multi-segmented exopodites, a well-developed and differentiated hypostome-labrum complex, and a pancrustacean-like topological configuration of frontalmost three pairs of appendages. Phylogenetic analysis resolves Acercostraca and Marrellida as stem-Mandibulata. The undifferentiated post-antennular appendages in Primicaris suggest a possibility that the head appendages acquired a crown-mandibulate configuration before their morphological specialization in mandibulate origin. The emergence of novel appendage morphotypes in Acercostraca and Marrellida reveals that the complexity of limb tagmatization evolved independently in different Euarthropoda clades.
Keywords: Computed tomography, Limb tagmatization, Euarthropod, Mandibulate, Chengjiang biota, Cambrian explosion
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| Overall morphology of Primicaris larvaformis. (a) NIGP 200783A, optical photo, ventral view. (b, c) Juvenile specimens. (b) NIGP 200792, optical photo, ventral view. (c) NIGP 200791A, optical photo, dorsal view. (d, e) NIGP 200812 (see also in Fig. 3). (d) Optical photo, dorsal view. (e) Tomographic maximum projection image, dorsal view. (f) NIGP 200798A, optical photo, ventral view. (g) NIGP 200809A, optical photo, dorsal view. (h) NIGP 200810, optical photo, dorsal view. (i) NIGP 200813A, optical photo, ventral view. (j, k) NIGP 200799. (j) Optical photo, dorsal view. (k) SEM-EDS map of carbon. (i, m) NIGP 200817. (l) Optical photo, dorsal view. (m) SEM-EDS map of iron. a1–14, post-antennular appendage 1–14; ab, anterior border; an, antennule; ax, axial region; ba, base of appendage; dv, gut diverticulae; eb, eye bulge; er, eye ridge; gt, gut; ls1–11, lateral spine of dorsal shield 1–11; pl, pleural region; ps, posterior spine of dorsal shield; rg, ridge of dorsal shield; st, setae. Scale bars represent: 0.5 mm (a–m). |
Systematic palaeontology
Phylum Euarthropoda Lankaster, 1904.
Order Acercostraca Lehmann, 1955.
Genus Primicaris Zhang, Han, Zhang, Liu and Shu, 2003
Primicaris larvaformis Zhang, Han, Zhang, Liu and Shu, 2003
Emended diagnosis: Small euarthropod with a body length less than 6 mm. Undivided dorsal shield with up to twelve pairs of lateral marginal spines and a pair of posterior spines. One pair of uniramous antennules is followed by up to 14 homonomous pairs of biramous appendages (emended from ref. 13).
Yao Liu, Han Zeng, Fangchen Zhao, Yuyan Zhu, Yimeng Li, Zongjun Yin and Maoyan Zhu. 2025. A tiny Cambrian stem-mandibulate reveals Independent Evolution of Limb Ttagmatization and Specialization in early Euarthropods. Scientific Reports. 15: 19115. DOI: doi.org/10.1038/s41598-025-03544-0 [31 May 2025]