Abstract
Fossilized invertebrate embryonic and later developmental stages are rare and restricted largely to the Ediacaran-Cambrian, providing direct insight into development during the emergence of animal bodyplans. Here we report a new assemblage of eggs, embryos and bilaterian post-embryonic developmental stages from the early Cambrian Salanygol Formation of Dzhabkan Microcontinent of Mongolia. The post-embryonic developmental stages of the bilaterian are preserved with cellular fidelity, possessing a series of bilaterally arranged ridges that compare to co-occurring camenellan sclerites in which the initial growth stages retain the cellular morphology of modified juveniles. In this work we identify these fossils as early post-embryonic developmental stages of camenellans, an early clade of stem-brachiopods, known previously only from isolated sclerites. This interpretation corroborates previous reconstructions of camenellan scleritomes with sclerites arranged in medial and peripheral concentric zones. It further supports the conjecture that molluscs and brachiopods are descended from an ancestral vermiform and slug-like bodyplan.
Highlights
Fossilized invertebrate embryonic and later developmental stages are rare and restricted largely to the Ediacaran-Cambrian, providing direct insight into development during the emergence of animal bodyplans
The new assemblage of phosphatized eggs, embryos, and juveniles from Mongolia was discovered in the topmost phospholithoclastic carbonates of the Salanygol Formation at Salanyi Gorge (GPS N46° 48′ 32.1′′ E095° 46′ 18.8′′) located at the Dzhabkan Microcontinent (Fig. 1)
The size range of the developmental stages is indicative of growth, which only occurs in post-hatching stages (Supplementary Fig. 1)
Summary
Fossilized invertebrate embryonic and later developmental stages are rare and restricted largely to the Ediacaran-Cambrian, providing direct insight into development during the emergence of animal bodyplans. Existing reports include almost complete cnidarian life cycles[2,4,5], a possible ctenophore embryo[6], scalidophoran embryos, and larvae[7,8,9,10], as well as possible mollusc embryos[4] which have impacted on macroevolutionary theories of developmental model, life history strategy, as well as the evolution of bodyplan symmetry[3]. The influence of this dimension of the fossil record is strictly limited by a paucity of sites where they are preserved. The juveniles may represent early stages in the life cycle of co-occurring tommotiids, such as Camenella mongolica and Camenella parilobata, in which case they would appear to corroborate the hypothesis of slug-like bodyplan for stembrachiopods[16,17,18,19,20,21,22,23] and, possibly, ancestral trochozoans
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