Exceptionally preserved early Cambrian bilaterian developmental stages from Mongolia

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.


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We describe and interprete a set of fossilized Cambrian invertebrate eggs, embryos, larvae, and juveniles which have hitherto not been described. For the systematic size measurements of specimens all individuals were measured and investigated.
We describe fossilized Cambrian camenellan juveniles and co-ocurring remains of the Tommotiid Camenella mongolica, which is interpreted as a stem-group representative of brachiopod lineage For microfossil extraction we sampled carbonatic sediments in regular distances (50 cm oder lower stratigraphical distance); alltogether 50 samples were collected and all processed and investigated for fossilized fauna; only 3 samples (nos. Sal 133,134,139) contained the new eggs, embryos and postembryonic stages The stratigraphical sediment sequence in Mongolia was measured and documented by Steiner & Yang Ben, partly supported by Hohl and Li Da. All sediment samples were cut by a diamond saw in the lab of FU Berlin, one small slab of each sediment sample was stored as a reference at FU Berlin, Dpt. Geosciences; all sediment samples were chemically processed for microfossil extraction as described in the article; all sample residues were hand-picked by Steiner and all embryos and postembryonic stages were systematically studied under SEM at FU Berlin; synchrotron radiation X-ray Tomographic Microscopy (  Samples were taken by crack out of a small amount of rock from a natural rock exposure with geological hammers; we tried to keep our work impact on natural soils and fauna and flora at a minimum. All waste produced during the camp operation was carried back from the natural habitate to Altai city.