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A major macroecological event happened long after the Cambrian Explosion and the Great Ordovician Biodiversification Event, the Devonian Nekton Revolution . It signifies the rapid occupation of the free water column with nektonic animals, an important component of all post-Silurian and Recent marine ecosystems. This event coincides with an explosive diversification of gnathostomes (jawed fishes), the initial radiation of ammonoids (extinct, externally shelled cephalopods), and the intensification of the radiation of mollusks with planktotrophic larvae or hatchlings (ammonoids, bivalves, gastropods; see also Frýda et al. 2008;Manda and Frýda 2010).Many externally shelled mollusk groups (ammonoids, dacryoconarids, gastropods, nautiloids, etc.) show a progressive coiling in their embryonic and/or post-embryonic conch in the Devonian

Ammonites are prominent in macroevolutionary studies because of their abundance and diversity in the fossil record, but their paleobiology and position in the marine food web are not well understood due to the lack of preserved soft tissue. We present three-dimensional reconstructions of the buccal apparatus in the Mesozoic ammonite Baculites with the use of synchrotron x-ray microtomography. Buccal mass morphology, combined with the coexistence of food remains found in the buccal mass, suggests that these ammonites fed on plankton. This diet may have extended to all aptychophoran ammonites, which share the same buccal mass morphology. Understanding the role of these ammonites in the Mesozoic food web provides insights into their radiation in the Early Jurassic, as well as their extinction at the end of the Cretaceous/early Paleogene.

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