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Abstract
Owing to their great diversity and abundance, ammonites and belemnites represented key elements in Mesozoic food webs. Because of their extreme ontogenetic size increase by up to three orders of magnitude, their position in the food webs likely changed during ontogeny. Here, we reconstruct the number of eggs laid by large adult females of these cephalopods and discuss developmental shifts in their ecologic roles. Based on similarities in conch morphology, size, habitat and abundance, we suggest that similar niches occupied in the Cretaceous by juvenile ammonites and belemnites were vacated during the extinction and later partially filled by holoplanktonic gastropods. As primary consumers, these extinct cephalopod groups were important constituents of the plankton and a principal food source for planktivorous organisms. As victims or, respectively, profiteers of this case of ecological replacement, filter feeding chondrichthyans and cetaceans likely filled the niches formerly occupied by large pachycormid fishes during the Jurassic and Cretaceous.
Highlights
The fate of individual groups of marine organisms at mass extinctions is of considerable interest (e.g., Jablonski & Raup, 1994; Jablonski, 2008)
Based on similarities in conch morphology, size, habitat and abundance, we suggest that similar niches occupied in the Cretaceous by juvenile ammonites and belemnites were vacated during the extinction and later partially filled by holoplanktonic gastropods
If we assume that the eggs and embryos continued to grow after they were laid, ammonoid fecundity would further increase, but evidence for this is missing in ammonoids (Mironenko & Rogov, 2016)
Summary
The fate of individual groups of marine organisms at mass extinctions is of considerable interest (e.g., Jablonski & Raup, 1994; Jablonski, 2008). Extinctions of entire communities or ecosystems are most conspicuous during the great mass extinctions (e.g., Foster & Twitchett, 2014), when usually vast new ecospace was freed and thereby, new ecological niches could form during recovery periods. It is not the most severe of the Big Five, the end-Cretaceous mass extinction is likely the most famous among those with the greatest severity (McGhee et al, 2013).
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