Key innovations in Mesozoic ammonoids: the multicuspidate radula and the calcified aptychus

Abstract

A nearly complete radula with seven elements per row preserved inside of an isolated, bivalved, calcitic lower jaw (= aptychus) of the Late Jurassic ammonite Aspidoceras is described from the Fossillagerst€ atte Painten (Bavaria, southern Germany). It is the largest known ammonite radula and the first record for the Perisphinctoidea. The multicuspidate tooth elements (ctenodont type of radula) present short cusps. Owing to significant morphological differences between known aptychophoran ammonoid radulae, their possible function is discussed, partly in comparison with modern cephalopod and gastropod radulae. Analogies between the evolution of the pharyngeal jaws of cichlid fishes and the ammonoid buccal apparatus raise the possibility that the evolution of a multicuspidate radula allowed for a functional decoupling of the aptychophoran ammonoid jaw. The radula, therefore, represents a key innovation which allowed for the evolution of the calcified lower jaws in Jurassic and Cretaceous aptychophoran ammonites. Possible triggers for this morphological change during the early Toarcian are discussed. Finally, we hypothesize potential adaptations of ammonoids to different feeding niches based on radular tooth morphologies.

The old and the new plankton: ecological replacement of associations of mollusc plankton and giant filter feeders after the Cretaceous?

“…It has been claimed by various authors that ammonites, particularly heteromorphs, lived in the water column ( Cecca, 1997 ; Guex, 2006 ), i.e., with direct access to meso- and microplankton. As pointed out above, a microphagous diet has been shown for several ammonite species including heteromorphs ( Jäger & Fraaye, 1997 ; Klug & Lehmann, 2015 ; Keupp, 2012 ; Kruta et al, 2011 ; Keupp & Schweigert, 2015 ; Keupp et al, 2016 ), thus making ammonites as predators feeding on juvenile ammonites in the Cretaceous likely.…”

“…This trophic relationship is further suggested by the extinction of this group synchronous with the demise of the Ammonoidea and Belemnitida but direct evidence is missing. Taking the direct fossil evidence from the Jurassic into account, it appears likely that ammonites also played a role as micropredators feeding on early juvenile ammonite offspring ( Jäger & Fraaye, 1997 ; Klug & Lehmann, 2015 ; Keupp, 2012 ; Kruta et al, 2011 ; Keupp & Schweigert, 2015 ; Keupp et al, 2016 ).…”

Recent advances in heteromorph ammonoid palaeobiology

“…(2013) rejected this interpretation after finding actual radular teeth in three specimens of R. halli . These in situ teeth match the size and morphology of radular teeth known from other aptychophoran ammonoids (Kruta et al ., 2015; Keupp et al ., 2016 a ). Landman et al .…”

“…Reports of fossil cephalopod stomach contents are rare (Table S3). Planktotrophy was suggested for ammonoids that have aptychus‐type, calcified lower jaws (Kruta et al ., 2011, 2015; Tanabe, 2011; Tanabe et al ., 2015 a ; Keupp et al ., 2016 a ). This is supported by finds of stomach contents associated with a radula in Baculites (Kruta et al ., 2011) and Allocrioceras (Wippich & Lehmann, 2004), a pelagic, aperture‐upwards drifter that probably fed on echinoderms (comatulid crinoids or ophiuroids; Fig.…”

First record of non-mineralized cephalopod jaws and arm hooks from the latest Cretaceous of Eurytania, Greece

“…Fossil cephalopods are mainly known from preserved mineralized parts such as aragonitic phragmocones (e.g., nautilids: Tajika et al 2020;ammonoids: Hoffmann et al 2019, coleoids: Klug et al 2016a, b, 2019Iba et al 2012;Wani et al 2018;Hoffmann and Stevens 2020), calcitic jaws (e.g., nautilids: Saunders et al 1978;Klug 2001;ammonites: Lehmann 1972;Morton & Nixon 1987;Engeser and Keupp 2002;Keupp and Mitta 2015;Tanabe et al 2015), and calcitic rostra (e.g., belemnites; Hoffmann et al 2016Hoffmann et al , 2019Hoffmann and Stevens 2020;Iba et al 2012Iba et al , 2014. While soft parts are rarely preserved (Klug et al 2015(Klug et al , 2019Donovan and Fuchs 2016;Clements et al 2016), originally chitinous body parts such as jaws, arm hooks, and radulae are occasionally found (Matern 1931;Mapes 1987;Fuchs 2006a;Landman et al 2010;Kruta et al 2011Kruta et al , 2020Klug et al 2005Klug et al , 2010aKlug et al , 2016aKlug et al , b, 2017Klug et al , 2020Keupp et al 2016;Fuchs and Hoffmann 2017;Mitta et al 2018). Naturally, such discoveries add important anatomical information to improve our understanding of cephalopod evolution…”