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1 Buccal mass structure and radular apparatus in Recent cephalopods. a Diagram of the buccal mass structure and radular apparatus in Octopus. Abbreviations. Spr: supraradular organ, sbr: subradular organ. Modified from Messenger and Young (1999). b Diagram of a radular ribbon showing the series of rows of teeth and the nomenclature for the teeth in a single row as well as the measurements of the teeth. The row of teeth has a central rachidian tooth ( R), a first and second lateral tooth ( L1 and L2 respectively) on each side of the rachidian tooth, marginal teeth ( M) and marginal plate ( MP). Measurements for the teeth provided in Table 11.1 have been done on the width ( w) and the height ( h) of the teeth. (Modified from Naef 1923)
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The radula is a molluscan feeding device that was also present in ammonoids. It consists of a ribbon with regularly arranged chitinous teeth disposed in transversal rows located between the upper and lower jaws. This chapter describes the radula in twelve ammonoid genera. The radula is composed of seven teeth per row: the rachidian tooth, two later...
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... Although SEMs offer excellent spatial resolution, they are still 2D images. Synchrotron imaging allows us to capture detailed three-dimensional (3D) representations of radular morphology (see alsoKruta et al., 2015 for an ammonoid example), offering the chance to observe several new features. Once obtained, these models may then be used in advanced downstream analyses that address each ofPadilla (2003), focal concerns.The field of geometric morphometrics has advanced significantly since 2003(Adams et al., 2013), allowing for the detailed quantification and analysis of organismal form in three dimensions. ...
Several families of neogastropod mollusks independently evolved the ability to drill through mineralized prey skeletons using their own mineralized feeding teeth, sometimes with shell‐softening chemical agents produced by an organ in the foot. Teeth with more durable tooth shapes should extend their use and improve predator performance, but past studies have described only the cusped‐side of teeth, mostly overlooking morphologies related to functional interactions between teeth. Here, we describe the three‐dimensional morphology of the central drilling tooth (rachidian) from four species of the neogastropod family Muricidae using synchrotron tomographic microscopy and assemble a three‐dimensional model of a multitooth series in drilling position for two of them to investigate their dynamic form. We find two new types of articulating surfaces, including a saddle joint at either end of the rachidian and a large tongue‐and‐groove joint in the center. The latter has a shape that maximizes contact surface area between teeth as they rotate away from each other during drilling. Articulating joints have not been described in Neogastropod radula previously, but they are consistent with an earlier hypothesis that impact forces on individual teeth during predatory drilling are dispersed by tooth–tooth interactions.
... Kruta et al. (2011) ) a lower jaw covered with two calcareous plates, 2) a smaller upper jaw (less than one-half the length of the lower jaw), and 3) a radula nestled between the upper and lower jaws consisting of nine small delicate teeth with a tall, sabrelike marginal tooth. Klug et al. (2012) documented the same features in their study of baculitid ammonites from the Upper Cretaceous of Germany (for recent discussions about the morphology of the buccal apparatus in ammonites, see Kruta et al. [2015] and Tanabe et al. [2015]). ...
We report the discovery of lower jaws of Baculites (Ammonoidea) from the Upper Cretaceous U.S. Western Interior. In the lower Campanian Smoky Hill Chalk Member of the Niobrara Chalk of Kansas, most of the jaws occur as isolated elements. Based on their age, they probably belong to Baculites sp. (smooth). They conform to the description of rugaptychus, and are ornamented with coarse rugae on their ventral side. One specimen is preserved inside a small fecal pellet that was probably produced by a fish. Another specimen occurs inside in a crushed body chamber near the aperture and is probably in situ. Three small structures are present immediately behind the jaw and may represent the remains of the gills. In the lower Maastrichtian Pierre Shale of Wyoming, two specimens of Baculites grandis contain lower jaws inside their body chambers, and are probably in situ. In both specimens, the jaws are oriented at an acute angle to the long axis of the shell, with their anterior ends pointing toward the dorsum. One of the jaws is folded into a U-shape, which probably approximates the shape of the jaw during life. Based on the measurements of the jaws and the shape of the shell, the jaws could not have touched the sides of the shell even if they were splayed out, implying that they could not have effectively served as opercula. Instead, in combination with the upper jaws and radula, they constituted the buccal apparatus that collected and conveyed food to the esophagus.
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... In many heteromorphs in which the hook is well developed, the body chamber culminates in an upturned aperture. This may have been an adaptation to feeding on small organisms in the water column, which is consistent with the shape of their jaws and radula (Kruta, Landman & Tanabe, 2015;Tanabe, Kruta & Landman, 2015). These ammonites may have lived near the bottom, occasionally swimming to remain in place or evade predators, with limited vertical and lateral migration (Landman et al., 2010;Landman, Cobban & Larson, 2012). ...
Radular teeth have to cope with wear, when interacting with ingesta. In some molluscan taxa, wear-coping mechanisms, related to the incorporation of high contents of iron or silica, have been previously determined. For most species, particularly for those which possess radulae without such incorporations, wear-coping mechanisms are understudied. In the present study, we documented and characterized the wear on radular teeth in the model species Loligo vulgaris (Cephalopoda). By applying a range of methods, the elementary composition and mechanical properties of the teeth were described, to gain insight into mechanisms for coping with abrasion. It was found that the tooth regions that are prone to wear are harder and stiffer. Additionally, the surfaces interacting with the ingesta possessed a thin coating with high contents of silicon, probably reducing abrasion. The here presented data may serve as an example of systematic study of radular wear, in order to understand the relationship between the structure of radular teeth and their properties.
Heteromorphs are ammonoids forming a conch with detached whorls (open coiling) or non-planispiral coiling. Such aberrant forms appeared convergently four times within this extinct group of cephalopods. Since Wiedmann's seminal paper in this journal, the palaeobiology of heteromorphs has advanced substantially. Combining direct evidence from their fossil record, indirect insights from phylogenetic bracketing, and physical as well as virtual models, we reach an improved understanding of heteromorph ammonoid palaeobiology. Their anatomy, buoyancy, locomotion, predators, diet, palaeoecology, and extinction are discussed. Based on phylogenetic bracketing with nautiloids and coleoids, hetero-morphs like other ammonoids had 10 arms, a well-developed brain, lens eyes, a buccal mass with a radula and a smaller upper as well as a larger lower jaw, and ammonia in their soft tissue. Heteromorphs likely lacked arm suckers, hooks, tentacles , a hood, and an ink sac. All Cretaceous heteromorphs share an aptychus-type lower jaw with a lamellar calcitic covering. Differences in radular tooth morphology and size in heteromorphs suggest a microphagous diet. Stomach contents of heteromorphs comprise planktic crustaceans, gastropods, and crinoids, suggesting a zooplanktic diet. Forms with a U-shaped body chamber (ancylocone) are regarded as suspension feeders, whereas orthoconic forms additionally might have consumed benthic prey. Heteromorphs could achieve near-neutral buoyancy regardless of conch shape or ontog-eny. Orthoconic heteromorphs likely had a vertical orientation, whereas ancylocone heteromorphs had a near-horizontal aperture pointing upwards. Heteromorphs with a U-shaped body chamber are more stable hydrodynamically than modern Nautilus and were unable substantially to modify their orientation by active locomotion, i.e. they had no or limited access to benthic prey at adulthood. Pathologies reported for heteromorphs were likely inflicted by crustaceans, fish, marine reptiles, and other cephalopods. Pathologies on Ptychoceras corroborates an external shell and rejects the endocochleate hypothesis. Devonian, Triassic, and Jurassic heteromorphs had a preference for deep-subtidal to offshore facies but are rare in shallow-subtidal, slope, and bathyal facies. Early Cretaceous heteromorphs preferred deep-subtidal to bathyal facies. Late Cretaceous heteromorphs are common in shallow-subtidal to offshore facies. Oxygen isotope data suggest rapid growth and a demersal habitat for adult Discoscaphites and Baculites. A benthic embryonic stage, planktic hatchlings, and a habitat change after one whorl is proposed for Hoploscaphites. Carbon isotope data indicate that some Baculites lived throughout their lives at cold seeps. Adaptation to a planktic life habit potentially drove selection towards smaller hatchlings, implying high fecundity and an ecological role of the hatchlings as micro-and mesoplankton. The Chicxulub impact at the Cretaceous/Paleogene (K/Pg) boundary 66 million years ago is the likely trigger for the extinction of ammonoids. Ammonoids likely persisted after this event for 40-500 thousand years and are exclusively represented by heteromorphs. The ammonoid extinction is linked to their small hatchling sizes, planktotrophic diets, and higher metabolic rates than in nautilids, which survived the K/Pg mass extinction event.
Cephalopoda is the only class of molluscs in which virtually all its modern representatives have a pair of powerful jaws. There is little doubt that jaws have contributed to the evolutionary success of cephalopods, but their origin still remains a mystery. Though cephalopods appeared at the end of the Cambrian, the oldest unequivocal jaws have been reported to date from the Late Devonian, though they were initially interpreted as phyllopod crustaceans of the suborder Discinocarina. After their relation with ammonoids was proven, they were considered as opercula, and only later their mandibular nature was recognized and widely accepted. Finds of discinocarins from Silurian deposits are still considered as opercula of ammonoid ancestors ‐ nautiloids of the order Orthocerida. However, according to modern ideas, there is no place within their soft body for the location of such large opercula. Moreover, the repeated appearance of very similar structures in the same evolutionary line at least twice, but in different places of the body and for different purposes seems highly improbable. A new hypothesis is proposed herein, in which the Silurian fossils, earlier assigned to Discinocarina, are not specialized opercula, but protective shields, to defend orthocerids not from the predators, but from their own prey. The chitinous plates around the mouth likely appeared in the Silurian orthocerids for protection from such damage and later, during Silurian and Devonian, most likely gradually evolved into the jaws.
In the last few decades, hook‐like structures have been reported in the Mesozoic ammonite family Scaphitidae. Despite their exceptional preservation and debates about their function, no detailed reconstruction has been available until now. For the first time, we describe the composition and details of the morphology of these structures found in the body chambers of six specimens of the Campanian ammonite Rhaeboceras halli (Meek & Hayden) using high resolution x‐ray imaging. The hook‐like structures are composed of a thin layer of brushite. The base of each hook is open on one side forming an internal cavity, now filled with sediment. The tips of the hooks end in one or two cusps or, rarely, exhibit a blunt end. We used geometric morphometrics to capture the morphological disparity of the bicuspidate morphotypes comprising 98% of the hooks. Principal component analysis revealed chirality among the hooks and a cluster analysis recognized five morphologies. Contrary to the previous interpretation of these structures, we conclude that they are not radular teeth. They are much larger and more variable in size and shape than any known ammonite radulae and completely out of proportion with respect to the size of the jaw. The chirality, the hook‐like shape, and the absence of a size relationship between the hooks and the body chambers in which they occur, lead us to propose that these hooks could represent elements of the brachial crown related to copulatory behaviour. If so, these would be the first reported remnants of brachial crowns in ammonites.
Review of 143 journal articles and book chapters dealing with Palaeozoic ammonoids published in 2014 to 2018.
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 Fossillagerstätte 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.
