‘Arm brains’ (axial nerves) of Jurassic coleoids and the evolution of coleoid neuroanatomy

Although patchy, the fossil record of coleoids bears a wealth of information on their soft part anatomy. Here, we describe remains of the axial nerve cord from both decabrachian (Acanthoteuthis, Belemnotheutis, Chondroteuthis) and octobrachian (Plesioteuthis, Proteroctopus, Vampyronassa) coleoids from the Jurassic. We discuss some hypotheses reflecting on possible evolutionary drivers behind the neuroanatomical differentiation of the coleoid arm crown. We also propose some hypotheses on potential links between habitat depth, mode of life and the evolution of the Coleoidea

Découverte d’un os intercalaire dans le neurocrâne d’un actinoptérygien ‘paléonisciforme’, avec une reconstitution virtuelle d’un nouveau neurocrâne de <i>Lawrenciella</i> Poplin, 1984 du Carbonifère de l’Oklahoma (États-Unis)

Nous décrivons ici un nouveau spécimen de Lawrenciella Poplin, 1984 du Carbonifère supérieur (Missourien, Pennsylvanien) de l’Oklahoma des Etats-Unis d’Amérique. Ce spécimen est préservé en trois dimensions dans un nodule phosphaté et a été scanné par microtomographie aux rayons X. Le neurocrâne présente quelques différences morphologiques par rapport à Lawrenciella schaefferi Poplin, 1984 qui peuvent traduire une variation individuelle pour ce taxon. Le parashénoïde ainsi qu’une paire d’intercalaires sont associés au neurocrâne. La présence d’intercalaires sous forme d’une excroissance antérieure recouvrant la fissure otico-occipitale n’avait encore jamais été documentée chez des « paleonisciformes » et pourrait comporter un signal phylogénétique. Une discussion sur le rôle évolutif des intercalaires dans la fermeture de la fissure otico-occipitale chez les actinoptérygiens modernes est également apportée.We describe here a new Lawrenciella Poplin, 1984 specimen from the Upper Carboniferous (Missourian, Pennsylvanian) from Oklahoma, USA. This specimen is three dimensionnally preserved in a phosphatic nodule. It was scanned by X-Ray microtomography. The neurocranium shows some morphological differences from Lawrenciella schaefferi Poplin, 1984, which might represent individual variability. The parasphenoid and a pair of intercalar bones are associated with the braincase. The presence of intercalars as an anterior extension covering the otico-occipital fissure have not been yet documented in “paleonisciforms” and may have a phylogenetic signal. A discussion about the evolutionary role of such intercalars in the closure of the otico-occipital fissure in modern actinopterygians is provided.</p

Injuries on nautilus jaws: Implications for the function of ammonite aptychi

Volume: 50Start Page: 241End Page: 24

A new approach for the determination of ammonite and nautilid habitats.

Externally shelled cephalopods were important elements in open marine habitats throughout Earth history. Paleotemperatures calculated on the basis of the oxygen isotope composition of their shells can provide insights into ancient marine systems as well as the ecology of this important group of organisms. In some sedimentary deposits, however, the aragonitic shell of the ammonite or nautilid is poorly or not preserved at all, while the calcitic structures belonging to the jaws are present. This study tests for the first time if the calcitic jaw structures in fossil cephalopods can be used as a proxy for paleotemperature. We first analyzed the calcitic structures on the jaws of Recent Nautilus and compared the calculated temperatures of precipitation with those from the aragonitic shell in the same individuals. Our results indicate that the jaws of Recent Nautilus are secreted in isotopic equilibrium, and the calculated temperatures approximately match those of the shell. We then extended our study to ammonites from the Upper Cretaceous (Campanian) Pierre Shale of the U.S. Western Interior and the age-equivalent Mooreville Chalk of the Gulf Coastal Plain. In the Pierre Shale, jaws occur in situ inside the body chambers of well-preserved Baculites while in the Mooreville Chalk, the jaw elements appear as isolated occurrences in the sediment and the aragonitic shell material is not preserved. For the Pierre Shale specimens, the calculated temperatures of well-preserved jaw material match those of well-preserved shell material in the same individual. Analyses of the jaw elements in the Mooreville Chalk permit a comparison of the paleotemperatures between the two sites, and show that the Western Interior is warmer than the Gulf Coast at that time. In summary, our data indicate that the calcitic jaw elements of cephalopods can provide a reliable geochemical archive of the habitat of fossil forms

Enigmatic hook‐like structures in Cretaceous ammonites (Scaphitidae)

International audienceIn 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

Adaptations to squid-style high-speed swimming in Jurassic belemnitids

Although the calcitic hard parts of belemnites (extinct Coleoidea) are very abundant fossils, their soft parts are hardly known and their mode of life is debated. New fossils of the Jurassic belemnitid Acanthoteuthis provided supplementary anatomical data on the fins, nuchal cartilage, collar complex, statoliths, hyponome and radula. These data yielded evidence of their pelagic habitat, their nektonic habit and high swimming velocities. The new morphological characters were included in a cladistic analysis, which confirms the position of the Belemnitida in the stem of Decabrachia (Decapodiformes)

Peut-on extraire un signal sclérochronologique des mâchoires d’ammonites ?

International audienceChez les ammonites, l’estimation de l’âge et du rythme de croissance des individus reste un sujet débattu. Les études proposées pour résoudre ces questions se sont toujours basées sur les éléments de croissance périodiques visibles des coquilles (stries, ornementation, septes) ou sur les compositions chimiques de la coquille externe aragonitique à faible teneur en Sr. Selon les taxons et les méthodes, les âges obtenus pour atteindre la maturité vont de 3 à 20 ans. Ces résultats ont été discutés car ils font tous intervenir des valeurs de vitesse de croissance hypothétiques ou issues de données connues pour le nautile. Les études sclérochronologiques menées sur les mollusques récents et fossiles fournissent un cadre méthodologique qui pourrait être appliqué aux mâchoires inférieures calcitiques des ammonites. Ces dernières possèdent en effet des structures de croissance (stries et incréments) bien visibles. Les observations microstructurales indiquent que des incréments de croissance fins (jusqu'à 2 μm d'épaisseur) sont bien préservés et qu'une partie de l'information paléobiologique y est conservée. Nous avons analysé l'espacement des lignes de croissance, la microstructure pour évaluer l’intensité de la diagenèse, ainsi que les variations du 18O et des rapports Mg/Ca au cours du développement. Les résultats préliminaires qui sont présentés soulèvent des questions quant à l’origine possible des variations observées et montrent que la prise en compte du mode de vie des ammonites est indispensable pour l’interprétation des résultats

Reading the age of an ammonite from its jaws

International audienceAssessing ammonites lifespan is a challenging issue for macroevolutionary studies. Age estimations based on growth features and chemical compositions of the low Sr aragonitic shell have been attempted on few exceptionally preserved specimens, but remain debated (Bucher et al. 1996). Compared to the aragonitic shell, the calcitic covering of the lower jaw (called aptychus) is poorly investigated although in Recent cephalopods, chitinous beaks are considered a relevant proxy for the age of the animal (Perales-Raya et al. 2014). Sclerochonological studies carried out on recent and fossil mollusks have provided a methodological framework that could be applied to ammonite lower jaws (Lartaud 2010, Bougeois 2013). Following these studies, well-preserved aptychi from the Campanian of Alabama (Mooreville Chalk Formation) were investigated in order to assess its potential for schlerochonological studies. Microstructural observations indicate that fine growth features (up to 2µm thick) are well preserved and that a part of paleobiological information is conserved. We analyzed growth lines spacing-out, microstructure, and variation of the chemical composition along the growth direction (∂18O and trace elements) in order to provide the first age estimations for ammonites obtained from non-shell structures

The first gladius-bearing coleoid cephalopods from the lower Toarcian “Schistes Cartons” Formation of the Causses Basin (southeastern France)

The fossil record of gladius-bearing coleoids is scarce and based only on a few localities with geological horizons particularly favourable to their preservation (the so-called Konservat-Lagerstätten), which naturally leads to strongly limited data on geographical distributions. This emphasizes the importance of every new locality providing gladius-bearing coleoids. Here, we assess for the first time the gladius-bearing coleoid taxonomic diversity within the lower Toarcian “Schistes Cartons” of the Causses Basin (southeastern France). The material includes two fragmentary gladii, identified as Paraplesioteuthis sagittata and ?Loligosepia sp. indet. Just with these two specimens, two (Prototeuthina and Loligosepiina) of the three (Prototeuthina, Loligosepiina and Teudopseina) suborders of Mesozoic gladius-bearing coleoids are represented. Thus, our results hint at a rich early Toarcian gladius-bearing coleoid diversity in the Causses Basin and point out the need for further field investigations in the lower Toarcian “Schistes Cartons” in this area. This new record of Paraplesioteuthis sagittata is only the second one in Europe and the third in the world (western Canada, Germany and now France). Based on these occurrences, we tentatively suggest that P. sagittata originated in the Mediterranean domain and moved to the Arctic realm through the Viking Corridor to eventually move even farther to North America