Morphogenetic Origin of Radial Lirae and Mode of Shell Growth in Calliphylloceras (Jurassic Ammonoidea)
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Morphogenetic origin of radial lirae and mode of shell growth in Calliphylloceras (Jurassic Ammonoidea) Autor(en): Bucher, Hugo / Chirat, Régis / Guex, Jean Objekttyp: Article Zeitschrift: Eclogae Geologicae Helvetiae Band (Jahr): 96 (2003) Heft 3 PDF erstellt am: 04.10.2021 Persistenter Link: http://doi.org/10.5169/seals-169034 Nutzungsbedingungen Die ETH-Bibliothek ist Anbieterin der digitalisierten Zeitschriften. Sie besitzt keine Urheberrechte an den Inhalten der Zeitschriften. Die Rechte liegen in der Regel bei den Herausgebern. Die auf der Plattform e-periodica veröffentlichten Dokumente stehen für nicht-kommerzielle Zwecke in Lehre und Forschung sowie für die private Nutzung frei zur Verfügung. Einzelne Dateien oder Ausdrucke aus diesem Angebot können zusammen mit diesen Nutzungsbedingungen und den korrekten Herkunftsbezeichnungen weitergegeben werden. 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Helv. 96 (2003) 495-502 DOI 10.1007/S00015-003-1103-4 Birkhäuser Verlag, Basel, 2003 Morphogenetic origin of radial lirae and mode of shell growth in Calliphylloceras (Jurassic Ammonoidea) Hugo Bûcher1, Régis Chirat2 & Jean Guex3 Key words: radial lirae. growth. Calliphylloceras, Ammonoidea ABSTRACT RESUME The microstructure of the shell wall of Calliphylloceras reveals the morphogenetic La microstructure de la paroi coquillière de Calliphylloceras révèle l'origine origin of radial lirae. a character commonly encountered in both morphogénétique des lirae radiales, caractère fréquemment observé chez diverses Paleozoic and Mesozoic ammonoids. The outer prismatic layer consists of projected ammonites jurassiques et crétacées. Chez Calliphylloceras, la couche prismatique and overlapping tile-shaped, radial slats. The free, arched and projected externe est disposée en lattes radiales imbriquées et projetées. L'extrémité adorale de adorai end of each slat implies that secretion must have been interrupted and chaque latte arquée et projetée est libre, impliquant une interruption de la sécrétion followed by a withdrawal of the periostracal groove of the mantle back to the suivie d'un retrait du sillon périostracal jusqu'à la ligne de jonction avec la latte junction line with the preceding slat. Secretion resumed with a new increment précédente. La reprise de la sécrétion se fait avec un nouvel incrément composé de of thin outer prismatic layer and periostracum. and the adorai secretion of the periostracum et de couche prismatique externe. la sécrétion adorale de la couche nacrée underlying, continuous nacreous layer proceeded only after this new cycle sous-jacente et continue ne se pouvant se produire qu'après le démarrage de ce initiated. The junction lines between consecutive and overlapping slats were nouveau cycle. Les lignes de contact entre les lattes successives sont moulées par la subsequently moulded by the nacreous layer, thus forming the radial lirae. These couche nacrée sous-jacente nouvellement sécrétée, formant ainsi les lirae radiales. must be seen as fabricational noise originating from the discontinuous secre- Ces dernières sont donc assimilables à du bruit fabricationnel résultant simplement uon of the outer prismatic layer and periostracum. The hitherto unclear de la sécrétion discontinue du periostracum et de la couche prismatique externe. La morphogenesis of these radial lirae is now unambiguously distinguished from that morphogenèse jusqu'ici mal comprise des lirae radiales est clairement différentiable of ordinary -and smaller- growth lines, which do not involve such discontinuities de celle des lignes de croissance ordinaires, bien plus petites, et n'impliquant pas de accompanied by partially overlapping increments of the outer prismatic telles discontinuités et imbrications de la couche prismatique externe. Les lirae layer. It also differs structurally from megastriae. which cut across the radiales se différencient également des mégastries, discontinuités plus importantes periostracum. outer prismatic and nacreous layers. Small and acicular fibro-radiated impliquant conjointement la couche prismatique externe et la couche nacrée. Des deposits in the cylindrical cavities of the outer prismatic layer of Calliphylloceras dépôts de structure aciculaire et tibro-radiée dans les cavités résultant de l'imbrication also suggest thai 'remote biomineralization' (i.e. without direct control of des lattes de la couche prismatique externe suggère la présence d'un mécanisme de mantle) occurred inside the shell wall. biominéralisation à distance, indépendant d'un contrôle direct par le manteau. 1. Introduction are the focus of this work. Arkell's definition of radial lirae Among the usual ornamental characters of the ammonoid appears somewhat equivocal because it refers indifferently to shells. Arkell (1957, L91) gave a first, rather loose definition of marked growth lines as well as to independent structures the lirae: "In many shells the growth lines are accentuated or intercalated between them. This ambiguous definition has led to raised as lamellae, which may be crinkled, or they appear as some confusion resulting in the interchangeable use of the fine radial lines (lirae) or they are accompanied by them'. terms growth lines or lamellae, with lirae (e.g. Doguzhaeva Following this first definition, Arkell (1957) also used the same 1982; Korn & Price 1987; Bond & Saunders 1989; Bûcher & term but in reference to the longitudinal or spiral lines that Guex 1990; Bucher et al. 1996). However, the distinction occur in some genera (e.g. Amaltheus). Any possible confusion between growth lines and radial lirae was later illustrated by between these two distinct features will be here avoided by Bucher et al. (1996), who emphasized the different scales of systematically specifying the radial orientation ofthe lirae that these structures, but without any further distinction pertaining Paläontologisches Institut und Museum. Universität Zürich. Karl Schmid-Strasse 4. CH-8006 Zürich. E-mail: [email protected] '¦ U.F.R. Sciences de la Terre, Université Claude Bernard Lyon 1. 27-43 Bd. du 11 novembre 1918. F-69622 Villeurbanne Cedex. E-mail: [email protected] ' Institut de Géologie et Paléontologie. BFSH2. Université de Lausanne. CH-1015 Lausanne. E-mail: [email protected] Radiai lirae in Calliphylloceras 495 to the mode of growth. Spacing of growth lines varies around sion with growth lines, that the lirate surface ofthe shell 1 pm, while spacing of radial lirae may range from 0.1 to 1.5 mm. corresponds to the outermost calcareous layer, namely the outer depending on the size of the specimen and on the whorl height prismatic layer. In fact, close examination of many specimens at which measurements are taken. If growth lines actually of Calliphylloceras reveals that a few of them have a lirae- reflect intermittent growth at the cellular level, a similar underlying bearing layer still partly covered with a thin, brittle and wavy morphogenetic process for the more widely spaced radial layer (Fig. IA and IB). Further preparation of shell sectors lirae is unlikely. Another difference between these two structures still embedded in limestone consistently indicates that this can be seen in their respective lateral continuity or extension: additional layer rests directly on the lirate surface of the shell all both are parallel to the growing edge but single growth around the whorl. Median sections also reveal that this layer is lines may not systematically extend all around the apertural preserved on the earlier whorls, where protected by whorl edge, being generally more numerous on the venter than on overlap (Fig. IC and ID). As illustrated in Figure IB. the flanks and with variable strength, as opposed to radial lirae. unusual morphology of this layer results from the juxtaposition which are continuous all around the aperture (except in of parallel tubular units whose trajectories are strictly parallel repaired shell breakages), of relatively uniform strength, and to those of the radial lirae. with an invariable one-to-one with a sharp outline in cross section. Although the shells of the correspondence. Following the spacing of lirae. the spiral length Early and Middle Jurassic Calliphylloceras described here are of the tubular units increases from the umbilical margin to the calcified, layer boundaries can be readily recognized and the venter. unusual preservation of an undescribed outer layer covering Closer examination of this outermost layer, where superficially the lirate surface lead us to infer a very distinctive morphogenetic cracked away (see Fig. IB) reveals that each unit is a process for these radial lirae. hollow structure filled with sparry calcite. This justifies the terrr 'tubular' as a first approximation for its general outline. Foi each unit, the enfolding layer is thin (from 10 to 50 pm) 2.