Morphologie und Bildung der frühontogenetischen Gehäuse bei conchiferen Mollusken

Zusammenfassung

Die Arbeit behandelt die frühontogenetische Mollusken-Entwicklung der Schale und der diese bildenden Epithelien. Speziell werden Archaeogastropoden, Ammoniten und Neritaceen behandelt und mit einigen ausgewählten höheren Gastropoden verglichen, Archaeogastropoden, Neritaceen und höhere Gastropoden sind paläontologisch gut trennbare Gruppen, wenn die Bildung und Morphologie der frühontogenetischen Schalen verglichen werden. Eine Verbindung zwischen Archaeogastropoden und Ammoniten zeigt sich in der Art der Mineralisierung der organischen Primärschälchen. Cephalopoden sind generell eine ziemlich einheitliche Gruppe der Mollusken, deren Vorfahren den Vorfahren der Archaeogastropoden nahe verwandt waren. Neritaceen und höhere Schnecken verbindet die innere Befruchtung sowie der Besitz einer echten Larve, die den Archaeogastropoden wie auch den Cephalopoden fehlt. Die Ergebnisse werden in einem Schema des Entwicklungsablaufes in der Ontogenese dargestellt und zur Phylogenese des Molluskenstammes in Bezug gebracht. Hierzu werden auch einige Entwicklungsabläufe bei Käferschnecken, Muscheln und Scaphopoden miteinbezogen. Es erweist sich, daß einige in der Systematik bisher noch bewertete Eigenschaften und Merkmale nur sehr mit Vorsicht genutzt werden dürfen, während andere bisher wenig beachtete Kriterien mehr Aussagekraft besitzen als bisher vermutet. Die Bildung der Schalendrüse erweist sich als die zentrale Erfindung der Mollusken, die vermutlich während des oberen Kambriums die Conchifera entstehen ließ. Mit vielen Beispielen wird belegt, daß innerhalb aller behandelten Molluskengruppen eine Ausdeutung fossiler Schalenreste nur dann zu sinnvollen Ergebnissen führt, wenn die Bildungsweise der Schale bei rezenten Verwandten gut untersucht ist. Umgekehrt zeigt es sich, daß Modelle der Evolution der Mollusken, die nur auf Rezentbefunden basieren, in der Regel wenig Ähnlichkeit mit dem tatsächlichen Ablauf der Geschehnisse aufweisen.

Summary

The development of the early ontogenetic shell and the epithelia forming it is traced in archaeogastropods, ammonites, and neritaceans and is compared with that of some selected higher gastropods. Results are integrated into a scheme of evolution of molluscs. Here developmental stages of ontogeny are related to the evolution of mollusc classes, especially those of the conchifers. To do so in a more general way polyplacophoran, bivalve and scaphopod ontogenies are described with few examples.

The archaeogastropods form their primary shell in a way that is different from all other gastropod orders (Chapter 2). Here a bilaterally symmetrical purely organic shell is mechanically deformed by force of the soft body from the inside and the outside. During the transition from the free swimming stage to the crawling young the shell is thus pressed into trochospiral shape and mineralized rapidly afterwards by growth of aragonitic needles in the organic shell. In contrast to current theories about gastropod body torsion the coiling of the shell of archaeogastropods is independent from twisting of the soft parts inside it. This is demonstrated by the cuplike patellacean archaeogastropods. Here a bilaterally symmetrical shell can be produced by a normally torted animal. Similar independence of shell and body torsion can also be observed in quite different and unrelated gastropod groups outside of the archaeogastropods. Torsion has nothing to do with the ability of the embryo to withdraw into its shell, but is the result of differential growth of epithelia, mainly those of the visceral mass. Due to it foot and mantle cavity are brought into the right position needed in benthic life.

The early ontogentic shell of fissurellacean archaeogastropods shows several features of the same systematic value as those used in taxonomy but usually suggesting a different system. The formation of a slit occurs late during ontogeny and place of occurrence as well as absence and presence should not be overemphazised in systematics of molluscs. Shell pores also occur in these gastropods and thus are not restricted to bivalves and polyplacophores. The primary shell of the archaeogastropod can be a tool to differenciate these from all other gastropods including the Neritacea. An account of different morphologies and sculptures of the primary shell is provided.

Secretion and mineralization of the early ontogenetic shell of two Mesozoic ammonites are described and reconstructed in detail (Chapter 3). The absence of growth lines indicates the formation of the whole embryonic conch in uninterrupted contact to the gland cells (periostracum cells). After detachment from the latter the whole outer shell became mineralized, while the inner conch walls remained of organic structure. These and the whole interior of the conch were covered by mineral layers afterwards. Tissue shell connection migrated from the inner surface of the inner lip to the surface of the first septum and from there to the internal side of the third chamber during formation of the siphuncular system. The construction of the first septum preceeded that of the original siphuncular rod, while following septa were produced in connection to each segment of the siphuncle. Results are compared with some of the more modern interpretations of ammonite embryonic shell construction and function. A comparison with the development of the embryonic shell of recentSepia, Spirula, andNautilus and some fossil cephalopods provides two general trends in cephalopods: First, that the shape and mode of mineralization of the primary shell is connected to egg size and not to systematic placement, and second, that recent and fossil cephalopods alike lack a true larval stage and have extremely yolk rich eggs if compared with most other molluscs. Shape, size and structure of embryonic shells of fossil cephalopods allow the reconstruction of the course early ontogeny took. Shape size and structure of the scars formed by the attachment of the body to the shell makes it possible to differentiate endocochleates from ectocochleates. The data presented indicate that original cephalopods and primary archaeogastropods had a common ancestor, living in the Upper Cambrian, with direct development, a feature still present in both groups. First cephalopods can be reconstructed a bit more in detail and with more functional reason than found in literature up to now (Chapter 3.6.3).

The Neritacea (Chapter 4) are an independent group of gastropods, neither archaeogastropod, nor of meso-neogastropod relation. This is shown in their anatomy in their embryonic development, and in their shell morphology. The latter is analyzed in detail and will allow in the future to differentiate fossil nertiaceans from other molluscs. The shell detaches from the cells of the mantle during early ontogeny, before the visceral mass is covered by it. Early mineralization and growth lines as well as growth around the slightly torted body characterizes the embryonic shell. Its mode of coiling differs from that of the strongly convolute larval shell. These characters separate neritaceans with free larvae from all other gastropods, but when the larval phase occurs within the egg capsule a separation from higher gastropods with similar development is probelmatic.

Within Neritacea as well as within other unrelated gastropod families and orders limpet-like species are found (Chapter 4.5). The change from the coiled early ontogenetic shell to the cup-like adult shell goes along with a rearrangement of tissue shell attachment and a loss of the operculum. The animal no longer withdraws into its shell, but rather pulls its shell down onto the substrate, when in danger. The result is an externally and internally similar shell which also may become extremely similar to shells of untorted molluscs, like recent and fossil monoplacophores. It is documented that neither morphology of the internal mould, nor muscle scars, or apex position provide sound systematic evidence. The later can be gathered only from the early ontogenetic shell, but here restrictions must be taken into account.

The course of the embryonic development of higher gastropods (Chapter 5.1) is quite variable because a true larval phase is developed. During this phase plankton is eaten or this normal food of the free larva is substituted by nutrients provided within the egg capsule. The mode of development of a free larva provides systematic information expressed by the size, shape and sculpture of the shell. Where larval food is present within the egg capsule, embryonic development is adapted to this, and systematic information is lost. A number of cases shows how liquid yolk, yolk grains, and nurse eggs are taken by embryos at different stages of development. Distance from marine way of life masks the indirect course of development strongly. The fresh-water and land snails in addition, provide a good model for the formation of internal shells, that can be applied to the cephalopods as well. Here three types of tissue, originating in the shell gland and characterizing the mantle, are produced simultaneously. The muscle mantle can close over the shell gland before shell formation, thus making the shell an internal one.

Differences present in the ontogeny of molluscs makes it possible to gather information regarding the course in which evolution proceeded (Chapter 5.2). Polyplacophores branched off prior to the invention of an embryonic shell gland. Bivalves, scaphopods, archaeogastropods, cephalopods and higher gastropods developed their specific way of early shell formation independently from untorted bilaterally symmetrical common ancestors, probably at the end of the Cambrian. The protostome or deuterostome development, the type of swimming larva and the mode and way of feeding are variable features and of little use for reconstructing the phylogeny. Direct development with a adult organs appearing without transitional organs is the original type, present in archaeogastropods and cephalopods. Larvae having to metamorphose transitional organs into adult ones and thus carrying out indirect development characterize all higher gastropods and have developed independently within the bivalves.