Inäquivalver Schalenbau Bei Crania Anomala

Abstract

As a result of attachment over its entire surface, the ventral valve of the brachiopod Crania anomala (Müiller) differs from the dorsal valve not only in growth form, time sequence of calcification, and distribution of endopuncta, but also (withoutap‐parent functional reason) in ultrastructure. The secondary layer of the dorsal valve grows together from flat crystals (up to 5μ. diameter) showing screw‐like dislocations. The same layer of the ventral valve is formed later and has an irregular structure. Spiral crystals are not secreted in the ventral valve until the adult growth‐stage is attained. The periostracum and primary layer are developed in both valves in a normal fashion. Fossil representatives, in which the ventral valve is partly or completely free, show the same ultrastructure in both valves. Infolge ganzflächiger Anheftung weicht die Ventralklappe von Crania anomala (Müller) nicht nur in Wuchsform, zeitlichem Ablauf der Kalzifikation und Verteilung der Endopuncta von der Dorsalklappe ab, sondern ohne erkennbare funktionelle Gründe auch in der Ultrastruktur. Während die Sekundärschicht in der Dorsalklappe aus spezifischen, bis 5μ. Großen Kristallen mit Schrauben‐versetzungen zusammenwächst, wird sie in der Ventralklappe verzögert und mit regelloser Struktur angelegt. Erst in adulten Wachstumsstadien können auch in der Ventralklappe spiralige Kristalle abgeschieden werden. Periostrakum und Primärschicht sind bei C. anomala normal entwickelt. Fossile Anpassungstypen der Craniacea, deren Ventralklappen nur punktförmig oder überhaupt nicht zementiert sind, zeigen in beiden Klappen dieselbe Ultrastruktur.

Evolutionary and Diagenetic Changes in the Chemico‐structure of the Shell of Cranioid Brachiopods

“…The most profound diagenetic changes occurred before the Late Cretaceous with proteins degrading into peptides that were dispersed, with the loss of less stable amino acids, during laminar recrystallization. Palaeozoic shells suffered further recrystallization but, even after pressure solution, the original laminar fabric was replicated long after it had lost its constraining organic membranes.KEY WORDS: organoclastic shell structures, shell degradation, protein diagenesis.T H E ultrastructure of the organocalcitic shells of cranioid brachiopods was ®rst described by Williams and Wright (1970) and Schumann (1970). The accounts dealt almost exclusively with the mineral components of the shell in relation to an outer organic cover, the periostracum, and an underlying secreting epithelium, the mantle, and its papillose outgrowths, caeca, that permeate the skeletal succession.…”

“…T H E ultrastructure of the organocalcitic shells of cranioid brachiopods was ®rst described by Williams and Wright (1970) and Schumann (1970). The accounts dealt almost exclusively with the mineral components of the shell in relation to an outer organic cover, the periostracum, and an underlying secreting epithelium, the mantle, and its papillose outgrowths, caeca, that permeate the skeletal succession.…”

Crystallography of craniid brachiopods by electron backscatter diffraction (EBSD)

“…Only dorsal valves of all taxa are used herein because ventral valves are more weakly developed and differentially mineralised (e.g. Williams & Wright 1970;Schumann 1970;Cusack & Williams 2001). Also, ventral valves of most craniid brachiopods only display primary layer fabric within the shell (Williams & Wright 1970;Schumann 1970).…”

“…Williams & Wright 1970;Schumann 1970;Cusack & Williams 2001). Also, ventral valves of most craniid brachiopods only display primary layer fabric within the shell (Williams & Wright 1970;Schumann 1970).…”

Growth of protein–doped rhombohedra in the calcitic shell of craniid brachiopods

“…The structure of the secondary layer of the calcitic shell of the living brachiopod Neocrania has been known since 1970 (Schumann 1970;Williams & Wright 1970). The layer consists of spirally growing tablets that coalesce into sheets (laminae) interleaved with membranes (Williams 1970).…”