The La Casita Formation in north‐eastern Mexico represents a period of increased terrigenous clastic supply that is characteristic for the entire northern and western margins of the Gulf of Mexico Basin during latest Jurassic and earliest Cretaceous time. A prograding delta complex developed at the south‐eastern margin of the Coahuila Peninsula. Delta plain, delta front, prodelta and shelf areas show distinct lithofacies associations. Some of the most characteristic elements are fining upward and coarsening upward grain size trends, syndepositional deformational features, storm layers, phosphorites, fossiliferous carbonate concretions and an anaerobic biofacies.
A possible break in sedimentation is inferred for parts of the Valanginian. This may be interpreted as a sequence boundary between second order cycles. The Kimmeridgian ‐ Berriasian cycle appears to correlate with the Cotton Valley cycle of the Texas Gulf Coast.
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.
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