Historical distribution patterns of trigonioidids (non-marine Cretaceous bivalves) in Asia and their palaeogeographic significance
The non-marine trigonioidid bivalves show five phases of radiation in the Cretaceous of Pal-Asia: pre-Aptian (?Valanginian/Hauterivian–Barremian), Aptian, Albian, Cenomanian and Turonian–Maastrichtian. Their distribution patterns show two distinct palaeo-river systems feeding trigonioidids. Before the Cenomanian, the river system occupied the southwestern–southern–southeastern Pal-Asian continental margin areas. During the Turonian–Maastrichtian, it extended along the line of southcentral China−eastern China−northeastern China−northern China and Mongolia−northwestern China–eastern Fergana Basin of Kyrgyzstan−western Tajikistan Basin of Tajikistan–Tashkent area of Kazakhstan−central Kyzylkum of northern Uzbekistan–Aral Sea area of Kazakhstan. Furthermore, the general trigonioidid distribution pattern demonstrates that Japan was probably attached to part of eastern China and/or Korea during the ?Valanginian/Hauterivian–Cenomanian stages.
Ontogenetic variations of the Early Cretaceous non-marine bivalveTrigonioides (T.) heilongjiangensis and the concept of fossil species
KurzfassungBeiTrigonioides (T.) heilongjiangensis aus der Unterkreide des östlichen Heilongjiang, NE China, wird der Winkel der V-förmigen Rippen auf der Flankenmitte während des Wachstums größer; die subradiären Rippen auf der vorderen Flankenhälfte variieren zwischen einfach und dabei kräftig bis schwach oder fehlend und einfach oder mehrfach aufspaltend und deutlich. Die Zahl der unter dem Wirbel gelegenen Zähne nimmt zu, und der Umriß der Art variiert kontinuierlich. Alle Individuen oder Populationen, deren Merkmale in einem Zeithorizont und/oder in einem bestimmten Zeitraum innerhalb einer biogeographischen Region kontinuierlich variieren, sollten als eine Art betrachtet werden. Demzufolge werden Individuen aus dem östlichen Heilongjiang, die bislang zu fünf verschiedenen Taxa(T. (T.) kodairai Kobayashi & Suzuki,T. (Wakinoa) wakinoensis intermedia Hase,T. (W.) subtrigonus Sha, T. (W.) cf.tamuraia Yang undTrigonioides sp.) gestellt wurden, als verschiedene ontogenetische Individuen vonTrigonioides (T.) heilongjiangensis Sha & Gu betrachtet und mit ihr synonymisiert.AbstractWith growth, the angle of V-ribs on the median flank ofTrigonioides (T.) heilongjiangensis enlarges; the subradial ribs on the anterior half vary from simple, but prominent, through feeble or vanished to bifurcate or multiple and clear; the number of submedian teeth increases and the variation in outline of the species is quite continuous. All individuals or populations which vary continuously in characters at one time horizon and/or in a certain time space within the same biogeographic region should be regarded as conspecific. Accordingly, the individuals of five taxa ofTrigonioides hitherto recognized in the eastern Heilongjiang area, namelyT. (T.) kodairai Kobayashi & Suzuki,T. (Wakinoa) wakinoensis intermedia Hase,T. (W.) subtrigonus Sha,T. (W.) cf.tamuraia Yang andTrigonioides sp., are regarded as representing various ontogenetic stages ofTrigonioides (T.) heilongjiangensis Sha & Gu. 263
Divaricate patterns in pelecypod shells
Shingled divaricate ribs occurring in several unrelated groups of pelecypods are highly functional structures related to specific burrowing processes. However, their obliqueness and ambivalence, seemingly the most aberrant features, are shared by a variety of colour, mineralization, and groove patterns both in the functional and non-functional field. Their basic form is assumed to be less controlled by adaptation and phylogeny than by a common principle of shell growth, the nature of which has still to be found out.