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Orientation of in situ radiolitid shells and possible implications on their functional morphology: a case study from the Adriatic Carbonate Platform

Numerous outcrops of radiolitid-bearing Cretaceous carbonates belonging to the Adriatic Carbonate Platform domain are exposed along northeastern coastal area of the Adriatic Sea (Croatia). Besides dominantly para-autochthonous and allochthonous congregations of radiolitid shells, there are rare localities which contain autochthonous (in situ) radiolitid communities. Although a preferentional orientation of in situ radiolitid shells has been previously observed both at the field and in the literature, it has been just sporadically discussed by another authors. Considering aforementioned statements there was a need and an opportunity for a numerical evaluation of the orientation. The best outcrops have been found within Upper Cretaceous carbonates on the island of Brač and Vis. A few communities, each of them consisting of a distinct taxa, have been analysed. Azimuth of a generally dorsal-ventral direction of a shell has been measured. The direction is commonly marked by some of a pronounced morphological parts of the shell. The measured directions are presented graphically for each of the community. Some of them show pronounced disorientation but another show generally preferential orientation. The question of the main reason for preferential orientation or disorientation of the community has been discussed. The observed preferential orientation, which characterizes some of the communities, implicates a functional morphology of the shells. Oppositely, the disorientation could be linked to an independence of the taxa to the presumably directional water currents. However, it could be also related to an environment characterized by a more turbulent and undirectional water-currents. There are two presumable functional implications of the radial bands, previously discussed by a few authors: a) the radial bands mark the sites of inhalant and exhalant currents, and b) the radial bands mark ejection sites of faeces and pseudofaeces. There is also a possibility of combination of these two functions. Both aforementioned functions need a water current for better results. It could be linked up with another function of the radial bands: to channelize water containing oxygen and/or food particles in suspension. The channelized water increases velocity of the water current and thus improves removing of faeces and pseudofaeces from commissural position. If these presumtions are correct, it could be important for radiolitids to be oriented, especially for communities which have been living within environments characterized by a low-velocity currents and/or food- and oxygen-poor environments. This research should be considered just as a pioneering work in this kind of radiolitid rudist palaeoecology that should be tested by a comprehensive investigation within another locations containing appropriately preserved in situ radiolitid communities. It can be speculated that possible afirmative results could promote the radiolitid communities, in combination with another indicators of palaeocurrent direction and/or a trend of a palaeocoastline, to a usefull tool in many aspects of a palaeoenvironmental investigations.

Rudists; Radiolitids; Functional morphology; Orientation

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