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Taphonomic selection among echinoids or why is Clypeaster the most abundant echinoid in the Miocene of Croatia

As POLJAK (1938) or MITROVIĆ-PETROVIĆ (1969) already reported, at least 75% of fossil echinoids in the Middle Miocene of Croatia and surrounding areas belong to the sand dollar genus Clypeaster. This statement seems to be correct when we look at the composition of briefly collected echinoid fossil faunas from the wide-spread Miocene outcrops in the continental part of Croatia. The large and heavy tests of Clypeaster can be easily found elsewhere where the terrain is build of shallow marine Miocene sediments of Carpathian and mostly Badenian stages. Released from sediment by weathering, these fossils are often incorporated in the composition of recent sediments as short- lived clasts, soluble in the meteoric water. Clypeaster shells could be preferably found in bioclastic corallinacean limestone of different type and less often in lithoarenite and lithic graywacke. Once used as an architectural and building stone, Miocene bioclastic limestone was a subject of intensive quarrying which left a lot of abandoned works (excavation pits and quarries) around northern Croatia suitable for fossil researches. In this study, we investigated some of the most cited Clypeaster-dominated outcrops in the northern Croatia where we thoroughly checked the sediments to reveal as much as possible of complete echinoid fauna. Positions of localities follow the marginal parts of Paratethys. Due to the relatively changeable firmness of the rocks, we used a microanalysis of weathered residual sediments and fresh rocks dissolved in the solution of hydrogen peroxide. Thus prepared material was sieved and analyzed on stereo microscope. The result of this research was a quite different composition of echinoid fauna from investigated localities when compared to the earlier reports and available fossil collections. The disproportion between fragmentary material and completely preserved echinoid tests was also observed. Discrepancy between completely preserved tests and fragments are the greatest in the case of Clypeaster ; its fragments are relatively rare. All other genera except Echinolampas are relatively rarely represented in localities with wholly preserved tests. Determinable fragments (isolated spines and plates) show completely different composition of echinoid fauna with presence of large amount of spatangoid, cassiduloid and clypeasteroid echinoids. Some of these echinoids are the first records in Croatian Miocene like the genus Pliolampas and Echinocyamus (Fig. 1A). The uncovered echinoid fauna is not totally composed of genus Clypeaster regarding almost all of the sites. For example, some of the most popular sites like Bizek quarry in the Medvednica Mt. are dominated by echinoid genus Conolampas, not by Clypeaster. Localities in Hrvatska Banovina (Sjeničak, Sjeničak Lasinjski) are mainly composed of spatangoids and small clypeasteroid Echinocyamus, also not by Clypeaster. Only one site, Sutinska vrela quarry in Medvednica Mt. were found as Clypeaster- dominated echinoid assemblage joined with some rare remnants of cidaroid regular echinoids. Clypeaster-dominated assemblages, in terms of completely preserved echinoid tests, are obviously a product of taphonomic selection, where the test of this echinoid has a large preservation potential (SEILACHER, 1979 ; NEBELSICK, 1995, 1999) opposite to all the other present echinoids. This could be partially explained by the structure of clypeasteroid tests which are reinforced with numerous pillars as adaptation to living in high- energy shallow-marine environments (NEBELSICK, 1992, 1995 ; Figs. 1B, 1C). On the other hand, selection existed also between different species of Clypeaster where the conical forms (e.g. C. campanulatus gr.) are surprisingly more abundant than the flat forms (e.g. C. folium) with stronger and more compact test. This suggest on few living niches affected by different taphonomic conditions, where the conical forms of Clypeaster may lived deeper, buried deeply in the sediment or lived out of destructive influence of the waves to survived early phase of taphonomic selection. This early phase was destructive for the most other echinoids like thin shelled spatangoids or unstable skeletons of regular echinoids what explain a minimum record of their complete tests. Favorable geochemical conditions through all the phases of diagenesis and type of sediment also supported preservation of the echinoid tests in carbonate deposits. Sedimentary environment in which the echinoids are preserved is also important for their preservation (NEBELSICK, 1995). The echinoids from Dvor na Uni (Zrinska gora Mt.) locality were found in fine-grained lithic graywacke and they are all fragmented with no preservation of complete echinoid test. Large sediment grains probably redirect or take over some part of the sediment pressure to the surrounding sediment mass. In the fine-grained sediments (like on the Dvor na Uni locality) this phenomena is missing and echinoids show a hydraulic cracking of its test (collapsed ambulacral zones, implosion of the test forming ‘pie slice’ fragments etc.). Analysis of the echinoid collections from Croatian Natural History Museum also confirms the dominance of genus Clypeaster in localities of northern Croatia where the genus Clypeaster makes 80% of total echinoid fauna. Some local town museums and private collectors also display only Clypeaster in their fossil collections, sometimes misidentified as a genus Scutella in case of flat forms of the test. The explanation of high percent of Clypeaster here also may lay on excellent perceptibility and size of its test which makes it an easy target for amateur collectors who often donates the material to museums and professional paleontologist. The fact that this kind of fossil material in Croatia was never researched in details also contributes to the explanation. As final conclusion, we can tell that seemingly high percent of Clypeaster tests in Croatian Miocene shallow-marine deposits is product of different taphonomic processes which strongly influenced the original composition of echinoid fauna.

echinoids; taphonomy; Clypeaster; Echinocyamus; Miocene; Croatia

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