Lithiotid Bivalves in Slovenia and Their Mode of Life

Irena Debeljak
1997 Geologija  
Lithiotid bivalves are a characteristic faunal element of the shallow marine fades of Lower Jurassic beds in southern Slovenia. The horizon containing Middle Liassic bivalves, which is up to 75 m thick, is called the "lithiotid horizon" and is attributed to the Pliensbachian or Domerian. In Slovenia the name lithiotid bivalves represents three morphologically similar genera or species of sessile monomyarian dysodont bivalves: Lithiotis problematica, Cochlearites loppianus and Lithiopema
more » ... Lithiopema scutata, which are systematically examined in this paper The term lithiotid bivalves does not have any taxonomic significance, since they are now classified in different families. The order is Pterioida. Lithiotid bivalves lived in an upright position on soft lagoonal bottoms in a tight aggregate of individuals crowded together which mutually supported one another and simultaneously competed for living space and light. The sedimentation of calcareous mud was fairly rapid, thus throughout their lives they grew constantly in a subvertical direction so that the small soft body space at the ventral end remained above the level of the surrounding substratum. Lithiotid bivalves had peculiar, variable shells adapted to the specific environment. They are very large, flat and distinctly dorso-ventrally elongated. Their inner surface is tripartite; in the middle is the central area, and at the sides there are feather-like areas. The mechanism of opening and closing their valves has not yet been fully explained. Kratka vsebina Litiotidne školjke so značilen favnistični element plitvomorskega razvoja spodnjejurskih plasti južne Slovenije. Do 75 metrov debeli horizont s srednjeliasnimi školjkami imenujemo "litiotidni horizont" in ga uvrščamo v pliensbachij oz. domeri] • Pod imenom litiotidne školjke v Sloveniji združujemo tri morfološko podobne rodove oziroma vrste sesilnih, monomiarnih, disodontnih školjk: Lithiotis problematica, Cochlearites loppianus in Lithiopema scutata, ki so sistematično obdelane v tem prispevku. Izraz litiotidne školjke nima taksonomskega pomena, saj jih danes uvrščamo v različne družine. Spadajo v red Pterioida. Litiotidne školjke so v pokončnem položaju živele na mehkem lagunskem dnu, v tesni združbi skupaj nagnetenih osebkov, ki so se med seboj podpirali in hkrati tekmovali za življenjski prostor Sedimentacija karbonatnega blata je bila precej hitra, zato so litiotidne školjke vse življenje enakomerno rastle v navpični smeri, tako da je njihov bivalni del na ventralnem koncu ostajal nad nivojem obdajajočega substrata. Litiotidne školjke so imele nenavadne, variabilne, specifičnemu okolju prilagojene lupine. So zelo velike, sploščene in izrazito razpotegnjene v višino. Njihova notranja površina je tridelna; v sredini je glavno oz. osrednje polje, ob straneh pa peresasti polji. Mehanizem odpiranja in zapiranja njihovih lupin še danes ni docela pojasnjen.
doi:10.5474/geologija.1997.001 fatcat:p3r23erplja7bire2gackfovme