A new occurrence of fossil acrothoracican cirripedes: Trypetesa polonica sp. n. in hermitted gastropod shells from the Korytnica Basin (Middle Miocene: Holy Cross Mountains, Central Poland), and its bearing on behavioral evolution of the genus Trypetesa

A new species of acrothoracican cirripedes, Trypetesa polonica sp. n., is established upon their borings emplaced in gastropod shells occurring in the Middle Miocene deposits (the Korytnica Clays) of the Holy Cross Mountains, Central Poland. The new species is recognized as an obligate commensal to the hermit crabs which inhabited some of the selected (thick-walled, high-spired, and/or spacious) gastropod shells. The morphology ofborings is comparable to that of the present-day species, especially of the pan-Atlantic species Trypetesa lampas (Hancock, 1849) and the Californian species Trypetesa lateralis Tomlinson,1953, to the latter of which the new species was . supposedly an ancestor. A peculiar feature of borings in some species of the genus Trypetesa, the newly established T. polonica sp. n. including, namely the presence of the accessory exit produced by the external mantle flap is interpreted as having been constructed to involve a passive flow of water (the so-called prairie-dog's effect). This was required by the Trypetesa specimens to ensure further food supplies and thus to survive in the case when the gastropod shell had been discarded by the hermit crab. The Trypetesa to hermit crabs relationship, recognized for the Miocene representatives, is postulated to have resulted from a refuge of one stock of acrothoracican cirripedes from their former endolithic mode of life. The behavior of the modem Trypetesa species, heritaged from their Miocene forerunners, is consequently dated back some over 15 m. y. ago (Langhian to Serravalian stage interval).