Abstract
In northeastern Mexico, the Upper Jurassic to lowermost Cretaceous (Kimmeridgian–Berriasian) hemipelagic La Casita Formation is widely known for its diverse and abundant assemblage of well-preserved ammonites. Here we present specimens of Kimmeridgian Idoceras sp. and Tithonian “Virgatosphinctes” aff. communis Spath covered by the epizoic oyster Liostrearoemeri (Quenstedt, 1843). Attachment to both sides of the ammonite shells, taphonomic and epizoan-to-ammonite orientation analyses and shell-size measurements indicate that the oysters attached to living hosts. We interpret the presence of distinct bivalve growth cohorts at this location to reflect two generations of epizoan settlement: an initial cohort of oysters settled on the juvenile whorls of ammonites and were followed by a second set of considerably smaller individuals on the outer whorls. A pseudoplanktonic mode of life of Liostrea roemeri attached to ammonites may reflect a strategic response to oxygen-depleted conditions near the seafloor of the Upper Jurassic La Casita Sea. L. roemeri was previously described from Europe, also attached to ammonites.
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