ORGANIC MATRIX IN CIRRIPEDS: SIMILARITIES TO MATRIX IN OTHER GROUPS AND ITS TAPHONOMIC POTENTIAL

Many invertebrate organisms have utilized calcium carbonate in their shells or skeletons, compensating for its limited strength and solubility by enveloping the crystallites in an organic matrix, essentially creating a composite material with resistance to dissolution. These and other functions are well established for the organic matrix in various classes of mollusks, where it is abundant not only in crystal sheaths but also as a fine gossamer within the crystals. A similar form of matrix is well represented among the brachiopods and bryozoans, and sheets of matrix are found within at least one foraminifer. All these matrices are physically similar. Arthropods have generally taken a different pathway in the formation of their exoskeletons, tending to use calcium carbonate sparingly, and more as an addition to their standard design than as a new approach. One exception to this is the humble barnacle, whose calcite plates are secreted to serve as a sort of secondary housing. These plates, whether forming a rigid wall as in the balanomorphs, or articulating armor as in the lepadomorphs, are quite strong, leading to the question of whether they incorporate an organic matrix. Cirriped shell sections prepared by polishing, etching, and critical point drying show that the calcite crystallites have organic matrix sheaths, with thicknesses and extent similar to other groups. It seems likely that the cirriped matrix acts as a cushion, both between the crystallites and between interlocking plates. It is unclear to what extent the studied matrices provide strength, as the matrix of the balanomorphs seemed very soft, while that of the single lepadomorph was more like that of mollusks. Whether this is an artifact or a significant aspect of design requires further investigation. It is also unclear whether the 'soft' matrix of the balanomorphs has preservation potential, as only problematic shreds were observed in those Pliocene and Miocene specimens so far examined.