Enigmatic polychaete jaws from the Silurian of Gotland, Sweden

The jaws of the new polychaetaspid polychaete, Oenonites? honki, from the S ilurian of Gotland, Sweden, differ from most Palaeozoic polychaete jaws. Th ey exhibit enigmatic microstructural features in that they appear rough and give a corroded, or weathered impression. The altered microstructure of th e jaws suggests a jaw chemistry and/or composition differing in some way fr om that of the co-occurring polychaete taxa. The jaws appear to have limite d preservational potential and/or were particularly susceptible to secondar y processes, resulting in microstructural alteration. Commonly, a row of di stinct pits occurs on the outer face, especially of the first right maxilla e (MIr). Because these pits are interpreted as associated with the dentary, the term 'denticle marks' is suggested. The pits may be the result of prim ary or secondary physical wear, or, more probably, secondary chemical alter ation of localized mineral deposits. The primary function of such mineral d eposits was to harden those parts of the surface that were exposed to great stress. The restricted occurrence of O.? honki, coupled with occasional in creases in abundance (especially in the Halla Formation, unit b), indicates a preference for shallow marine, high-energy environments, particularly in reefal pockets with calcilutitic sediments. Highest frequency coincides wi th faunas characteristically containing a few labidognath species also disp laying high frequencies.