Sheltered preservation in Ordovician trilobites

Articulated holaspid specimens of Placoparia Hawle and Corda, 1847 and Eoharpes Raymond, 1905 entombed inside cephalopod conchs and under the remains of large illaenid, asaphid, cyclopygid and dalmanitid trilobites from the Middle Ordovician Šárka Formation of the Prague Basin (Czech Republic) are described and discussed. Two such samples were also found in the overlying Dobrotivá Formation of Middle/Late Ordovician age. Four articulated juvenile exoskeletons of Placoparia preserved under a cephalon of the cyclopygid trilobite Degamella Marek, 1961 represent the first record of shelter strategy of non-holaspid trilobites. The sheltered preservation of trilobites could be explained by a hiding behaviour associated with the danger of predation, storm disturbances, seeking for food or high vulnerability after moulting. It is obvious that Placoparia and Eoharpes deliberately entered the restricted space under skeletal parts of large trilobites or inside cephalopod conchs. These exceptional finds provide a new insight in the life strategy of some Ordovician benthic trilobites and are classified as cases of “frozen” behaviour.