STRATIGRAPHIC TRENDS IN TRILOBITE ENROLLMENT

Enrolled trilobites are iconic fossils, known for the beauty of their intricate articulation and locking mechanisms. Familiar North American examples, such as Flexiclaymene meeki or Eldredgeops rana, are post-Cambrian, and belong to derived trilobite clades. Although examples of enrolled Cambrian trilobites have long been known, they are relatively rare, and the morphologies of some Cambrian trilobites are thought to have precluded encapsulated enrollment. However, rare examples of Cambrian assemblages that did preserve abundant enrolled trilobites draw attention to the fact that enrollment was physically possible for the majority of trilobite morphotypes common in the Cambrian. Why then, are such occurrences, rare in the Cambrian? To assess this we have examined some structural and taphonomic features associated with trilobite enrollment throughout the history of the group. Firstly, we find that the proportion of enrolled trilobites in assemblages bearing abundant articulated trilobites rose significantly in the post-Cambrian. Secondly, particular styles of enrollment changed through time, with those demanding the most precise coordination of sclerite shape and movement becoming more common later in trilobite evolutionary history. This rise was paralleled by an increase in the number of structural features associated with guiding and locking enrolled posture. The enhanced preservation of enrolled trilobites may partly reflect the increased structural integrity of enrollment in later trilobites. However, this seems unlikely to be the sole cause, because the preservation of articulated trilobites in prone posture, which almost invariably accompanied enrollment, implies rapid burial that was subsequently undisturbed. The fact that prone specimens are commonly preserved throughout the record might also suggest that depositional regimes favoring the preservation of enrolled specimens became more common in the post-Cambrian.