STRUCTURAL COMPARISON OF EARLY CAMBRIAN AND LATER TRILOBITE EXOSKELETON SUGGESTS A SELECTIVE PREMIUM UPON GREATER STRENGTH THROUGH TIME

The evolutionary history of trilobites witnessed profound changes in physical and biotic environments, global temperature, and ocean and atmospheric chemistry paralleled by episodes of biotic innovation including more sophisticated and effective predators. Trilobite exoskeletons responded to these changes through changes in exoskeletal composition and structure as well as the evolution of more effective protective strategies. The preservation of cuticle in early Cambrian trilobites is extremely rare, but the exceptionally well preserved exoskeleton of the olenelloid Schmidtiellus mickwitzi from rocks aged 520 Ma of the Lükati Formation in Estonia reveals important structural differences from that the general cuticular character shared among derived groups. Major differences include: 1) Schmidtiellus exoskeletal thickness is approximately 30 to 80μm, as opposed to the more common 200 to 400μm cuticle in more derived trilobites; 2) Schmidtiellus cuticle has a 10μm thick dense outer layer covering a more porous inner layer composed of laminated sheets of skeletal material, apparently succeeded in two specimens by a further dense layer; 3) examined layers are composed of calcium phosphate (as opposed to the low magnesium calcite that characterizes more derived trilobites). It is presently unclear whether the original mineralogy of the exoskeleton of S. mickwitzi was phosphatic, or whether this is an alternation product and it was originally aragonitic or some other mineral phase. The notably thin cuticle of this relatively large trilobite is striking and strongly suggests a selective premium for thicker and stronger cuticle by middle Cambrian times, presumably in response to escalating predatory pressure. The next step is to consider the structural properties of the olenelloid cuticle in comparison to those of later trilobites.