TOWARDS A PHYLOGENETIC CLASSIFICATION OF TRILOBITA: SYNAPOMORPHIES AND STRUCTURE OF THE ORDER OLENIDA ADRAIN, 2011

Despite progress at lower taxonomic levels, our understanding of higher trilobite phylogeny is surprisingly weak. Although peak global species richness was attained during the Cambrian, trilobites of this age are classified in a small number of orders widely considered either paraphyletic or polyphyletic. Very little modern phylogenetic work has been carried out on any of these groups. Post-Cambrian trilobites are classified in a greater number of orders, most of which are clearly monophyletic. The relationship of these groups to each other, or to any potential Cambrian sister taxa, is largely unknown and hence the basic structure of Trilobita remains unresolved. Our basic ignorance of higher trilobite phylogeny even has a name: Stubblefield, in 1959, introduced the term "cryptogenesis" to refer to the unknown origins of post-Cambrian orders.

Secondarily silicified trilobites yield new data on ventral structures and developmental morphologies, and have revealed the surprising presence of a complex articulation of the librigenal anterior projection and the cranidium in several groups of trilobites, many of which have not been considered related to one another. We propose the term "stylidion" for this structure. It involves a series of tiny pillars running dorsally from the inner edge of the librigenal doublure and articulating in a series of small pits on the inner (ventral) surface of the cranidium. These pits are often expressed on the dorsal (external) surface of the cranidium as tiny tubercles. The pillars are matched on the outer (ventral) surface of the librigenal doublure by another series of small pits. This morphology has not previously been documented. Most trilobites have a simple inner doublural margin and no cranidial modifications in this region. We hypothesize that the stylidion is an unreversed synapomorphy of the recently proposed Order Olenida. While available data are scant, all firmly associated larvae and early growth stages of olenide species have a characteristic morphology involving transversely complete glabellar furrows (most trilobites have the furrows expressed only abaxially), anteriorly placed eye ridges, and tuberculate fixigenae. These will likely prove to be additional synapomorphies of the order, supplementing the stylidion, which is developed later in ontogeny.