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