ONTOGENY AND RELATIONSHIPS OF TRINUCLEOIDEA (TRILOBITA)

Protaspides and later growth stages are described and discussed for th e Ordovician trinucleid trilobite cryptolithus tesselatus Green and th e raphiophorid Lonchodomas chaziensis shaw. A small triangular rostral plate is described for a single protaspid instar in both Cryptolithus and Lonchodomas. The presence of this small sclerite in the ontogeny of these taxa supports the origin of the Trinucleoidea from the Ptycho pariida through reduction in and then fusion or loss of the rostral pl ate. Earlier and later growth stages have fused ventral sclerites, wit h no signs of connective sutures. All members of the Trinucleoidea wit h known ontogenics share similar small, ovoid asaphoid protaspides, wi th distinct axes and varying numbers of sharp, conical to subtubular, submarginal spines on the dorsal exoskeleton and marginal spines on th e hypostome. Two protaspid instars are identified in both Cryptolithus and Lonchodomas, sharing many characters that indicate homologous lev els of development. A single origin for the median suture of the Asaph ida is supported, with an anterior rostellum as its precursor.