Trinodus

Trinodus is a very small to small (about ) blind trilobite, a well known group of extinct marine arthropods, which lived during the Ordovician (Tremadocian to early Hirnantian), in what are now the Yukon Territories, Virginia, Italy, Czech Republic, Poland, Denmark, Sweden, Svalbard, Ireland, Scotland, Wales, Iran, Kazakhstan and China. It is one of the last of the Agnostida order to survive.
Etymology.
"Trinodus" is derived from the Latin tri (three) and nodus (node).<br>
"Arthrorhachis" is derived from the Greek ἄρθρον ("árthron", "joint") and ῥάχις ("rháchis", meaning axis, spine, ridge or backbone).
Taxonomy.
"Trinodus", "Arthrorhachis" and "Geragnostus" are closely related and it may be appropriate to assign their species to just one genus. All species in these three genera have virtually identical cephalons, but of "T. agnostiformes", the type species of "Trinodus" only one poorly preserved cephalon was known. However, relatively recent, pygidia assignable to "T. agnostiformes" were found. Although this material is distorted or incompletely preserved, it is very similar to the pygidium of "Arthrorhachis tarda". Species with a rear rhachis lobe longer than the postaxial region are henceforth combined in "Geragnostus", all others are assigned to "Trinodus".
Development.
"Trinodus elspethi", which - as an agnostoid - only has two thorax segments, has at least nine
larval stages (or instars), three meraspid and six holaspid, in its life. So it molted at least eight times.