Trinodus


Trinodus is a very small to small blind trilobite, a well known group of extinct marine arthropods, which lived during the Ordovician, 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 and nodus.
Arthrorhachis is derived from the Greek ἄρθρον and ῥάχις.

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.

Species previously assigned to ''Trinodus''

Distribution

  • T. agnostiformes was identified in the Upper Ordovician of Ireland.
  • Trinodus danicus occurs in Ordovician of Svalbard.
  • Trinodus elspethi is present in the Upper Ordovician of the USA.
  • T. hupehensis has been found in the Middle Ordovician of China.
  • Trinodus knockerkensis has been collected from the Ordovician of Ireland.
  • Trinodus pragensis is present in the Upper Ordovician of the Czech Republic. A. pragensis precedes A. tardus in Czech Republic.
  • Trinodus tardus is present in the Upper Ordovician of Denmark, of Sweden ; Varvsberget, Skogastorp, Plantaberget of Norway of the Czech Republic, of Italy, of Iran, and of Kazakhstan.
  • Trinodus sp. has been collected from the Ordovician of Czech Republic.
  • Trinodus sps. are found in the Ordovician of Canada, of China, of Poland, of Wales and Scotland.

Development

Trinodus elspethi, which - as an agnostoid - only has two thorax segments, has at least nine
larval stages, three meraspid and six holaspid, in its life. So it molted at least eight times.