This is a museum-quality example of the trilobite Flexicalymene retrosa from the Ordovician of Ohio. Flexicalymene retrorsa is a member of the Order Phacopida, Family Calymenidae.
Phacopida ("Lens-face") is an order of trilobite that lived from the Ordovician to the Devonian. It is made up of a morphologically diverse group of related suborders.
The origin of the Phacopida order is uncertain. It comprises three suborders (Phacopina, Calymenina, and Cheirurina) which share a distinctive protaspis (developmental stage lacking segments) type. The Cheirurina and Calymenina retain a rostral plate (an apomorphy) but in virtually all Phacopina the free cheeks are yoked as a single piece. This sort of similarity in development suggests phylogenetic unity. The suborder Calymenina is the most primitive of the Phacopida order and shares some characteristics with the order Ptychopariida, though it is not included in the subclass Libristoma.
Phacopida had 8 to 19 thoracic segments and are distinguishable by the expanded glabella, short or absent preglabellar area, and schizochroal (Phacopina) or holochroal (Cheirurina and Calymenina) eyes. Schizochroal eyes are compound eyes with up to around 700 separate lenses. Each lens has an individual cornea which extended into a rather large sclera. The development of schizochroal eyes in phacopid trilobites is an example of post-displacement paedomorphosis. The eyes of immature holochroal Cambrian trilobites were basically miniature schizochroal eyes. In Phacopida, these were retained, via delayed growth of these immature structures (post-displacement), into the adult form.
Calymenidae is an extinct family of nektobenthic carnivores, meaning these forms of marine life exist just above the ocean bottom and occasionally rest on it.
Flexicalymene is used as an index fossil for the Ordovician. The location from which this trilobite was collected is now closed, so specimens of this quality will be harder and harder to find.
This specimen measures approximately 1 inch long on a matrix measuring 2 ¾ inches by 2 ¾ inches by 1 inch thick.