page_title: Triarthrus
section_title: Exoskeleton
hierarchical_section_title: Triarthrus / Description / Exoskeleton
caption_attribution_description: English: Triarthrus eatoni, a trilobite, family Olenidae, order Ptychopariida, exposed from the ventral side, collected from the Frankfort Shale, Sixmile Creek in Cleveland's Glen, near Rome, Oneida Co., New York, USA, from the Upper Ordovician (Caradoc, 458 to 448 mjo) showing the biramous appendages and antennae deposited as pyrite.
context_page_description: Triarthrus is a genus of Upper Ordovician ptychopariid trilobite found in New York, Ohio, Kentucky, and Indiana, eastern and northern Canada, China and Scandinavia. It is the last of the Olenid trilobites, a group which flourished in the Cambrian period. The specimens of T. eatoni that are found in the Beecher's Trilobite Bed, Rome, New York area are exquisitely preserved showing soft body parts in iron pyrite. Pyrite preservation has given scientists a rare opportunity to examine the gills, walking legs, antennae and digestive systems of trilobites, which are rarely preserved. Triarthrus is therefore commonly used in science texts to illustrate trilobite anatomy and physiology.
context_section_description: Triarthrus is an average size trilobite (up to about 5 centimetres or 2.0 inches) and its moderately convex body is about twice as long as wide (excluding spines). Like in all Olenidae, the headshield (or cephalon) of Triarthrus has opisthoparian sutures, and the right and left free cheeks that they define are yoked. The cephalon in Triarthrus is semicircular. The central raised area (or glabella) is approximately quadrate, and considerably wider than the posterior margin of the fixed cheeks (or fixigenae). The front of the glabella is close to the anterior border (or the preglabellar field is short), but the border furrow is absent in front of the glabella. The facial suture crosses in front of the glabella on the top of an inflated rim. The posterior end of the glabella consists of the occipital ring that is defined by a furrow crossing over the midline. The occipital ring carries a small node at its centre point, and may be adorned with a backward directed spine as long as the glabella. In front of the occipital furrow two pairs of lateral furrows emerge from the axial furrow almost perpendicularly and curve backwards as they cut toward the midline, but without reaching it. Anteriorly, one or two further furrows are isolated shallow slits or depressions. The fixigenae are very narrow (less than ¼ of the width of the glabella at the back of the eye, and less than ½ at the occipital ring). The librigenae are narrow, and with or without spines. The border is prominent elsewhere, and the eyes of small to medium size, sitting at the end of small palpebral lobes next to the frontal half of the glabella. The thorax consists of 13 to 16 segments, with the axis wider than pleural regions, obliquely truncated or rounded pleural tips, and the fulcrum placed very close to the axis. The tailpiece (or pygidium) is small, with 3 to 5 axial rings, distinctly segmented pleural fields, and an entire, evenly rounded margin.
caption: Pyrite preserved Triarthrus eatoni - ventral side
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