Type-Cincinnatian Triarthrus specimens previously assigned to T. eatoni (Hall) were recently assigned to T. beckii by Kim et al., 2009
Triarthrus is an extinct genus of fast-moving low-level epifaunal carnivores.
Identification in Hand Sample:
- Glabella straight sided with distinct lateral furrows.
- Exoskeleton up to 3 cm in length.
- Cephalon is semicircular in outline; pleural areas are slightly narrower than the axis.
- Pygidium is small and lenticular in outline.
- Eyes small, holochroal.
- Thorax with 13 to 16 segments.
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- Presumed pelagic taxa that are found in Ohio. Pelagic trilobites may have been filter-feeders or perhaps predators that fed on small planktonic organisms. Glabella straight sided and has distinct lateral furrows.
- Has an exoskeleton up to 3 cm in length. The cephalon is semicircular in outline; pleural areas are slightly narrower than the axis. The glabella is low, parallel sided, has three pairs of lateral glabellar furrows, and generally has a distinct pair of pits in the axial furrow at the anterolateral (front, side) corners of the glabella. Eyes are small and holochroal. Facial sutures are opisthopaian; librigenae are narrow and low; genal areas are developed into blunt angles or long, narrow spines. The thorax has 13 to 16 segments; pleural areas are narrower than the axis; each pleura is straight and terminates bluntly. The pygidium is small and lenticular in outline. The axis has three to five pairs of pleural furrows.
- Two species of Triarthrus have been identified from Middle and Upper Ordovician rocks of Ohio. The more common species, T. spinosus Billings, has elongate genal spines that are covered with terrace lines. Triarthrus is known both from surface exposures of the Kope Formation and from subsurface rocks recovered through coring. The morphologies of Triarthrus species, their widespread distributions in North America (see Ludvigsen and Tuffnell, 1983), and their typical occurrence in black or dark-gray shales suggest that they were pelagic trilobites. In Ohio, Triarthrus commonly is found in association with Proetidella parviusculus in shales representing oxygen deficient bottom conditions.
- Some slabs bearing Triarthrus comprise mostly headless specimens. These are interpreted to represent molted exoskeletons. Whether such occurrences represent trilobites that molted in one small area at about the same time or current-concentrated molts, is an open question.
Treatise on Invertebrate Paleontology, Part O (1959):
- Exoskeleton elongate. Cephalon semicircular; glabella considerably wider than posterior areas of fixigenae, separated from front border furrow or by short preglabellar field, with 2 to 4 pairs of lateral furrows, usually 1 or 2 anterior pairs separated from axial furrows. Fixigenae very narrow, especially anterior and palpebral areas; anterior sections of facial sutures converging from eyes to front border; librigenae extremely narrow, spineless or (rarely) with slender, curved genal spines. Thorax of 13 or 16 segments, with axis wider than pleural regions obliquely truncated or rounded pleural extremities, and fulcrum placed very close to axis. Pygidium small, with 3 to 5 axial rings and entire, evenly rounded posterolateral margin.
McFarlan (1931) :
- Elliptical. Cephalon semicircular. Glabella large and well defined with straight sides and rounded front, marked by three deep furrows extending toward the center from each side. Eyes small. Central axis of thorax wider than lateral lobes; furrows of axis not continuous with those of pleura. Thoracic segments 14-16. Pleural segments grooved. Pygidium with 6 segments in the axis and with entire margin. (Grabau and Shimer, 1910, p. 286)