Triarthrus beckii

Phylum:  Arthropoda
Class: Trilobita
Order: Ptychopariida
Family: Olenidae
Species: Triarthrus beckii (Green, 1832)

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Note: Type-Cincinnatian Triarthrus specimens that were previously assigned to T. eatoni (Hall) were recently assigned to T. beckii (Green) by Kim et al., 2009
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Stratigraphic Occurrences

T. eatoni

Geographic Occurrences

Map point data provided by iDigBio.
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Sequences (Formations)

  • C1 Sequence (Clays Ferry/Kope: Southgate, Economy/Fulton)

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Identification in Hand Sample

  • Narrow with a long thorax.
  • Pygidium small.
  • Notable feature on the cephalon is the presence of a series of glabellar furrows.
  • Eyes are slender, and difficult to discern except on well preserved specimens.
  • Commonly found preserved by pyritization, which allows for detailed analysis.

Triarthrus beckii (eatoni) from Hudson River Shale, New York (CMCIP 17753), Utica Black Slate, New York (CMCIP 17751)

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Kim et al. (2009):

  • Finally, we also include two populations of the closely similar, younger species Triarthrus eatoni from Ontario. These specimens provide additional context in which to gauge the variation among our T. beckii collections. This youngest olenid differs from T. beckii primarily in having relatively larger eyes that extend farther toward the back of the cranidium (Ludvigsen & Tuffnell 1983, 1994). Cisne et al. (1980) referred their Mohawk Valley material to both of these Triarthrus species, however, Ludvigsen & Tuffnell (1983, 1994) concluded that all of the Triarthrus specimens from Utica Shale of New York state studied by Whittington (1957) and Cisne et al. (1980) should be included in T. beckii. Our results (described below) support this conclusion. Similarly, our work suggests that the Triarthrus specimens that occur in the lower Kope Formation of Ohio and Kentucky (Ross 1979) should be included within T. beckii. All exhibit a continuous range of morphologic variation that is consistent with genetic continuity among the populations and contradicts their separation into morphologically distinct species.

Cisne (1973):

  • Triarthris eatoni is common in the Frankfort Shale and elsewhere in Upper Ordovician rocks of the Appalachian region. Adults (Fig. 1), which show no obvious indications of sexual dimorphism, were epifaunal deposit feeders (Cisne, in press). They may have moved over the surface of the sediment and swam about near the bottom. Except perhaps for its long legs and narrow pleural regions, Triarthrus eatoni is like most other trilobites. But because the morphology of trilobites, apart from that of their dorsal exoskeletons, is so incompletely known, it is difficult to say how typical this or most other trilobites might actually be. Triarthrus was a cosmopolitan genus in later Ordovician marine environments and is particularly associated with deep water habitats. It is the last genus of the superfamily Olenacea, a group especially common in Upper Cambrian and Lower Ordovician rocks.

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