Isotelus gigas

Classification
Phylum: Arthropoda
Class: Trilobita
Order: Asaphida
Family: Asaphidae
Genus: Isotelus
Species: Isotelus gigas (DeKay,1824)

Stratigraphic Occurrences

I.gigas_strat

Geographic Occurrences

Map point data provided by iDigBio.

Stratigraphic Description

Sequence (Formation)

  • C5 Sequence (Whitewater, Liberty, Waynesville)
  • C4 Sequence (Arnheim)
  • C3 Sequence (Mt. Auburn, Corryville)
  • C2 Sequence (Fairview)
  • C1 Sequence (Clays Ferry/Kope)

Notes:
Ross (1967)

  • 5 feet above base of the Salvisa Bed of the Lexington Limestone, 20 ft below base of Clays Ferry
    Grier Limestone Member of the Lexington Limestone, 75-78 feet below base of Clays Ferry Formation

Identification in Hand Sample

  • Pygidium is distinctly subtriangular.
  • Cephalon similarly subtriangular.
  • Lacks genal spines in all cases except for the smallest specimens.
  • Can be distinguished from the much more common I. maximus by the lack of genal spines altogether, or the presence of shorter genal spines on small specimens.
  • Pygidial and cephalic margins more triangular than I. maximus.

Isotelus gigas from Utica Group of New York (CMCIP 33070, left) and unknown location (CMCIP 0339)

Published Description

McFarlan (1931):

  • A large species commonly attaining a length of 200-250 mm., at times practically double the figure. Width somewhat over half the length. Axis forming about one-half the width. The most distinctive feature is the subtriangular form of the pygidium with length about 3/4 width. Cephalon similarly subtriangular. Genal spines short, present only in the smaller specimens.
    Mohawkian and Cincinnatian.

DeKay (1824):

  • Isotelus gigas:
    Head representing a spherical triangle, surface punctate, convex descending from between the eyes to the anterior border, which has a narrow raised rim: posterior extremity concave and corresponding to the articulation of the abdomen. Eyes elevated, prominent, sub-pedunculated; cornea oblong, lunated, highly polished. Abdomen with eight distinct articulations, the middle lobe double the size of the lateral one; these latter are continuous with the middle lobe, have a deep furrow impressed on their upper surfaces, which becomes gradually effaced towards their narrow free extremities. These lateral lobes are rounded. at their extremities, and flattened in such a manner as to allow each lobe to slide easily under the lobe immediately preceding. Tail subtriangular, convex, equaling the head in size, with the posterior termination rounded. On the centre of its surface, when accidentally decorated, a slight elevation may be traced, if the specimen be held in a certain light, which appears to be a continuation of the middle lobe; this extends to within short distance of the posterior angle of the tail, when it is either entirely laced, or terminates in an abrupt truncation. Another elevation runs parallel to, and at a short distance from the edge of the tail. These elevations are connected by obscure parallel lines, imitating the spaces between the lateral lobes. When the tail is fractured on the borders, a semi-lunar depression is visible, exhibiting concentric striz. The whole surface of the animal has a jet black polish.
    Total length, 6-12 English inches.
    Cabinet of the Lyceum.