Genus: Isotelus Dekay, 1824
Cincinnatian Species: Isotelus gigas, Isotelus maximus
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Middle – Late Ordovician
Isotelus is an extinct, fast-moving, low-level, epifaunal deposit feeder.
Identification in Hand Sample:
- Carapace oblong with broad axial lobe.
- Eyes large, prominent.
- Thorax with 8 segments.
- Cranidial fragments, pygidial fragments, and thoracic segments generally flattened.
- Labrum, genal spines bear conspicuous terrace lines that resemble a fingerprint pattern.
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Fossils of Ohio (1996):
- “Isotelus Dekay is a large trilobite — its exoskeleton may exceed 50 cm in length. The cephalon is subtriangular in outline and smooth; pleural areas are narrower than the axis. The glabella is relatively low, poorly defined, tapering forward to the position of the eyes, then expanding forward; the lateral glabellar furrows are faint. Eyes are moderately long, holochroal, and slightly elevated. Facial sutures are opisthoparian; librigenae are moderately wide; genal angles are rounded, bluntly pointed, or have genal spines. The thorax has eight strap-like segments; pleural areas are defined by broad, shallow axial furrows; pleural areas are narrower than the axis; each pleura is slightly curved and terminates bluntly. The pygidium is large, subequal in size to the cephalon, subtriangular in outline, and smooth. The axis is poorly defined and has many poorly defined axial rings and a terminal piece. Pleural fields have many poorly defined pairs of pleural furrows. The labrum is large and forked and has distinct terrace lines that parallel the margin. The doublure around the margin of the cephalon and pygidium is wide, flat, and covered with terrace lines that parallel the margin of the exoskeleton.”
- “Separated sclerites are distinctive because of their large size and characteristic golden to dark brown color. Cranidial fragments, pygidial fragments, and thoracic segments are generally flattened and rather smooth. Portions of the undersurface, including the doublure, labrum, and genal spines, bear conspicuous terrace lines that resemble a fingerprint pattern. In Isotelus and other benthic trilobites, terrace lines on the undersurface probably aided in stabilizing the animals during feeding by gripping the sediment at the margins of the exoskeleton.”
Caster, Dalve & Pope (1961):
- The exclusively marine trilobites, related to the crabs, spiders, and insects, (Phylum, Arthropoda) abounded in the Paleozoic seas. They appear among the earliest well-preserved fossils found in the Lower Cambrian (Waucobian), and continue into the Permian, although diminished in numbers after Devonian times. The Cincinnatian rocks are filled with trilobite fragments, and good specimens of the common genus, Flexicalymene, are abundant. Two other genera, Isotelus and Cryptolithus, somewhat rarer than Flexicalymene, and fragments of extremely rare genera may be found here.
Treatise on Invertebrate Paleontology, Part O (1959):
- Cephalon and pygidium mostly with poorly defined flattened border. Frontal Area moderately long, cephalic axis ill-defined, almost obsolete in several species, slightly expanding in front of medium-sized eyes situated somewhat behind transverse mid-line of cranidium; no posterior border furrow; genal angles rounded, pointed, or with short genal spines. Hypostoma almost parallel-sided, posterior margin with broad (tr.), deep notch; anterior lobe short. Thoracic axis broad, poorly defined, almost obsolete in several species; pleural fields smooth or very faintly ribbed.
- Cephalic and caudal shields of nearly equal size with broad infolded margins; glabella nearly smooth, not lobed; free cheeks large, sometimes meeting in front of the cranidium; eyes prominent holochroal; hypostoma deeply forked; thoracic axis wide; pygidium obscurely lobed, segmentation often obsolete at maturity. This genus is distinguished from Asaphus by the absence of the lobation of the glabella, the distinct segmentation of the pygidial axis and the wider thoracic axis.