Phylum Arthropoda
Class Trilobita
Order Ptychopariida
Superfamily Asaphacea
Family Asaphidae
Subfamily Isotelinae
Genus and species: Isotelus latus

Middle Ordovician
Trenton Series
Cobourg Formation
Bowmanville, Ontario, Canada

Isotelus latus is one of the Ordovician Period's large trilobites. This particular specimen is 10 inches (25 cm) long; many have been found 15 or more inches long! This specimen was found on a block of limestone approximately the size of a car. The negative was on an adjacent car-sized block! If not for a concrete cutoff saw, a lot of superglue, and many hours of hard labor, this bug would have taken a journey to the crusher instead of home with me. To find such a trilobite, it isn't enough for the quarry to blast out a rock with the trilobite attached. The large trilobites must survive a blast that must break the rock so the trilobite remains on an unbroken slab, either covered with matrix, or with the negative nearby. Those who collect southern Ontario's quarries can relate how many fragmented giants they have seen.

What makes this specimen even more interesting is the healed wound on the pygidium. From the shape of the injury, this is probably a bite mark from a predator.

Generally, to tentatively identify the large Ontario Isotelus, 2 features should be observed. If the cephalon and pygidium are rounded with long genal spines--Isotelus maximus*; with blunt or no genal spines--Isotelus latus*. If the cephalon and pygidium are triangular with genal spines, odds are it is an Isotelus mafritzi, with no genal spines, odds are it is an Isotelus gigas*. There are other rarer species present, but this is a simple method to establish what Isotelus species you probably have.

* indicates this species can exceed 10 inches (25 cm) in length.

Bibliography:

Iellamo, John, 1986, "Middle Ordovician Trilobites of Southern Ontario - an Overview", MAPS Digest Expo VIII Edition, pg. 15-24.

Ludvigsen, Rolf, 1979, Fossils of Ontario, Part 1: The Trilobites, Royal Ontario Museum Publications in Life Sciences, pg. 42.

Moore, Raymond C., 1959, Treatise of Invertebrate Paleontology, Part O, pg. O334-O340.