Isotelus rex

Isotelus is a genus of asaphid trilobites from the middle and upper Ordovician period, fairly common in the Northeastern United States, northwest Manitoba, southwestern Quebec and southeastern Ontario. One species, Isotelus rex, is currently the world's largest trilobite ever found as a complete fossil.

A specimen of Isotelus rex, from Churchill, Manitoba, is the largest complete trilobite ever found. Discovered by Dave Rudkin (Royal Ontario Museum), Robert Elias (University of Manitoba), Graham Young (Manitoba Museum) and Edward Dobrzanske (Manitoba Museum) in 1999, it measures 720 millimetres (28 in) in length, 400 millimetres (16 in) in maximum width (across the cephalon) and 70 millimetres (3 in) in maximum height (at the posterior midpoint of the cephalon).

A number of species have been described: • I. aktchokensis Weber, 1948 • I. bradleyi Amati, 2014 • I. copenhagenensis Ross, Jr. & Shaw 1972 • I. frognoensis Owen, 1981 • I. gigas Dekay, 1824 • I. harrisi Raymond, 1905 (synonym I. platymarginatus Raymond, 1910) • I. iowensis Owen, 1852 • I. kimmswickensis Bradley, 1930 • I. maximus Locke, 1838 • I. megistos Locke, 1842

• Very similar to Homotelus • "Double" posterior • Head and tail shields are semi-circular and very similar in shape (isopygus) • Thorax has 8 segments and a slightly raised middle lobe between the outer pleurae