|Among the evolutionary trends of trilobites,
gigantism yielded some spectacularly large species, expressed maximally
in the orders Asaphida, Redlichiida, and Lichida. Below, depicted in scale
alongside a familiar object, are some of the largest recorded specimens:
Isotelus rex, the largest known species of trilobite, at nearly
three quarters of a meter in length (720 mm), was found in Canada recently
in a nearly complete state, and can be seen at the Manitoba Museum in Winnipeg.
The second and third largest species Uralichas hispanicus (from Spain) and Terataspis grandis (from New York), in contrast, have not been found at full size as complete specimens, but large disarticulated parts result in reconstructions of the sizes indicated below. However, many large specimens of Paradoxides (Acadoparadoxides) briareus (from Morocco) at or near 450 mm have been found, and the large asaphoid Isotelus brachycephalus has likewise been found as a complete specimen over 300 mm in length. The total length of Isotelus rex is not augmented by long spines, as in Uralichas and Terataspis.
Comparison Chart of the Largest Trilobites
Isotelus rex is a moderately effaced asaphid trilobite lacking
terminal spines or prolongations, and the holotype specimen was found in
a carbonate unit showing little evidence of distortion or compaction. All
dorsal sclerites of the holotype are closely articulated, suggesting this
is not an exuvium. At about 720 mm long, 400 mm in maximum width (across
the cephalon), and 70 mm in height (at the posterior midpoint of the cephalon),
it is the largest complete trilobite specimen ever found. Large representatives of Isotelus
occur elsewhere in the Late Ordovician succession of
|Examples of large non-asaphide
trilobites include redlichiide Cambropallas (Holmidae, 230 mm) and
Acadoparadoxides (Paradoxididae, 390 mm), out of the Cambrian
of Morocco (Geyer, 1993). Many large specimens emerge each year out of Morocco,
but they are often at least partially restored. However, some very large
complete specimens have been prepared.
||Terataspis grandis (model)
|Persistent reports of large
lichid trilobites are based on fragmentary remains or large disarticulated
sclerites preserved in shaly, compacted facies (e.g., Reimann, 1942; Rabano,
1989; Whittington in Kaesler, 1997). The widely cited maximum length
of 660 mm for the Ordovician Uralichas hispanicus from the
G. 1993. The giant Cambrian trilobites of
M. C. 1989. Large Isotelus found:
R. L. (ED.). 1997. Treatise on Invertebrate Paleontology, Pt.
O, Arthropoda 1, Trilobita, Revised, Volume 1. Geological
Rudkin, D.M., G.A. Young, R.J. Elias, & E.P. Dobrzanski.
2003. The world’s biggest trilobite—Isotelus rex new species from
the Upper Ordovician of Northern Manitoba,