I have a couple of chunks of slate shale around here with some trilobites I collected years ago near Delta, Utah. The biggest is about the size of a fingernail, and most are much smaller. I was very impressed to see this monster, Isotelus rex, an Ordovician trilobite that was over two feet long.

Depending on your screen resolution, this image is about a third the size of the real thing, which is 700mm long by 400mm wide.

World's biggest known trilobite, holotype of Isotelus rex n. sp. (MMMN I-2950), articulated dorsal exoskeleton showing broken posterior margin of pygidium; Late Ordovician (Richmondian), Churchill River Group, near Churchill, northern Manitoba; note scale bar.

The paper describing this beast also had some discussion of other arthropod giants. In the figure below, #4 is the giant spider crab, and #5 is a large lobster. Those are the only two species in the drawing that are still extant, unfortunately.

Scaled outline drawings of large fossil and living arthropods; note scale bar. 1, Isotelus rex n. sp. from Late Ordovician of northern Manitoba, based on holotype (MMMN I-2950; 720 mm long, with reconstructed pygidial margin). 2, Reconstruction of pterygotide eurypterid from Silurian of New York State, based on illustrations in Clarke and Ruedemann (1912), restored to approximate length (anterior margin of prosoma to posterior tip of telson) of 1.3 m. 3, Simplified reconstruction of Arthropleura armata from Carboniferous of Scotland, based on illustrations in Rolfe and Ingham (1967), restored to approximate length (anterior margin of hypothetical head to end of posterior trunk segment) of 1 m. 4, Macrocheira kaempferi, drawn to approximate carapace length of 300 mm. 5, Generalized homarid lobster, drawn to approximate length (anterior margin of carapace to posterior tip of uropods) of 500 mm.

The first animal up there is Isotelus rex, and it's comparable in size to the lobster; in fact, the largest lobster on record would have had about the same body length as I. rex. The second beast is a eurypterid, or sea scorpion—a marine predator that feasted on trilobites. And the third is an arthropleuran, a kind of millipede-like giant. Some fossil trackways from these guys suggest that they might have reached up to 8 feet in length…imagine that crawling out from under your kitchen sink.

I don't think I've had enough seafood lately, because looking at that picture has me drooling at the idea of one of those eurypterids on a big platter, drenched in butter. Too bad they've been extinct since the Permian.

Rudkin DM, Young GA, Elias RJ, Dobrzanski EP (2003)The world's biggest trilobite—Isotelus rex. New species from the upper Ordovician of Northern Manitoba, Canada. J. Paleo. 77(1):99-112.