The giant extinct invertebrate Arthropleura resembled some modern millipedes, but could grow to be more than one-and-a-half feet wide, and may sometimes have been more than six feet long.
During the Pennsylvanian and earliest Permian periods (about 320 to 290 million years before present), much of present-day North America and Europe was located close to the equator and was covered by vast, richly vegetated swamps. The remains of this vegetation ultimately formed the great coal deposits that fuelled the Industrial Revolution and to this day remain a key energy resource. These ancient swamps were home to many large arthropods including early dragonfly relatives with wingspans in excess of two feet and the subject of this blog, the giant millipede Arthropleura. One species of Arthropleura (“jointed rib”) is the largest known land-dwelling invertebrate of all time.
The flattened body of Arthropleura is composed of approximately 30 jointed segments, each of which was covered by two side plates and one center plate. The ratio of pairs of legs to body segments was approximately 8:6, similar to some present-day millipedes. Typically, the body armor of Arthropleura fell apart after the death of the animal, and only individual segments or plates were preserved as fossils.
Unfortunately, nobody has yet found a complete large individual of Arthropleura. One partial body fossil from southwestern Germany has a length of 90 cm (3 ft.). A trackway ascribed to a large Arthropleura on a Pennsylvanian-age sandstone surface from Nova Scotia (Canada) comprises two parallel rows of small imprints and is 50 cm (19.7 in.) wide. It is estimated that the maker of this track was at least 1.7 m (5.6 ft.) long. Similar trackways have also been discovered in the United States and in Scotland. The size of some isolated armor segments indicates that Arthropleura adults could attain a length of at least 2 m (6.6 ft.). The only even larger arthropod was the aquatic Early Devonian “sea scorpion” Jaekelopterus, which, based on one isolated chelicera (pincer-like mouth part), reached an estimated length of 2.5 m (8.2 ft.).
As no complete fossils of large Arthropleura are known, the interpretation of their structure has been difficult. In the last few years, two German researchers–Otto Kraus, an expert on present-day millipedes, and Carsten Brauckmann, a specialist on ancient arthropods–have undertaken a detailed re-examination of the known fossils. Many older reconstructions of Arthropleura showed a large rounded “head end,” but this appears to be the first armor plate, known as the collum, and the actual head capsule was tucked under the collum, as it is in present-day millipedes. Another interesting result of the new research is the discovery that the sturdy-looking body armor is only a few millimeters thick and was not reinforced by calcium carbonate (as, for example, in crustaceans). Considering their size, adult Arthropleura would have had few if any enemies in the Pennsylvanian coal swamps and therefore no need for heavy armor.