Arthropleura, genus of exceptionally large extinct millipede-like arthropods that thrived from the Viséan Age of the Carboniferous Period to the Asselian Age of the Permian Period (346.7 million to 293.52 million years ago) and are known from their fossilized exoskeletons. Members of Arthropleura are the largest known arthropods in Earth’s history, with an estimated length exceeding 2.6 meters (8.5 feet)—roughly equivalent to the width of a school bus or the length of an adult female American alligator in the class Diplopoda, and they were similar to modern millipedes in appearance. The exoskeleton of Arthropleura was composed of some 30 jointed segments, each of which was covered with three plates—one on each side and one spanning the center. It had approximately eight pairs of jointed legs for every six body segments and between 32 and 64 legs in total. The word arthropleura is Greek for “rib joint.”

　　Fossil discoveries Fossilized remains of Arthropleura have been known to science for some 170 years. Specimens have been extracted from 60 different sites within 13 countries, most findings occurring in North America, continental Europe, and the U.K. During the early Carboniferous Period and the early Permian Period these locations would have been in the tropics. The majority of specimens that have been found are made up of scattered portions of molted legs and exoskeletons belonging to adults, along with a handful of nearly complete juvenile bodies. Arthropleura’s wide fossilized trackways—which measure more than 50 cm (20 inches) across—have been found in sedimentary rocks in Canada, France, Scotland, and the U.S.

　　The largest Arthropleura fossil on record, and one of the most complete, was discovered at Howick Bay in Northumberland, England, in 2018. Paleontologists suggest that this specimen, which is made up of 12–14 segments that collectively measure 76 cm (30 inches) long by 36 cm (14 inches) wide and weigh more than 80 kg (176 pounds), represents only part of the upper half of the animal’s exoskeleton.

　　Diet and habitat Scientists note that Arthropleura was, like modern millipedes, a herbivore that consumed woody plant debris, leaf litter, dead plant matter, and nuts and seeds scattered on the forest floor. Evidence of such plant material may appear in a fossil found in Scotland. An initial analysis of what was thought to be the specimen’s gut showed pteridophyte spores (which are produced by ferns) and lycophytes (club mosses), and what were taken to be its fossilized feces (which are called coprolites) suggested that Arthropleura most likely ate vegetation. Upon a second look, however, such plant material was thought to have been accidentally associated with the animal’s shed exoskeleton, and thus the evidence was deemed to be inconclusive. In addition, some researchers contend that Arthropleura could not have grown so large without consuming heartier fare, such as other invertebrates and possibly amphibians. The first Arthropleura fossils to be found were embedded in the shale roof layer above coal seams, which led paleontologists to posit that the millipede resided in humid Carboniferous coal swamps. The Howick Bay specimen, however, was discovered in layers of river sediment, which suggests that Arthropleura also lived in open woodlands with rivers and streams near coastal beaches.

　　Earth’s atmosphere during the Carboniferous Period was rich in atmospheric oxygen, with levels roughly one-third greater than today’s. This gave Arthropleura and other arthropods more energy to fuel their tremendous growth (see also evolution of the atmosphere). However, the Howick Bay specimen, which is the oldest Arthropleura fossil on record, was discovered in rocks dating to a time when atmospheric oxygen levels were only slightly higher than the present day’s, suggesting that other factors were involved in helping Arthropleura achieve its massive size.