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.
Reconstruction of the giant millipede Arthropleura from the Pennsylvanian and earliest Permian of North America and Europe. The head capsule (marked by an asterisk) is shown tucked under the first plate (collum), as in present-day millipedes. Courtesy and copyright of Dr. Elke Gröning (Technische Universität Clausthal-Zellerfeld).
By Hans-Dieter Sues
As a kid I enjoyed watching those old sci-fi movies (like the 1954 classic Them!) where giant ants and spiders, created by fallout from atomic explosions, laid waste to cities and towns. While most of us are not overly fond of “creepy-crawlies” invading our homes, many people love the frisson of learning that there were once really big animals like these.
Fortunately, the laws of nature impose tight limits on the maximum size that arthropods can attain. The arthropod body is completely encased in an exoskeleton. The legs are made up of jointed tubes that contain the muscles necessary for their movement. As the animal’s size increases, the walls of these leg tubes rapidly increase in thickness, and operating the limbs soon would be impossible if the animal grew too big.
Another constraint faced by large arthropods is breathing. Small forms such as insects can breathe through tubes (tracheae) that open on the outside of the body. The body then absorbs the oxygen into the haemolymph (blood) through specialized soft membranes. The surface area of a body increases in proportion to the square of its dimensions, but the body’s volume increases as the cube. Thus if the size of an animal doubles, its body volume (which needs to be supplied by oxygen) increases eightfold. This geometrical relationship significantly constrains size increase. Thus, no monster bugs will ever menace humanity!
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.
How did Arthropleura breathe? There are no traces of a tracheal system, and gas exchange through the body surface would have been insufficient for the oxygen needs of such a large animal. There are paired, pocket-like features on the underside of each body segment, and these pockets have a peculiar granulated surface. It has been suggested that a thin layer of air covered these surfaces and oxygen could be absorbed by diffusion through them. Geochemical modeling by Robert Berner (Yale University) suggests that the oxygen content of Earth’s atmosphere was much higher during Pennsylvanian times (30 to 35%) than today (21% free oxygen), so large arthropods could have breathed more easily than they would have today.
What did Arthropleura eat? An earlier study reported possible gut contents in a specimen from Scotland. These contents were composed of debris from the tree-like club mosses (lycophytes) that formed a major component of the coal swamp vegetation. Restudy of the fossil in question by Kraus, however, indicates that this is an accidental association of a shed skin of an Arthropleura with some plant fragments. Kraus believes that Arthropleura indeed fed on plants but thinks that the enormous quantities of spores shed by swamp plants including lycophytes as well as early growth stages of these plants would have been rich sources of food. Most present-day millipedes feed on dead plant matter, and it is reasonable to assume that Arthropleura did likewise.
The extinction of Arthropleura is probably related to the climatic changes during the Permian Period when increasingly drier conditions led to the disappearance of the coal swamps. The work by Kraus and Brauckmann and other researchers indicates that Arthropleura may be most closely related to the present-day Penicillata, a group including the tiny bristle millipedes (Polyxenus), which are widespread in drier habitats in eastern North America. What a relief that we no longer have to worry about tripping over six-foot millipedes on our hikes through the forest!
Hans-Dieter (Hans) Sues is a vertebrate paleontologist based at the National Museum of Natural History in Washington, D.C. He is interested in the evolutionary history and paleobiology of vertebrates, especially dinosaurs and their relatives, and the history of ecosystems through time. A former member of the National Geographic Committee for Research and Exploration, Hans has traveled widely in his quest for fossils and loves to share his passion for ancient life through lectures, writings, and blogging.
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