The Carboniferous period (roughly 360 to 300 million years ago) was home to Arthropleura, the largest terrestrial invertebrate that ever walked the Earth. This massive, extinct arthropod was a relative of modern-day millipedes, yet it dwarfed them considerably. Paleontologists estimate the largest specimens reached lengths of up to 8.5 feet (2.6 meters). Its imposing size immediately raised a fundamental question: what kind of diet was necessary to sustain such a massive metabolism? For decades, the sheer scale of the animal led to speculation that it must have been a predator, but fossil evidence reveals a different story.
Defining the Creature and its Environment
Arthropleura possessed a segmented body covered in a hard, armored exoskeleton, giving it a distinctive millipede-like appearance. Its body structure featured diplosegmentation, meaning its trunk segments were fused, each bearing two pairs of legs.
The creature thrived in the tropical, equatorial regions of the Carboniferous world, a time known for its high humidity and lush, dense vegetation. This environment was characterized by the sprawling “coal forests,” though Arthropleura favored more open habitats such as coastlines and floodplains. The forest floor was rich with leaf litter, spores, and other plant debris, providing an abundance of detrital material.
Hypotheses Based on Fossil Evidence
The scientific understanding of Arthropleura’s diet has relied heavily on two primary types of fossil evidence: trace fossils and preserved body parts. Trace fossils, known as ichnofossils, are wide trackways (Diplichnites cuithensis) left behind in the ancient sediment, sometimes measuring over 20 inches (50 cm) across. The morphology of these tracks indicates a slow, lumbering locomotion, suggesting the animal was not built for the rapid pursuit associated with active predation.
Initial hypotheses often leaned toward a carnivorous diet, partly due to its size and the scarcity of preserved head fossils. Rare findings of plant material near the gut region were later dismissed as inconclusive, likely being plant debris that fossilized adjacent to the animal’s shed exoskeleton (a taphonomic artifact).
The most definitive evidence came from micro-computed tomography analysis of well-preserved juvenile head fossils found in France. This breakthrough revealed the structure of its feeding apparatus: a small mandible tucked beneath the head. Crucially, the head lacked the venomous fangs (forcipules) and the robust, tearing mouthparts characteristic of predatory arthropods like centipedes.
Resolving the Dietary Question
The analysis of the physical evidence has largely settled the question, leading to a strong scientific consensus that Arthropleura was not the terrifying predator often depicted in popular culture. The creature is now widely classified as a detritivore or herbivore, primarily consuming decaying plant matter, spores, and the nutritious seeds and nuts found in the deep leaf litter. This detritivorous diet aligns with the habits of most modern millipedes, which play a similar role in recycling plant material on the forest floor.
The small size and structure of the mandibles were designed for processing soft material, such as grinding down vegetation, rather than tearing flesh or crushing bone. Its slow-moving nature, inferred from the trackways, further supports this lifestyle, as agility is not necessary for foraging on stationary detritus.
By feeding on the vast amounts of decaying vegetation, Arthropleura served as a primary recycler in its ecosystem, earning it the nickname “the cow of the Carboniferous.” The lack of specialized predatory structures and the evidence of its slow movement moved the scientific community away from the carnivorous hypothesis. Arthropleura’s true role was that of a giant, armored scavenger, consuming the organic waste that defined its lush, ancient world.