Arthropleura

Arthropleura Temporal range: Early Viséan-Sakmarian, 346–290 Ma PreꞒ Ꞓ O S D C P T J K Pg N [1]
Fossil of A. armata at the Senckenberg Museum of Frankfurt
Juvenile specimen
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Myriapoda
Class:	Diplopoda
Subclass:	†Arthropleuridea
Order:	†Arthropleurida Waterlot, 1933
Family:	†Arthropleuridae Zittel, 1885
Genus:	†Arthropleura Meyer, 1854
Species[2]
A. armata Meyer, 1854 A. cristata Richardson, 1959 A. fayoli Boule, 1893 A. maillieuxi Pruvost, 1930 A. mammata Salter, 1863
Synonyms
A. affinis Goldenberg, 1873 A. britannica Andrée, 1910 A. moyseyi Calman, 1914 A. zeilleri Boule, 1893

Arthropleura (Greek for 'jointed ribs') is a genus of massive millipedes that lived in what is now North America and Europe around 345 to 290 million years ago,^[2][3] from the Viséan stage of the lower Carboniferous Period to the Sakmarian stage of the lower Permian Period.^[2][4] The species of the genus are the largest known land invertebrates of all time, and would have had few, if any, predators.

A. armata grew to be up to 2.5 metres (8 ft 2 in) long.^[5] Tracks from Arthropleura up to 50 centimetres (20 in) wide have been found at Joggins, Nova Scotia.^[6] In 2021 a fossil, probably a shed exoskeleton (exuviae) of an Arthropleura, was reported with an estimated width of 55 centimetres (22 in), length of 1.9 metres (6 ft 3 in) to 2.63 metres (8 ft 8 in) and body mass of 50 kg (110 lb).^[3][2] It is one of the largest arthropods ever known, as large as the eurypterid Jaekelopterus rhenaniae, whose length is estimated at 2.33–2.59 metres (7 ft 8 in – 8 ft 6 in).^[7] Arthropleura was able to grow larger than modern arthropods, partly because of the greater partial pressure of oxygen in Earth's atmosphere during the lower Carboniferous and partly because of the lack of large terrestrial vertebrate predators.^[8] However, large-sized specimens of Arthropleura are described from the Serpukhovian stage, during which the oxygen pressure was only a bit higher than modern Earth at around 23 percent, suggesting that high oxygen pressure might not have been a primary reason for its gigantism.^[2]

• 
  Diagrammatic reconstruction of A. armata
• 
  Reconstruction of juvenile specimen
• 
  Detail of mouthpart in juvenile specimen
• 
  Anterior morphology of A. armata
• 
  Modern millipede anatomy for comparison
• 
  Leg and associated structures

The 2024 study reported the complete head and trunk of a juvenile specimen of Arthropleura sp. (MNHN.F.SOT002123) from Kasimovian (~305 Ma) Montceau-les-Mines lagerstätte, which revealed multiple previously unknown features.^[1] Arthropleura had large, flattened ventral sclerites and a pair of antennae with seven antennal articles at the front of its head.^[1] The trunk anatomy of Arthropleura is characterized by a series of well-developed 28-32 tergites (dorsal exoskeleton)^[2] having three lobes like a trilobite, with dorsal surfaces covered by many tubercles.^[1] Juvenile specimens have fewer numbers of tergites at 20-24, suggesting its anamorphic development, number of tergites increase as it shed.^[1]

The antennae and mandibles identify it as a millipede, but the presence of a pair of legs on its collum, which is absent in present day species, reveals it as the sister group of the millipede crown group. Other parts of the head show traits found in centipedes, like a pair of leg-like second maxillae, but there is no forcipules. Unlike any living myriapods, they had stalked compound eyes, which despite also being known from the extinct myriapod group Euthycarcinoidea, appears to be a derived trait evolved independently from the euthycarcinoids.^[1][9] Around each walking leg pair, there were three pairs of ventral plates located alongside the median sternite, namely K-, B- and rosette plates, and either the B- or K-plates were thought to be respiratory organs.^[10][11][12] The body terminated with a trapezoidal telson.^[11]

All fossils of Arthropleura available are believed to be exuviae (molting shells) instead of carcasses.^[12] The good preservation of its thin exuviae, buttressing plates around the leg base, and evidence of 3 cm deep trackway fossils (namely the ichnotaxon Diplichnites cuithensis^[13][14]) altogether suggests that they had a sturdy exoskeleton and roamed the land.^[2] Arthropleura was once thought to have lived mainly in coal forests.^[11] However, it probably lived a forest-independent life, as fossils of the trackway were found in more open areas and fossils were found even after the Carboniferous rainforest collapse.^[2]

There is no solid evidence for the diet of Arthropleura, as the fossils that were once considered coprolites, including lycopod fragments and pteridophyte spores,^[15] are later considered to be merely coexistence of plant fossils and exuvia remains.^[10] Nonetheless, the interpretation of a herbivorous diet is still accepted, and it is estimated that Arthropleura ate not only spores but also sporophylls and seeds, based on its enormous size that possibly required ample nutrition.^[12]

When it comes to the Arthropleura's diet, it was also found that Arthropleura had a well-preserved gut region which was packed with plant remains, mainly lycopod fragments such as Lepidodendron. This information helped in concluding that Arthropleura was not only herbivorous but also possibly omnivorous.^[16] The complete head and trunk anatomy of Arthropleura suggested that it was likely a detritivore: Arthropleura did not have the forcipules or post-mandibular cephalic limbs of predatory centipedes and arachnids, and its trackways, diplosegmentation and short, millipede-like locomotory limbs imply that it was a slow-moving animal.^[1]

Through careful observation of the Arthropleura's fossils, the likeliest modes of behaviour of the Arthropleura were determined. The Arthropleura would invade exposed environments and retreat to levee forests during wet seasons. Thin cuticles on the arthropleurids indicate that these creatures were exposed to a terrestrial habit.^[16]

Previously, the extinction of Arthropleura was attributed to the decrease of coal forest.^[17] However, many fossils have been discovered even after the Carboniferous rainforest collapse, and it is estimated that Arthropleura also throve outside a forest environment. A more recent proposal is that the diversification of tetrapods and the desiccation of the equatorial regions caused it to become extinct.^[2][12]

• Diplichnites
• Largest prehistoric animals

Wikimedia Commons has media related to Arthropleura.

• Larson, Christina (2024-10-09). "Scientists recreate the head of this ancient 9-foot-long bug". Phys.org. Retrieved 2024-10-10.
• Lhéritier, Mickaël; Edgecombe, Gregory D.; Garwood, Russell J.; Buisson, Adrien; et al. (2024-10-09). "Head anatomy and phylogenomics show the Carboniferous giant Arthropleura belonged to a millipede-centipede group". Science Advances. 10 (41). American Association for the Advancement of Science. doi:10.1126/sciadv.adp6362.
• Lyall I. Anderson; Jason A. Dunlop; Carl A. Horrocks; Heather M. Winkelmann; R. M. C. Eagar (1998). "Exceptionally preserved fossils from Bickershaw, Lancashire UK (Upper Carboniferous, Westphalian A (Langsettian))". Geological Journal. 32 (3): 197–210. doi:10.1002/(sici)1099-1034(199709)32:3<197::aid-gj739>3.0.co;2-6.