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Giant Arthropods Were Not Just a Carboniferous Phenomenon
Fossil of the griffinfly Meganeura monyi (Gaëlle Doitteau & Sandra Daillie, CC BY 4.0)
How often have you heard this?: during the Carboniferous Period (358.86-298.9 MYA), oxygen levels in the atmosphere were so high that it allowed insects and other arthropods to grow to large sizes. This is because of the way arthropods breathe, using a system of tubes called tracheoles that run through their body, where gas is exchanged through pores called spiracles that run across the sides of their abdomen. This gas exchange becomes more limited at larger sizes, as it is harder for oxygen to permeate into the body... at least that is the case today, with lower oxygen levels than the Carboniferous. As such, the earlier conditions enabled land arthropods to grow to their biggest sizes ever! Dragonflies the size of hawks! Millipedes as long as cars! Enormous killer scorpions!
I'm sure you can judge by the way I'm framing things that you're going to be in for some serious debunking~
A Brief History of Oxygen & CO2 Levels
Oxygen levels across the Phanerozoic Eon; the lighter blue highlights ranges from different models, while the dark blue line is the composite mean (Snelling, et al. 2026)
The amount of oxygen in the atmosphere is correlated with other gases, like carbon dioxide. As a general rule, the more oxygen in the atmosphere, the cooler the average global climate becomes, while more carbon dioxide leads to a warmer average global climate. Over the course of the Earth's history, these two gases have fluctuated with each other in response to events on the Earth's surface. Geologists can learn about past oxygen vs. CO2 levels through studies of isotopes in the shells of marine plankton in sediment cores, ice cores, and from other sources, reading the proportions of the isotopes like a "palaeothermometer". These findings are compared between sources to give estimates of global climate in the past (Benton, 2023).
In Precambrian times (prior to 538.8 MYA), CO2 levels were extremely high and there was no free oxygen in the atmosphere. By 2.45 BYA, photosynthesizing bacteria evolved which could convert sunlight and CO2 into food, which produced free oxygen as a byproduct. From that point, there was a gradual rise in oxygen levels (Holland, 2006). Earth's geology and biology was impacted in significant ways, from the evolution aerobic organisms to the creation of banded iron formations (as dissolved iron in the seas rusted and sank to the bottom) to a series of major ice ages around 717-635 MYA. The shifting of tectonic plates and mountain weathering, along with photosynthesis and oceanic sinks, captures CO2 and keeps it out of the air, while emissions from volcanic eruptions, decay and/or burning of biological matter, and the natural cycle of carbon release CO2 into the atmosphere. It is this play between the two that has shaped the subsequent Phanerozoic Eon: the last 538.8 million years of Earth history (Benton, 2023).
The rise of various algae and seaweeds in the oceans & freshwater habitats and the evolution of land plants has been a major contributing factor to the state of CO2 vs oxygen in their climatic roles (Berner, 1998). So much so that it is argued a series of mass extinctions at the end of the Devonian period (372.15-358.86 MYA) were a result of access nutrient runoff from plant life on land (Benton, 2023). By the Carboniferous and Permian periods (358.86-251.9 MYA), the supercontinent of Pangaea had formed, which caused an increase in weathering and erosion that additionally captured CO2. This drop in CO2 and rise in oxygen from photosynthesis led to such a cooling in the Earth's climate that another ice age occurred in the southern hemisphere by the late Carboniferous.
Extreme volcanism events at the end of the Permian (251.9 MYA) and Triassic (201.4 MYA) periods were to severe that they also seem to have caused mass extinction events (Benton, 2023), but they also released so much CO2 that the following Jurassic Period (201.4-143.1 MYA) remained an age of low oxygen levels and very humid-to-arid climates (Dai, et al. 2022). By this point Pangaea had already broken up, but there was still much continental rifting occurring (e.g. the breakup of the supercontinent Gondwana), so oxygen levels rose again in the Cretaceous (143.1-66 MYA) and the Earth - while still quite warm - could support temperate conditions and sea ice at higher latitudes (Davies, et al. 2009).
The bolide-impact at the K-Pg boundary (66 MYA) unleased a brief but tremendous burst of CO2 in to the atmosphere, and high CO2 levels carried into the Paleogene with a peak during the Paleocene-Eocene Thermal Maximum ~55 MYA (Bowen, et al. 2014). From that point, tectonic activity contributed to an overall decline in global CO2 and rise in oxygen through to the Holocene epoch (beginning 11.7 KYA), where burning of fossil fuels and other anthropogenic activities are the primary source of the rise in CO2 levels over the past 300 years.
Today, oxygen levels in the atmosphere are at 20.95% of dry air atmospheric composition (209.5 ppm), while CO2 is 0.043% (~428 ppm as of August 2026), a level not seen since the late Miocene, 14 MYA.
Carbon dioxide levels in the atmosphere over the last 420 million years, including various models for future levels due to anthropogenic climate change (Foster, et al. 2017)
With this history of oxygen levels in your back-pocket, let's examine the evidence for evolution of large size in arthropods on land to see if there is a correlation between high oxygen levels and gigantism.
Giant Bugs Through Time
Hercules beetle resting on arm (Novita Estiti, CC BY 2.0)
The best way to have a frame of reference for what constitutes a "giant terrestrial arthropod" is to survey what the largest species are today, by weight, length, and windspan.
The heaviest recorded insect is Deinacrida heteracantha, the Little Barrier giant wētā of North Island, New Zealand at 2.5 oz (71 g), followed by various several species of beetles within the 1.4-1.7 oz (40-50 g) range, including the titan beetle Titanus giganteus and actaeon beetle Megasoma actaeon; the larvae (grubs) of actaeon beetles and Goliath beetles (Goliathus spp.) weigh more than all of these, maxing 8.04 oz (228 g) and 2.5-3.5 oz (70-100 g), respectively (Guinness World Records, 2011; 2017). The wētās of New Zealand are especially known for being a lineage of generally sizable insects, but it is a misnomer to consider them as ecological equivalents of rodents (Griffin, et al. 2011).
The longest recorded insect has been claimed to be a species of stick insect Phryganistria chinensis, known in the wild from Guangxi Zhuang Autonomous Region, China, but this particular individual was bred at Insect Museum of West China and spanned 25.19 in (640 mm) with its legs outstretched. But given that technicality, perhaps the title should go to another stick insect, Phobaeticus chani, whose body length alone is 14 in (355 mm), while P. chinensis' body only measures 13.78 (350 mm) (GWR, 2008).
A number of living insects sport impressive wingspans: including the dragonfly Megaloprepus caeruleata, 7.5 in (19.1 cm); the cockroach Megaloblatta longipennis, 8 in (20 cm); the giant water bug Lethocerus maximus, 8.5 in (21.6 cm); the emperor bush-cricket, 10.25 in (27.4 cm); the Queen Alexandra's birdwing butterfly Ornithoptera alexandrae, 11 in (28 cm); and various moths span the 11-13 inch (28-30 cm) range, like the Atla moth Attacus atlas & owlet moth Thysania agrippina (GWR, 2001; Carwardine, 2007).
Other terrestrial arthropod groups, like arachnids and myriapods, are sizable organisms. The largest spiders - various tarantulas (Theraphosidae) and the huntsman spider Heteropoda maxima - have 10-12 inch (25.4-30.48 cm) legspans; the longest scorpion Heterometrus swammerdami spanned 11.2 in (29.4 cm); the Africa giant black millipede can reach 15.25 in (38.7 cm) long, while the Amazonian giant centipede reaches 13.25 in (35 cm) long (Carwardine, 2007).
So it seems that insects and other land arthropods today can reach impressive sizes, comparable with small mammals and birds! How does this compare to prehistoric times?
Gigatitan vulgaris, a titanopteran of the Triassic Period (Nuryahyaya, CC BY-SA 4.0)
A typically large-bodied group of insects were the titanopterans, which evolved in the Carboniferous Period but survived by the late Triassic. The largest species Gigatitan vulgaris is estimated to have a wingspan of 15.74 in (40 cm) and would've been a predator of smaller animals which they dispatched with mantis-like raptorial arms (Park, et al. 2022). One specimen of Clathrotitan preserves a single forewing which was 5.4 in (13.8 cm) in length (McKeown, 1937). Within a similarly significant range was an extinct group of lacewings called kalligrammatids which lived during the Jurassic & Cretaceous periods; the genus Makarkinia was found with partial wing which may have spanned 3.93-6.3 in (10-16 cm) long and preserves eye-spots (Bechly & Makarkin, 2016).
Bojophlebia, an early lineage of flying insects of the Carboniferous, may have had a 17.7 in (45 cm) wingspan (Kukalová-Peck, 1985). Another early group which evolved in the middle Carboniferous but died out at the end of the Permian were the palaeodictyopterids. These are considered some of the most important insect herbivores of their time. One genus, Mazothairos, reached a wingspan of 22 in (55.88 cm). Such a size is rivaled by the griffinflies or Meganisoptera/Protodonata, which also spanned the Carboniferous to the Permian. Meganeura is probably the most familiar genus, having appeared in popular books and documentaries as a staple of the Carboniferous (and often misidentified as a dragonfly), but the Permian species Meganeuropsis permiana is larger, with a 28 in (71 cm) wingspan and an estimated 3.5 oz (100 g) weight. In fact, it is the largest insect which ever lived by all measures (Grimaldi & Engel, 2005; Snelling, et al. 2026).
Collection of largest flying insects through time, to scale (Zyoute, CC BY-SA 4.0, click image link for species identifications)
Even these record-breakers are dwarfed by some of the non-insect arthropods.
True land-living scorpions (Scorpiones) evolved in the Silurian Period, and by the Devonian & Carboniferous they were already reaching sizes of 11.81-27.55 in (30-70 cm) in length; Pulmonoscorpius kirktonensis was at the large-end of that range (Jeram, 1998). Then there's Arthropleura, which has often been classified as a millipede but has recently been found to lay outside that clade. Fossils of these myriapods span the Carboniferous and into the early Permian by 290 MYA, and hold the record for the largest terrestrial arthropod that ever lived, measuring 8 feet 7.5 in (2.63 m) long and 110.23 lbs (~50 kg) in weight. Even particularly small specimens were only less than a meter (Davies, et al. 2021; Lhéritier, et al. 2024).
In examining so many examples of species and genera, a curious observation emerges. Besides a few outliers like the aforementioned Arthropleura, the size of these giant arthropods all fall within the same dimensions: some of the recorded wingspans, for example, are comparable between species living today and back in the Carboniferous or Jurassic Periods (see the image above). Which doesn't make sense if high oxygen levels are a supposed to be a prerequisite for large size in land arthropods...
Is There A Link?
Trackway attributed to Arthropleura, with human for scale (Keith Burns, CC BY-SA 2.0)
Clearly that does not appear to be the case. Some of these large arthropods - like kalligrammatids - existed during periods of the Earth when oxygen levels were low in the atmosphere, while others - like titanopterans - existed across oxygen-rich and oxygen-poor conditions. Some of these lineages evolved their giant sizes during periods when oxygen levels were much lower or not much higher than today: Arthropleura, for example, is known at the earliest by 344 MYA, when estimated oxygen composition was ~23%, compared to today's 20.95% (Davies, et al. 2021). Other evidence shows an actual decrease in average insect sizes during periods of high oxygen, like the Cretaceous (Clapham & Karr, 2012). So the connection between oxygen & large body size can be severed. Such ideas of "high oxygen = big animals" are now often a misconception that gets shared around some circles, but by this point I'm sure I've made it clear to my readers that this is wrong.
Where do we stand on the model that gas exchange in the respiratory system was a factor in limiting size in land arthropods? This idea stems from previous work during the 1960s-70s onwards, which also argued that flight capabilities in insects could reach a certain limit as the body adapted to become larger (Dudley, 1998).
In a very recent paper by Edward P. Snelling and colleagues, this was tested through a close examination of living insect tracheoles - the tubes within the respiratory system where gas exchange occurs - in comparison to the griffinfly Meganeuropsis. What was found that, when scaled up, the space within the tracheoles only increases a marginally small amount as the body-size increases - on the order of "1.8-fold over a 10,000-fold body mass range", even given the ability for such organs to evolve that way. This same study also tested the "flight-limitation" hypothesis and found it wanting as well: the increase in tracheoles near the wing muscles had no compromising effect on the ability for giant insects to fly (Snelling, et al. 2026).
A takeaway from that paper, which the authors end their discussion on, is that just because the anatomy of arthropods could allow them to become giants, doesn't mean that they necessarily will do so. And this is the crux of my blog post.
It seems that the best explanation for the enormous sizes of terrestrial arthropods past and present is that there were opportunities within their ecosystem that they took advantage of.
In the absence of vertebrate animals on land or in the air during the Silurian and Devonian, there were more than enough niches available for the various arthropods who colonized the land independently. The rising biodiversity of vascular plants encouraged a similar biodiversity in arthropods, and because stegocephalians (e.g. Ichthyostega) were still primarily aquatic animals, there were no sizable predators to effect the new arrivals in any ecological way. Without big vertebrate predators or herbivores to occupy their niches, insects, arachnids, and myriapods were at the mercy of each other, and they could respond by becoming large. This remained the case well into the late Carboniferous (or Pennsylvanian), when the great coal forests formed. Flying insects had the skies to themselves, and some grew enormous as a result.
While oxygen levels do not trend with land-arthropod size, what does trend is the rise and spread of land vertebrates, among other environmental changes. Arthropleura was extinct by the end Permian when the humid forested environments they lived in dried out and terrestrial tetrapods moved in (Davies, et al. 2021). Griffinflies and palaeodictyopterids of large size persisted through the Permian but died out during the End-Permian Mass Extinction Event; in fact the only time that insects suffered during a mass extinction (Ponomarenko, 2016). Giant insects returned in the Triassic - as evidenced by groups like the surviving titanopterans - but soon the pterosaurs would become the first vertebrates to evolve powered flight. As well, by the early Cretaceous, stem birds like enantiornithes would also take to the air, and we see a subsequent decline in insect size following this. But this was not the end of giant insects and their kin, because many groups like beetles and cockroaches were more terrestrial in their habits (Clapham & Karr, 2012). Thus, in the present day, the distribution of giant arthropods reflects their relationship with other organisms. The atlas moth, for example, only lives for one or two weeks as an adult, not feeding but focused on finding a mate, and mimics a writhing snake when attacked by predators (Pavid). Such adaptations help them survive in a world of similarly-sized flying organisms.
Having set the record straight, perhaps it would be good to ask, why is a land arthropod being big so important to us? It has gotten to a point where the prehistory of insects in media is often reduced to just the giants of the Carboniferous Period. It is clear, among the million-or-so arthropods alive today (not to mention the countless multitudes in the past), that very few species have attained the size of small mammals or birds. Overall, being an insect or a scorpion means being small, and it is this strategy that has certainly ensured their survival. Perhaps, instead of focusing solely on the giants, we should direct our curiosity towards the evolutionary success of being tiny.
Postscript: Because the topic of this article is specifically debunking the idea that giant arthropods only existed in the Carboniferous, I didn't address the wider discussion about the overall limits on size for arthropods, like just how big they could get in a speculative context. Arthropleura and the aquatic eurypterids represent the known upper-limits for size, but whether even bigger forms could evolve is unclear. Reading through articles featuring physiologist Jon Harrison, it seems the most research done on this front is clarifying that larger exoskeletons on land arthropods are not thicker than smaller ones, so perhaps that's an important thing to note when thinking of speculative biology.
Book References
Michael J. Benton. Extinctions: How Life Survives, Adapts, & Evolves (Thames & Hudson, 2023)
Mark Carwardine. Animal Records (Natural History Museum, 2007)
David Grimaldi & Michael S. Engel. Evolution of the Insects (Cambridge University Press, 2005)
Paper & Article Citations
Günter Bechly & Vladimir N. Makarkin, 2016. A new gigantic lacewing species (Insecta: Neuroptera) from the Lower Cretaceous of Brazil confirms the occurrence of Kalligrammatidae in the Americas (Cretaceous Research)
Robert A. Berner, 1998. The carbon cycle and carbon dioxide over Phanerozoic time: the role of land plants (The Royal Society Publishing, Philosophical Transactions)
Gabriel J. Bowen, et al. 2014. Two massive, rapid releases of carbon during the onset of the Palaeocene–Eocene thermal maximum (Nature Geoscience)
Matthew E. Clapham & Jered A. Karr, 2012. Environmental and biotic controls on the evolutionary history of insect body size (PNAS)
Xianduo Dai, et al. 2022. Middle Triassic to Late Jurassic climate change on the northern margin of the South China Plate: Insights from chemical weathering indices and clay mineralogy (Palaeogeography, Palaeoclimatology, Palaeoecology)
Andrew Davies, et al. 2009. Late Cretaceous seasonal ocean variability from the Arctic (Nature)
Neil S. Davies, et al. 2021. The largest arthropod in Earth history: insights from newly discovered Arthropleura remains (Serpukhovian Stainmore Formation, Northumberland, England) (Journal of the Geological Society)
Robert Dudley, 1998. Atmospheric Oxygen, Giant Paleozoic Insects and the Evolution of Aerial Locomotor Performance (Journal of Experimental Biology)
Gavin L. Foster, et al. 2017. Future climate forcing potentially without precedent in the last 420 million years (Nature Communications)
Melissa J. Griffin, et al. 2011. Exploring the concept of niche convergence in a land without rodents: the case of weta as small mammals (New Zealand Journal of Ecology)
Guinness World Records, 2017. Heaviest insect larva
Guinness World Records, 2011. Heaviest insect
Guinness World Records, 2008. Longest insect body length
Guinness World Records, 2001. Largest butterfly
Guinness World Records, 2001. Largest cockroach
Guinness World Records, 2001. Largest dragonfly
Guinness World Records, N/A. Largest moth
Heinrich D. Holland, 2006. The oxygenation of the atmosphere and oceans (The Royal Society Publishing, Philosophical Transactions)
Andrew J. Jeram, 1998. Phylogeny, classification and evolution of Silurian and Devonian scorpions (British Arachnological Society)
Jarmila Kukalová-Peck, 1985. Ephemeroid wing venation based upon new gigantic Carboniferous mayflies and basic morphology, phylogeny, and metamorphosis of pterygote insects (Insecta, Ephemerida) (Canadian Journal of Zoology)
Mickaël Lhéritier, et al. 2024. Head anatomy and phylogenomics show the Carboniferous giant Arthropleura belonged to a millipede-centipede group (Science Advances)
Keith C. McKeown, 1937. New fossil insect wings (Protohemiptera, family Mesotitanidae) (Records of the Australian Museum)
Tae-Yoon S. Park, et al. 2022. A new titanopteran Magnatitan jongheoni n. gen. n. sp. from southwestern Korean Peninsula (Cambridge University Press)
Katie Pavid, N/A. Spotlight: the atlas moth (Natural History Museum)
A. G. Ponomarenko, 2016. Insects during the time around the Permian—Triassic crisis (Paleontological Journal)
Edward P. Snelling, et al. 2026. Oxygen supply through the tracheolar–muscle system does not constrain insect gigantism (Nature)
I'm so glad you got to experience A Single Molecule Of Dog. They are my all time favorite appointments and calling them a molecule never fails to make the owner laugh.
And yeah, the husky hair coalesces later and forms a litter of brand new husky puppies
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I may have skipped last Avian August, but this year I'm back! This list looks like it's filled to the beak with amazing, colorful birds and I look forward to drawing them all :>
last week i woke up from one of the most fucked up nightmares ive ever had with that middle panel burned into my brain. like the exact wording and the exact apartment and the exact squidward. i feel like if i didnt make it real something bad wouldve happened. anyway todays upload is spunchbob comic oc
a rough timeline of events (from memory, there may be inaccuracies, feel free to correct me if I'm wrong)
Cambridge professor of sociology Jason Arday is accused of plagiarism by another academic
the accuser is a "race realist" who believes that in a true meritocracy all university professors would be white. this is somehow not acknowledged much during the entire ordeal
this leads to several days of media coverage demanding an investigation, or that Arday is expelled from his post immediately
somehow this accusation that has no effect on the lives of 99.99% of the population becomes FRONT PAGE NEWS in all the newspapers for multiple days, above anything else happening in the UK or the world at large
following intense media scrutiny, Arday resigns his post as a Cambridge professor
today, august 14th, days after resigning, Arday is found dead at his home in Battersea
the British tabloid press remains, and will always be, fucking evil
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today i learned there are like 60+ fics in the philomena cunk tag on ao3 and i am going to have to exhibit some SERIOUS willpower to not plough through all of cunk on [fandom]
big fan of Anglish (so this is right up my alley. (or right up the opposite of my alley so to speak. (etymological horseshoe theory and all.) ) ), so I though I'd take a stab at it.
cholesterol -> fortibile
translate -> metaphor (looks like that one's taken)
describe -> dysgraph
helicopter -> rotopen
circumstance -> peristat
contradiction -> catalogue (cutting it really close)
architect -> magistructor
prescription -> prographysis
i should have done more mixed ones
after doing all that can say that its about as hard as it sounds, but it was really fun. :D
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