The Enigmatic World of Carboniferous Giants

Imagine a landscape dominated by towering ferns and clubmosses, where the air hummed with the wings of dragonfly-like creatures spanning up to 70 centimeters. During the late Carboniferous period, roughly 300 million years ago, Earth hosted some of the largest insects ever known, including griffinflies of the order Meganisoptera, such as Meganeura and Meganeuropsis permiana. These aerial predators, with wingspans rivaling those of modern hawks, ruled swampy forests teeming with life.

The Carboniferous (358 to 299 million years ago) marked a pivotal era in Earth's history. Vast coal-forming swamps covered continents, trapping carbon and boosting atmospheric oxygen levels to peaks of 30-35%, compared to today's 21%. This hyperoxic environment coincided with arthropod gigantism not just in flying insects but also millipedes like Arthropleura, which stretched over 2 meters long. For decades, scientists linked this phenomenon directly to oxygen abundance, arguing it enabled passive diffusion through insects' unique tracheal systems to fuel massive bodies.

These giants weren't anomalies; fossils from sites like Kansas and Europe reveal dozens of species exceeding modern limits. A single Meganeuropsis specimen boasted a 71 cm wingspan and weighed around 100 grams—ten times heavier than the largest living dragonfly. Yet, as oxygen dipped in the Permian, gigantism faded. This temporal alignment fueled the dominant narrative, embedded in textbooks and popular media.

Unpacking the Long-Standing Oxygen Hypothesis

Insects breathe differently from vertebrates. Air enters through spiracles (valved openings) and branches into a tracheal network, ending in fluid-filled tracheoles that hug muscle cells. Oxygen diffuses passively across thin walls to mitochondria, without a circulatory pump like blood. Larger bodies mean longer diffusion distances, potentially starving tissues unless external oxygen partial pressure rises.

The hypothesis crystallized in the 1990s. A landmark 1995 Nature paper modeled tracheal diffusion limits, concluding modern 21% oxygen caps insect size at observed maxima. Hyperoxia (up to 35%) allegedly allowed gigantism by steepening concentration gradients. Subsequent studies reinforced this, correlating Phanerozoic oxygen fluctuations (from GEOCARB models) with fossil body sizes. Amphibian giants like 1.5m Proterogyrinus were invoked too, as skin-breathers facing similar diffusion woes.

This view dominated, explaining why today's insects rarely exceed 15 cm despite abundant food. It even inspired experiments: rearing fruit flies in hyperoxia yielded marginally larger offspring, hinting at physiological constraints. However, inconsistencies emerged—some gigantism peaked amid stable oxygen, and certain modern high-altitude insects thrived in low-oxygen niches.

Emerging Doubts and the Push for New Evidence

By the 2010s, cracks appeared. Vertebrate paleontologists noted pterosaurs and birds achieved flight in low-oxygen eras without tracheal limits. Insect respirometry showed metabolic scaling that buffered diffusion shortfalls. A 2012 study proposed predation by early birds post-150 million years ago shrank insects, despite rising oxygen.

Critically, no direct measurements existed for ancient tracheolar density. Fossils preserve impressions, not microstructures. Researchers called for nanoscale analysis of preserved muscle tissues. Enter electron microscopy: could it reveal if griffinflies packed more tracheoles per gram of muscle?

The 2026 Nature Study: A Methodological Masterstroke

Published March 25, 2026, in Nature, "Oxygen supply through the tracheolar–muscle system does not constrain insect gigantism" delivers that evidence. Led by Edward P. Snelling from the University of Pretoria's Faculty of Veterinary Science, the team analyzed 1,320 electron micrographs from 44 modern flying insect species across 10 orders, spanning 10,000-fold body mass range.Read the full study here.

Tracheolar volume density hovered at ~1% or less, rising just 1.8-fold with size—trivial compensation. Extending models to M. permiana (100g body), density remained under 2%. By contrast, vertebrate heart capillaries claim 10% volume, underscoring insects' spare capacity. Sensitivity analyses confirmed tripling tracheoles boosts oxygen flux massively with minimal power costs.

"If atmospheric oxygen really sets a limit... there ought to be evidence of compensation at the tracheoles," Snelling noted. "There is some... but it is trivial." Co-author Roger Seymour (University of Adelaide) added insects hold "great evolutionary potential" untapped.

Spotlight on the Research Team and Institutions

Snelling, an associate professor specializing in comparative physiology, bridges veterinary science and evolutionary biology. With a PhD from Adelaide and expertise in locomotor energetics, his lab at UP's Centre for Veterinary Wildlife Studies employs cutting-edge microscopy.UP press release.

Collaborators span continents: Susana Clusella-Trullas and John Terblanche (Stellenbosch University, South Africa) on ecophysiology; Jon Harrison (Arizona State) on insect respiration; international inputs from Greifswald, Trinity Dublin, Auckland, Witwatersrand. Adelaide's Seymour brought modeling prowess. This global effort exemplifies modern higher ed collaboration, funded by national research councils.

UP and Adelaide shine as hubs for integrative biology, training PhDs in paleophysiology—a niche blending fossils, physiology, and tech.

Beyond Griffinflies: Other Carboniferous Behemoths

Griffinflies weren't alone. Arthropleura, a 2.5m millipede, grazed lush lycopod forests. Giant scorpions and centipedes prowled. All shared exoskeletons, tracheal breathing. Post-study, oxygen's role diminishes; buoyancy in humid swamps or nutrient-rich detritus may explain terrestrial giants. Aquatic arthropods like eurypterids (sea scorpions up to 2.5m) faced different physics, thriving pre-Carboniferous sans hyperoxia.

What Sparked the Gigantism Boom?

• Ecological Vacuums: Pre-vertebrate aerial predators; insects filled apex niches.
• Resource Abundance: Decaying plants fueled high-nutrient diets, accelerating growth.
• Physical Factors: Lower gravity effects? No; but denser air (from CO2) aided lift.
• Exoskeleton Scaling: Chitin strength scales sub-linearly, permitting larger frames temporarily.

Predation pressure intensified with amphibian evolution, then reptiles and birds. A 2012 UCSC study tied insect decline to avian rise, independent of oxygen.

Modern Parallels and Lessons

Today's giants—Goliath beetles (11cm), helicopter damselflies—hint at untapped potential. Climate change alters oxygen solubility in water, stressing aquatic larvae. Hyperoxia experiments yield subtle size boosts, but behavior trumps. For higher ed, this underscores microscopy's power in paleo-research.

Photo by James Wainscoat on Unsplash

Future Horizons in Paleoentomology

The study reopens debates: syncytial muscles? Larval bottlenecks? Genomics of ancient tracheae via eDNA? UP's lab eyes vertebrate comparisons. For students, fields like this demand interdisciplinary skills—histology, modeling, fieldwork—fueling careers in academia.

As oxygen fluctuates amid climate shifts, insights inform biodiversity forecasts. This "final nail" in the myth coffin propels bolder questions about life's scaling laws.