The question of how large spiders grew in prehistoric times often conjures images of monsters, but the answer spans hundreds of millions of years of Earth’s history. The “prehistoric” period saw dramatic changes in atmospheric conditions and animal life. These changes, particularly in the air we breathe, directly influenced the maximum size an arachnid could attain. To understand the true dimensions of ancient spiders, scientists rely on a fragmented fossil record that requires careful interpretation.
Interpreting the Ancient Evidence
Determining the exact dimensions of ancient spiders presents a challenge because their soft bodies rarely fossilize well. Paleontologists typically rely on two main forms of preservation. The most common is the compression fossil, where the spider’s body is flattened between layers of sedimentary rock, often volcanic ash. These fossils provide an outline, but the crushing process frequently distorts the specimen, making precise size and morphological measurements difficult.
A much rarer method of preservation is in amber, which is fossilized tree resin. Amber specimens offer a three-dimensional view of the spider’s entire body, including fine details of their respiratory organs and appendages. This level of detail allows scientists to make much more accurate estimations of body mass and overall size. By analyzing both the distorted and the pristine fossils, researchers establish a reliable size range for many extinct arachnids.
The Largest Known Prehistoric Arachnids
The largest confirmed fossil spider currently known is the extinct species Mongolarachne jurassica, which existed during the Middle Jurassic period. This species was originally misidentified, but further study established its own distinct family lineage. A female specimen had a body length of approximately 24.6 millimeters (just under an inch). Its front legs measured about 56.5 millimeters, giving it a modest leg span compared to some modern descendants.
For context, the largest spider alive today by mass is the Goliath Bird-Eater tarantula (Theraphosa blondi), which dwarfs the fossil record holder. The Goliath Bird-Eater can have a body length of up to 13 centimeters (five inches) and a leg span reaching 30 centimeters (nearly a foot). While Mongolarachne was certainly a large spider for its time, it was not the dinner-plate-sized behemoth that popular culture sometimes imagines. The truly massive ancient arthropods, such as giant dragonflies and millipedes, were insects and myriapods, not spiders.
Why Spiders Grew So Large
The primary scientific explanation for the increased size of some arthropods lies in the composition of the Earth’s atmosphere. During the Carboniferous and Permian periods (roughly 360 to 250 million years ago), atmospheric oxygen levels were significantly higher than today. Concentrations may have reached 35 percent, compared to the current 21 percent. This hyperoxia provided an environment that enabled gigantism in various invertebrate groups.
Spiders, like all arachnids, rely on passive respiratory systems, such as book lungs and tracheal tubes, to exchange gases. Unlike mammals, they do not actively pump air throughout their bodies. Instead, oxygen diffuses directly into the body tissues. The distance oxygen can effectively travel through passive diffusion acts as a strict cap on maximum body size.
Elevated oxygen levels effectively increased the efficiency of passive diffusion, allowing oxygen to penetrate deeper into larger body masses. With the size constraint momentarily lifted by the hyperoxic atmosphere, spiders and other arthropods were able to evolve significantly larger body sizes. This phenomenon explains why many of the largest fossil arthropods date back to this specific geological time window.
Modern Size Constraints
The current size of spiders is a direct result of the planet’s return to lower oxygen levels following the Permian period. With atmospheric oxygen stabilized at 21 percent, the physical limits imposed by passive diffusion have reasserted themselves. A spider with the same physical proportions as a Goliath Bird-Eater could not sustain its internal organs. The respiratory system simply cannot move enough oxygen to the center of a large body to support its metabolic needs.
Beyond respiratory limitations, the arthropod exoskeleton also imposes a physical ceiling on size. The external skeleton must be shed through molting, which becomes progressively riskier as the animal grows larger. During molting, the spider is temporarily soft and vulnerable, and the sheer weight of a large body can lead to structural collapse or suffocation. These twin constraints—respiratory inefficiency and exoskeletal weakness—explain why even the largest modern spiders represent the upper limit of size today.