Insects of a Lost World

The Devonian period, often called the "Age of Fishes," was also the time when insects first appear in the fossil record. While vertebrate life was still largely confined to the seas, small hexapods were already colonizing the land alongside the first forests. The Devonian represents the very beginning of insect history — a time when life on land was still sparse and the ecosystems that would later support giant insects were only just forming.

The World of the Devonian

During the Devonian, the continents were grouped differently than today. The southern supercontinent Gondwana dominated the Southern Hemisphere, while smaller landmasses like Laurentia, Baltica, and Siberia lay nearer the equator. Sea levels were high, and much of what is now dry land was covered by shallow tropical seas.

On land, the Devonian saw a dramatic transformation. Early in the period, terrestrial vegetation was limited to small, low-growing plants. By the Late Devonian, the first true forests had appeared, composed of trees like Archaeopteris, which could reach heights of 30 meters. These forests fundamentally changed the land surface, creating soil, altering river systems, and generating new habitats that arthropods could exploit.

Atmospheric oxygen during the Devonian was lower than today, hovering around 15%. Carbon dioxide levels were significantly higher. The climate was generally warm, with no evidence of polar ice for much of the period, though a glaciation event occurred in the Late Devonian.

The Earliest Insects

The oldest insect fossil currently known is Rhyniognatha hirsti, found in the famous Rhynie Chert deposit in Scotland. This specimen dates to approximately 410 million years ago, placing it in the Early Devonian. Only the mandibles (jaws) of Rhyniognatha have been preserved, but their structure suggests this was already a relatively advanced insect, possibly even a winged one, though this interpretation remains debated.

If Rhyniognatha was indeed winged or related to winged lineages, it pushes the origin of insect flight much further back than previously thought. However, most paleontologists are cautious about this interpretation given the limited fossil material.

Beyond Rhyniognatha, the Devonian record for insects is extremely sparse. A few other hexapod body fossils and trace fossils (such as trackways) from this period suggest that small, wingless insects were present in Devonian forest floors and leaf litter. These were likely detritivores, feeding on decaying plant material in the newly forming terrestrial ecosystems.

Other Arthropods of the Devonian

While insect fossils are rare in the Devonian, other terrestrial arthropods are better represented. Mites, springtails (Collembola), and early arachnids are known from Devonian deposits. The Rhynie Chert, in particular, has yielded a remarkable window into early terrestrial life, preserving mites, harvestmen, and the trigonotarbids (an extinct group of arachnids) in extraordinary cellular detail.

These early land arthropods give context for the world insects evolved in — a land surface that was still being colonized, with simple food webs and limited competition compared to what would come later.

Why So Few Fossils?

The scarcity of Devonian insect fossils is likely due to several factors. Insects in this period were probably small and soft-bodied, making them poor candidates for fossilization. The terrestrial fossil record overall is much thinner for the Devonian than for later periods, because the conditions that produce good insect fossils — fine-grained lake sediments, amber, and anoxic swamp deposits — were less common or less well-preserved from this time.

It is likely that insects were more diverse in the Devonian than the fossil record suggests. Molecular clock studies (which estimate divergence times using DNA mutation rates in living species) consistently place the origin of several major insect groups in the Devonian or even the Silurian, well before their first fossil appearances.

Significance

The Devonian is the starting point of insect history. Although the fossil record is thin, the specimens that do exist — particularly Rhyniognatha — show that insects had already evolved key features like dicondylic mandibles by the Early Devonian. This sets the stage for the explosive diversification that would occur in the Carboniferous period, when oxygen-rich atmospheres and vast swamp forests created the conditions for insects to grow to extraordinary sizes and become the dominant land invertebrates.