Vertebrates, particularly the first tetrapods, are often credited with conquering land.(72 kb)
Yet invertebrates were the first true invaders, having crept onto the continents long before the vertebrates.
Clues left behind in the form of trace fossils tell us that some invertebrates made timid excursions out of the water as early as Ordovician time. It was in the Silurian Period, however, that significant numbers of small arthropods were evolving in the open air after developing respiratory structures. They included spiders, acarids (mites), springtails (so-called garden fleas) and millipedes. Their emergence followed soon after plants began to spread beyond the aquatic world, and fossilized arthropod excrement and digestive systems reveal that many of the arthropods ate these first land plants, including their spores. In doing so, they assisted in the decomposition of organic matter and helped to form the first soils.
The emergence of arthropods onto land occurred more than just once. Evidence from fossils demonstrates that separate groups made the transition from their ancestral aquatic home at different times. Beginning as small creatures highly dependent on damp environments at the start of the Devonian, arthropods became more impressive by the Middle Devonian. The fossil record from that time includes land scorpions, primitive spiders called trigonobartids, large millipedes measuring several centimetres long, and even the first insects, which resemble todays silverfish.
The first land snails also appeared during the Devonian, and were the only group of molluscs to conquer land.
The inventory of the small animals that once populated the shores of the ancient Miguasha estuary is still incomplete. We know with certainty that scorpions and millipedes were among them, but others have only left partial evidence: acid dissolution has revealed abundant pieces of chitin from as yet unidentified arthropods in specific layers of the Escuminac Formation.