According to research published in 2012, the first footprints on land were the footprints of euthycarcinoids. Euthycarcinoids are arthropods that lived approximately 500 million years ago. Scientists were uncertain of the first animal that set foot on land and suspected amphibians or centipedes for some time. However, recent research shows that it was actually euthycarcinoids of the Cambrian period that first made the transition from sea to land. It was the discovery of fossils preserving both euthycarcinoids and protichnites, that is walking imprints,that allowed scientists to make the connection.
Actually, the idea that arthropods may have been the first to walk on land had emerged all the way back in 1852 by Sir Richard Owen. He based the idea on fossils of footprints in Quebec. However, the idea could not be proven until the fossils of the animal responsible for the footprints could be found. Hence, the theory was proven only recently when the segmented tail of a euthycarcinoid was identified in one of the footprint fossils.
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Aside from euthycarcinoids, one of the first verified land animals was a one-centimeter myriapod. Present-day examples of myriapods include millipedes and centipedes. This myriapod, discovered in 2003 in Scotland and named Pneumodesmus newmani, is dated to 428 million years ago. Paleontologists can tell it lived on land because its fossil shows it possessed spiracles; holes that insects, spiders, rays, and sharks use for breathing air. Prior to the discovery of newmani, the oldest known air-breathing creature was a spider-like organism from 410 million years ago.
The first land-walking animals are often incorrectly cited as Devonian transitional forms called “fishapods” because they are intermediate between fish and true tetrapods. An example is the fish Tikaalik, which lived approximately 375 million years ago, during the Devonian period. It is remarkable that such organisms are so frequently cited as the first land animals when land animals from more than 50 million years before, such as Pneumodesmus newmani, are now widely known. The effect may have something to do with a bias in favor of the more familiar vertebrates over invertebrates.
The earliest land animals probably lived in oxygen-poor shallow pools near land. As the first vascular plants developed, they would have choked the areas around these pools with weeds, making it evolutionarily advantageous to climb over and around them via quick forays onto land areas. The land at that time would have been much more nutrient-rich than the water, as plants colonized the land before animals and left their decaying plant matter everywhere. Bacteria and fungi broke down much of the plant matter but it still would have been appealing to a hungry fish. Around 365 million years ago, some fish (so-called “fishapods”) developed limbs and climbed onto the land. The appearance of the first true trees about 370 million years ago would have helped this along, by depositing more nutrients into the soil and making the environment more habitable.