A euthycarcinoid fossil from Blackberry Hill, Wisconsin.
According to research published in 2012, the first traces on earth were the traces of euthycarcinoids. Euthycarcinoids are arthropods that lived approximately 500 million years ago. Scientists weren’t sure about the first animal to walk on land and suspected amphibians or centipedes for some time. However, recent research shows that it was actually the Cambrian euthycarcinoids that made the first transition from sea to land. It was the discovery of fossils that preserve euthycarcinoids and protichnites, that is, walking impressions, that allowed scientists to make the connection.
Fossilized tracks can reveal information about the behavior of an extinct animal.
In fact, the idea that arthropods may have been the first to walk on land arose in 1852 by Sir Richard Owen. He based the idea on fossil footprints in Quebec. However, the idea could not be tested until fossils of the animal responsible for the footprints were found. Thus, the theory was recently proven, when the segmented tail of a euthycarcinoid was identified in one of the footprint fossils.
Centipedes are myriapods.
In addition to euthycarcinoids, one of the first verified land animals was a centimeter-long myriapod. Current examples of myriapods include millipedes and centipedes. This myriapod, discovered in 2003 in Scotland and named Pneumodesmus newmani, dates back to 428 million years ago. Paleontologists can tell that it lived on land because its fossil shows that it had spiracles; holes that use insects, spiders, rays and sharks to breathe air. Before Newmani’s discovery, the oldest air-breathing creature was a spider-like organism from 410 million years ago.
First appearing during the Silurian Period, vascular plants are a more advanced plant species, with two types of transport tissue: xylem and phloem.
The earliest land animals are often incorrectly cited as Devonian transitional forms called “pezapods” because they are intermediates between fish and true tetrapods. An example is the Tikaalik fish, which lived about 375 million years ago, during the Devonian period. It is remarkable that such organisms are so often cited as the first land animals when land animals from more than 50 million years earlier, such as Pneumodesmus newmani, are now widely known. The effect may have something to do with a bias in favor of more familiar vertebrates over invertebrates.
The first land animals probably lived in shallow, oxygen-poor pools near the ground. As the first vascular plants developed, they would have clogged the areas around these ponds with weeds, making it evolutionarily advantageous to climb and get around them by quick forays onto land areas. The land at that time would have been much richer in nutrients than water, as plants colonized the land before animals and left their decaying plant matter everywhere. Bacteria and fungi have destroyed much of the plant matter, but it would still be attractive to hungry fish. About 365 million years ago, some fish (so-called “fishapods”) developed limbs and climbed onto land. The appearance of the first true trees around 370 million years ago would have helped with this, depositing more nutrients into the soil and making the environment more habitable.