The evolution of the first animals on land, 500 million years ago, has been revealed. The ancestors of millipedes, called euthycarcinoids, evolved in warm tidal nursery pools from juvenile arthropods. Individuals that acquired sexual maturity early and survived the harsh tidal zone, passed on their genes to successive generations, leading to arthropods that could crawl onto land when the tidal pools dried up.
Reconstruction of the first animals on land, an ancient tidal flat at Blackberry Hill (500 million years ago), Wisconsin. Stranded jellyfish (1 m wide), the euthycarcinoid Mosineia (bottom left, 15 cm long), phyllocarid crustacean Arenosicaris (bottom right, green) and large slug‐like mollusc (70 cm long). Some euthycarcinoids occupy a tidal pool before it dries up. Image credit: Todd Gass.
All life first evolved in the sea. Fossils of early land animals are very rare because dead bodies decay easier on land than in the sea.
Arthropods — creepy-crawlies such as spiders, crabs and insects with a segmented body, jointed limbs and a hard outer exoskeleton — were the first animals to move onto land.
The oldest body fossil of a land animal is a millipede called Pneumodesmus newmani from the Late Wenlock Epoch of the Silurian period in Scotland, around 428 million years ago (Ma).
Millipedes, centipedes and their relatives are called myriapods and comprise around 12,000 living species.
There is another type of fossil that provides clues on when ancient animals first stepped out of the sea — trace fossils.
They represent the remains of an organism’s activity and include animal tracks and burrows.
Trace fossils allow us to breath life into extinct animals, revealing snap-shots in time of their behavior and interactions.
Fossil burrows from Pennsylvania (445 Ma) and fossil trails from Cumbria, England (450 Ma) suggest that myriapods lived on land 22 Ma before their earliest body fossils.
The oldest tracks on dry land were made by the ancestors of myriapods, called euthycarcinoids, on ancient coastal dunes (in New York State and Ontario) and tidal flats (in Quebec and Wisconsin) around 500 Ma.
It may have been one small step for a bug but it was a giant leap for life on Earth.
The euthycarcinoids were 4 to 15 cm long (possibly up to 30 cm long based on their fossil trackways), and lived from 500 Ma to 225 Ma. They look like a pill bug (woodlouse), but with a tail spine.
In the Cambrian period marine arthropods called fuxianhuiids lived in the shallow seas.
Euthycarcinoids resemble juvenile fuxianhuiids, suggesting that an evolutionary process called neoteny (i.e. the retention of juvenile traits in descendant species) was involved in the evolution of these earliest land animals.
Euthycarcinoids spawned in warm tidal pools, possibly to protect their eggs from marine predators and speed up the development of their larvae.
In these harsh tidal conditions, those individuals that acquired sexual maturity earlier and were able to survive and pass on their genes to successive generations, so juvenile traits were gradually selected in the population.
Cambrian euthycarcinoids had a barrel-shaped body, short legs and six tail segments.
They gradually lost tail segments as they evolved, from six to five in the Silurian to Carboniferous periods, and then four in some later species. They also developed fine spines on their legs.
One group of euthycarcinoids, called sottyxerxids, look more myriapod-like, with a long multi-segmented trunk and similar length legs.
The kampecarids, a rare extinct group of myriapods, may represent an evolutionary link between the sottyxerxids and myriapods, based on their shared legless neck and tail segments.
Euthycarcinoids were shortly followed by sea scorpions (eurypterids), the ancestors of the scorpions, leading to the widespread invasion of the land by animals.
Our own (vertebrate, i.e. backboned) ancestors, called tetrapods, first lumbered ashore 130 million years after the euthycarcinoids, in their own breath-taking adventure.
As Walter Garstang eloquently put it: “The facts are much the same. An arthropod with yolky eggs, whatever be it’s name, must own aquatic ancestors that once laid smaller eggs, and hatched as tiny larval forms with many fewer legs. So to those fry prophetic of our millipedes and spiders (and centipedes and insects surely cannot be outsiders).”
This paper appears in the Neues Jahrbuch für Geologie und Paläontologie.
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Braddy, S.J. 2024. Euthycarcinoid ecology and evolution. Neues Jahrbuch für Geologie und Paläontologie, doi: 10.1127/njgpa/2024/1199