What was the first animal to emerge from the sea and walk on land? Most reference sources still give this honor to fish or amphibians, although some recent ones suggest a yet-to-be-discovered species of centipede or scorpion. When did this take place? The answers have ranged from Devonian times (roughly 350 million years ago) to Ordovician (around 450 million years ago). What lured these animals out of the water? Exploitation of new food sources is the prevailing thought.
A group of researchers from the United States has now unearthed evidence confirming that the textbooks need to be re-written. Fossils from a quarry in a region of central Wisconsin known as Blackberry Hill show that the first footprints on land were made by an extinct arthropod known as a euthycarcinoid, and this occurred in the Cambrian period, roughly 500 million years ago. The authors of the study, Joseph Collette of the University of California – Riverside, Kenneth Gass, a researcher from Wisconsin, and James Hagadorn of the Denver Museum of Nature and Science, published their findings in the May 2012 issue of the Journal of Paleontology.
The suggestion that extinct arthropods had been walking about on land in what is now called Cambrian times is not a new one. Sir Richard Owen had published that idea in 1852, based on fossil footprints that he named Protichnites from Cambrian beach sandstone of Quebec. The problem was that no fossils of the animal itself had yet been discovered, and that remained true for the next 150 years. Finally, in 2010, Collette and Hagadorn published fossils of euthycarcinoids found in the same quarries in Blackberry Hill and Quebec that also preserve Protichnites, strongly suggesting these animals to be the trackmakers. What was still missing was the smoking gun that tied those animals to the footprints.
Right around that time, fossils turned up at a quarry in Blackberry Hill that would end the speculation. As Collette and his team would announce, these fossils show impressions of the segmented tail of a euthycarcinoid caught in the act of making Protichnites trackways. It was the smoking gun. The team supported their postulate experimentally by showing that a model of the euthycarcinoid found at Blackberry Hill can produce the key parts of the Protichnites trackways found at the same quarry.
The new fossils and experiments also opened the door to a new interpretation of how those tracks were made and suggested a different reason for emerging from the sea. In 2009 Hagadorn and Adolph Seilacher of Yale University published a variety that they named Protichnites eremita, again from the same quarry. They pointed out that the peculiar markings of those trackways were consistent with impressions of a mollusk shell, as though the trackmaker was using the shell as hermit crabs do. If true this would have pushed hermit behavior back 290 million years earlier than previously believed. But the new fossils and experiments show that the same kind of markings can be produced with the euthycarcinoid’s own tail. The authors now suggest it is more likely that Protichnites eremita resulted not from hermit behavior, but from mating behavior. They argue that impressions appearing to come from a spiral shell held to the side may have instead resulted from the female euthycarcinoid’s segmented tail held to the side to keep it from interfering with fertilization. They also interpret other markings within those trackways to be impressions of the tail from the male animal.
The euthycarcinoid Mosineia macnaughtoni came ashore a half billion years ago, presumably to mate and lay its eggs where they were less likely to be eaten.
Ideas about the identity of the first landwalker have thus come full circle: from an extinct group of arthropods in Cambrian times, to various other animals much later on, and right back to an extinct group of arthropods in the Cambrian. What is new is that solid evidence supporting this conclusion is finally at hand. It shows that the first landwalkers were euthycarcinoids that came ashore for reasons that apparently included mating and laying eggs in a brand new environment where their eggs were less likely to be eaten. In a very real sense, the euthycarcinoids’ step toward the colonization of land paved the way for most of the living creatures that we see all around us, including mankind itself.
You can read Collette, Gass and Hagadorn’s study in the Journal of Paleontology.
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