Fossils show millipede and centipede ancestors evolved legs underwater

The myriapoda group of arthropods includes the many-legged centipedes and millipedes that most people are familiar with. Although myriapods are all terrestrial creatures, researchers are unclear about when and how they evolved their many legs.

Now, fossils found in the Silurian Brandon Bridge Formation in Waukesha, Wisconsin have revealed that a long-lost ancestor of myriapods already had many unbranched legs while still living underwater. The finding, described in a new study published in Proceedings of the Royal Society B: Biological Sciences, contradicts the idea that this particular type of limb was an adaptation for land.

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Waukesha fossils reveal a new aquatic arthropod

Paleontologists discovered 35 fossils of a myriapod-like arthropod in Waukesha's Silurian Brandon Bridge Formation dating from around 437 million years ago. The Silurian Brandon Bridge Formation is a geological formation known as the Waukesha Lagerstätte—an exceptional deposit of soft-bodied fossils from the Llandovery epoch. The area's finely laminated dolomitic mudstones preserve a diverse, shallow marine community, including early arthropods.

The new arthropod, called Waukartus muscularis, resembles modern myriapods with a head and long segmented body featuring many sets of legs. Many of the fossils were well-preserved, with evidence of uniramous, or single-branched, limbs, muscle tissue, and a cuticular endoskeleton. Although some specimens were incomplete, the research team says that among the fossils there is no evidence for less than 11 segments in Waukartus muscularis. They say it also features multiple "head appendages," which increase in size toward the rear of the head.

"The head appendages of Waukartus appear to resemble those of the trunk, but details are not preserved. Their shorter length may indicate that they were not involved in walking but specialized for sensory or feeding functions, but the mode of feeding is unknown. The trunk of Waukartus was flexible as evidenced by curved specimens and variation in the nature of the overlap between successive segments," the study authors write.

A place in the family tree

Phylogenetic relationships were analyzed by the team to determine where exactly Waukartus lands in the family tree of arthropods. The team found that Waukartus fits just outside the crown group of myriapods, which requires at least 17 pairs of limbs. Another close relative, euthycarcinoids, had been identified in another study as stem-group myriapods, and were found to be an unusual group of aquatic, or potentially amphibious, arthropods.

"Stem-group Myriapoda remained unknown until euthycarcinoids were assigned to the clade; their presence in the Cambrian extends the myriapod stem-group to that period. Waukartus, however, sits just stemward of euthycarcinoids and living myriapods," the study authors write.

Walking underwater

Sitting so close in the family tree to myriapods, Waukartus has helped to shed some light on when and how terrestrial traits evolved in myriapods, since there had been a gap in the fossil record for early myriapod relatives. In particular, Waukartus appears to have been aquatic, while possessing uniramous appendages instead of branched limbs, called exopods, which are common in aquatic arthropods. In other words, Waukartus appears to have been equipped with legs already adapted to live on land, but hadn't quite made it out of the sea.

"The uniramous limbs of Waukartus comprise an endopod alone, an anatomical trait interpreted as a terrestrial adaptation in myriapods that is convergently shared with terrestrial insects and arachnids. The inferred marine life habit of Waukartus indicates that the loss of the exopod occurred before terrestrialization and is not an adaptive change.

"The loss of exopods is also evident in euthycarcinoids and in the aquatic sister group to arachnids and is likely to be an exaptation—an evolutionary change that occurs under one selective regime but is subsequently coopted for a different function," the study authors explain.

Written for you by our author Krystal Kasal, edited by Gaby Clark, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.

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