TL;DR
Fossils of a 437-million-year-old arthropod indicate that millipede and centipede ancestors developed legs underwater, predating terrestrial adaptation. This discovery reshapes understanding of myriapod evolution.
Fossils of Waukartus muscularis, a 437-million-year-old aquatic arthropod, have shown that early ancestors of millipedes and centipedes already possessed legs suited for underwater life, contradicting previous theories that such limbs evolved primarily for terrestrial adaptation.
The fossils, discovered in the Silurian Brandon Bridge Formation in Waukesha, Wisconsin, include well-preserved remains of a myriapod-like creature with many unbranched, uniramous limbs. These limbs, along with muscle tissue and a cuticular endoskeleton, indicate an aquatic lifestyle. The fossils lack evidence of exopods, limb branches common in aquatic arthropods, suggesting that the loss of these features predates the move onto land.
Phylogenetic analysis places Waukartus just outside the crown group of myriapods, which require at least 17 limb pairs, indicating it was a close ancestor rather than a direct descendant. Its limb structure, with uniramous limbs, is interpreted as an adaptation for terrestrial life, but the fossils themselves show it was still aquatic.
Why It Matters
This discovery challenges the long-held view that the limbs of myriapods evolved solely as adaptations for terrestrial environments. Instead, it suggests that some limb features predate land colonization and may have originally been adaptations for underwater life, with limb loss or modification later facilitating terrestrial transition.
The finding provides new insights into the evolutionary timeline of myriapods, indicating that aquatic ancestors already had limb structures similar to those seen in land species, which could influence how scientists understand arthropod evolution overall.
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Background
Prior to this discovery, the fossil record for early myriapod relatives was sparse, with the earliest known stem-group myriapods like euthycarcinoids dating back to the Cambrian period. These early forms were known to be aquatic or amphibious, but their limb structures and evolutionary significance remained unclear. The new fossils fill a gap, showing that complex limb structures existed before the terrestrial transition, and challenge the idea that limbs only evolved for land adaptation.
“The loss of exopods in Waukartus and related groups appears to be an exaptation, occurring before the shift to land, and not as a direct adaptation for terrestrial life.”
— Lead researcher in the study
“Waukartus provides evidence that early myriapod ancestors already possessed limb structures suited for underwater environments, which were later co-opted for terrestrial life.”
— Study authors
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What Remains Unclear
While the fossils clearly show aquatic adaptations, it remains unclear exactly how widespread such limb structures were among early myriapod ancestors. The precise nature of their feeding and sensory functions is also still unknown, as well as how limb modifications varied across different lineages.
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What’s Next
Further fossil discoveries are expected to clarify the diversity of early myriapod ancestors and their limb adaptations. Researchers aim to find more specimens to better understand the transition from aquatic to terrestrial environments and to refine the evolutionary timeline of limb development in arthropods.
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Key Questions
What does this discovery tell us about the evolution of millipedes and centipedes?
It suggests that their ancestors developed legs while still living underwater, indicating that limb evolution was not solely driven by terrestrial adaptation but began earlier in aquatic environments.
Why is the loss of exopods significant in this context?
The loss of exopods appears to have occurred before the move onto land and may have been an exaptation, meaning limbs evolved for one purpose (aquatic life) and were later used for another (terrestrial movement).
How does this change current understanding of arthropod evolution?
It shifts the timeline, showing that complex limb structures existed in aquatic ancestors well before terrestrial colonization, challenging the idea that limbs only evolved for land adaptation.
Are these fossils the oldest evidence of aquatic myriapod ancestors?
They are among the oldest known fossils showing limb structures in aquatic relatives of myriapods, dating back approximately 437 million years, but further discoveries could extend this record.
What are the next steps for researchers studying early arthropods?
Researchers plan to search for additional fossils to better understand limb evolution, feeding behaviors, and the transition from aquatic to terrestrial life in early arthropods.
