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Newly discovered fossil reveals aquatic ancestors of centipedes and millipedes

The majority of terrestrial , including insects, spiders, and millipedes, constitute a significant presence on land today. Despite their current predominantly terrestrial habitats, their ancestors had aquatic origins. A fossil dating back 411 million years provides a glimpse into the aquatic life of one of these ancestral groups.

Arthropods, the highly successful group encompassing insects, spiders, millipedes, and crabs, trace their roots to the Cambrian Period around 541 million years ago. During this period, these creatures were primarily small and fully aquatic, coexisting in oceans and freshwater with various emerging animal lineages. However, the transition of these early arthropods into the diverse land-living groups seen today has not been clearly defined.

The origin of centipedes and millipedes, collectively known as the Myriapoda, has been a significant gap in the arthropod fossil record. The discovery of aquatic myriapods in from the Rhynie Chert rock formation, dating to around 410 million years ago, addresses this gap. Researchers, including Dr. Greg Edgecombe, a palaeontologist at the Museum specializing in arthropods, meticulously examined these fossils, offering a detailed insight into the early relatives of centipedes and millipedes.

Dr. Edgecombe reflects on the significance of this discovery, stating, “It's something I have been wrestling with for a couple of decades now and is one of the great holes in the arthropod fossil records. This is the first opportunity to see what the animals looked like in that gap.” The findings have been published in the Proceedings of the National Academy of Sciences of the U.S., contributing valuable information to our understanding of the evolutionary journey of arthropods.

The image of the fossil arthropod head, showing in incredible detail how the mouthparts are similar to those of centipedes and millipedes. Credit: Edgecombe et al. 2020

The first animals on land

Arthropods, constituting approximately 80% of all current animal species, stand as one of the most successful animal groups on Earth. They achieved a remarkable feat by transitioning from aquatic environments to terrestrial ones, marking a crucial moment in evolutionary history around 420 million years ago—preceding even the amniotes in this significant move.

Terrestrial arthropods can be broadly categorized into three main groups: hexapods (including insects), arachnids (such as spiders), and myriapods (comprising centipedes and millipedes). Interestingly, rather than a singular event, each of these arthropod groups independently underwent the transition to land.

“There were three major independent terrestrialization events in Arthropoda,” explains Greg, a researcher in . “All of them had to deal with the same basic challenges because land was a hostile .” Each group confronted common hurdles, including preventing desiccation, supporting their bodies for terrestrial movement, developing excretory mechanisms, and adapting to new respiratory strategies.

A reconstruction of the aquatic myriapod. Credit: Edgecombe et al. 2020

The appearance of the first arthropods dates back to the Early Cambrian, around 541 million years ago, with fossils found in deposits like the Burgess Shale. By the Early Devonian, they had successfully established themselves on land. The interim period between these milestones, however, has remained less certain in scientific understanding.

Unraveling the specifics of this transitional phase offers insights into the resilience and adaptability of arthropods as they navigated the challenges of a terrestrial environment, ultimately becoming a dominant force in the world's ecosystems.

The first terrestrial ecosystems

The Rhynie Chert, nestled in Aberdeen, Scotland, serves as a remarkable repository of one of the earliest terrestrial ecosystems, encapsulating the genesis of land colonization by plants and animals.

During the formation of these rocks, the region resembled a network of pools and springs, akin to present-day features in Yellowstone National Park, albeit less extreme. These pools fostered thriving freshwater plants and animals, with tentative explorations onto the surrounding land.

Within the Rhynie Chert rocks, Dr. Christine Strullu-Derrien, collaborating with Greg and using advanced microscopy methods, unveiled intricate details of tiny fossil arthropods. The high-resolution imaging of mouth parts provided by Christine revealed distinct organs in the head of these fossils, aligning with the mouth regions of contemporary myriapods. This finding strongly suggests that these were early, aquatic ancestors to present-day centipedes and millipedes.

Interestingly, these early myriapods, known as euthycarcinoids, have surfaced in the fossil record before. Their bodies have been found in rocks spanning from the Cambrian to the Triassic, and their trackways have been discovered on tidal flats and coastal sand dunes. However, past interpretations misclassified them as early crustacean or arachnid relatives due to insufficient fossil details.

Greg and his colleagues have now rectified this misidentification, highlighting that these fossils belong to an entirely distinct group of arthropods, thereby addressing a longstanding gap in the fossil record.

Beyond this taxonomic clarification, the research emphasizes the power of modern technology in enhancing insights into ancient specimens. Greg expresses awe at the beauty of these fossils, noting that the level of detail revealed surpasses his initial expectations. The captivating image of the specimen, in his words, stands as “one of the most sublime fossils” he has ever worked on, showcasing the intersection of paleontological expertise and cutting-edge imaging techniques.

Source: Natural History Museum

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