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A fossil sample erroneously categorized since its unearthing during the Civil War and accurately recognized by a University of Michigan researcher indicates an evolutionary advancement: It signifies the inaugural instance of this lineage of marine organisms venturing from the ocean.

U-M postdoctoral associate Richard J. Knecht discovered that the specimen, thought to be a sea worm for more than a century, is, in fact, a type of invertebrate known as a lobopod. According to Knecht, lobopods are soft-bodied invertebrates resembling a “worm with legs,” and he directed the research during his tenure at Harvard University.

Lobopods are regarded as the forebears of all arthropods—including insects, arachnids, and crustaceans—and researchers have long presumed that lobopods, which first emerged in the fossil record more than 500 million years ago, were restricted to aquatic habitats.

Currently, Knecht proposes that the specimen, dubbed Palaeocampa anthrax, most likely thrived in a freshwater setting. He also postulates that the specimen displayed chemical defenses, indicating it might have spent at least part of its life on land. Supported by the National Science Foundation, the study was published in Communications Biology.

“This is the first proof we have that this group indeed made a foray away from the ocean. Transitioning from a saline environment, even to a freshwater one, is a significant evolutionary stride,” Knecht stated. “When a group is discovered to have undergone this transition, it’s not only enlightening but also pivotal from an evolutionary standpoint to comprehend how this transpired.”

Knecht asserts that his research also highlights the necessity of reexamining fossil collections, even those previously classified, with renewed perspectives and modern methodologies. Since its discovery, this specimen has been misidentified variously as a caterpillar, marine worm, and millipede—organisms belonging to vastly divergent evolutionary branches. Even contemporary studies have referred to images of the fossil without acknowledging its authentic identity.

“When the specimen was initially found, lobopods had not even been recognized as a distinct group. This exemplifies the value of revisiting older specimens,” remarked Knecht, a researcher in the U-M Department of Ecology and Evolutionary Biology and Museum of Paleontology. “Even specimens that are iconic and highly recognized can lead to major breakthroughs simply by reevaluating them using new techniques and fresh insights.”

Palaeocampa anthrax and innovative methods

Knecht first encountered the specimen while examining a collection of millipedes in the Invertebrate Paleontology Department at the Museum of Comparative Zoology at Harvard University. He promptly identified it as a lobopod species—not a millipede, as labeled in the drawer.

Paleozoic lobopods can be challenging to classify, Knecht noted. This particular species superficially resembles woolly bear caterpillars with papillae and spikes emerging from its body. Some lobopods from this time bore armored plates, while others featured forward-facing appendages protruding from their heads. One species appears devoid of distinctive traits, resembling a smooth straw-like body with slender legs.

However, upon examining the Palaeocampa specimen, Knecht identified several distinct divergences from other lobopod species. For instance, most lobopodian spines exhibit a cone-in-cone growth pattern, resembling a stack of traffic cones tapering to a point at the tip. In contrast, Palaeocampa’s spines resemble a series of rigatoni extending from its trunk, segmented like bamboo, crowned with a design that resembles a castle turret, Knecht explained.

Subsequently, the research team observed orange halos at the tips of the preserved spines. The team speculated whether the lobopod might have released a fluid or chemical from the tips of its spines.

To explore this, Knecht collaborated with Nanfang Yu, a physicist at Columbia University. Yu employed a technique known as FTIR spectroscopy and infrared light to confirm the presence of fossilized molecules at the tips of the specimen’s spines that were not observable elsewhere along the spine or within the rock. This indicates, according to Knecht, that a unique chemical was indeed emanating from the apex of the spine.

This feature provides the most compelling evidence suggesting that the lobopod was likely amphibious, Knecht stated. Most marine or aquatic creatures with spines and chemical defenses possess spikes that can inject into predators. If their defenses originated from an exposed tip, their chemical protection would be less effective and would merely disperse in the water.

“If I were to place a stake in the ground, I’d assert this organism was probably more terrestrial or amphibious, specifically due to that tip. It aligns better with an aerial environment rather than an underwater one,” he remarked.

Statistical wonders

Lobopodians are scarce in the fossil record, particularly after the Cambrian Period, Knecht explained. As invertebrates lack bones, their soft bodies decompose rapidly post-mortem. A rare set of conditions must exist for fossilization: they must die and be swiftly buried before scavengers or decay take over, often occurring in aquatic settings with fine sediments or mud and minimal oxygen.

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All identified lobopodians originate from Lagerstätten—remarkable fossil locales with the unique geological conditions required to maintain soft-bodied creatures in remarkable detail and quantity. Palaeocampa anthrax was located at both Mazon Creek, Illinois, and Montceau-les-Mines, France, both Lagerstätten celebrated for their fossils.

This specimen represents one of two lobopodians from the Carboniferous, the concluding period of the Paleozoic—and signifies the most recent fossil lobopodian species identified. It also serves as a transitional fossil, exemplifying a species that marks an evolutionary advancement.

Locating transitional fossils is exceedingly challenging, Knecht remarked. They frequently appear during a brief geological interval. Discovering transitional fossils of invertebrates is even more arduous; their lack of bones inhibits preservation in the fossil record. Finding one is akin to hitting the jackpot. Capturing the Palaeocampa fossil in such intricate detail, documenting chemicals emanating from its extremities, indicates that its preservation must have been nearly instantaneous, Knecht noted. It was likely entombed alive in the mud.

“It’s akin to winning the lottery not once, but three times. First, that the site was present at all; second, achieving this degree of preservation; and third, to then rediscover it over 300 million years later,” he stated. “It’s a sequence of statistical wonders.”

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