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Specimen found in Texas was confused with crustacean since 1985, but new analysis revealed ancestral animal with 24 legs, possible aquatic and terrestrial life and anatomy close to modern insects in Nature study.
An insect fossil found in West Texas and mistaken for a juvenile crustacean for nearly 40 years has been identified as a 324-million-year-old animal, equipped with 24 legs and possible adaptations for both water and land.
Insect fossil remained classified as crustacean since 1985
The specimen was collected in 1985 near Rough Creek Ranch in Brewster County, north of Big Bend National Park. Flattened and preserved in dark shale, the fossil appeared to be just a stain on the rock.
Since it was found in the same fossil bed as Tesnusocaris goldichi, a crustacean with multiple body segments, researchers initially interpreted the animal as a juvenile of that species.
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The classification began to be questioned when Erik Tihelka, a paleoentomologist at the University of Cambridge, came across an old drawing of the specimen in a used Russian textbook. The image raised doubts about the long-held identification.
After gaining access to the fossil, Tihelka examined the material under cross-polarized light. The technique revealed anatomy different from what was expected for a juvenile crustacean.
The animal had six legs on its thorax and another 18 along its abdomen. With a total of 24 legs, it combined features currently associated with both insects and crustaceans.
The study identifies the species as Chosha praecursor and presents it as a likely ancestor of insects. According to Tihelka, the fossil “fills a significant gap in the fossil record.”
Anatomy reveals adult female with 24 legs
The team found a thorax divided into three parts, a body organization characteristic of insects. The animal also had an abdomen made up of 11 segments, a pair of long antennae, and a central filament at the tail.
Nine abdominal segments contained pairs of articulated legs. Added to the six thoracic legs, these structures raised the total number of identified legs in the specimen to 24.
The appendages located at the rear ended in flattened, ridged structures resembling paddles. This shape is a key clue about the possible relationship of the animal with shallow aquatic environments.
The body measured approximately 3.2 centimeters in length. Including the tail filaments, the animal reached nearly 5 centimeters.
Another identified element was an ovipositor, a structure used for laying eggs. Its presence indicates that the fossil likely corresponds to an adult female, rather than a juvenile animal as previously believed.
The anatomy offers a rare record from a period prior to the establishment of the body plan of modern insects. Currently, these animals are defined by the presence of six legs, a condition that was not yet fully established in Chosha.
Analysis positions species close to living insects
To investigate the evolutionary position of the animal, the researchers compared Chosha with 22 other taxa of living and fossil arthropods. In total, 58 anatomical characteristics were classified and used to construct the evolutionary tree.
The most well-supported tree positioned Chosha just outside the group that includes living insects. The analysis indicates that the animal was not a crustacean, contradicting the interpretation adopted after its discovery.
However, scientists did not rule out a position somewhat deeper at the base of all hexapods. Since there is only one specimen, which is also incomplete, the exact position of the species in the tree of life remains uncertain.
The research also reclassified three other enigmatic fossils. Among them is Leverhulmia mariae, a Scottish species approximately 405 million years old placed in the basal group of insects.
With this classification, Leverhulmia mariae has become the oldest insect recognized by the study. Two fossils of many-legged animals, originating from Mazon Creek in Illinois, were also included in this broader group.
These fossils are about 306 million years old. Together with Chosha, they help fill a gap of approximately 80 million years between early Devonian hexapods and the diversity of insects found later in the Carboniferous.
Rudder-like legs may indicate life between water and land
The flattened abdominal legs are the main clue about the lifestyle of Chosha. In position and shape, they resemble gill structures found in some modern mayflies.
The authors suggest that these appendages could have been used for breathing, swimming, or allowing movement over mud in shallow waters. There is also the possibility that they served simultaneously both respiratory and swimming functions.
The thoracic legs, on the other hand, had structures suitable for walking on land. The ovipositor may have allowed the female to deposit her eggs in protected crevices.
This combination supports the possibility of a gradual transition between coastal aquatic environments and land. Thus, the animal would possess adaptations related to both water and terrestrial movement.
The exact environment occupied by the species was not determined. The Tesnus Formation, where the fossil was found, is considered an ancient shallow marine, deltaic, or coastal environment.
The specimen also did not preserve spiracles, the openings connected to the tracheal system of insects. Therefore, it was not possible to confirm if the abdominal appendages functioned as gills, swimming structures, or performed both functions.
The identification has been tested in multiple phylogenetic analyses, but researchers still rely on the discovery of new fossils to strengthen the interpretation. For paleobiologist William Shear, the finding “opens many doors for the study of insect evolution.”
