The phylum Athropoda is incredibly diverse. There are over a million different species of these guys on Earth—and that’s only counting the ones scientists have discovered. Arthropods are invertebrates (meaning they lack a backbone) that have an external skeleton; a body composed of a number of segments, and limbs that bend at joints. They range from microscopic to a few meters long, live anywhere from deep in the sea to high in the trees, and include such commonly recognized creatures as spiders, crustaceans, insects, and worms.

The versatility of Arthropoda intrigues paleontologists, but makes plotting out the phylum’s evolutionary ancestry a daunting task. The arthropod head is of particular interest to scientists studying evolutionary relationships—the morphology of their nervous systems, as well as the way they develop and function, can help determine which organisms came from which and when. A recent fossil finding in China is offering new evidence towards solving the evolution of the mysterious arthropod head.

In the Yunnan province in Southwestern China, a 520 million-year-old arthropod fossil known as a fuxhianhuiid was discovered. Living in the early Cambrian period, fuxhianhuiids are the predecessors of Euarthropods—in Latin, “true arthropods”—arthropods with hardened cuticles and jointed appendages. The team of researchers who studied the fossil; Javier Ortega-Hernández and Nicholas Butterfield from the University of Cambridge, and Jie Yang and Xi-guang Zhang from Yunnan University; found nine arthropod specimens in total, representing two species: Chengjiangocaris kunmingensis and Fuxianhuia xiaoshibaensis.

Paleontologists often have difficulty studying the morphology of the internal organs of fuxhianhuiids because their shell fossilizes and blocks out the details of the body and head. Of the nine specimens, one of them was exceptionally well preserved. This arthropod seemed to have been flipped before being fossilized, so the details of its organs are discernible. Due to this unusual preservation, the research team found that the nervous system of this fuxhianhuiid extended beyond the head—the earliest instance of such a nervous system in the history of arthropods.

Most interesting was the discovery of specialized limbs that extended from either side of the fuxhianhuiid mouth, right under the head. The researchers believe the appendages were used for sweeping the ocean floor in search of food, which could then be easily transferred to the arthropod’s mouth. The fossils were some of the oldest arthropods ever discovered; according to Ortega Hernandez: “This is as early as we can currently see into arthropod limb development."

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The study is published in the journal Nature.

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