A fossilized sea creature that lived more than 500 million years ago is offering fresh insights into the evolutionary history of arthropods—the most diverse and successful group of animals on Earth.

The study, published in Nature Communications, addresses one of the last major enigmas of arthropod evolution: the division between mandibulates (insects, crustaceans, centipedes, and millipedes) and chelicerates (spiders, scorpions, and related species).

Researchers have determined that Jianfengia multisegmentalis, a segmented marine creature, represents an early ancestor of mandibulates, overturning earlier assumptions that it belonged to the chelicerate lineage.

Revealing the Brain of Jianfengia

Led by Regents Professor Nicholas Strausfeld from the University of Arizona’s Department of Neuroscience, the team reconstructed minute details of Jianfengia’s fossilized nervous system. Their findings show a brain structure closely resembling that of modern crustaceans such as shrimp and crayfish.

“These megacheirans didn’t have antennules, which are antenna-like appendages that are common to crustaceans, insects and centipedes. Instead we see these strange, quite sturdy head appendages that were specialized for reaching and clasping things,” Strausfeld said.

These specialized appendages, known as “great appendages,” had long been thought to indicate a relationship with chelicerates. However, the new neurological evidence instead places Jianfengia firmly within mandibulate ancestry.

A Tale of Two Fossils

Two exceptionally well-preserved fossils, Jianfengia multisegmentalis and Alalcomenaeus, both dating to around 525 million years ago, were traditionally grouped together as chelicerates.

The new study, however, distinguishes their evolutionary roles:

• Jianfengia is now identified as a stem lineage of mandibulates.
• Alalcomenaeus retains features consistent with chelicerates.

“Our results demonstrate that close examination of fossilized neural remains can provide powerful data indicating evolutionary relationships impossible to obtain just from features of the exoskeleton,” Strausfeld noted.

Ancient Brains, Modern Connections

Co-author Frank Hirth, professor at King’s College London, emphasized the remarkable continuity revealed by these fossils:

“The organization of their fossilized brains perfectly aligns with that of living arthropods, suggesting that their ancient genetic and developmental constituents are extraordinarily robust, yet diverse, which may explain why arthropods are the most successful inhabitants of this planet.”

The study also confirmed that the “great appendages” in Jianfengia were precursors of the segmented antennules seen in modern mandibulates, while in Alalcomenaeus they evolved into the pincer-like fangs of spiders and scorpions.

Fossil Evidence From Yunnan, China

The first Jianfengia fossil was discovered in 1984 by co-author Xianguang Hou at the Yunnan Key Laboratory for Palaeobiology, Yunnan University. The Cambrian fossil beds near Kunming, China, are among the world’s richest sources of early animal life.

Strausfeld highlighted the exceptional preservation of neural structures:
“What we saw was unexpected: the brain looks really modern, comparable to that of a living crustacean. In one specimen we even could peer into the compound eyes and look down some of its facets to see fossilized ‘cone cells’ that supported the photoreceptors.”

Confirming Evolutionary Placement

To further validate Jianfengia’s classification, co-author David Andrew of Lycoming College constructed evolutionary trees based on neural traits. His analyses repeatedly placed Jianfengia at the base of mandibulate ancestry, while Alalcomenaeus belonged within chelicerates.

“In chelicerates, these ‘great appendages’ shrunk, so they eventually became the spider fangs. In mandibulates, evolution modified them into segmented antennules,” Strausfeld explained.

Evidence from living ostracods, small marine crustaceans with antennules tipped with claspers, further supports this evolutionary link.

Summary of Key Findings

• Jianfengia multisegmentalis, a 525-million-year-old fossil, has been reclassified as a mandibulate ancestor.
• Brain structure analysis revealed striking similarities to modern crustaceans.
• The long-debated “great appendages” evolved differently in the two main arthropod lineages:
  • Into antennules in mandibulates (Jianfengia).
  • Into fangs in chelicerates (Alalcomenaeus).
• Exceptional fossil preservation in Yunnan, China, enabled the discovery of neural structures rarely seen in Cambrian specimens.
• Findings highlight the importance of neural fossils in resolving evolutionary relationships

Original Publication
Authors: Nicholas J. Strausfeld, David R. Andrew, Xianguang Hou and Frank Hirth.
Journal: Nature Communications
DOI: 10.1038/s41467-025-62849-w
Method of Research: Data/statistical analysis
Subject of Research: Not applicable
Article Title: Brain anatomy of the Cambrian fossil Jianfengia multisegmentalis informs euarthropod phylogeny
Article Publication Date: 28-Aug-2025

Original Source: https://news.arizona.edu/news/ancient-signpost-minute-fossils-tell-big-story-about-arthropod-evolution

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