These bizarre Cambrian swimmers are challenging our understanding of how chordates first evolved

Nature · Published 3 October 2026 Swipe more like this →

In short: Vetulicolians were once dismissed as stem arthropods, but their strange, two-part bodies and gill-like openings suggest they belong to the chordate lineage. These ancient, swimming creatures lacked eyes and appendages, leaving scientists to debate whether they were filter-feeding planktivores or selective deposit-feeders navigating the seafloor millions of years ago.

First described in 1987, Vetulicolia comprises a group of extinct marine animals from the Cambrian period. Their body plan is distinct, featuring a voluminous anterior forebody and a segmented, tail-like posterior section. Because they lacked any appendages—no legs, feelers, or eyes—researchers initially struggled to classify them. While early theories placed them among stem arthropods, the 2001 discovery of pharyngeal gill slits shifted the consensus toward them being stem deuterostomes or chordates.

The evolutionary position of these organisms remains a subject of intense scientific scrutiny. While many researchers favor placing them as stem-group chordates, others argue for a sister-group relationship with tunicates. A 2024 analysis by Mussini and colleagues suggests they may actually be a paraphyletic grade of stem-chordates. This shift is partly driven by the discovery that the basal ambulacrarian clade Cambroernida shares the vetulicolian lack of a post-anal tail, a trait once thought to be a defining feature of the last common ancestor of all deuterostomes.

The debate extends to their lifestyle and internal anatomy. Because their mouths lacked grasping or chewing features, they were likely planktivores or selective deposit-feeders. Some fossils show notochord-like structures, though evidence is often contested. Furthermore, the inclusion of the class Banffozoa within the group is debated, as some researchers suggest these specimens lack the serialized gill slits found in other vetulicolians and might instead belong to the Protostomia. As research continues, the classification of these enigmatic creatures remains a fluid, evolving puzzle in early vertebrate history.

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