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Meet Our Weirdest Ever Cousins

A school of vetulicolians swimming in the Cambrian ocean

Our strangest relatives? A school of vetulicolians swimming in the Cambrian ocean, 515 million years ago. Credit: Katrina Kenny

By Diego García-Bellido & Michael Lee

Strange-sounding and even stranger-looking, vetulicolians are close relatives of vertebrates.

“Vetulicolians” sounds like a new race out of Star Trek. Indeed these 500 million-year-old fossils are so bizarre that they could be mistaken for aliens. They were tough-skinned blind creatures shaped like a figure-of-eight, with a drum-shaped head-end and a segmented tail. They were also important elements of Cambrian ecosystems. Larger than most of their fellow creatures, perhaps rendering them less prone to predation, they are some of the most abundant animals in Cambrian fossil sites after trilobites.

Yet their unfamiliar body plan has long challenged palae­ontologists. First discovered in 1911, they were put in the “too hard” basket and not described formally until new specimens were found in 1997 – and yet remained enigmatic.

The vetulicolians were a diverse group, with 14 species in four different families, they were 8–15 cm in length and spread over several continents: North America, China, Greenland and Australia.

Their lifestyle has been debated. It had been suggested that they grubbed about in the mud, though most people now think they swam in the water column.

Their affinities are even more uncertain: some workers noted their crustacean-like carapace and segmented tail and thus linked them to arthropods, but others emphasised the presence of fish-like gill slits in the anterior region of their bodies, suggesting they were related to vertebrates.

A new twist in this debate comes from our ongoing excavation on Kangaroo Island. The site, known as Emu Bay Shale, has produced thousands of new fossils and over 50 different species, including trilobites and soft-bodied arthropods, worms, molluscs and sponges. The preservation is so good that the fine sediments replicate structures as delicate as skin, eyes, muscle and even guts that still contain their last meals.

Among the discoveries were over 150 specimens of a large vetulicolian. This turned out to be a distinct new species we called Nesonektris (“island swimmer”) aldridgei (in honour of our late colleague Dick Aldridge). These new fossils shed light on how these animals lived, and finally revealed crucial aspects of their anatomy that clarified their true evolutionary affinities.

The vetulicolian fossils at Emu Bay are usually preserved as contorted – often ripped – bodies in siltstone beds. This normally happens when animals swimming in the water column are engulfed, smothered, weighted down and finally buried by a load of sediment, as would happen when a heavy storm generates lots of muddy runoff that is transported into deeper waters.

We are thus reasonably confident that Nesonektris inhabited the water column. They probably fed in a similar way to modern-day planktivores such as whale-sharks and manta rays: by opening their mouths while actively swimming through plankton and organic matter, letting the excess water out through the lateral gill openings and directing the food towards the posterior of their body for digestion. In contrast, bottom-dwelling creatures like trilobites are preserved in the much-finer “background” sediment that continuously falls to the ocean floor.

The underwater mud storms that preserved Nesonektris also provided unprecedented insights in the anatomy of these strange creatures. We noticed that many specimens preserved a stiff, rod-like structure in the tail region that resembles a notochord – the cartilaginous precursor of the backbone. The notochord is found only in vertebrates and their closest relatives (such as lancelets and sea squirts), a phylum appropriately termed Chordata. In primitive chordates, such as the tiny eel-like lancelets, the stiff notochord anchors the body muscles and enables them to generate undulatory swimming movements of the tail.

Importantly, the notochord in these forms consists of a cylindrical stack of disks, which tend to separate during decay. A few specimens of Nesonektris, preserved at exactly the right time, showed precisely the same arrangement of stacked, partially separated blocks. We were thus confident that the rod-like element in the vetulicolian tail was a true notochord used for posture and propulsion, and not a gut or some other organ. The fossils of Nesonektris therefore reveal that vetulicolians were active swimmers propelled by powerful tail movements generated by muscles acting against a stiff notochord.

Incidentally, in humans the notochord persists as the cartilaginous discs between our bony vertebrae – not unlike those in Nesonektris. These are still essential to our movement, as anyone unfortunate enough to suffer a slipped disc can attest.

The presence of a notochord also solves the riddle of vetulicolian affinities, placing them as members of our phylum Chordata. Within this phylum they seem most closely related to tunicates, which include sea squirts and their floating relatives, the salps. Vetulicolians resemble scaled-up versions of salps and the tadpole-like larvae of sea squirts, and their thickened skin (previously mistaken for arthropod cuticle) was probably made of the same material found in the hard outer “sac” or “tunic” that encases typical adult sea squirts. Like sea squirts and salps, they are therefore strange relatives of vertebrates, including ourselves.

In his book Wonderful Life: The Burgess Shale and the Nature of History, the great evolutionary biologist Steven Jay Gould emphasised the rarity and insignificance of our phylum Chordata during the early history of animals. He used the amazing fossils of the Burgess Shale (from the Cambrian of Canada) to stress the role of contingency in evolution: how a scientist in the Cambrian would never have been able to predict the eventual success of chordates, which today comprise more than 75,000 species and include fish, amphibians, reptiles, birds and mammals as well as more obscure groups like tunicates and lancelets.

At the time, vetulicolians had yet to be properly described, let alone recognised as chordates. However, Gould’s intuition was of such a calibre that when one of us (Diego) asked him in 1994 to sign a copy of Wonderful Life, and knowing that Diego would be studying Burgess Shale fossils that summer, Gould wrote: “Try Banffia!”, referring to the only vetulicolian known from the Burgess Shale.

Ironically, the new revelation that these large, abundant and highly conspicuous Cambrian animals are chordates changes Gould’s scenario somewhat. Our phylum was a much more important player during the Cambrian explosion than he could possibly have envisioned, and the eventual success of the vertebrates – though by no means guaranteed – is less surprising.

Diego García-Bellido is an ARC Future Fellow at the University of Adelaide and an Honorary Research Associate of the South Australian Museum. Michael Lee is a senior Research Scientist at the South Australian Museum and the University of Adelaide.