Ancient 'squid' turned out to be something else entirely

Ancient marine fossils from North Greenland have shed light on a major misidentification. A creature once thought to be an early squid relative is not a cephalopod at all. New research shows it is actually linked to arrow worms, or chaetognaths. These creatures swam in Earth’s seas more than 500 million years ago.

The study was led by researchers from the University of Bristol, the Korean Polar Research Institute, and the University of Copenhagen.

The team examined 25 fossils of a mysterious creature called a nectocaridid. Once believed to be an ancient cephalopod, it had a squid-like shape that puzzled scientists for years.

Arrow worm fossil found in Greenland

The fossils came from Sirius Passet, a site known for exceptional preservation. Located in North Greenland, this fossil bed holds delicate remains from the Early Cambrian period, around 518 million years ago.

“Sirius Passet is a treasure trove of fossils from the Cambrian Explosion. We not only find delicate soft-bodied fossils but also their digestive systems, musculature and sometimes even their nervous system,” noted Dr. Jakob Vinther from the University of Bristol.

Fifteen years ago, a paper based on Burgess Shale fossils proposed that nectocaridids were cephalopods related to octopuses and squids. That claim confused many in the field.

“It never really made sense to me, as the hypothesis would upend everything we otherwise know about cephalopods, and their anatomy didn’t closely match cephalopods when you looked carefully,” said Dr. Vinther.

Nervous system holds the answer

As more fossils emerged, researchers noticed something unique. Many specimens had their nervous systems preserved in mineral form. This was a key breakthrough.

“We discovered our nectocaridids preserve parts of their nervous system as paired mineralised structures, and that was a giveaway as to where these animals sit in the tree of life,” said Dr. Vinther.

Soon after, the team found fossils of arrow worms, which helped solve the mystery. One clear feature made the connection possible: the ventral ganglion.

Ganglion links to arrow worm fossil

This ventral ganglion is a nerve mass found on the underside of living arrow worms. It is unique to their group and sometimes gets replaced with phosphate minerals during decay, making it fossilize well.

“These fossils all preserve a unique feature, distinct for arrow worms, called the ventral ganglion,” noted Dr. Tae-Yoon Park from the Korean Polar Institute.

That discovery gave the team the final clue they needed. “We now had a smoking gun to resolve the nectocaridid controversy. Nectocaridids share a number of features with some of the other fossils that also belong to the arrow worm stem lineage,” said Park.

Arrow worm fossil only looked like squid

Despite their similarity to squids, nectocaridids were not related. Their streamlined shape was simply an adaptation to swimming.

“Many of these features are superficially squid-like and reflect simple adaptations to an active swimming mode of life in invertebrates, just like whales and ancient marine reptiles end up looking like fish when they evolve such a mode of life,” Park explained.

This means evolution shaped their bodies for speed and stealth, and similarities with squids were not due to a shared ancestor or genetic heritage.

Eyes, antennae, and stealth

One of their most remarkable features is their eyes. These ancient creatures had complex eyes whereas today’s arrow worms can barely detect light direction.

“Nectocaridids have complex camera eyes just like ours. Living arrow worms can hardly form an image beyond working out roughly where the sun shines,” said Dr. Vinther.

“Our fossils can be much bigger than a typical living arrow worm and, combined with their swimming apparatus, eyes and long antennae, they must have been formidable and stealthy predators.”

Evidence of carnivorous diet

The team found direct proof of their predatory nature. Some fossils had remains of Isoxys, a type of swimming arthropod, in their digestive tracts. This shows they hunted and consumed other marine animals.

The new species has been named Nektognathus evasmithae. It honors Professor Eva Smith, Denmark’s first female law professor and a defender of human rights.

“My decision to name our fossil after Eva, is that this animal was a smart and stealthy fighter just like she is,” Dr. Vinther concluded.

Ultimately, this discovery changes our view of early marine food webs and the evolution of modern sea predators.

The study is published in the journal Science Advances.

Image Credit: Bob Nicholls

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–