Nectocidides are enigmatic paleozoic animals with a controversial position. These creatures have been adapted to swimming, fins, a head -of -the -head region in the eyes of the traced camera and twin tentacles. Previous hypotheses have placed them in their own crustacean, chordés, cephalopods or radioiodontes type phylum. An analysis of new fossils in northern Greenland shows that nectocidides are in fact early descendants of arrows, also called chaetognathes. This discovery means that fairly simple marine arrows have ancestors with much more complex anatomies and a predatory role higher in the food chain.
Reconstruction of the life of Nektognathus evasmithae. Image credit: Bob Nicholls.
“About 15 years ago, a research document, based on fossils from the famous burgess shale, said the nectocidides were cephalopods,” said the Paleontologist of the University of Bristol, Jakob Vinther.
“It never really had meaning for me, because the hypothesis would upset everything we know otherwise on the cephalopods and their anatomy did not correspond closely to the cephalopods when you looked carefully.”
In new research, Dr. Vinther and his colleagues have described Nektognathus evasmithaeA new species of Nectocaridid du Past Sirius, 519 million, the Passet de Sirius, from northern Greenland.
By analyzing 25 fossil samples of Nektognathus evasmithae, They were able to determine where the nectocidides are part of the tree of life.
“We have discovered that our nectocidids retain parts of their nervous system such as mineralized paired structures, and it was a gift for the place where these animals are in the tree of life,” said Dr. Vinther.
Nektognathus evasmithaeHolotype. Image credit: Vinther and al., TWO: 10.1126 / SCIADV.adu6990.
Recently, paleontologists have discovered fossils in Sirius past -up to another branch of the animal tree – a small group of swimming worms called to arrows or chaetognaths.
“These fossils all preserve a unique characteristic, distinct for arrow worms, called the ventral lymph node,” said Dr. Tae-Yoon Park, paleontologist at the Corée Polar Institute.
The ventral lymph node is a large mass of nerves located on the belly of living arrows, which is unique to this type of creature.
The unique anatomy of the organ combined with the special preservation conditions means that it is sometimes replaced by phosphate minerals during decrease.
“We now had a firearm to resolve the controversy with nectocidides,” said Dr. Park.
“Nectocidides share a number of features with some of the other fossils which also belong to the line of arrow verse rod.”
“Many of these features are superficially similar to calmar and reflect simple adaptations to an active life lifestyle in invertebrates, just like whales and old marine reptiles end up looking like fish when they evolve such a way of life.”
“Nectocidides have complex camera eyes like ours,” said Dr. Vinther.
“The verses of living arrows can barely form an image beyond the training roughly where the sun is shining.”
“So, the ancestors of arrow worms were really complex predators, as well as calmars that only evolved 400 million years later.”
“So we can show how arrow worms occupied a much higher role in the food chain.”
“Our fossils can be much larger than a typical living arrow worm combined with their swimming apparatus, their eyes and their long antennas, they had to be great and stealthy predators.”
“As an additional proof that the nectocidids are carnivores swimming, we have found several specimens with the shells of a swimming arthropode, called isoxys, inside their digestive tract.”
The study was published this week in the journal Scientific advances.
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Jakob Vinther and al. 2025. A fossilized ventral lymph node reveals a chaetognath affinity for Cambrian nectocidides. Scientific advances 11 (30); Two: 10.1126 / Sciadv.adu6990
