It may have looked like a squid, but the beautifully preserved fossil of a creature with 10 arms is the oldest known ancestor of another group of animals that includes octopuses and vampire squid, US scientists say.
"We have soft tissues that are preserved in such good detail that we can actually see individual suction cups on the arms," said Christopher Whalen of the American Museum of Natural History.
The torpedo-shaped creature with fins on its back swam in the tropical waters that covered North America around 328 million years ago.
Its fossilised body was excavated more than three decades ago and put away on a museum shelf until Dr Whalen and his colleague Neil Landman stumbled upon it.
Dubbed Syllipsimopodi bideni, it has joined the pool of species named in honour of US presidents.
The fossil, described today in the journal Nature Communications, pushes back by 82 million years the time when "vampyropods" evolved.
Squid-like but not a squid
Vampyropods are one of three groups of soft-bodied cephalopods that contain vestiges of internal shells.
The group includes the enigmatic vampire squid — which is neither a blood-sucker nor a squid — and is related to octopuses.
Dr Whalen said the fossil provides insight into the point when vampyropods split away from their squid cousins.
"It's very interesting showing you the earliest stages of that evolutionary line."
It had long been thought that ancestral vampyropods had 10 arms like today's squids and cuttlefish, but this is the first time it has been found in the fossil record.
"That's never been documented before in vampyropod," Dr Whalen said.
Along each arm appeared to be suckers.
"This helps confirm that suckers are a very old adaptation," he said.
Two of the ancient animal's arms were longer than the other eight, similar to squid and cuttlefish, which use the longer arms as hands to grab food.
It is this feature that inspires the name of the creature's genus: Syllipsimopodi, which is Greek for "prehensile foot".
By the beginning of the age of dinosaurs, about 80 million years after S. bideni appeared, vampyropod fossils looked more like today's vampire squids, which have eight arms and two long filaments.
Eventually, octopuses evolved with just eight arms.
"For whatever reason, those elongated arms were no longer useful, so they were evolutionarily reduced until they were completely lost in modern octopuses," Dr Whalen said.
But what really sets this animal apart from its squid cousins is what's happening inside its body.
The ancient animal had a long, triangular-shaped, semi-transparent feature made out of chitin called a gladius.
The gladius, which is named after a Roman sword, is the soft outer remnant of the hard shell found inside species such as cuttlefish.
The hard shell, known as a cuttlebone, is important for buoyancy in cuttlefish, a member of the group Decabrachia which includes squid and their relatives.
A few other members of the squid family also have remnants of the hard shell that can only be seen through a microscope, but most just have a gladius.
Vampyropods, such as vampire squid, have ditched the shell altogether and only have a gladius, which is shaped slightly differently to those in the squid family.
In octopuses, the soft gladius is reduced even further to two tiny rods that support the fins.
"Having an organism this early on that's lost the shell on the vampyropod side indicates that they developed an alternative means of buoyancy control relatively early in their evolutionary history," Dr Whalen said.
"Today, vampryopods mainly control their buoyancy through the [regulation of chemicals in their] tissues rather than any hard structure."
Very rare fossils
Dr Whalen said it was rare to find vampyropods in the fossil record.
"You can usually only get them preserved in rather special settings."
The fossil was found in the Bear Gulch Lagerstätte formation in Montana, which was once a tropical sea.
"Part of the reason we think that this fossil is preserved in such great detail is the monsoons that have been predicted to have occurred in this area," Dr Whalen said.
When this animal died, it sank to the sea floor and was quickly covered by silt washed into a prehistoric bay by the monsoonal rains, which preserved its soft tissues including the gladius and arms.
But whether those arms actually have suckers is still up for debate, said Patrick Smith, a palaeontologist at the Australian Museum who studies cephalopod fossils such as giant nautiloids.
"They [the US researchers] do present images which possibly represent suckers, but it's a bit equivocal either way," Dr Smith said.
Dr Smith said interpreting soft-bodied fossils can be extremely challenging.
For instance, there has been a lot of controversy surrounding a previous contender for the earliest known ancestor of octopuses, known as Polsepia, which lived around the same time as S. bideni.
Many scientists now believe Polsepia was not even a cephalopod.
"It's always difficult to interpret any fossil that is made out of soft material," Dr Smith said.
"As soon as a structure has undergone even slight decay, you lose features that are subtle features that are useful for identification."
Dr Smith said S. bideni's internal features were reminiscent of vampire squid and octopus, and the age of the fossil aligned with the timing of squid and vampyropod families splitting in the DNA record of modern cephalopods.
But he said it was "always good to see additional specimens to be certain about bold claims".
A name for a president
S. bideni was named in honour of the inauguration of President Joe Biden.
"I submitted my paper fairly soon after his inauguration … and it seemed like a natural way to celebrate that moment," Dr Whalen said.
"I was particularly encouraged by some of the plans that he put forward to counter climate change."
The ancient cephalopod is not the first animal species named after recent presidents.
Donald Trump has Dermophis donaldtrumpi, a blind amphibian from Panama that buries its head in the sand, along with a moth (Neopalpa donaldtrumpi) and a fossil sea urchin (Tetragramma donaldtrumpii).
While Barack Obama has at least nine species named after him including an extinct prehistoric lizard (Obamadon gracilis), a trapdoor spider (Aptostichus barackobamai), a bird (Nystalus obamai), and several invertebrates including a blood fluke (Baracktrema obamai) and some fish.
