Is it a plant? Is it a squid? No, it’s a dead fish

The oldest squid in the fossil record has turned out to be a fish.

A paper in the Journal of Palaeontology addresses the correct identification of a fossil unearthed in the US state of Kansas some 70 years ago. The fossil has been misidentified twice, and thus serves as a demonstration of the tunnel vision that can sometimes arise when researchers drill down deep into their own narrow fields.

The specimen was first examined in 1948 by a pair of US paleobotanists. Looking at the roughly 45-centimetre by 25-centimetre slab of rock they felt confident in identifying hexagonal plates and threadlike filaments and concluding therefore that what they were looking at was an example of fossilised algae. They named it Platylithophycus cretaceus.

This classification was then challenged in 1968 when two other researchers, specialists in cephalopods, compared striations on the surface to those of modern-day cuttlefish and pronounced the fossil to be a squid – at that point, the earliest ever discovered.

And there things sat, perhaps a little uneasily, until Allison Bronson and John Maily from the American Museum of Natural History opted to take another look at the 85 million year-old evidence. The first thing the pair did was to conduct a test to determine exactly what the fossil remains were made of. Remarkably, neither the paleobotanists nor the cephalopod boffins had done such a thing.

Applying a little dilute acid to the specimen, Bronson and Maily quickly discovered that the remains were made of calcium phosphate. Squid and algae fossils, in contrast, would be made of calcium carbonate.

With this information on board, the pair then used a combination of close-quarters visual inspection and scanning-electron microscopy to study the structures previously described. The hexagonal plates, they determined, were tessellated calcified cartilage, and the filaments were most likely gill arches.

Platylithophycus cretaceus, it turns out, was a fish, closely related to modern day sharks and rays.

“There are many examples of temporarily misplaced taxa in paleontological history, including ferns that were once thought to be sponges and lungfish teeth thought to be fungi,” says Bronson.

“In this case, the misidentification didn’t happen because of a lack of technology at the time – scientists familiar with cartilage structure could easily see this was a chondrichthyan fish. The researchers used reasonable arguments for their interpretations, but didn’t look outside of their own fields.”

The researchers speculate that the fossil may be from a species similar to a modern manta ray, but because it does not contain teeth or other features needed to make a definite identification, it cannot be formally renamed. The fish, therefore, whatever it might have once been, will forever carry the name of a plant.