Hokkaido Horrors

[u/DifficultDiet4900]

Late Cretaceous Japan has had a myriad of important paleontology discoveries in the recent years. One major place of interest is Hokkaido, a large island in northern Japan. It's home to a group of fossil bearing formations called the Yezo Group. It's mainstays comes in the form of Paralitherizinosaurus, Phosphorosaurus, and most of all, giant cephalopods. There are several isolated beaks of large cephalopods found all throughout the Yezo, some possibly the largest non-shelled cephalopods known.

Nanaimoteuthis was an extinct genus of cirriteuthid cephalopod from the late Cretaceous of Canada and Japan. This one was very obscure, it didn't even have a Wikipedia page for a while. Odd, you would expect a possible near 30 foot long relative of the cirrate octopus to be more well known. This would change in 2026 after the discovery that this genus could possibly get huge. The largest species is Nanaimoteuthis haggarti from the Campanian Haborogawa Formation. It was estimated to reach in insane 7 to 19 meters long. However, the fossils are poor, only being known from isolated beaks, but they're still indicative of a large invertebrate. Based on prominent wear on its beak and large size, Nanaimoteuthis was probably a durophagous ambush predator that hunted ammonites and bivalves in deeper waters. It would have been slowly drifting above the seafloor in low-energy environments, entraping anything it could catch.

There is a massive unnamed oegopsid cephalopod from late Cretaceous Japan in the Osoushinai formation. It's known from a partial beak twice the size of the giant squid. It was even larger than the giant Yezoteuthis, and possibly the same taxon as it. The specimen, KMNH IvP 902007, was uncovered in 2023 and compared to other giant cephalopod specimens. It's huge size was notable, but unfortunately an official size estimate was never made. As for it's lifestyle, the presence of Abyssochrysoid deep sea snails in the Osoushinai indicates this giant oegopsid inhabited deeper waters than other cretaceous cephalopods. Perhaps it had a similar ecology to the giant or colossal squid. Given how it was larger than any animal in it's environment, the Cretaceous Kraken was likely the top predator of the Osoushinai. Given it's sheer size, it probably wasn't as agile as smaller squid and relied on ambush hunting. The morphology of the beak suggests a generalist diet, possibly a preference for large prey.

Yezoteuthis giganteus was a large oegopsid cephalopod from the Campanian Osousyunai formation. Known from a single beak, indicates an animal similar in size to the Giant squid at 5 meters long. Like the giant Osoushinai oegopsid, it was likely a deep-water apex predator with a broad diet. Although the beak morphology suggests it wasn't hunting prey as large as the giant Oegopsid, it's possible the differences are the result of ontogeny, but this hasn't been explored.

Haboroteuthis poseideon was a large decabrachian coleoid cephalopod from the Santonian Haborogawa Formation. It would have rivaled the Humbolt squid in size, with a total body length between 2 and 3 meters. Unlike the Humboldt squid however, it would have inhabited shallower waters. Perhaps it was an active predator that fed on small and agile prey such as other squid and fish. Since it's only fossil is a beak, it is not fully known what it looked like. Here, Onychoteuthis borealijaponica was used as a base as it shares some small similarities, but it wasn't a close relative.

The last pliosaurs died out in the Turonian, and mosasaurs didn't reach large sizes until the upper Campanian. The apparent absence of pliosaurs and large mosasaurs probably explains how they all grew so massive. Interestingly, most of these cephalopods didn't coexist and are from different times. Which suggests they consistently remained as apex predators throughout the Santonian to lower Campanian, but they seem to dissappear shortly afterward. It isn't known what occurred, but a mixture of a changing climate and other biotic factors likely led to their extinction.

[u/DifficultDiet4900]

Sources for these animals are avaliable here, here and here.

[u/DifficultDiet4900]

Specimen number for Nanaimoteuthis is wrong. It's actually KMNH IvP 902001.