When did squids begin to thrive in the sea? Scientists have struggled to answer this question. This is because soft-bodied creatures like squids are difficult to preserve as fossils. Most of the previously discovered squid fossils date back to after 45 million years ago, and even then, they were only the minute structures called 'statholiths' used for maintaining the squid's balance.
Researchers from Japan's Hokkaido University and the Synchrotron Radiation Research Institute announced on the 27th in the international journal 'Science' that they revealed the history of squids from 100 million years ago using a new method called 'digital fossil excavation.'
The research team ground the carbonate rocks excavated in Japan from the Mesozoic Cretaceous period (from 146 million to 66 million years ago) to a thickness of within 10 µm (micrometers; 1 µm is one-millionth of a meter) and took high-resolution images of each plane. They digitally reconstructed the entire rock in three dimensions by layering thousands of these images. As a result, they could restore 263 fossilized squid mouth parts that were previously invisible within the rock in three dimensions.
Creating a three-dimensional image of the fossil this time is similar to taking computer tomography (CT) images. CT involves dividing the human body into hundreds of high-resolution flat X-ray images. Each X-ray image shows a cross-section of tissue. By assembling these, one can view the internal structure of the body in three dimensions.
CT can also provide a three-dimensional view of fossils within rocks. However, it is in black and white. In contrast, this method provided color three-dimensional images since it involved polishing the rock while taking and combining flat images.
The rocks where the fossils were found are from strata about 100 million to 70 million years old, indicating that squids had already diversified and reigned as major predators in the ecosystem since the early to mid-Cretaceous period. Until now, the lack of early fossils rendered the hypothesis that squids only began to thrive after the extinction of dinosaurs 66 million years ago plausible, but this study has pushed that timeline significantly earlier.
The researchers noted, "Squids were larger and more common than contemporaneous soft-bodied ammonites or fish ancestors, and they show that squids were already forming the backbone of the marine ecosystem as intelligent and fast predators from that time."
Additionally, it was found that the two major groups of squids, Oegopsida and Myopsida, appeared 15 million years and 55 million years earlier than previously thought, respectively. Oegopsida refers to oceanic squids, while Myopsida refers to coastal squids. Oegopsida had anatomical features different from today, while Myopsida already resembled modern squids.
Professor Iba Yasuhiro of Hokkaido University stated, "Fossil research has previously relied on physical specimens, which diminished accessibility and reproducibility, but in this study, we processed the entire process from excavation to analysis digitally and made all the data public. This method is crucial for ensuring research integrity and will facilitate groundbreaking discoveries globally."
References
Science(2025), DOI: https://doi.org/10.1126/science.adu6248