Why Did Squid Survive the Dinosaur Extinction? — The 100-Million-Year Deep Sea History Reveals the "Fuse of Evolution"

Squids Were the "Survivors"

When people hear about the extinction of dinosaurs, they often think of the end of gigantic land creatures. However, the same era's massive upheaval also relentlessly struck the oceans. About 66 million years ago, during a global mass extinction event, marine life was significantly affected. Among them, the ancestors of modern squids and cuttlefish, which now dominate the seas, seem to have quietly survived. Recent research strongly suggests that the stage for their survival drama was the deep sea.

Squids and cuttlefish are peculiar animals, with features like color-changing skin, agile jet propulsion, and advanced nervous systems. Yet, their evolutionary history was surprisingly unclear. The reason is simple: their soft bodies rarely fossilize, and their genomes are large and complex. In this study, researchers added three new high-quality genomes and integrated them with existing genome and transcriptome data to reconstruct the phylogenetic relationships of the Decapodiformes, which include squids and cuttlefish.

The Origin Was Not Shallow Waters, But the Deep Sea

Generally, evolution is thought to occur in shallow seas or coastal areas, where there is light, high productivity, and intense competition among organisms. However, the current analysis shows that the main lineages of squids and cuttlefish began to rapidly diverge in the deep oceanic environment around the mid-Cretaceous period, about 100 million years ago. The abstract of the paper also states that the divergence of major extant groups is consistent with rapid divergence in the deep ocean.

This point resonates with another study published in Science in 2025. This study visualized numerous squid beak fossils through "digital fossil mining," showing that squids appeared and rapidly diversified by about 100 million years ago. In other words, both fossil and genomic perspectives are converging on the idea that "the significant starting point for squids was about 100 million years ago." This study further connects this with an ecological narrative of "deep-sea origin" and "post-extinction coastal expansion."

Why They Survived the Extinction

So, why were they able to survive a global catastrophe? The research team strongly considers the existence of oxygen-rich "refuges" scattered in the deep sea. The shallow seas at the end of the Cretaceous likely experienced severe impacts from ocean acidification and environmental degradation, making it harsh for shelled cephalopods. In contrast, the deep sea had relatively stable oxygen conditions, and by retreating there, ancestral lineages could have survived.

What is interesting here is the idea that the deep sea was not just a harsh frontier but may have served as a "safety net" for evolution. Environments that usually go unnoticed can become the last safe havens during crises at the scale of Earth's history. While deep-sea research is often romanticized, this study reimagines the deep sea as an "evolutionary refuge." The dark depths of the ocean may not just be habitats for strange creatures but buffers that pass biodiversity into the future. This is more a significant implication drawn from the data than a strict conclusion of the paper.

Evolution Didn't Explode Immediately

Another striking aspect of this study is the concept of a "long fuse." While the main lineages of squids and cuttlefish had already diverged during the Cretaceous, their diversity didn't immediately flourish. According to the research team, there was a period with few major branchings, and only during the recovery period after the K-Pg mass extinction did they rapidly expand into new niches in coastal and shallow seas.

This framework revises the image of evolution as a series of "sudden mutations." Major changes don't necessarily occur abruptly without preparation. Lineages diverge beneath the surface, wait for environmental changes, and when conditions are right, they rapidly disperse. Squids were likely a prime example of this. The world after the extinction of the dinosaurs offered new seats not only for mammals but also for marine predators.

Creatures That Had Shells and Then Shed Them

When looking at squids and cuttlefish as a whole, it's easy to understand them merely as "soft-bodied creatures." However, one focus of the research is on how they have transformed their internalized shells. The rounded shell of cuttlefish, the elongated gladius of typical squids, the spiral shell of ram's horn squid, and even the loss of shells in some shallow-water species demonstrate the incredible diversity of internal structures in Decapodiformes. This phylogenetic analysis provides a foundation for understanding the order in which these morphological differences arose.

Thus, this study not only addresses "where squids came from" but also connects to the question of "why they have such peculiar bodies." The molecular evolution that led to their characteristics—fast swimming, camouflage, advanced nervous systems, and unique sensory organs—can now be more precisely tracked. Researchers at OIST also noted that with this phylogenetic relationship established, it will be easier to compare the evolution of novel organs and behaviors in cephalopods.

What Resonated on Social Media

The reason this topic is gaining attention on social media is not just its academic significance. Following the text of public posts, what is strongly shared is the surprise that "the origin of squids was the deep sea." In OIST's Facebook posts and ScienceX's Threads posts, the diversification that began in the deep sea about 100 million years ago and the expansion after a long stagnation are prominently highlighted.

Next, the narrative that "squids survived the extinction that wiped out the dinosaurs" stands out. Headlines and Facebook posts from Discover Magazine emphasize the point that they escaped to oxygen-rich refuges in the deep sea, and similar angles are shared on the Mastodon-based Biodiversity Channel. For general readers, the drama of "how they survived that global catastrophe" is likely more intuitively appealing than the phylogenetic tree itself.

Furthermore, announcements by the researchers themselves and related parties on X can be confirmed. Posts by the first author Gustavo Sanchez and co-author Fernando Fernández-Álvarez, as well as third-party dissemination posts, directly shared the paper's title and OIST's press release. Based on search results, the current social media reaction seems to focus more on surprise and intellectual curiosity, such as "Was the deep sea a refuge?" and "Is the evolutionary history of squids that old?" rather than controversy or strong skepticism. However, this is a limited observation from initial reactions that can be picked up in public searches and is not a comprehensive analysis of social media as a whole.

More Interesting Than the Numbers Is How the Ocean's Perception Changes

On the Nature paper's page, the Altmetric score was 72 shortly after publication, indicating relatively fast dissemination outside the research community. However, the truly interesting part is not the numbers themselves. This research has shifted the perception of ocean history from a "story of competition occurring on the surface" to a "story where possibilities preserved in the deep sea reshape the surface world."

Squids didn't just survive stubbornly. They kept the flame of evolution alive in the deep sea for a long time, ventured into the coasts after Earth's environment stabilized, and grew into one of the most fascinating and peculiar groups of creatures in today's oceans. Extinction is both an end and a beginning for other lineages. This study shows the rich and brutal history of Earth through the unexpected protagonist of squids.

Source URLs

1. ScienceDaily
Summary of the research, the story of deep-sea origins, and diversification after the K-Pg mass extinction.
https://www.sciencedaily.com/releases/2026/03/260331001100.htm

2. OIST Research Update Page
Explanation from the research institution itself. Used for the addition of new genomes, divergence about 100 million years ago, oxygen-rich refuges in the deep sea, and the long fuse model.
https://www.oist.jp/news-center/news/2026/3/30/100-million-years-ago-evolutionary-fuse-was-lit-deep-ocean-sparking-squid-diversification

3. Nature Ecology & Evolution Published Paper
Original research paper. Used for phylogenetic relationships, rapid divergence in the deep ocean, post-K-Pg coastal expansion, and checking Altmetric values.
https://www.nature.com/articles/s41559-026-03009-1

4. PubMed Information on the 2025 Science Paper
Previous related research. Used for background on the appearance and dispersion of squids by about 100 million years ago.
https://pubmed.ncbi.nlm.nih.gov/40570114/

5. Hokkaido University Related Research Introduction
General explanation of the 2025 fossil research. Used for supplementary background on digital fossil mining and ancient squid fossils.
https://www.global.hokudai.ac.jp/news/22419/

6. ScienceX/Phys.org Threads Post
Used to confirm how the point "diversification began in the deep sea about 100 million years ago" is shared on social media.
https://www.threads.com/@sciencex.physorg/post/DWgLdd9lpkF/genomic-analysis-indicates-that-squid-and-cuttlefish-began-diversifying-in-the/

7. OIST Facebook Post
Used to confirm the presentation of deep-sea origins and diversification after mass extinction in the research institution's own social media announcement.
https://www.facebook.com/oistedu/posts/new-mapping-suggests-deep-sea-origins-and-mass-extinction-triggered-diversificat/1251207693874234/

8. Discover Magazine Article
Reconstructed article for the general public. Used to confirm the angle of "oxygen refuges in the deep sea" and "squids survived the dinosaur extinction" shared on social media.
https://www.discovermagazine.com/66-million-years-ago-squid-survived-the-dinosaur-killing-extinction-in-deep-sea-oxygen-refuges-then-rapidly-evolved-48890

9. Discover Magazine Facebook Post
Used to confirm which expressions are prominently featured in social media for general readers.
https://www.facebook.com/DiscoverMag/posts/for-tens-of-millions-of-years-squid-and-cuttlefish-barely-changed-then-almost-su/1309007781090366/

10. Biodiversity Channel Mastodon Post
Used as an example of how the angle "squids endured the dinosaur extinction crisis in the deep sea" is shared on public social media.
https://newsmast.community/@biodiversity

11. First Author Gustavo Sanchez's X Post
Used to confirm the researcher's own announcement of the paper's publication.
https://x.com/CephaGus/status/2038640748544041148

12. Co-author Fernando Fernández-Álvarez's X Post
Used to confirm the dissemination of publicity by research associates.
https://x.com/cefafalopodo/status/2038582218251997464