https://x.com/ren_wenyu/status/2096963110854951010

Art Credit to Mario Lanzas

There's a popular paleontology aphorism that “sharks are older than trees.” It's memorable and evocative, but it's also one of those statements whose truth depends almost entirely on what you are willing to call a “shark” and what you are willing to call a “tree.” Devonian forests containing genuinely tree-sized plants existed long before anything resembling a modern shark, while many of the extraordinarily ancient animals routinely presented to the public as “sharks” occupy positions on the opposite end of the Chondrichthyan family tree. However, that changes if we move forward into the Early Carboniferous, something much more interesting begins to happen. By roughly 340–330 MYA, the enormous ctenacanthiform Saivodus striatus was swimming through the sea, making it older than the earliest generally accepted fossil record of true gymnosperms that appeared around 320 MYA. More importantly, Saivodus was not merely some vaguely shark-shaped Paleozoic fish. It belonged to a radiation of chondrichthyans that was considerably closer to the evolutionary lineage of modern sharks and rays than many of the bizarre animals normally used to illustrate the phrase “ancient shark.”

Helicoprion, despite appearing constantly in books and documentaries about prehistoric sharks, was a eugeneodont and is now securely interpreted as a stem holocephalan, as such, its closest living relatives are ultimately ratfish, not sharks, same with Cladoselache. Even Stethacanthus, perhaps the single most iconic Devonian “shark,” belonged to the Symmoriiformes, a group that several modern phylogenetic analyses likewise recover on the holocephalan side of the shark–chimaera split. Ctenacanthiforms are different. Although their exact internal relationships remain messy, and “Ctenacanthiformes” itself may not represent one perfectly clean monophyletic group, they are commonly interpreted as total-group elasmobranchs, lying on the evolutionary side of Chondrichthyes that ultimately produced sharks and rays. Saivodus therefore occupies a particularly fascinating position: not a modern shark, not even a crown-group shark, but something far more legitimately describable as a shark relative than many of the Paleozoic creatures habitually given that label. If Helicoprion and Stethacanthus represent spectacular experiments undertaken by branches whose surviving legacy lies with the chimaeras, Saivodus belonged to the side of the family tree whose descendants would linger in the minds of many a man as the paramount embodiment of oceanic death.

Saivodus lived in tropical Carboniferous seas spread across what are now North America, Europe, and North Africa. Its fossils have been reported from places including Britain, Ireland, Belgium, Austria, Morocco and the United States, demonstrating that this was not some obscure predator confined to one unusual lagoon, but a truly cosmopolitan feeder. Saivodus belonged to a broadly distributed marine fauna at exactly the time ctenacanthiform diversity was exploding. A 2024 review describes the Viséan–Serpukhovian interval of the Mississippian as the point of the group's greatest diversification, with numerous species coexisting in shallow marine environments shortly before the assembly of Pangaea.

And Saivodus was enormous.

For most of its scientific history, the animal was frustratingly known almost entirely from teeth. Some Saivodus teeth have bases exceeding 6 cm across, immediately suggesting an animal radically larger than most contemporary chondrichthyans. Then Mammoth Cave National Park produced something extraordinary: actual cranial cartilage associated with Saivodus teeth, including a lower jaw roughly 60 cm long. That discovery gave paleontologists something considerably better than isolated teeth with which to constrain the animal's proportions. Published work conservatively suggests individuals of at least 4–5 meters or more, while extrapolations from the largest known teeth have produced estimates in the 6–8 meter range, and National Park Service reconstructions have proposed still larger individuals. One NPS assessment has suggested 8–10 m for exceptionally large Mammoth Cave animals and potentially around 11 m from enormous Irish teeth, although those upper estimates should be regarded as considerably more speculative than the basic conclusion that this was a very large predator.

This is why I think Saivodus has a legitimate claim to being the first elasmobranch for which "apex predator" becomes essentially unavoidable as an ecological interpretation. Earlier chondrichthyans were certainly predators, and some were impressive animals, but they inhabited Devonian oceans containing gigantic placoderms. With Saivodus, we are dealing with a post-Devonian hunter potentially larger than a Great White, equipped with a huge jaw and a specialized predatory dentition in ecosystems where there simply is not an obvious larger vertebrate predator sitting above it.

Interestingly, though, its teeth were not miniature great-white teeth. Saivodus possessed the classic multi-cusped grasping architecture associated with early ctenacanths: a prominent central cusp accompanied by lateral and intermediate cusplets. Work on later Saivodus material interprets this dentition primarily as a puncturing-and-grasping apparatus, suited to seizing prey rather than carving enormous pieces out of it with serrated blades. Think something more comparable to a colossal Sand Tiger rather than a Great White per se. The prey would then have been manipulated and swallowed rather than dismantled through repeated sawing bites.

The well-established S. striatus record belongs primarily to the Mississippian, especially the Viséan and Serpukhovian. Yet teeth assigned to the genus have also been reported from the Permian. Most spectacularly, Saivodus sp. occurs in the Kaibab Formation of Arizona, alongside a surprisingly diverse assemblage of Permian ctenacanthiforms, and keep in mind that this formation is approximately 270 MYA. If those Permian teeth genuinely represent the same genus-level lineage as the giant Mississippian Saivodus, we may be looking at a lineage stretching from roughly the mid-Carboniferous into the Permian, roughly 60–70+ million years of geological history.

We tend to treat the Paleozoic as a strange prelude populated by "primitive sharks," after which the real story begins with hybodonts and eventually modern neoselachians. But by the Early Carboniferous, the basic ecological phenomenon we associate so strongly with sharks was already here: large, fast, wide-ranging elasmobranch predators occupying the upper levels of marine food webs. Sure, they wouldn't look anatomically identical to a modern shark with their characteristic dorsal-fin spines; however, the broader ecological pattern is eerily familiar, and the lineage would never really surrender the ocean again.

Ctenacanthiforms themselves eventually vanished. Hybodont-grade elasmobranchs flourished afterward. Crown sharks diversified later still, and by the Cretaceous the Lamniformes & Carcharhiniformes would start moving into the roles we still see them in today. The individual branches changed, mass extinctions repeatedly rearranged the players, and the anatomical machinery evolved enormously. Overall, the ecological experiment that becomes unmistakable with animals like Saivodus has proved unimaginably durable.

Works Cited

Clark, Mary Ann, et al. “26.1 Evolution of Seed Plants.” Biology 2e, OpenStax, 28 Mar. 2018, openstax.org/books/biology-2e/pages/26-1-evolution-of-seed-plants. Accessed 7 Sept. 2026.

Coates, Michael I., et al. “A Symmoriiform Chondrichthyan Braincase and the Origin of Chimaeroid Fishes.” Nature, vol. 541, 2017, pp. 208–211. https://doi.org/10.1038/nature20806.

Duffin, Christopher J., and Michał Ginter. “Comments on the Selachian Genus Cladodus Agassiz, 1843.” Journal of Vertebrate Paleontology, vol. 26, no. 2, 2006, pp. 253–266. https://doi.org/10.1671/0272-4634(2006)26[253:COTSGC]2.0.CO;226%5B253:COTSGC%5D2.0.CO;2).

Hodnett, John-Paul M., et al. “Ctenacanthiform Sharks from the Permian Kaibab Formation, Northern Arizona.” Historical Biology, vol. 24, no. 4, 2012, pp. 381–395. https://doi.org/10.1080/08912963.2012.683193.

Hodnett, John-Paul M., et al. “New Ctenacanth Sharks (Chondrichthyes; Elasmobranchii; Ctenacanthiformes) from the Middle to Late Mississippian of Kentucky and Alabama.” Journal of Vertebrate Paleontology, vol. 43, no. 3, 2024, article e2292599. https://doi.org/10.1080/02724634.2023.2292599.

Hodnett, John-Paul M., et al. “Sharks in the Dark: Paleontological Resource Inventory Reveals Multiple Successive Mississippian Subperiod Cartilaginous Fish (Chondrichthyes) Assemblages within Mammoth Cave National Park, Kentucky.” Parks Stewardship Forum, vol. 40, no. 1, 2024, pp. 53–67. https://doi.org/10.5070/P540162921.

Schnetz, Lisa, et al. “Rise and Diversification of Chondrichthyans in the Paleozoic.” Paleobiology, vol. 50, no. 2, 2024, pp. 271–284. https://doi.org/10.1017/pab.2024.1.

Tapanila, Leif, et al. “Jaws for a Spiral-Tooth Whorl: CT Images Reveal Novel Adaptation and Phylogeny in Fossil Helicoprion.” Biology Letters, vol. 9, no. 2, 2013, article 20130057. https://doi.org/10.1098/rsbl.2013.0057.

“A Peculiar Ridge” - An illustration of a pair of Europejara curiously looking at the hump of an almost sleeping female Concavenator, which I made for a commission.

Its coloration was inspired by a Path of Titans (game) skin, at the client’s request

The Concavenator was a Carcharodontosauridae theropod from the Barremian (Cretaceous – approximately 125 million years ago) of what is now Spain. Like Europejara, it was found at Las Hoyas, a Lagerstätte (a fossil deposit characterized by exceptional preservation) of the La Huérguina Formation, in Cuenca, which preserves a continental subtropical environment consisting of wetlands, channels, and floodplains.

Its fossil includes an almost complete skeleton, scale impressions on the tail and feet, and controversial marks on the arms, interpreted by some as muscle attachment sites and by others as possible feather attachment sites or similar structures.

Its most distinctive feature is the large dorsal hump, whose function remains unknown, with hypotheses including sexual display and thermoregulation.

Stegosaurs were the flailman of the Jurassic period. Wielding a dangerous Spike with unmatched agility and range, they could bring down the biggest and most vicious predators of their day.

Two stegosaurs stand Head and shoulders above the rest as the largest of their kind and among the largest of all armored dinosaurs.

Stegosaurus ungulatus was the largest of its genus. some 8 to 9 m in length and Weighing close to seven tons, it was one of the largest armored dinosaurs to ever live in North America. It had massive meter tall plates on its back and was among the most formidable prey animals of its day.

Dacentrurus, both the type species armatus and the unnamed Iberian species, rivaled stegosaurus ungulatus is size. Measuring 9 m in length and weighing over 7 tons. The type species armatus is alone 9 m in length and the Iberian species has giant femurs referred to it meaning it was just as big. Unlike stegosaurus which was taller and more display oriented, dacentrurus was far spikier and porcupine like.

These two were the largest of their kind and among the most formidable armored dinosaurs and dinosaurs of the Jurassic period. And amazingly they were not separated from each other. Both coexisted on the island of Iberia at the same time in the late Jurassic some 150 million years ago.

In the lourinha formation of Portugal, stegosaurus ungulatus was discovered some 20 years ago. The giant dacentrurus has been known from the formation for decades now.

They both lived on the same island and probably crossed each other's paths although they likely prefer different environments. Stegosaurus was likely an inland dweller while dacentrurus was closer to the coast.

https://x.com/DanPalaeon1/status/1753028055050674267

https://www.geol.umd.edu/~tholtz/dinoappendix/HoltzappendixWinter2011.pdf

https://doi.org/10.1098/rsbl.2014.0984

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6853308

https://doi.org/10.1671/039.029.0420