No single fish ruled the Cretaceous. The seas were shared by several kinds of big fish, from giant sharks to fanged bony fish the size of a small boat. Many of them lived in a vast shallow sea that once split North America in two, which is why Kansas chalk is famous for their fossils.1New Mexico Museum of Natural History and Science Bulletin. Overview of the Late Cretaceous fishes of the northern Western Interior Seaway The group that dominates the oceans today was still a supporting player then, and only took over after the extinction that ended the dinosaurs.2PubMed Central. New Age of Fishes initiated by the Cretaceous−Paleogene mass extinction
The Top Predators Cretoxyrhina and Xiphactinus
Cretoxyrhina was a powerful shark that grew up to about 7 metres long.3PubMed Central. Evidence for the Cretaceous shark Cretoxyrhina mantelli feeding on the pterosaur Pteranodon from the Niobrara Formation It was likely one of the top predators of its sea, filling a role like today’s great white and tiger sharks.4PubMed Central. Evidence for the Cretaceous shark Cretoxyrhina mantelli feeding on the pterosaur Pteranodon from the Niobrara Formation Fossils show it ate large animals, including big bony fish, the marine reptiles called mosasaurs, and plesiosaurs.5FHSU Scholars Repository. Paleobiology of the Late Cretaceous Shark, Cretoxyrhina Mantelli (Lamniformes: Cretoxyrhinidae), from Kansas One fossil suggests it even bit into the flying reptile Pteranodon.6PubMed Central. Evidence for the Cretaceous shark Cretoxyrhina mantelli feeding on the pterosaur Pteranodon from the Niobrara Formation Even the hunter’s own body became food: after death it was often eaten by a smaller shark, Squalicorax.7FHSU Scholars Repository. Paleobiology of the Late Cretaceous Shark, Cretoxyrhina Mantelli (Lamniformes: Cretoxyrhinidae), from Kansas
Xiphactinus was a bony fish with a body shaped like a modern tarpon, a crescent tail and large fang-like teeth, all signs of a fast, active hunter.8Journal of Vertebrate Paleontology. Evidence of endothermy in the extinct macropredatory osteichthyan Xiphactinus audax (Teleostei, Ichthyodectiformes) It reached roughly 5 to 6 metres, about the length of a car and a half. One famous fossil holds an entire 2-metre fish inside it, proof it could catch large, quick prey.9Journal of Vertebrate Paleontology. Evidence of endothermy in the extinct macropredatory osteichthyan Xiphactinus audax (Teleostei, Ichthyodectiformes) Cretoxyrhina could eat Xiphactinus-sized fish, so the hunters were sometimes prey too. Even very young Xiphactinus already had teeth for eating fish, so it hunted fish its whole life.10Historical Biology. The smallest recorded specimen of Xiphactinus audax from the Niobrara Chalk of Kansas and its implications for juvenile ichthyodectid ecology Like tarpon, the young probably grew up in shallow coastal water where they were safer.11Historical Biology. The smallest recorded specimen of Xiphactinus audax from the Niobrara Chalk of Kansas and its implications for juvenile ichthyodectid ecology
Enchodus the Saber-Toothed Herring
Enchodus was a widespread part of sea life around the world in the Late Cretaceous.12Wikipedia. Enchodus Its large front fangs earned it the nickname “saber-toothed herring,” though it was not a herring.13Wikipedia. Enchodus It had more fangs on the roof of the mouth.
Its fangs, long sleek body and big eyes point to a predator.14Wikipedia. Enchodus Most were modest in size, roughly like a large trout. Long fangs are good for gripping slippery, struggling prey and stopping it from wriggling free, rather than for slicing it up. Big eyes suggest it hunted by sight, likely in open water where small fish gathered.
As a common mid-sized fish, Enchodus was both hunter and frequent prey for bigger animals, a link between small fish and the giant predators. Its teeth are among the most common Cretaceous fish fossils collectors find.15Wikipedia. Enchodus
Giant Filter Feeders and Shell Crushers
Some of the biggest fish ate the smallest food. Bonnerichthys was a giant bony fish that fed by straining tiny food from the water.16Journal of Palaeogeography. The first record of the pachycormid fish Bonnerichthys in Eurasia marks a global distribution of Late Cretaceous suspension-feeding giants It swam with its mouth open and filtered plankton through gill structures, as today’s whale sharks and basking sharks do.
Bonnerichthys was long thought to live only in what is now the USA.17Journal of Palaeogeography. The first record of the pachycormid fish Bonnerichthys in Eurasia marks a global distribution of Late Cretaceous suspension-feeding giants A find in Eurasia shows it lived more widely, as other big filter-feeding fish of its family did.18Journal of Palaeogeography. The first record of the pachycormid fish Bonnerichthys in Eurasia marks a global distribution of Late Cretaceous suspension-feeding giants
At the other extreme, Ptychodus was a shark with flat, pavement-like teeth for crushing hard prey.19PubMed Central. New insights on the shell-crusher shark Ptychodus decurrens Agassiz, 1838 (Elasmobranchii, Ptychodontidae) based on the first known articulated dentition from the Upper Cretaceous of Croatia Instead of points, its jaws held rows of flat or bulging teeth like cobblestones, and collectors often find single teeth with ridged tops. The shape works like a nutcracker rather than a knife. It is the only known fast, open-water shark with a shell-crushing diet.20New Scientist. Exquisite fossils of Cretaceous shark solve mystery of how it hunted So it likely ate well-armoured animals like large ammonites and sea turtles.21New Scientist. Exquisite fossils of Cretaceous shark solve mystery of how it hunted
Ammonites and turtles were plentiful, and few predators could break their shells, so there was less competition. On reefs, deep-bodied pycnodont fish used rounded, thick-enamelled teeth to crush shelled invertebrates,22Encyclopedia Britannica. Pycnodontiformes so crushing was a way of life in several unrelated groups.
Sawfish Rays, Freshwater Coelacanths and Regional Differences
Sclerorhynchids were rays with long toothed snouts, like today’s sawfish but only distant relatives.23PubMed Central. Evolutionary origins and development of saw-teeth on the sawfish and sawshark rostrum (Elasmobranchii; Chondrichthyes) The snout was probably used to slash sideways through schools of fish or to dig prey out of the sediment, depending on the species.
Some, like Ischyrhiza, had thorny backs typical of slow, bottom-dwelling rays.24Historical Biology. Paleobiology of the Late Cretaceous sclerorhynchid sawfish, Ischyrhiza mira (Elasmobranchii: Rajiformes), from North America based on new anatomical data Schizorhiza, though, seems to have been a medium-sized open-water ray, so not all were bottom dwellers.25PubMed Central. Early development of rostrum saw-teeth in a fossil ray tests classical theories of the evolution of vertebrate dentitions It spread from the Middle East to North Africa and North America.26PubMed Central. Early development of rostrum saw-teeth in a fossil ray tests classical theories of the evolution of vertebrate dentitions Rays and sharks have skeletons of cartilage, which rots away easily, so what usually survives is teeth and the tiny toothlike points in the snout.
Coelacanths, now known only from the deep sea, also lived in Cretaceous rivers and lakes.27PubMed Central. The last known freshwater coelacanths: New Late Cretaceous mawsoniid remains (Osteichthyes: Actinistia) from Southern France They were thought extinct until one was caught in 1938, and today’s two species live deep in the Indian Ocean. A fossil from southern France is the youngest known freshwater coelacanth.28PubMed Central. The last known freshwater coelacanths: New Late Cretaceous mawsoniid remains (Osteichthyes: Actinistia) from Southern France Mawsonia, a freshwater and brackish coelacanth, lasted at least 50 million years.29PLOS ONE. The first late cretaceous mawsoniid coelacanth (Sarcopterygii: Actinistia) from North America: Evidence of a lineage of extinct ‘living fossils’
The mix of fish also changed from place to place. In the northern inland sea, low-oxygen bottom water left fewer shell-crushing sharks and rays than in the south.30New Mexico Museum of Natural History and Science Bulletin. Overview of the Late Cretaceous fishes of the northern Western Interior Seaway Shell-crushers eat bottom-dwelling animals, which struggled in the low-oxygen water, so they had less to eat.
What Happened at the End of the Cretaceous
The extinction 66 million years ago did not hit all sharks and rays equally.31PubMed. Global impact and selectivity of the Cretaceous-Paleogene mass extinction among sharks, skates, and rays Rays and shell-crushing species lost more than 72% of their kinds.32PubMed. Global impact and selectivity of the Cretaceous-Paleogene mass extinction among sharks, skates, and rays Sharks with other diets fared better, as did species with large ranges.33PubMed. Global impact and selectivity of the Cretaceous-Paleogene mass extinction among sharks, skates, and rays
Shell-crushers depended on prey like ammonites that collapsed. A shark that relied on one kind of prey had no fallback, while one that ate many things could shift to what was left. Species spread over a large range had a better chance that some population lived somewhere less badly hit. Sharks survived but never regained giant Cretaceous predators like Cretoxyrhina.
Modern bony fish were already diversifying in the Cretaceous.34Palaeogeography, Palaeoclimatology, Palaeoecology. Correlation between environment and Late Mesozoic ray-finned fish evolution But spiny-finned fish, the group that includes perch and tuna, stayed low in variety of body shapes through the period.35PubMed Central. Explosive morphological diversification of spiny-finned teleost fishes in the aftermath of the end-Cretaceous extinction Their variety jumped sharply right after the extinction and has changed little since.36PubMed Central. Explosive morphological diversification of spiny-finned teleost fishes in the aftermath of the end-Cretaceous extinction They filled roles emptied by the extinction, beginning an “age of ray-finned fishes.”37PubMed Central. New Age of Fishes initiated by the Cretaceous−Paleogene mass extinction Most fish on a fishmonger’s counter today are spiny-finned fish, and their success started in those emptied seas.
Since the Late Cretaceous, shooting the jaws forward to snatch prey has grown more common and extreme in coastal fish.38PubMed. The Rise of Jaw Protrusion in Spiny-Rayed Fishes Closes the Gap on Elusive Prey The mouth extends like a tube, creating suction in a fraction of a second that pulls in water and prey together. It helps catch quick, hiding prey without swimming right up to it, and many reef fish use it today. The first such fish could extend their jaws about 8% of their body length,39Science News. Fish have had telescoping jaws for 100 million years so the strong, tube-like versions seen now developed gradually over millions of years.