Plesiosaurs, a diverse group of extinct marine reptiles, commanded the ancient oceans for over 135 million years, from the Late Triassic until the end of the Cretaceous Period. Recognized by their unique body plans, these creatures evolved to fill various ecological roles within marine ecosystems. Understanding what they ate provides insights into their adaptations and the prehistoric food webs they inhabited.
Their Primary Prey
Plesiosaurs were carnivores, with diets varying by species and adaptations. Fish formed a significant part of the diet for many plesiosaurs, including bony fish and smaller sharks. The presence of fish vertebrae and scales in fossilized stomach contents directly supports this.
Cephalopods, such as belemnites and ammonites, were also common prey for many plesiosaurs. Their strong jaws were likely capable of crushing the hard shells of these invertebrates. Some larger plesiosaurs, particularly the short-necked pliosaurs, were apex predators that hunted other marine reptiles, including smaller plesiosaurs, and occasionally pterosaurs or juvenile mosasaurs.
Some studies suggest that certain long-necked elasmosaurids, typically thought of as fish and squid eaters, also consumed bottom-dwelling invertebrates. Evidence from fossilized gut contents includes clams, snails, and crustaceans, indicating a broader dietary range for some species than previously assumed.
Clues from the Fossil Record
Scientists piece together the diets of plesiosaurs using various types of fossil evidence. Direct evidence, though rare, comes from preserved stomach contents (gastric residues or precoprolites). These finds can contain identifiable remains of a plesiosaur’s last meal, such as cephalopod hooklets, fish teeth, and vertebrae. For example, the plesiosaur Umoonasaurus demoscyllus and a specimen nicknamed “Eric” both contained fish bones in their gut regions, confirming a fish-heavy diet.
Coprolites, or fossilized faeces, offer another form of direct evidence. While it is often difficult to definitively link a coprolite to a specific plesiosaur species, these fossils can reveal general dietary components, frequently containing fish scales and bones. Their shape and contents provide insights into digestive processes and prey types.
Tooth morphology (the study of tooth shape and structure) provides indirect evidence about a plesiosaur’s diet. Plesiosaur teeth were generally sharp and conical, well-suited for piercing and gripping slippery prey like fish and cephalopods, rather than chewing. Wear patterns also indicate prey type and feeding method.
Gastroliths, or stomach stones, are frequently found associated with plesiosaur skeletons, particularly in elasmosaurids. Their exact purpose remains debated; they may have aided in grinding food, acted as ballast for buoyancy control, or served both functions. Gastroliths found alongside fish bones support their role in digestion.
Hunting Styles and Dietary Specializations
The diverse physical characteristics of plesiosaurs suggest a variety of hunting styles and dietary specializations across different groups. Long-necked plesiosauromorphs, like elasmosaurids, typically had smaller heads and numerous slender, interlocking teeth. These adaptations suggest ambushing smaller, soft-bodied prey like fish and cephalopods. Their elongated necks might have allowed them to sneak their heads into schools of fish without moving their large bodies, effectively functioning as a “fish trap”.
Earlier depictions showed them raising heads above water, but research indicates their necks were not flexible, likely used for rapid underwater strikes. Some long-necked plesiosaurs may have also bottom-fed, probing the seabed for invertebrates. Upward-positioned eyes in some elasmosaurids could have aided ambushing prey from below.
In contrast, short-necked pliosauromorphs, like Liopleurodon and Pliosaurus, possessed massive heads and powerful jaws equipped with large, conical teeth. These features indicate they were powerful apex predators, hunting and dismembering larger prey, including other marine reptiles and substantial fish. Their robust skulls and deeply rooted teeth suggest they could withstand the forces involved in “twist-feeding,” a method used to tear apart large carcasses.
Different pliosaur species also showed specialized feeding adaptations. For instance, Peloneustes appears to have been a piscivore, while Liopleurodon likely targeted large, hard-boned prey. Simolestes, with its exceptionally wide and deep skull, may have predominantly consumed invertebrates, possibly cephalopods, potentially necessitating salt-secreting glands due to high salt content. These varied dental and cranial morphologies reflect the broad range of ecological niches occupied by plesiosaurs.