Plesiosaurs were a diverse group of marine reptiles that dominated the oceans during the Mesozoic Era, characterized by their four large flippers and generally long necks or massive heads. Determining the diet of these extinct predators presents a significant challenge, as direct evidence is incredibly rare and often fragmented. Scientists must rely on a combination of fossilized gut contents, the presence of stomach stones, and detailed analysis of their anatomical features to reconstruct their feeding habits.
Direct Evidence from Stomach Contents and Gastroliths
The most concrete data regarding the plesiosaur diet comes from the remnants of their last meal preserved within some fossils. Instances of preserved stomach contents, known as precoprolites, are rare but provide an unambiguous link between predator and prey. These remains often include small, hard parts that resist digestion, such as the tiny hooklets from the tentacles of coleoid cephalopods, like squid and belemnites.
Fossilized fish remains, including scales, teeth, and vertebrae from small teleost fish, have also been found in the gut region of several plesiosaurs. A juvenile plesiosaur nicknamed “Eric,” for example, was found to have the vertebrae of at least 17 small, undescribed fish species in its gut. In one Japanese specimen, the jaw apparatus of several ammonites, hard-shelled cephalopods, was concentrated in the stomach area.
Another common feature associated with plesiosaur fossils is the presence of gastroliths, or polished stomach stones, often found in large numbers near the rib cage. While some paleontologists suggested these stones acted as ballast to stabilize the marine reptile, the weight of the stones relative to the animal’s massive size likely made this effect negligible.
Evidence suggests a digestive function, where the stones, along with powerful stomach muscles, helped grind up the ingested food, particularly the bones and shells of their prey. The stones themselves are often composed of rock types not found locally, indicating the plesiosaurs traveled great distances to ingest them, possibly from coastal areas or rivers.
Inferring Diet from Jaw and Tooth Structure
When direct gut evidence is unavailable, scientists turn to the functional morphology of the plesiosaur skull and teeth to infer feeding strategies. The dental structure of plesiosaurs typically featured teeth not designed for chewing, meaning prey had to be swallowed whole, limiting the size of the animal they could consume. However, the shape and placement of the teeth varied significantly, reflecting different dietary niches.
Many plesiosaurs possessed long, slender, and interlocking teeth, sometimes described as needle-like, which were well-suited for a piscivorous diet. These teeth formed a cage-like trap, perfect for seizing and holding onto small, fast-moving prey like fish and squid. Some species also exhibited heterodonty, meaning they had different tooth shapes along the jawline, often with larger, more robust teeth at the front of the mouth for initially piercing and securing the prey.
In contrast, the short-necked plesiosaurs, or pliosaurids, evolved much more robust jaws and teeth, indicative of a powerful, tearing bite. Their teeth were often larger, more conical, and sometimes featured ridges that would have strengthened the tooth against torsional forces. This powerful jaw structure, sometimes associated with an expanded mandibular symphysis, suggests a feeding style that involved subduing and consuming larger, struggling prey, possibly including a specialized technique known as “twist-feeding” to dismember victims.
The Role of Plesiosaur Morphotypes in Diet
The overall body plan (morphotype) of plesiosaurs correlates strongly with their hypothesized diet, dividing the group into specialized feeders. The Elasmosaurids, known for their exceptionally long necks and relatively small heads, likely employed a stealth-based hunting strategy. Their long, flexible necks may have allowed them to maneuver their small heads rapidly into schools of fish or cephalopods without displacing much water, reducing the chance of scaring the prey away.
These long-necked forms generally specialized in a diet of smaller, soft-bodied marine organisms, utilizing their needle-like teeth to impale and secure fish and invertebrates. The long neck may have also served as a visual barrier, allowing the predator to approach from below, ambushing prey silhouetted against the ocean surface. The small, delicate nature of their prey aligns with the less robust jaw structure seen in this group.
Conversely, the Pliosaurids, characterized by their short necks, massive heads, and immense body size, were the apex predators of their marine ecosystems. Their powerful, triangular skulls and deeply rooted, robust teeth equipped them to tackle much larger and more formidable animals than their long-necked relatives. Fossil evidence, including bite marks found on the bones of other marine reptiles like ichthyosaurs and even smaller plesiosaurs, confirms their role as macropredators. They specialized in a diet of large fish, sharks, and other marine reptiles, relying on sheer power and a bone-crushing bite.