Jurassic food webs: who ate what in the Morrison Formation?

The Morrison Formation of western North America preserves a famous Late Jurassic dinosaur fauna, including Allosaurus, Diplodocus, Stegosaurus, Camarasaurus and Camptosaurus. But a geological formation is not one moment frozen in rock. Its many outcrops and layers formed across a broad region and through time. A list of genera from the whole formation does not prove that every species met, competed or fed together.

Food webs are reconstructed from several kinds of evidence. Skeletons identify the animals present in a layer. Teeth, wear and bite marks suggest feeding actions. Plant fossils and sediment help describe habitats. Chemical measurements may reveal resource differences that anatomy alone cannot show. The Jurassic Period guide provides the wider context for this regional ecosystem.

What a fossil association can establish

Fossils found in the same quarry are associated in the geological record, but they may not all have died together. Bones can be transported by rivers, concentrated during floods, exposed and reburied, or accumulated over a long interval. Sediment grain size, orientation, abrasion and the condition of bones help test these possibilities. A bonebed may represent one event, repeated events or a time-averaged sample.

This matters when describing predators and prey. A large theropod and a sauropod from the same formation could have overlapped in a region, but that does not prove a specific encounter. Tooth marks on prey bones provide a closer link, especially when tooth shape and mark geometry match. Even then, the mark can show feeding contact without identifying whether the animal was killed by the predator or scavenged after death.

Plants are also unevenly preserved. Fossil leaves, wood, pollen and spores come from particular environments and may have been carried before burial. The Morrison records river channels, floodplains, soils and changing water conditions. Those settings shaped where plants grew, where animals fed and which remains entered the fossil record. The Jurassic Period overview places the Morrison's habitats in the wider history of the period, not as a claim that all taxa shared one site.

Many herbivores, more than one feeding strategy

The formation contains several large sauropods as well as smaller plant-eating dinosaurs. Their size differences encouraged a simple explanation: long-necked species fed at different heights. Height may have mattered, but it is not the only possible dividing line. Different sauropods had different teeth and skulls, and they could have selected different plant parts, feeding motions or patches of vegetation. Smaller herbivores such as Camptosaurus used a different body plan and dental arrangement from the giant sauropods.

A 2025 study of calcium isotopes in tooth enamel from five taxa at Carnegie Quarry offers a direct test of resource use. The authors found isotope differences consistent with Camarasaurus and Camptosaurus using different resources, while Diplodocus overlapped with both. Their analysis also found that the plant tissues consumed may influence the isotope signal more strongly than feeding height alone. This makes niche partitioning plausible, but it does not deliver a complete menu or a map of each animal's daily movements.

Stable isotopes are chemical measurements, not labels for particular plants. Interpreting them requires assumptions about how a signal moves from food into enamel and how soils, plant physiology and preservation affect it. The sample is also small and tied to one quarry. The results therefore support a pattern among the animals tested, not a universal rule for every Morrison sauropod or every season.

Predators and the evidence for meals

Allosaurus is the best-known large theropod in many Morrison assemblages. Its teeth and skull indicate a carnivorous animal, and bones bearing theropod bite marks document feeding on large vertebrates. A 2025 isotope study found that sampled Allosaurus enamel was consistent with an opportunistic predator drawing on the herbivores represented at Carnegie Quarry. The same data did not support substantial bone consumption for the sampled individuals.

That conclusion complements, rather than replaces, other evidence. Bite marks show physical contact; healed injuries may record survival after an attack; stomach contents, when preserved, provide unusually direct dietary clues. Each can answer a different question. A predator's anatomy indicates what it could do, while a mark or gut content records a particular event or meal.

Food webs include smaller animals and decomposers that are less visible in popular reconstructions. Insects, fish, amphibians, mammals, turtles and crocodile-line reptiles occur in Morrison deposits. They could have consumed plants, invertebrates, eggs, carrion or one another. The fossil record samples these groups unevenly, so a diagram with only giant dinosaurs is a useful simplification, not a complete ecosystem.

Why the Morrison was not one uniform habitat

The Morrison Formation extends across several states and records different local settings. River channels shifted across floodplains. Some intervals were wetter, others more seasonal or dry. Soils preserve episodes of exposure; channel sandstones record moving water; mudstones can preserve quieter floodplain conditions. These changes influenced plant cover and water availability, and they altered how bones were buried.

Climate reconstructions combine sediment, fossil plants, soils and geochemistry. They support a landscape with substantial environmental variability, not a single endless lush jungle or a bare desert. Water and vegetation were patchy. Large animals may have moved between feeding areas, but migrations are difficult to establish from a single bonebed.

Some iconic quarries are unusually rich because preservation concentrated remains. A dense bonebed tells us that many fossils came together in one place; it does not automatically mean a herd gathered there or that a predator hunted in packs. Age structure, sedimentology, bone orientation and evidence of transport must be considered before a social story is attached to the assemblage.

How to read a Jurassic food web

A useful food-web reconstruction distinguishes observations from links inferred between them. A plant fossil is direct evidence of a plant in a depositional setting. Tooth wear can constrain how a jaw processed food. Isotope values can reveal differences in resource use among sampled animals. A bite mark may connect a consumer to a carcass. A drawn arrow between two species is a hypothesis unless a specific trace supports it.

The Morrison record suggests that multiple herbivores could share a broad landscape while using resources in partly different ways, and that large predators fed opportunistically on available animals. It does not reveal an exact, stable food chain for the entire formation. Communities changed as channels, climate and species distributions changed, and fossils preserve only a portion of those interactions.

This cautious picture is still rich. It replaces the idea of a simple pyramid with a network of plants, herbivores, predators, scavengers and small animals, each known from different evidence. The best food web is not the most crowded illustration. It is one that shows which connections are measured, which are supported indirectly, and which remain reasonable possibilities.