The Rise of Dinosaurs Through the Food Web: Digestive Fossils Reveal Ecological Advantage and Opportunity

Dinosaurs represent one of the most successful evolutionary lineages in Earth's history. Even today, more than ten thousand living dinosaur species—birds—inhabit the planet and dominate the skies. Yet despite this remarkable success, the early radiation of dinosaurs remains one of the most puzzling events in vertebrate evolution. Exactly how dinosaurs rose from relatively minor components of Triassic ecosystems to the dominant animals of terrestrial environments has long remained unclear.

A recent study approached this question from an unusual source of evidence: fossilized digestive remains. Researchers examined more than five hundred digestive fossils from the Polish Basin, collectively known as bromalites. These fossils include coprolites (fossilized feces), regurgitated material, and preserved digestive contents from ancient digestive tracts. By analyzing the remains of food preserved inside these fossils, scientists were able to reconstruct ancient diets and food webs, comparing the feeding habits of dinosaurs with those of other animals living in the same ecosystems. Through this approach, the study attempts to identify the ecological factors that allowed dinosaurs to gain a long-term advantage over their contemporaries.

Bromalites produced by various Triassic animals（圖片來源：Qvarnström, M et al. (2024)，採用 CC BY 4.0 授權）

Earlier Hypotheses About Dinosaur Success

Current fossil evidence indicates that dinosaurs first evolved during the Middle Triassic. During this early phase, dinosaurs played only minor roles within terrestrial ecosystems and were far from dominant. Only about thirty million years later, during the Early Jurassic, did dinosaurs come to occupy nearly all ecological niches on land and emerge as the most influential vertebrates in terrestrial ecosystems.

During this transitional period, many groups of non-dinosaur terrestrial vertebrates gradually disappeared. Because of this pattern, the rise of dinosaurs has often been viewed as a classic example of large-scale biological replacement in macroevolution. Two major hypotheses have traditionally been proposed to explain this event.

The first is the competitive replacement hypothesis. According to this idea, dinosaurs possessed superior physiological characteristics or anatomical innovations that enabled them to outcompete other animals occupying similar ecological roles.

The second is the opportunistic replacement hypothesis. This explanation emphasizes the role of chance environmental events. Extinctions or ecological disruptions may have eliminated competing groups, leaving ecological niches empty. Dinosaurs then expanded into these available niches and diversified rapidly.

Although both ideas have been widely discussed, neither fully explains the pattern seen in the fossil record. The new study therefore attempts to approach the problem from a different angle by examining fossil evidence that directly records feeding interactions.

Five Fossil Communities Across Time

The fossils examined in the study come from the Polish Basin and represent five successive vertebrate communities spanning the Late Triassic to the earliest Jurassic. These communities provide a rare opportunity to trace ecosystem change through time within a single geographic region.

The earliest community corresponds to the Carnian stage of the Late Triassic. At this stage, close relatives of dinosaurs existed but occupied only minor ecological roles.

The second community, from the Norian stage, records the diversification of the first carnivorous dinosaurs.

The third community, dating from the late Norian to the earliest Rhaetian, shows that dinosaurs had begun to play a more significant ecological role within their environments.

The fourth community, from the Rhaetian stage, documents the appearance of the first large herbivorous dinosaurs.

Finally, the fifth community spans the late Rhaetian and earliest Jurassic. By this time herbivorous dinosaurs had diversified and expanded across most terrestrial ecosystems, forming the dominant components of the land vertebrate fauna.

By examining these five communities in chronological order, researchers were able to reconstruct how ecological structures and food webs changed through time.

Ecosystems Before Dinosaurs Became Dominant

In the earliest community, dinosaurs were represented only by the silesaurids, close relatives of dinosaurs that recent studies often interpret as early members of the ornithischian lineage. At this time the top predators of the ecosystem were large pseudosuchians—crocodile-line archosaurs related to the ancestors of modern crocodilians. Other important animals included temnospondyl amphibians and various synapsids belonging to the therapsid lineage, which includes mammals and their ancestors.

In the second community, dinosaurs were mainly represented by small to medium-sized basal theropods or early saurischians. These early dinosaurs fed primarily on fish and insects. Members of the Silesauridae were still present, forming part of the broader dinosauriform fauna of the time.

By the third community, major ecological shifts had begun. Large pseudosuchian predators were disappearing, while carnivorous dinosaurs—especially theropods—were increasing in body size. Fossil footprints attributable to ornithischian dinosaurs also began to appear, and the prey size of theropods increased as well. Interestingly, sauropodomorph dinosaurs had not yet appeared in the Polish Basin during this stage, even though they were already present in many other regions of the world.

The Arrival of Herbivorous Dinosaurs

In the fourth community, sauropodomorph footprints finally appear in the Polish Basin. Fossil tracks of small ornithopod dinosaurs and both small and large theropods are also present. Crocodylomorphs and some temnospondyl amphibians continued to exist in these ecosystems, although most typical Triassic animal groups had already disappeared.

From the perspective of herbivorous animals, the Triassic ecosystems had previously been dominated by dicynodonts, synapsid herbivores such as Lisowicia. Bromalite fossils indicate that these animals had highly specialized diets focused mainly on conifer plants. This dietary specialization contrasts sharply with the feeding habits of early herbivorous dinosaurs.

Both ornithischians and sauropodomorphs appear to have consumed a far wider range of plant material. Their digestive remains include many types of plant fragments and even charcoal. This evidence indicates that early herbivorous dinosaurs were capable of feeding on a wide variety of plant resources.

Expansion of Dinosaur Food Webs

In the fifth community, which already belongs to the Jurassic period, dinosaur diversity and body size increased further. Coprolites attributed to large theropods contain bone fragments from large prey animals, including other predators such as crocodylomorphs. This suggests that large theropods occupied a particularly broad ecological role as apex predators.

Theropod coprolites also contain plant pollen from plant species that did not grow in the Polish Basin. This suggests that these animals either migrated across large distances or inhabited extremely large territories.

Herbivorous dinosaurs show similar ecological flexibility. Their digestive fossils contain a wide range of plant remains, including material originating outside the local region. Together, these patterns indicate that dinosaurs were generally less selective in their diets than many of the other animals with which they coexisted.

Climate Change and Ecological Opportunity

Paleoenvironmental evidence indicates that the Late Triassic and Early Jurassic were periods of repeated and rapid climatic change. One major event was the Carnian Pluvial Episode, during which global rainfall increased dramatically and climates shifted from seasonal dryness toward more humid conditions.

These environmental changes repeatedly altered plant communities. Herbivores such as dicynodonts had relatively specialized diets and therefore struggled to adapt to rapidly changing vegetation. As herbivore populations declined, the predators that depended on them were also affected.

Dinosaurs, by contrast, appear to have possessed more flexible feeding strategies. This ecological flexibility allowed them to exploit newly available food sources and ecological niches created by environmental instability and extinction events.

At the same time, sauropodomorph dinosaurs migrated into the Polish Basin once environmental conditions allowed it. Together with ornithischians, they became the dominant herbivores of terrestrial ecosystems. As herbivorous dinosaurs increased in size and abundance, carnivorous dinosaurs also evolved larger body sizes.

The rise of dinosaurs therefore appears to have resulted from a combination of environmental opportunity and biological advantages.

The gradual rise of dinosaurs to higher trophic positions across five communities（圖片來源：Qvarnström, M et al. (2024)，採用 CC BY 4.0 授權）

A Gradual Rise to Dominance

The fossil material used in this study represents more than twenty-five years of continuous collection efforts, providing an unusually detailed record of digestive fossils. Although the study focuses on a single geographic region—the Polish Basin—the patterns observed in these fossils match skeletal evidence from other parts of the world.

Scientists therefore believe that the ecological process reconstructed in this study reflects a broader global pattern. Dinosaurs did not suddenly dominate terrestrial ecosystems. Instead, their rise occurred gradually through a series of ecological transitions spanning roughly thirty million years. Climatic instability created opportunities, while the biological adaptability of dinosaurs allowed them to take advantage of those opportunities.

Through this combination of environmental change and evolutionary flexibility, dinosaurs eventually became the dominant vertebrates on land, a position they maintained for the remainder of the Mesozoic era.

Author: Bai Leng

Reference:

Qvarnström, M., Wernström, J. V., Wawrzyniak, Z., Barbacka, M., Pacyna, G., Górecki, A., Ziaja, J., Jarzynka,A., Owocki, K., Sulej, T., Marynowski, L., Pieńkowski, G., Ahlberg, P. E., Niedźwiedzki, G. (2024). Digestive contents and food webs record the advent of dinosaur supremacy. Nature.