A New Migration Route in the Evolution of European Hadrosauroids: The Bulgarian Land Bridge

During the Late Cretaceous, global sea levels were much higher than today. As a result, much of Europe—being relatively low-lying—was fragmented by shallow seas into a vast archipelago composed of numerous islands. Such a landscape fostered unusual ecosystems. Island environments often impose strong ecological constraints, including limited space and restricted resources. Under these conditions, large animals frequently evolve smaller body sizes as an adaptation to island life, a phenomenon known as island dwarfism.

A modern analogy can be seen in the Formosan black bear of Taiwan, which is considerably smaller than the Asian black bears living on the Asian mainland. The opposite pattern may also occur: smaller species living on islands may evolve larger body sizes because competition is reduced and large predators are often absent. This evolutionary trend is known as island gigantism, as illustrated by the dodo, which was much larger than its closest relatives.

Within the Late Cretaceous European archipelago, small hadrosauroid dinosaurs—often referred to as “duck-billed” dinosaurs—were among the most abundant and diverse herbivores. Because of this ecological prominence, fossils of European hadrosauroids provide valuable insight into the evolutionary history of the region's vertebrate faunas.

Dwarf hadrosauroid Tethyshadros insularis from Italy（Image source：Slate Weasel，CC0 1.0 ）

The complex evolutionary history of European hadrosauroids

The evolutionary history of hadrosauroids in Europe is remarkably complex. Some lineages were native groups that had already inhabited the European islands since the middle to late Cretaceous. Others arrived later as immigrants from Asia or North America during the Late Cretaceous. Evidence suggests that these migration events may have occurred multiple times.

Understanding this evolutionary history is challenging because dinosaur fossils from Europe prior to the Campanian stage are relatively scarce. The limited fossil record makes it difficult to reconstruct the early stages of hadrosauroid diversification in the region.

Recently, however, a research team analyzed hadrosauroid fossils discovered in Bulgaria. Their findings have helped fill some of the gaps in our understanding of these dinosaurs.

Phylogenetic tree of hadrosauroids（Image source：Debivort， CC BY-SA 3.0 ）

Bulgaria lies in southeastern Europe and would be expected to play an important role in reconstructing migration routes of Mesozoic dinosaurs. Yet for many years paleontologists believed that dinosaur fossils were unlikely to be found there because of local geological conditions. Over the past decade, however, discoveries and descriptions of dinosaur fossils from Bulgaria have gradually overturned this assumption. Although the number of fossils discovered so far remains limited, excavation and research efforts continue to expand.

The fossils examined in this study belong to a hadrosauroid dinosaur discovered in Bulgaria.

A small dinosaur from a Late Cretaceous island

During the Late Cretaceous, the Bulgarian region formed part of the European island archipelago and represented a relatively small island environment. If this were the case, local dinosaur populations might be expected to show evidence of island dwarfism.

Initial estimates based on the fossil bones suggested that the Bulgarian hadrosauroid measured approximately 3.5 meters in body length. However, this estimate was based only on bone size and did not take into account the microscopic structure of the bones. Without histological analysis, it is impossible to determine whether such a small size represents a dwarf adult or merely a juvenile individual.

The primary goal of the study was therefore to analyze the bone tissues of the fossil specimen in order to determine its age at death and clarify the reason for its relatively small size.

Histological analysis

Microscopic examination of the bone tissues revealed that the bones contained abundant blood vessels and lacked clear growth-arrest lines. These characteristics indicate that the individual had not yet reached skeletal maturity and was still growing rapidly.

Comparisons with other hadrosauroid fossils suggest that the Bulgarian specimen likely died at about two years of age, during a phase of rapid growth corresponding to a late juvenile or subadult stage. Researchers estimate that if the animal had survived to adulthood, it might have reached a body length of approximately six to seven meters.

This size is comparable to that of related hadrosauroids living on the Asian mainland. However, when the growth patterns of the Bulgarian specimen were compared with mainland species, researchers found that the Bulgarian dinosaur exhibited slower bone growth rates. This difference may reflect adaptation to island conditions.

A possible early stage of island dwarfism

Sedimentary rocks from the fossil site indicate that the area was a shallow marine environment during the Late Cretaceous. Combined with Bulgaria's geographic position at the southeastern edge of the European archipelago, this suggests that the region may have served as a migration corridor connecting Asia with the European islands.

The shallow-water deposits imply the existence of either an intermittent land bridge or a chain of shallow marine environments that allowed terrestrial animals—including hadrosauroids—to disperse from Asia into Europe. Once these dinosaurs reached the islands, they could establish local populations.

The relatively slow growth rate observed in the Bulgarian specimen may therefore represent an early evolutionary response to island conditions. In other words, the population may have consisted of recent immigrants from the mainland that were beginning to undergo the evolutionary process leading toward island dwarfism.

Conclusion

Although the study of dinosaurs in Bulgaria is still in its early stages, this discovery demonstrates that the region likely played a significant role in the migration and evolution of dinosaurs within the Late Cretaceous European archipelago. Future fossil discoveries will be crucial for testing the proposed migration route and for clarifying how hadrosauroids spread across Europe and adapted to island environments.

Author: Bai Leng

Reference:

Nikolov, V., Dochev, D., Brusatte, S. L. (2024). The ontogenetic status of a small hadrosauroid dinosaur from the uppermost Cretaceous of Bulgaria, and implications for the paleobiogeography and assembly of European island faunas. Cretaceous Research.

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