Cretaceous Forests – The Ecology Beneath Tree Bark

Understanding the interactions among species within an ecosystem is essential for reconstructing how ecological systems function and evolve. Fossils, however, rarely preserve such information. Most fossil records capture the morphology of individual organisms rather than the relationships those organisms maintained with other species in the same environment.

In this study, researchers uncovered a miniature ecosystem that once existed beneath tree bark. The entire system was preserved inside amber from the Kachin deposits of northern Myanmar, dating to the Late Cretaceous.

In recent years, Kachin amber has yielded numerous remarkable discoveries that reveal predator–prey interactions among Cretaceous invertebrates. For instance, larvae of rove beetles, mantises, and lacewings possess specialized structures adapted for capturing prey. At the same time, other organisms such as lacewing larvae, stick insects, and certain orthopterans evolved mimicry as a strategy to avoid predation.

Yet most of these fossil examples illustrate only one side of ecological interactions. Within any ecosystem, predators are rarely secure from danger themselves; many are also prey for larger organisms. The new genus described in this research displays both offensive and defensive adaptations simultaneously, offering a clearer glimpse into the complexity of these ancient ecological networks.

The Genus Rutrizoma

Rutrizoma represents a newly described genus of trogossitid beetles within the order Coleoptera. These beetles possessed a metallic blue structural coloration. Their bodies were elongated and slender, with shortened elytra, a form well suited for moving through narrow crevices within wood. Their robust mandibles were particularly effective for attacking prey.

At the same time, the posterior dorsal plate of the abdomen formed a sloping structure. In modern beetles, similar structures serve as defensive barriers against predators, suggesting that the same function likely applied here. Thus, Rutrizoma probably used this specialized structure to defend itself from larger predators.

At present, two species are recognized within this genus.

Rutrizoma donoghuei

Rutrizoma pisanii

Photograph of the specimen of Rutrizoma donoghuei（Image source：Li, Y.-D et al. 2025， CC BY 4.0 ）

Photograph of the specimen of Rutrizoma pisanii（Image source：Li, Y.-D et al. 2025， CC BY 4.0 ）

The two species differ mainly in body size and in the shape of the sloping plate at the end of the abdomen. R. donoghuei is smaller, measuring about 4.1 mm in body length, and its posterior plate bears leaf-like projections along the margin. R. pisanii is larger, reaching about 5.4 mm in length, and the margin of its abdominal plate is smooth and simple.

A Complex Ecosystem

Amber specimens containing Rutrizoma frequently also include other small organisms. Among them is the wood-boring beetle genus Poinarinius, a common component of Kachin amber. In four separate amber pieces, Poinarinius was found preserved alongside Rutrizoma.

These wood-boring beetles possess a sloping structure similar to that of Rutrizoma. However, in their case the slope occurs at the end of the elytra and is known as an elytral declivity.

The body of Rutrizoma was highly specialized for movement through narrow cracks within wood, which explains the reduction of its elytra. Because the elytra no longer covered the abdomen, the defensive slope evolved instead on the abdominal plate, representing a case of convergent evolution with the elytral declivities seen in wood-boring beetles.

In modern bark beetles, the elytral declivity functions as a defensive structure that helps block tunnels against intruders. Considering the body size and feeding habits inferred for these insects, Poinarinius was likely prey for Rutrizoma. At the same time, Rutrizoma itself was probably vulnerable to larger predators.

The sloping structures found on the posterior body segments of both Rutrizoma and Poinarinius are common among burrow-dwelling arthropods. Such structures act like a plug or barrier, sealing the tunnel entrance and preventing predators from entering.

Modern bark beetles often form complex tunnel systems beneath tree bark, where numerous organisms coexist. These communities involve many different ecological interactions, ranging from mutualistic relationships to direct predation.

Bark beetles may feed not only on the phloem of trees but also on fungi growing within their tunnels. As the beetles move through wood, they carry fungal spores with them, allowing the fungi to spread to new locations. Some fungi are edible for the beetles, while others compete with these beneficial species, creating additional ecological interactions within the tunnels. Beetles of the genus Poinarinius likely encountered similar challenges.

Another component found in the amber specimens is phoretic mites. These mites have complex relationships with their beetle hosts. They often use beetles as transportation, attaching to their bodies to move between habitats. Some mites feed on beetle eggs or larvae, while others consume harmful fungi. Through these activities, they can directly or indirectly influence beetle populations within the ecosystem.

Rutrizoma, Poinarinius, and phoretic mites preserved together in a single piece of amber（Image source：Li, Y.-D et al. 2025， CC BY 4.0 ）

The exact diet of the mites preserved in the amber remains uncertain. However, two distinct morphological types can be recognized. One type possesses an elongated body and slender legs, representing a dispersal form adapted for attaching to hosts. The other type has a shorter body and thicker legs, representing a more sedentary form. Most of the mites preserved in these amber pieces belong to the dispersal type.

Conclusion

This discovery shows that by the Cretaceous period, complex multi-layered ecological systems had already developed beneath tree bark.

Ecological reconstruction of Rutrizoma, Poinarinius, and phoretic mites within the same habitat（Image source：Li, Y.-D et al. 2025， CC BY 4.0 ）

Many fossils of tree-dwelling insects have been discovered in amber deposits, yet the use of these specimens to reconstruct ancient ecological communities is only beginning. Continued discoveries of well-preserved amber fossils will undoubtedly provide further insight into the intricate ecological networks that existed in prehistoric forests.

Author: Bai Leng

Reference:

Li, Y.-D., Leschen, R.A.B., Kolibáč, J., Engel, M.S., Zhang, Z.-Q., Yu, Y., Huang, D. & Cai, C. (2025). Specialized bark-gnawing beetles reveal phragmotic defence and subcortical ecology in the Cretaceous. Proceedings of the Royal Society B.

（Paid content. Unauthorized reproduction or use is prohibited.）