In a small limestone cave in southwestern Dominican Republic, the fossil record is written not only in bones but inside them. The empty tooth sockets of extinct rodents, and even the pulp cavity of a sloth molar, are packed with tiny clay capsules: the brood cells of ground-nesting bees that chose dead mammals as a place to raise their young.
Most people know bees for their role as pollinators or think of honeybees and hives. In reality, the majority of bee species are solitary and nest in the ground. They dig burrows, build individual brood cells, seal in pollen and nectar, and leave the larvae to grow on their own. These nests almost never fossilize, but when they do, they give us something rare in paleontology - direct window into behavior of long-gone organisms, not just bones or shells.
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| Life reconstruction of the trace-making bee nesting inside a cave and using bone cavities as containing chambers for some of the brooding cells. Copyright: Jorge Mario Macho (Machuky Paleoart). |
Our material comes from Cueva de Mono, a cave in the dry karst of southwestern Dominican Republic. The cave floor is made of red clay packed with fossils of extinct mammals, reptiles, and birds. Most of the bones were probably brought in by an owl that used the cave as a roost and dropped or regurgitated its prey there. When we looked closely at some of the rodent jaws and other bones, we noticed that a few of the empty tooth sockets were filled with tiny, smooth-walled capsules of sediment, neatly shaped and clearly different from the surrounding clay.
To see what these structures really were, we turned to micro-CT scans. On the computer screen, the bone and the infilling sediment separate into different shades of gray and we can rotate the specimen in three dimensions. Inside several mandibles and skulls of the extinct rodent Plagiodontia araeum, and in the vertebral canal of another rodent and a sloth tooth, we found small, ellipsoidal capsules with a rounded end and a narrow opening. In some sockets, several of these capsules were stacked on top of each other, like a column of overlapping cups inside the same cavity. That pattern is exactly what you expect from generations of brood cells built in the same place.
The shape, size, and construction of these cells match what we know from ground nesting bees. The inner walls are smooth and multilayered. Under the scanning electron microscope, you can see that they are made of very fine clay grains packed tightly together, with a slightly different texture than the outer sediment. This kind of lining is typical of bees that smooth their brood cells and add an organic, water-resistant coating. The trace-maker was probably a medium sized solitary bee, likely related to modern halictid bees that nest in soil.
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| CT scan and photograph images of left dentary of Plagiodontia araeum (MNHNSD.FOS 25.5282) and type specimen of the ichnofossil Osnidum almontei. |
What makes this behavior special is the choice of nesting site. Instead of excavating fresh burrows in open ground, these bees used preexisting cavities inside bones that had accumulated in the cave sediment. The tooth sockets and vertebral canals acted as natural molds, giving the brood cells mechanical protection and a fixed shape. Micro CT shows that some cavities were reused over and over, with up to six generations of cells preserved in a single rodent alveolus. That kind of repeated use points to strong nest site fidelity and to a long-term nesting aggregation inside the cave.
Why would a bee do this? The landscape around Cueva de Mono is a rugged limestone karst where soil is thin or absent on much of the surface. Deeper, better developed red clays tend to collect inside caves and sinkholes. If you are a ground nesting bee, searching for stable, fine-grained sediment to dig into, those underground pockets become very attractive. In this case, the bees appear to have gone one step further and taken advantage of the readymade cavities provided by accumulated bones in the cave floor.
We also looked at the cells at much smaller scales. Scanning electron microscopy (SEM) and palynomorph analysis shown traces of bacterial and fungal communities on the inner walls, tiny mineral crystals, and a few scattered plant microfossils. The cells glow faintly under ultraviolet light, which suggests remnants of organic coatings. Most of the original pollen provisions have probably been eaten by the larvae and then broken down by microbes over thousands of years, but the structure of the cells and their context are still clear enough to reconstruct how they were built and used.
Because these are trace fossils, not body fossils of the bees themselves, we cannot say exactly which species made them. Based on size and architecture, we argue that a halictid bee or a similar ground nesting form is the best candidate. What is certain is that this behavior is extreme by modern standards. Bees are known to nest in soil, wood, plant stems, snail shells, and even abandoned wasp nests. Using the tooth sockets and vertebral canals of extinct mammals in the dark zone of a cave is something new.
For me, the larger story is about how much ecological information can be hidden in a handful of small fossils. In these bee cells you see the imprint of late Quaternary Caribbean ecosystems: owls hunting rodents, caves trapping bones, soils enriched with phosphate, and solitary bees solving a nesting problem by turning dead mammals into shelter for their larvae. You also see the value of careful collecting and curation. Without detailed excavation, micro-CT scanning, and museum access, these structures would remain anonymous bits of clay in old bones.
This blog post is based on our open access article in Royal Society Open Science, where we formally name the new ichnogenus and ichnospecies Osnidum almontei and explore its implications for bee evolution, cave ecology, and Caribbean paleontology (free access here).
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| Juan N. Almonte at the entrance of Cueva del Mono, Dominican Republic. Photograph of Lázaro W. Viñola López. |
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Note: The trace fossil Osnidum almontei is a
generic name that combines the Latin os or ossis (bone) and nidum (nest),
a reference to bees that chose bone cavities instead of soil burrows to build
their brood cells. The species name honors Juan N. Almonte Milán, curator of
the paleobiology collection at the Museo Nacional de Historia Natural in Santo
Domingo, whose careful fieldwork, curation, and long-term care of the Cueva de
Mono material made this study possible.
This project was led by paleontologist Lázaro W. Viñola López, whose discovery, insight and attention to detail guided the study from fieldwork to publication. I am deeply grateful for his invitation to join this remarkable collaboration.
Recommended citation:
Viñola-López, L. W., Riegler, M., Olson, S. L., Orihuela, J., & García, J. A. (2025). Osnidum almontei ichnogen. et ichnosp. nov.: A new bee trace fossil from the Quaternary of the Dominican Republic. Royal Society Open Science, 12(251748). https://doi.org/10.1098/rsos.251748
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