Could the strangest bee nursery ever recorded in history have existed inside the skeleton of another creature? This is exactly what paleontologists have discovered in a limestone cave on the Caribbean island of Hispaniola, a discovery that marks a first, where fossilized burrowing bee nests have been found inside the skeletons of vertebrates.
The find was made in the Cueva de Mono, a cave in the Dominican Republic that has long been famous for its fossil riches from the Late Quaternary period. However, the team, led by Lazaro Viñola López from the Field Museum of Natural History, was conducting their search for vertebrates to learn more about the impact of human colonization and the effects of climate change on past island extinctions. The floor was already thick with thousands of hutia bones, the guinea pig-like large rodents, sloths, birds, reptiles, and even crocodiles. The accumulation had taken place over thousands of years due to the hunting and subsequent defecation by the extinct giant barn owls (Tyto ostologa), which spit out undigested pellet remains that decomposed to form the dense fossil bed.
During the cleaning process of a hutia mandible, a peculiar thing caught the eye of Viñola López: the sedimentation inside an empty tooth socket was not like normal soil accumulation. The sedimentation was smooth and curved, and it resembled fossilized cocoons he remembered seeing years ago. The fossilized cells would have been damaged if the team examined them up close. The team decided to perform a CT scan to produce three-dimensional images of the inside of the cavity without damaging the fossil. The result exposed ellipsoidal brood cells that were smaller than a pencil head eraser and consisted of compacted soil coated with a waxy material. Some cells even retained fossilized pollen grains, which were the food source for the larvae brought by the bee mothers.
By process of elimination, wasps could be eliminated because their nest architecture was characterized by a rough texture comprising plant fibers laced with saliva. However, the smooth walls pointed towards a burrowing bee. This was most likely associated with Halictidae because these bees are generally solitary. They would excavate their nesting holes in soil; however, in this new environment, they took advantage of the alveoli in fossilized jaw bones, vertebras, as well as the pulp chamber in a sloth’s tooth. There was also evidence of six generations of wasp nesting in a single hutia mandible as indicated by CT scans.
This behavior of the bees was also a result of the unique environment provided by the cave. The neighboring karst environment is characterized by sharp limestone with minimal natural topsoil; hence, ground digging is not easy. On the other hand, the fine clayey silt of optimal excavation quality flowed into the caves over the years. Pre-formed cavities from the fossil bones of owls acted as favorable habitats devoid of flooding and predators. According to scientists, this is the second event documented where burrowing bees make use of a cave, and the first event where the bees use pre-formed cavities of fossilized animal bones.
Microscopic and chemical observations further contributed to the information gathered. Scanning Electron Microscopy results showed the presence of organic materials in the cell walls, as well as fossilized microbial assemblages of cyanobacteria, and fungi such as Canalisporium and Acrodictys, which thrive in damp, decomposing conditions, typical of what an owl pellet would induce. The low presence of pollen residues, below 1% of the samples, suggested that the larvae were successful in consuming their food sources prior to pupation.
These fossil nests have been systematically identified as a new ichnospecies, Osnidum almontei, in recognition of the Dominican paleontologist, Juan Almonte Milan, who identified the importance of this cave site. It is unclear whether the bees are an extinct relative or ancestor of the bees that are found in the Caribbean today; this is due to the fact that the cave conditions are hot and humid, which would not preserve a body as delicate as that of a bee.
This discovery is leading to a new way of excavation. As Viñola López observes, “It changed how we look at and prepare fossils from these cave deposits… Now we take much more care before cleaning them to make sure we don’t destroy any other interesting behavior of ancient insects hiding in the sediment inside the fossils.” The team is also examining other specimens from Cueva de Mono, hoping to gain new knowledge about prehistoric environments on the island, where the interactions of predators and insects led to one of the most unusual fossil records of bee behavior in history.
