Archaeologists are perplexed by the discovery of two ancient human infants who were buried while wearing the skulls of other children on their heads.

These morbid “helmets,” which were found in Ecuador, represent “the only examples of this funeral rite in the world,” according to a new paper about the odd find in the journal Latin American Antiquity.

The babies were interred in a burial mound at Salango, an archeological site on the central Ecuadorian coast, sometime around the year 100 BCE. Their remains were uncovered during excavations that ran from 2014 to 2016, and resulted in the discovery of 11 human burials.

Lead author Sara Juengst, an archaeologist at UNC Charlotte, wasn’t at the site when the infants were excavated, but she said the initial reaction of co-author Richard Lunniss, an archaeologist at Ecuador’s Universidad Técnica de Manabí who was at the dig, was “surprise.”

“He quickly recognized that there were two layers of skull and removed the burials with the surrounding soil matrix intact to help preservation,” Juengst explained in an email. “When I analyzed the remains in 2017, we actually finished excavating the remains in the lab, which led to the more detailed discoveries about age of the primary individuals and the extra crania.”

Juengst and her colleagues think that the juvenile skulls probably still had flesh on them when they were removed from the children and placed on the infants’ heads. Skin and tissue likely held the skulls together as they were attached to the infants prior to their burial.

One of the specimens also featured a small hand bone and a shell that appeared to be deliberately placed between the infant skull and the helmet. The headgear of this infant was oriented “such that the primary individual’s face looked through and out of the cranial vault of the second,” according to the study.

It’s almost certain that this arrangement was part of a ritual ceremony, but the purpose of the skull stacking is unclear. Though many ancient South American cultures venerated human heads and skulls, often burying them in isolation, this particular version of ritual skull manipulation is more difficult to interpret because of its novelty.

Juengst’s team speculates that the unusual rite may have been a symbolic way of offering protection to the infants in the afterlife. The authors wrote that the arrangement may have represented “an attempt to ensure the protection of these ‘presocial and wild’ souls” which is supported by the presence of stone ancestor figurines near the skeletons.

The presence of lesions on all four skulls hints at the possibility of malnutrition or infectious diseases. These symptoms may have been linked to food disruptions after a volcanic eruption known to have occurred before the burials, but the team said it will take more evidence to definitively determine a link between the eruption and the burials.

To that point, future research into the biochemical properties of the skulls may shed more light on the circumstances that led to the bizarre helmet rite.

“We hope to do future studies on isotopes, which allow insight into diet and geophysical location on the landscape, and DNA, which might clarify the relationships between the infants and their accompanying crania, and others buried in the mound,” Juengst said.