Archaeologists have unearthed thousands of animal bones as well as human remains while excavating an ancient stone monument in Saudi Arabia.

The discoveries suggest that people gathered at stone structures to perform rituals, depositing animal horns and skulls, in Saudi Arabia about 7,000 years ago. 

Thousands of prehistoric rectangular stone structures called mustatils have been found in Saudi Arabia, but when they were built and for what purpose has remained a mystery. 

The Royal Commission for Al-'Ula, which was founded to to preserve and develop the 2,000-year-old archaeological and historical site of Al-'Ula in north-western Saudi Arabia, launched a project to study mustatils and other archaeological remains in the region. The project has been ongoing since 2018.

The recently excavated mustatil measures 131 by 39 feet (40 by 12 meters) and has stone walls going up up to 6.6 feet (2 meters) thick, but the original height of the walls is unclear as they have eroded over time. 

There is a courtyard within the mustatil, and in the center, there is a structure that holds two hearths and may have functioned as a shrine where ceremonies took place, according to archaeologists in an August paper published in a supplement to the journal Proceedings of the Seminar for Arabian Studies.

Archaeologists also found more than 3,000 fragments of animal remains in the mustatil, that weigh about 55 pounds (25 kilograms) combined. The animal remains include hundreds of horns and heads of animals, such as cattle and goats. 

Cattle heads and horns have been found at other prehistoric sites in the Middle East, including a site in Yemen where a ring of cattle skulls was displayed, lead study author Wael Abu-Azizeh, a junior professor of archaeology at Lumière University Lyon, told science news outlet Live Science. The animal bones were deposited between 5300 BCE and 5000 BCE, according to the archaeologists.

The human bones found in the mustatil belonged to at least nine people including two infants, five adults, an adolescent or young adult, and a child. The human remains are also younger than the animal remains, dating to a few centuries after the animal bones were placed in the mustatil.

"It's a collective burial," although it is unclear if the people buried there are related to the builders of the mustatil, Abu-Azizeh said.

Olivia Munoz, an archeo-anthropologist at the French National Center for Scientific Research (CNRS), was not involved in the study but praised the research and is hopeful that more details about the human remains will be published. 

"It would be interesting to know the distribution by bone type to help understand whether individuals were deposited whole or whether parts of the already decomposed skeleton could be brought into the mustatil," Munoz told Live Science.

Religious meaning

Although there are theories, it is currently unknown why the mustatil was built and why it holds so many human and animal bones.

A 2021 paper published in the journal Antiquity suggested that the mustatil may have been used by a "cattle cult" in the region. Abu-Azizeh disagrees with this idea, however, noting that cattle bones accounted for only a small proportion of the animal remains from the site, with caprines making up the most.

The open-air courtyard design of the mustatil indicates that it was a place for crowds to gather, possibly for religious purposes.

The presence of animal horns and bones suggests the two hearths were likely a shrine, and the fact that some of the animal bones were burnt suggests that the religious rituals took place which involved burning the bones. 

The site is very well preserved, especially compared to similar ancient sites. 

"All too often open-air structures such as this, wherever they are found, are badly disturbed,"  Anne Porter, an assistant professor emerita of Near Eastern archaeology at the University of Toronto, in an email to Live Science.

The environment in the region where the mustatil were built would have been much wetter and rainier than it is today, according to an email sent to Live Science from Gary Rollefson, a professor emeritus of anthropology at Whitman College in Washington. 

He agreed the mustatil were used for spiritual and religious reasons, and stated that the animal horns and heads may have been burnt as "votive offerings." 

Nomadic groups that dispersed and traveled throughout the year may have gathered at the mustatil near the end of the rainy season, Rollefson said.

The mustatil will be a part of the Al-'Ula World Archaeology Summit, which will take place Sept. 13-15.