In an ancient, dried-up drinking well, a graveyard of rhinos, horses, and giraffes was discovered.

 Thousands of desperate giraffes, rhinos, horses, and sabertooth cats flocked to a watering hole in what is now Spain nine million years ago, first as a shelter, then as a final resting place.

According to new research published in a September issue of a journal and available online July 15, dozens of animals died of starvation, dehydration, and mired in the dwindling watering hole during three different periods of drought in the late Miocene. When the rains returned, the animals' remains were quickly covered in mud, largely undisturbed by scavengers or weathering.

They are exceptionally maintained, despite being almost 9 million years old," study leader David Martin-Perea, a palaeontologist at Madrid's National Natural Sciences Museum, stated. Martin-Perea and his colleagues discovered frog, mouse, and bird remnants, as well as two fetal horses, at the site.

A Miocene hotspot

The area south of what is now Madrid was a combination of woods and grassland throughout the late Miocene, with watering holes carved out of the underlying limestone and mudstone. Miners unearthed a wealth of bones in one of these old drinking holes in 2007.

Thousands of bones have been discovered buried over nine sites 19 miles (30 kilometres) west of Madrid since then, according to palaeontologists. One of those sites, Batallones-10, was the focus of the new study. Three unique layers of petrified bones have been discovered at the site, which was formerly a watering hole. There have been around 9,000 fossils discovered, representing dozens of species. Extinct horses, mastodons, rhinoceroses, musk deer, and cattle were among the 15 big creatures discovered in the mix.

Two species of sabertooth cats, a hyena related, a mustelid (a relative of modern-day weasels, badgers, and otters), and an ailurid were all carnivorous (an extinct relative of modern-day red pandas).

Death and drought

The presence of amphibians and tortoises at the site suggests that it once served as a wetland oasis in the surrounding grassland. The bones exhibited little evidence of predation, scavenging, or trampling, implying that they were buried promptly after the animals died.

A scientist  and his colleagues determined that drought was the cause of mortality after putting these indications together, as well as the fact that the animals died in three distinct times. The head of the team told the science journalists. 

This is a "classic example" of a drought-caused collection of fossils.

First, based on investigations of animal teeth that show specifics about what they were eating and drinking across time, the site is in an area that would have undergone periods of seasonal drought. Second, a large number of animals died in a short amount of time near a water source, and the fossils show that several species that would not ordinarily be found together gathered in one location, indicating that they were all hunting for moisture. Other geological signs, such as semi-arid mineral deposits, suggest that this was a drought-prone location.

The animals were also biased young, which makes sense in the context of drought: young animals have fewer reserves to depend on when times go difficult, and they are the first to die in modern drought observations, according to the researchers.

Many of these young kids most likely died of malnutrition rather than dehydration. As alternative sources of water became scarce, more animals flocked to the Batallones oasis. They would have chewed down the adjacent foliage until there was little forage left, as they were unwilling to travel far from this water source.

Some, weak from hunger and thirst, would have gone farther into the dwindling watering hole, only to get stuck in the mud. They would have drowned in shallow water if they had not been too weary to flee. During modern-day droughts, these types of miring deaths are common, according to the researchers. According to the experts, the die-offs occurred over for weeks or months.

Run-off from the surrounding land, which had been stripped of vegetation, would have filled the bottom of the watering hole once the rains returned, burying the buried animals in a layer of sediment and safeguarding their bodies.

Bones from animals that died along the shoreline would have washed down into the watering hole as well. Extremely delicate fossils, such as the two pregnant horses who perished together with their moms, were preserved thanks to the prompt burial.

The next stage, according to the head person of the team, is to dig even deeper. Similar sites nearby include deeper strata of predator-dominated fossils, and Ballatones-10 could still contain more sabertooth cats and other carnivores.