Someone — maybe an adolescent, maybe someone older — rushed across the edge of Lake Otero, slipping as they walked but moving steadily ahead. Evidence suggests this person was carrying a child approximately 3 years old, setting the child down for just a moment in at least three separate places along the journey before continuing on.
While this person was gone, an enormous proboscidean — a Columbian mammoth or a mastodon — lumbered across that path, stepping on a couple of the footprints. In fact, potentially three proboscideans moved across that landscape, cutting across the tracks left by the human.
Time is hard to determine, but at another point, a giant ground sloth happened to be making its way near Lake Otero as well. Its tracks indicate a decided awareness of the human — a change in behaviour — where it may have lifted up on two feet to smell the air, ascertain its own safety, and determine what lay ahead, before quickly changing direction and moving away.
The same person (or perhaps a different person) walked back next to the initial trackway at some point later on, but the footprints indicate they were no longer carrying something. If the same person were returning from whence they came, perhaps the child was left behind.
These scenes are described in a remarkable paper published earlier this month in Quaternary Science Reviews, and they interpret actions taken by humans and other animals that lived in what is now New Mexico at least 10,000 years ago. Today, that area is White Sands National Park.
Many people know it for the dramatic white sand dunes that prompted its name, but paleontologists recognise White Sands National Park for the unbelievable wealth of ichnofossils — fossil footprints, in this case — that it preserves. And while many of these footprints can easily be seen by the naked eye, it is White Sand’s unique “ghost tracks” that make the site even more unique.
David Bustos, resource program manager at White Sands and co-author on this recent paper, laughed when he recalled taking scientists to see an area he knew for sure contained fossil footprints, only to find them completely invisible when they arrived. The environmental circumstances, he explained by phone to Gizmodo, have to be just right for them to appear: not too dry and not too wet. Their great visibility on one day and complete lack thereof on the next is what has led people to refer to these ichnofossils as ghost tracks. This vanishing and reappearing act happens time and time again all over the park.
During the Pleistocene, White Sands was home to an enormous lake, now referred to as Lake Otero. Evidence suggests it may not have always existed as one body of water but rather many smaller lakes that, when flooded, made up a much bigger entity. It is around this paleo-lake that so many of the footprints are found. Footprints of proboscideans, giant ground sloths, canids, bison, camels, felids, and humans all have been discovered here, sometimes in association with each other.
“We have about [32,300 hectares] of Lake Otero in the Park,” Bustos explained. “And across those [32,000 hectares], we are finding fossil prints everywhere. I think that’s what’s so incredible. And they go for long distances. Because they’re so long, you can see interactions you can’t see in other trackways from around the world.”
This trackway described in the new paper extends for 1.5 kilometres, but other trackways, such as one by an extinct species of camel, extend for over 3 kilometres. The length and number of these trackways offer incomparable insights into ancient life.
Bustos, lead author Matthew Bennett, and co-author Sally Reynolds were part of the team who in 2018 described the phenomenal interaction between a giant ground sloth and several humans, in which footprints indicated the sloth was being followed and perhaps taunted by the group.
Another paper in 2019 outlined the technology used to research many of the tracks, as well as yet another trackway in which an ancient human and a proboscidean, perhaps a Columbian mammoth, walked in the same general area, crossing over each other’s footsteps.
Bustos has been working at White Sands for 15 years. Speaking with him, one comes away with two very strong impressions: He is remarkably grateful to the scientists who have been working in the park, and he really knows these footprints and the stories they might tell.
“If you find one set of prints, you usually find the others in the same area,” Bustos said. “And those are the humans, the mammoths, the giant ground sloth and camel both. Those four are very common across the [32,000 hectares]. This is important because they were formed under the same conditions and time frame. We have human and megafauna prints made of clay, sand, and dolomite, depending on where you are in the lakebed.”
Those of us who are not near White Sands have a somewhat narrow view of what the park entails. Each new paper brings with it wondrous surprises and fascinating details. But Bustos has seen some of this behaviour in other locations and knows so much more about the type of ichnofossils contained within the park.
“What we often see are giant ground sloths changing their movements in response to humans. They’ll stand up. And they’ll turn around. We’ve got several in other places throughout the park. Any time there are human [prints] around a sloth, [the sloth will] start spinning in circles or doing funny things. But when there are not human [tracks] present, the [sloths] just kind of meander around and walk in straight lines. They don’t start turning around in circles or start spinning. It’s really interesting. The mammoths,” he said and chuckled, “they don’t seem to care one way or the other.”
Paleontologist Melissa Macias noticed that aspect as well in reviewing this latest paper. “The mammoth just did not care and just kept going,” she said by phone to Gizmodo. “But the sloth turned around. That says so much about the behaviour of those animals.”
Three species of ancient sloth are known from that time period and geographic region: Megalonyx, Nothrotheriops, and Paramylodon. But of the three, only two — Nothrotheriops and Paramylodon — have a pedolateral foot, meaning that they walked by putting most of their weight on the sides of their feet. The authors suggest that Paramylodon is the most likely track maker in this particular instance, and of the three, it’s the largest species. Some estimate that Paramylodon was about 3 meters long and could weigh up to or more than a ton. Nothrotheriops, by comparison, was a slightly smaller giant ground sloth, weighing perhaps a quarter of a ton.
Macias suggested that the smaller Nothrotheriops may have been more afraid of humans, avoiding them entirely, which could explain the lack of human and Nothrotheriops prints together. “We might not ever know,” she said.
Interpreting these tracks is the domain of Matthew Bennett of Bournemouth University, who has worked on footprints for most of his career and created DigTrace, software designed to help analyse tracks. Scientists have studied fossil footprints using ground-penetrating radar, as well as 2D and 3D analysis.
“A geologist has to use what they have,” Bennett wrote in an email to Gizmodo, “but to make inferences and firm predictions on just a few tracks is wrong. You need a lot to correctly characterise a person.”
This research is what helped them understand how fast the person was walking, how old they might have been, whether or not they were carrying a child, the age of the child, and the relative timing in which these human footprints were intersected by ancient megafauna. It is the length of the track that offers so much valuable data.
Ashleigh Wiseman, postdoctoral researcher at the Royal Veterinary College in the UK, helped contextualize the data and analyse it for this paper, offering insight into whether the trackmaker was carrying a child.
“Seeing the toddler-sized footprints was very exciting,” Wiseman expressed in an email to Gizmodo, “particularly knowing that the tracks were cross-cut shortly after by mammoth and giant ground sloth tracks. Being able to see a glimpse into megafaunal interactions with humans over 13,000 years ago was extraordinary.”
“For Sally and I, the trackway has a very personal dimension,” Bennett wrote, referring to co-author (and co-parent) Reynolds. “I was working on it while Sally was at home in the first stages of pregnancy with our daughter; in fact I saw my first scan picture while excavating the trackway. So, we have long called the trackway ‘Zoe’s trail’ because of this, and it is the informal name the team now uses for the child tracks… Scientists are not supposed to have personal connections with science, but this piece does.”
When asked if this most recent paper and all of the papers since 2018 are just the tip of the iceberg, Bustos said: “Yes. There are so many stories we are excited to share still, and I am sure so many more [are] yet to be told.”
These papers might be considered chapter one of the book that is ancient White Sands, and we are all waiting with bated breath for the next instalment.