On a typical day in the Karoo basin of South Africa about 260 million years ago, a small animal retreated into its burrow, probably trying to stay cool on a hot afternoon.

But that wasn’t the only reason it was going underground.

This little herbivore was Diictodon feliceps a member of the group of ancient mammal-relatives called therapsids, and it wasn’t alone down there.

Fast forward to the 2010s, and the other occupants - tiny articulated skeletons of the same species - were uncovered, in not just one, but two different burrows.

Researchers had hypothesized that these burrows were used as ‘brood chambers,’ where Diictodon young might’ve been born and raised, and now there was finally fossil evidence to support that idea.

Because, within each burrow, entombed together for millions of years, was what may have been an ancient family - an adult with its offspring.

And while we might be tempted to think that the adult was female, because, well, that’s who we tend to associate parental care with, the adults discovered in these two burrows were actually male.

Diictodon lived during the Late Permian period, sticking around for about 12 million years and ranging from the Karoo of South Africa, to Zambia, to as far away as China.

They had big heads with toothless beaks for clipping tough vegetation and some had two tusks.

They also had strong limbs and adaptations that made them capable diggers, so it’s no surprise that we find them associated with burrows.

What was a surprise, though, were the extra occupants of those South African burrows.

Both burrow casts were initially prepared by hand using a tool called an air scribe, which chips away at the surrounding rock to reveal the fossils.

When the preparator saw tiny, fragile bones start to emerge from the rock, preparation was stopped and the fossils were taken to France for high-powered x-ray scanning.

These scans allowed the researchers to see what was inside the burrow cast without damaging the fossils.

Inside the first burrow cast, they found the skull and skeleton of an adult Diictodon, and a tiny humerus belonging to an infant of the same species.

The scans of the second burrow revealed more than just a humerus: not one, but two tiny Diictodon, again found alongside an adult.

One was a partial skeleton, including a skull and mandible, and the other just a solitary jaw bone.

They were similar in size and likely in age - probably from the same brood.

But how do we know that the adults found alongside the tiny bones in the two burrows were male?

Well, we’ve found a lot of fossils of Diictodon.

They’re abundant enough in the fossil record that we can look at the variation in anatomy between individuals and come up with some tell-tale differences between the sexes.

When males and females within a species differ in things like size and appearance, we call it sexual dimorphism.

And we can tell that Diictodon was sexually dimorphic.

For example, the females are often smaller.

And the males tend to have a pair of tusks, while the females don’t.

There are also other features of the skull that are different, like the morphology of the bone surface surrounding what’s called the pineal foramen - a small opening that a nerve would’ve passed through in life.

In males, the opening is surrounded by an elevated ring of bone called a boss, while in females it’s smooth.

And the adults discovered in these burrows had large tusks and a boss around the pineal foramen – so, we’re pretty sure they were male.

The discovery of two burrows, from the same place and time period, both with adult males entombed with infants support the idea that Diictodon used burrows to breed... And maybe even that the males stayed behind to play a role in feeding and protecting their young.

But why would they do that?

And why would it be surprising for this species to have had male parental care?

We mentioned before that Diictodon is a therapsid, the group that includes living mammals and their extinct relatives.

And when we think of living mammals, we often think of females as the primary caregivers, because they carry the young and provide milk to sustain them early on.

Male parental care is only present in about 10% of mammal species.

Because!

Investing in your offspring comes at a cost, including risking your own survival through things like increased risk of predation and reduced mobility, and potentially jeopardizing future mating opportunities.

But on the flip side, investing in your offspring improves their survival chances and helps ensure that your genes are carried into the next generation.

In terms of evolution, parenting theoretically should only evolve when the benefits outweigh the costs.

And what we see in living mammals is that male parental investment is often linked to the mating system of a species.

For example, male parental care is more common in socially monogamous species where the offspring is more likely to be the male’s.

Males will then work to ensure the survival of their offspring and shorten the intervals between broods.

Male parental care in non-monogamous species is much more rare, but it still exists, in most cases to improve their breeding chances, resulting in more potential offspring.

Now, we don’t know what kind of mating system Diictodon had - though its tusks might point to competition between males - so it’s difficult to infer why males might be more invested in caring for young than we expect.

And we also have our own biases in interpreting parental care as a female trait.

But, in living mammals, sexual dimorphism is often associated with mating systems where males compete for mates and don’t usually invest in parental care – which makes the idea of Diictodon dads caring for their young pretty surprising!

What we can say is that these fossils potentially extend mammal-like male parental care behaviors back to their distant, ancient cousins.

They show that caring for young is a behavior even older than the earliest mammals and something that wasn’t exclusive to female Diictodon.

And maybe this was part of the reason that Diictodon was one of the few survivors of the Great Dying - the mass extinction event at the end of the Permian Period that took