It's not easy tracing the footsteps of extinct beasts. Even more so when they happen to be about 250 million years old.

We don't quite know why, but at that time a mass extinction killed off about 96% of species on Earth, the so-called Permian Mass Extinction. A second extinction may have occurred several million years later.

With no competitors standing in their way, those that survived were able to rapidly diverge.

These included a large group called archosauriforms or "ruling lizards" in Greek, which included crocodiles, pterosaurs and many lizards. This same group would eventually give rise to the dinosaurs.

As they expanded and dominated the land, they changed from lizard-like creatures to more like the crocodiles we know today, but there were some remarkable differences too.

They evolved larger skulls, some had very long necks, they were heavier and several even stood upright in an almost erect stance.

But fossil traces of these creatures are hard to come by so researchers are turning to ancient footprints of these ruling beasts, giving new clues into their evolution.

If the few known archosauriforms did not survive, today we would not see birds flying over our heads

This way they can begin to understand "a common language" between two very different sets of data, skeletal fossils and fossil traces such as footprints, says Massimo Bernardi of the Muse Science Museum in Trento, Italy. These two sets of data are usually discussed separately, which is nonsense, he adds.

But by combining them, the researchers have been able to show that archosauriforms were more diverse than expected. 

As they managed to survive an era of extinction, leading from the Permian to the Triassic period, their surprising variation can tell us more about the dinosaurs they would eventually become.

"Having a better picture of archosauriforms diversity before and after this event is crucial for understanding why this group not only survived but, as we demonstrate, radiated during or immediately after the event.

"If the few known archosauriforms did not survive the Late Permian extinction, today we would not see birds flying over our heads and palaeontology books would not mention dinosaurs at all," Bernardi told BBC Earth.

That's because all birds alive today descended from one group of bird-like dinosaurs, the theropods.

Bernardi and colleagues also found that these creatures, both before and after the extinction events, lived at very low latitudes. This opposes other research which has previously stated that all reptiles would have escaped from these low, tropical areas due to the intense heat.

Looking at the Bletterbach Geopark in Italy, Bernardi, his team even discovered new footprints.

"We attribute these footprints to archosaurifoms with a very advanced anatomy. These footprints were left by an archosauriform which looked very much like those that we commonly find in the Triassic, some 10 million years later."

That these existed earlier now pushes back the time period when this group of archosaurs diverged. The new analysis, published in journal PLos One, proposes that the group began to rapidly flourish before – or possibly during – the mass extinction, not after as had been suggested previously.

This is interesting in itself, says Bernardi, as there is still a lot of debate about the immediate consequences of mass extinction events, especially those that occurred on land.

It might be that all these advanced features helped them survive the extinction, one of the most severe biological crisis of all time.

But of course, "a bit of good luck" could also have come into play, he adds.