Enlarge / Upper left: a reconstruction of Diadcetes. Below: false color images of its foot and tail prints. Right: the section of the tail that left the print. (credit: Voigt et. al./Urweltmuseum GEOSKOP.)

Their feet left copious traces in muddy Permian floodplains, leaving tracks scattered across ancient sediments. But in one slab of such trackways, scientists uncovered something more: the trace of an animal’s tail as it dragged across the ground. Strikingly, these tail prints come complete with scale impressions—at 300 million years old, they’re among the earliest scale impressions we have.

This may seem small, but it shows us that some of the hardened skin structures necessary for our ancestors to survive on land had evolved much earlier than previously suspected. A paper published in Biology Letters this past May describes this discovery in detail.

A rare find

The particular slab holding these traces was discovered in 2020 at the Piaskowiec Czerwony quarry in Poland. Mining had stopped to enable paleontologists to search the red sandstone rocks for fossils. Gabriela Calábková described climbing upon “a huge pile of rubble” only to discover a sizable slab of fossil tracks at the very top. There, among one set of footprints, was something new.

Enlarge / The echidna, an egg-laying mammal, doesn't develop teeth. (credit: Yvonne Van der Horst)

Outliers among mammals, monotremes lay eggs instead of giving birth to live young. Only two types of monotremes, the platypus and echidna, still exist, but more monotreme species were around about 100 million years ago. Some of them might possibly be even weirder than their descendants.

Monotreme fossils found in refuse from the opal mines of Lightning Ridge, Australia, have now revealed the opalized jawbones of three previously unknown species that lived during the Cenomanian age of the early Cretaceous. Unlike modern monotremes, these species had teeth. They also include a creature that appears to have been a mashup of a platypus and echidna—an “echidnapus.”

Fossil fragments of three known species from the same era were also found, meaning that at least six monotreme species coexisted in what is now Lightning Ridge. According to the researchers who unearthed these new species, the creatures may have once been as common in Australia as marsupials are today.