Strange egg-laying mammals once ruled Australia, then lost their teeth

Enlarge / The echidna, an egg-laying mammal, does not develop teeth.

Monotremes, outliers among mammals, lay eggs rather than giving birth to live young. There are only two types of monotremes, the platypus and the echidna, but about 100 million years ago there were more species of monotremes. Some of them may be even stranger than their descendants.

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

Fossil fragments of three known species from the same era were also found, meaning that at least six monotreme species coexisted on 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.

“[This is] the most diverse set of monotremes ever recorded,” they said in a study recently published in Alcheringa: An Australasian Journal of Paleontology.

Echidnapus emerges

Called Opaliums spend, “echidnapus” shows similarities with both platypuses (the platypus and similar species) and tachyglossids (echidna and similar species). It is believed to have evolved earlier than the common ancestor of any of the extant monotremes.

He O. splendid The holotype had been fossilized in opal like the other Lightning Ridge specimens, but unlike some, it is so well preserved that the internal structure of its bones is visible. Every Lightning Ridge mammal fossil has been identified as a monotreme based in part on its peculiarly large dental canals. While fossil evidence suggests that the jaw and snout of O. splendid They are narrow and curved, similar to those of an echidna, at the same time showing platypus characteristics.

So what relates the echidnapus to the platypus? Although its jaw appears at first glance similar to that of an echidna, its dentary, or the part of the jaw that carries the teeth, is similar in size to that of the platypus’s ancestor. Ornithorhynchus anatinus. Other features more closely related to the platypus than the echidna have to do with its ramus, or the part of the jaw that attaches to the skull. It has a short ascending branch (the posterior end) and a twisted horizontal branch (the front end) seen in other platypuses.

Another characteristic similar to the platypus of O. splendid is the flatness of the front of its lower jaw, which is consistent with the flatness of the platypus’s snout. Its jaw size also suggests a body size closer to that of a platypus. Although the echidnapus had characteristics of the two surviving monotremes, neither of them has the teeth found in this fossil.

My goodness, what teeth you don’t have

Cretaceous monotremes may not have had as many teeth as echidnapus, but they all had some teeth. The other two new monotreme species living among the Lightning Ridge fauna were Dawn of Dharragarra and Parvopalus clytiei, and the jaw structure of each of these species is closer to that of the platypus or the echidna. dawn It has the slightly crooked jaw and enlarged jaw canal that are characteristic of a platypus. It could even be on the branch that gave rise to the platypus.

P. clytiei It is the second smallest known monotreme (after another extinct species called Teinolofos trusleri). It was more of the echidna type, with a snout that was curved and deep like that of a tachyglossid rather than flat like that of a platypus. It also had teeth, although fewer than the echidnapus. Why did these teeth end up disappearing completely in modern monotremes?

Toothless monotremes came onto the scene when the platypus (Ornithorhynchus anatinus) It appeared during the Pleistocene, which began 2.6 million years ago. Researchers believe that competition for food caused the platypus’s teeth to disappear; The spread of the Australo-New Guinea water rat may have affected the prey that platypuses hunted. Water rats eat mainly fish and shellfish along with some insects, which are also believed to have been part of the diet of ancient platypuses. Turning to softer foods to avoid competition may explain why the platypus evolved to be toothless.

As for echidnas, tachyglossids are thought to have lost their teeth after they diverged from platypuses near the end of the Cretaceous. Echidnas are insectivores and grind the hard shells of beetles and spiny ants inside their mouths, so they don’t need teeth.

Although there is some idea of ​​what happened to their teeth, the fate of the various species of Cretaceous monotremes, which not only had teeth but were mostly larger than modern platypuses and echidnas, is unknown. The end of the Cretaceous brought a mass extinction caused by the Chicxulub asteroid. Clearly some monotremes survived, but no monotreme fossils from that time have yet turned up.

“It is unclear whether the diverse monotreme fauna survived the end-Cretaceous mass extinction event and persisted thereafter,” the researchers said in the same study. “Filling this mysterious interval of monotreme diversity and adaptive development should be the main focus of research in the future.”

Alcheringa: Australasian Journal of Palaeontology, 2024. DOI: 10.1080/03115518.2024.2348753

Leave a Comment