Outliers among mammals, monotremes lay eggs instead of giving birth to live young. Only two types of monotremes still exist, the platypus and the echidna, but there were more species of monotremes around 100 million years ago. Some of them may be even weirder than their offspring.
Monotreme fossils found in waste from the Lightning Ridge opal mines in 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 be a mixture of a platypus and an echidna – “echidnapus”.
Fossil fragments of three known species from the same period have also been found, meaning that at least six monotreme species coexisted in what is now Lightning Ridge. According to the researchers who discovered the new species, the creatures may once have been as common in Australia as marsupials are today.
“[This is] the most diverse monotreme assemblage in history,” they said in a study recently published in Alcheringa: An Australasian Journal of Paleontology.
Echidnapus appears
Appointed Opalios spends, “echidnapus” shows similarities to both ornithorhynchoids (platypus and similar species) and tachyglossids (echidna and similar species). It is thought to have evolved before the common ancestor of the single extant monotreme.
The O. splendens the holotype was 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 mammal fossil from Lightning Ridge has been identified as a monotreme based in part on their unusually large dental canals. While fossil evidence suggests a jaw and snout O. splendens they are narrow and curved, similar to those of the echidna, at the same time showing platypus features.
So what does the echidnapus have to do with the platypus? Despite its jaw being similar to an echidna at first glance, its tooth, or the part of the jaw that bears the teeth, is similar in size to the platypus ancestor. Ornithorhynchus anatinus. Other features more closely related to the platypus than the echidna have to do with its frame, or the part of the jaw that attaches to the skull. It has a short ascending ramus (posterior end) and a twisted horizontal ramus (anterior end) found in other ornithorhynchoids.
Another platypus O. splendens is the flatness of the front of its lower jaw, which is consistent with the flatness of the platypus’ snout. The size of its jaw also suggests a body size approaching that of a platypus. Although Echidnapus had characteristics of both extant monotremes, neither has the teeth found on this fossil.
My, what teeth you don’t have
Cretaceous monotremes may not have had as many teeth as Echidnapus, but they all had some teeth. Two other new monotreme species that lived among the Lightning Ridge fauna were Dharragarra Northern Lights and Parvopalus clytiei, and the jaw structure of each of these species is either closer to a platypus or an echidna. D. aurora it has a slightly twisted jaw and a widened canal in the lower jaw that are characteristic of an ornithorhynchoid. It can also be on the branch that gave rise to the platypus.
P. clytiei is the second smallest known monotreme (named after another extinct species Teinolophos trusleri). It was more of an echidna type, with a snout that was curved and deep like a tachyglossid, rather than flat like an ornithorhynchoid. It also had teeth, though fewer than echidnapus. Why did these teeth eventually disappear completely in modern monotremes?
Monotremes without teeth came on the scene when the platypus (Ornithorhynchus anatinus) appeared during the Pleistocene, which began 2.6 million years ago. Scientists believe that competition for food caused platypus teeth to disappear – the spread of the Australian-New Guinea water rat may have influenced which prey platypuses hunted. Water rats eat mostly fish and crustaceans along with some insects, which are also thought to have been part of the diet of ancient ornithorhynchoids. A shift to softer food to avoid competition may explain why the platypus evolved to be toothless.
As for echidnas, tachyglossids are thought to have lost their teeth after diverging from ornithorhynchoids at the end of the Cretaceous. Echidnas are insectivores, hard-shelled beetles and ants with spines inside their mouths so they don’t need teeth.
Although there is some idea of what happened to their teeth, the fate of the diverse species of Cretaceous monotremes, which were not only toothed but mostly larger than the modern platypus and echidna, remains unknown. The end of the Cretaceous brought a mass extinction caused by the Chicxulub asteroid. It is clear that some monotremes survived, but no monotremes fossils from that time have yet been found.
“It is unclear whether a diverse monotreme fauna survived the end-Cretaceous mass extinction event and subsequently persisted,” the researchers said in the same study. “Filling in this mysterious interval of monotreme diversity and adaptive evolution should be a primary focus of future research.”
Alcheringa: An Australasian Journal of Palaeontology, 2024. DOI: 10.1080/03115518.2024.2348753