Outliers among mammals, monotremes lay eggs instead of producing live young. Only two species of monotremes still exist, the platypus and the echidna, but about 100 million years ago there were more monotreme species. Some of them may be even stranger than their descendants.
Monotreme fossils found in waste from the opal mines of Lightning Ridge, Australia, have now revealed the opal jawbones 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 be a mix of a platypus and an echidna – an ‘echidnapus’.
Fossil fragments of three known species from the same era were also found, meaning at least six monotreme species coexisted in what is now Lightning Ridge. According to the researchers who unearthed this new species, the creatures were once as common in Australia as the marsupials are today.
“[This is] the most diverse monotreme assemblage ever recorded,” they said in a study recently published in Alcheringa: An Australasian Journal of Paleontology.
The Echidnapus emerges
Called Opalios spends, the “echidnapus” shows similarities to both ornithorhynchoids (the platypus and similar species) and tachyglossids (echidna and similar species). It is believed to have evolved before the common ancestor of both extant monotremes.
The Oh, splendor The holotype was fossilized in opal like the other Lightning Ridge specimens, but unlike some specimens it is so well preserved that the internal structure of the bones is visible. Every mammal fossil from Lightning Ridge has been identified as a monotreme, based in part on their particularly large dental canals. While the fossil evidence suggests the jaw and snout Oh, splendor are narrow and curved, similar to that of an echidna, but at the same time exhibit platypus characteristics.
So what connects the echidnapus to a platypus? Despite its jaw resembling an echidna at first glance, its dentary, or the part of the jaw that bears the teeth, is similar in size to that of the platypus ancestor. Ornithorhynchus anatinus. Other features more closely related to the platypus than to the echidna have to do with its ramus, or the part of the jaw attached to the skull. It has a short ascending ramus (the back) and a twisted horizontal ramus (the front) seen in other ornithorhynchoids.
Another platypus-like characteristic of Oh, splendor is the flatness of the front of the lower jaw, which is consistent with the flatness of the platypus’ snout. The size of its jaw also suggests a body size closer to that of a platypus. Although the echidnapus had features of both surviving monotremes, neither has the teeth found on this fossil.
My, what kind of 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 that lived among the Lightning Ridge fauna were Dharragarra aurora And Parvopalus clytiei, and the jaw structure of each of these species is closer to the platypus or echidna. D. aurora has the slightly twisted jaw and enlarged canal in the lower jaw characteristic of an ornithorhynchoid. It could even be due to the branch from which the platypus emerged.
P. clytiei is the second smallest known monotreme (after another extinct species called Teinolophos trusleri). It was more of an echidna type, with a snout that was curved and deep like that of a tachyglossid rather than flat like that of an ornithorhynchoid. It also had teeth, although fewer than the echidnapus. Why did those teeth eventually disappear altogether in modern monotremes?
Monotremes without teeth came onto the scene when the platypus (Ornithorhynchus anatinus) appeared during the Pleistocene, which began 2.6 million years ago. The researchers think that competition for food caused the disappearance of the platypus’ teeth; the spread of the Australo-New Guinean water rat may have influenced the prey that platypuses hunted. Water voles eat mainly fish and crustaceans, along with some insects, which are also believed to have been part of the diet of ancient ornithorhynchoids. Choosing softer food to avoid competition could explain why the platypus has become toothless.
As for echidnas, tachyglossids are thought to have lost their teeth after diverging from ornithorhynchoids near the end of the Cretaceous. Echidnas are insectivores, grinding the hard shells of beetles and ants with spines in their mouths, so they do not need teeth.
Although there is some idea of what happened to their teeth, the fate of the various species of Cretaceous monotremes, which were not only toothy but usually larger than the modern platypus and echidna, remains unknown. The end of the Cretaceous period brought about a mass extinction caused by the asteroid Chicxulub. It is clear that some monotremes survived, but no monotreme fossils from that time have yet emerged.
“It is unclear whether the diverse monotreme fauna survived the end-Cretaceous mass extinction and subsequently persisted,” the researchers said in the same study. “Filling this mysterious interval of monotreme diversity and adaptive development should be a primary focus of future research.”
Alcheringa: An Australasian Journal of Paleontology, 2024. DOI: 10.1080/03115518.2024.2348753