Outliers amongst mammals, monotremes lay eggs as a substitute of giving start to reside younger. Solely two kinds of monotremes, the platypus and echidna, nonetheless exist, however extra monotreme species have been round about 100 million years in the past. A few of them may presumably be even weirder than their descendants.
Monotreme fossils present in refuse from the opal mines of Lightning Ridge, Australia, have now revealed the opalized jawbones of three beforehand unknown species that lived throughout the Cenomanian age of the early Cretaceous. Not like trendy monotremes, these species had enamel. In addition they embrace a creature that seems to have been a mashup of a platypus and echidna—an “echidnapus.”
Fossil fragments of three identified species from the identical period have been additionally discovered, which means that a minimum of six monotreme species coexisted in what’s now Lightning Ridge. In line with the researchers who unearthed these new species, the creatures might have as soon as been as widespread in Australia as marsupials are at this time.
“[This is] essentially the most various monotreme assemblage on file,” they stated in a examine just lately printed in Alcheringa: An Australasian Journal of Paleontology.
The Echidnapus emerges
Named Opalios spendens, the “echidnapus” exhibits similarities to each ornithorhynchoids (the platypus and related species) and tachyglossids (echidna and related species). It’s thought to have advanced earlier than the widespread ancestor of both extant monotreme.
The O. splendens holotype had been fossilized in opal like the opposite Lightning Ridge specimens, however in contrast to some, it’s preserved so nicely that the inner construction of its bones is seen. Each mammalian fossil from Lightning Ridge has been recognized as a monotreme primarily based partly on their peculiarly massive dental canals. Whereas the fossil proof suggests the jaw and snout of O. splendens are slender and curved, much like these of an echidna, it concurrently shows platypus options.
So what relates the echidnapus to a platypus? Regardless of its jaw being echidna-like at first look, its dentary, or the a part of the jaw that bears the enamel, is analogous in measurement to that of the platypus ancestor Ornithorhynchus anatinus. Different options associated extra carefully to the platypus than the echidna need to do with its ramus, or the a part of the jaw that attaches to the cranium. It has a brief ascending ramus (the rear finish) and twisted horizontal ramus (the entrance finish) which can be seen in different ornithorhynchoids.
One other platypus-like function of O. splendens is the flatness of the entrance of its decrease jaw, which is in keeping with the flatness of the platypus snout. The dimensions of its jaw additionally suggests a physique measurement nearer to that of a platypus. Although the echidnapus had traits of each surviving monotremes, neither of these have the enamel discovered on this fossil.
My, what enamel you don’t have
Cretaceous monotremes may not have had as many enamel because the echidnapus, however all of them had some enamel. The opposite two new monotreme species that lived among the many Lightning Ridge fauna have been Dharragarra aurora and Parvopalus clytiei, and the jaw construction of every of those species is both nearer to the platypus or the echidna. D. aurora has the marginally twisted jaw and enlarged canal in its mandible which can be attribute of an ornithorhynchoid. It would even be on the department that gave rise the platypus.
P. clytiei is the second smallest identified monotreme (after one other extinct species named Teinolophos trusleri). It was extra of an echidna kind, with a snout that was curved and deep like that of a tachyglossid relatively than flat like that of an ornithorhynchoid. It additionally had enamel, although fewer than the echidnapus. Why did these enamel find yourself disappearing altogether in trendy monotremes?
Monotremes with out enamel got here onto the scene when the platypus (Ornithorhynchus anatinus) appeared throughout the Pleistocene, which started 2.6 million years in the past. The researchers assume competitors for meals brought on the disappearance of enamel within the platypus—the unfold of the Australo-New Guinean water rat might have affected which prey platypuses hunted for. Water rats eat principally fish and shellfish together with some bugs, that are additionally thought to have been a part of the food regimen of historical ornithorhynchoids. Turning to softer meals to keep away from competitors might clarify why the platypus advanced to be toothless.
As for echidnas, tachyglossids are thought to have misplaced their enamel after they diverged from ornithorhynchoids close to the top of the Cretaceous. Echidnas are insectivores, grinding the onerous shells of beetles and ants with spines inside their mouths, so haven’t any want for enamel.
Though there may be some concept of what occurred to their enamel, the destiny of the various species of Cretaceous monotremes, which weren’t solely toothy however principally bigger than the trendy platypus and echidna, stays unknown. The tip of the Cretaceous introduced a mass extinction triggered by the Chicxulub asteroid. Clearly, some monotremes survived it, however no monotreme fossils from the time have surfaced but.
“It’s unclear whether or not various monotreme fauna survived the end-Cretaceous mass extinction occasion, and subsequently endured,” the researchers stated in the identical examine. “Filling this mysterious interval of monotreme range and adaptive growth needs to be a major focus for analysis sooner or later.”
Alcheringa: An Australasian Journal of Palaeontology, 2024. DOI: 10.1080/03115518.2024.2348753
