Outliers amongst mammals, monotremes lay eggs as a substitute of giving beginning to reside younger. Solely two forms 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 would possibly 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 in the course of the Cenomanian age of the early Cretaceous. Not like fashionable monotremes, these species had tooth. In addition they embrace a creature that seems to have been a mashup of a platypus and echidna—an “echidnapus.”
Fossil fragments of three recognized species from the identical period have been additionally discovered, that means that at the very least six monotreme species coexisted in what’s now Lightning Ridge. In keeping with the researchers who unearthed these new species, the creatures might have as soon as been as widespread in Australia as marsupials are at present.
“[This is] probably the most numerous monotreme assemblage on document,” they stated in a examine lately revealed 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 developed 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 not like some, it’s preserved so nicely that the interior construction of its bones is seen. Each mammalian fossil from Lightning Ridge has been recognized as a monotreme based mostly partly on their peculiarly giant dental canals. Whereas the fossil proof suggests the jaw and snout of O. splendens are slim and curved, just 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 tooth, is comparable in measurement to that of the platypus ancestor Ornithorhynchus anatinus. Different options associated extra intently to the platypus than the echidna should 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 are seen in different ornithorhynchoids.
One other platypus-like characteristic of O. splendens is the flatness of the entrance of its decrease jaw, which is according to 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 tooth discovered on this fossil.
My, what tooth you don’t have
Cretaceous monotremes won’t have had as many tooth because the echidnapus, however all of them had some tooth. 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 are attribute of an ornithorhynchoid. It’d even be on the department that gave rise the platypus.
P. clytiei is the second smallest recognized 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 quite than flat like that of an ornithorhynchoid. It additionally had tooth, although fewer than the echidnapus. Why did these tooth find yourself disappearing altogether in fashionable monotremes?
Monotremes with out tooth got here onto the scene when the platypus (Ornithorhynchus anatinus) appeared in the course of the Pleistocene, which started 2.6 million years in the past. The researchers assume competitors for meals precipitated the disappearance of tooth 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 weight loss program of historic ornithorhynchoids. Turning to softer meals to keep away from competitors might clarify why the platypus developed to be toothless.
As for echidnas, tachyglossids are thought to have misplaced their tooth after they diverged from ornithorhynchoids close to the tip of the Cretaceous. Echidnas are insectivores, grinding the arduous shells of beetles and ants with spines inside their mouths, so haven’t any want for tooth.
Though there may be some thought of what occurred to their tooth, 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 top 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 numerous 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 variety and adaptive growth must be a main focus for analysis sooner or later.”
Alcheringa: An Australasian Journal of Palaeontology, 2024. DOI: 10.1080/03115518.2024.2348753
