The Key Takeaways about Egg-Laying Montremes
- Monotremes are mammal species that do not have a womb and lay their eggs. These include animals like platypuses and echidnas. The egg-laying monotremes are likely to have retained traits of their reptilian relatives, including egg-laying traits and even venomous characteristics. The monotremes of Oceania are now extinct, although they may have once lived in South America. The monotremes in Oceania are so different from other mammals that it has confused evolutionary biologists over the years. When compared to other mammals, the platypus and echidnas are the only ones who give birth by live birth. These creatures are not only different because they lay eggs. They also have venom – a trait that is almost unheard of in mammals – and lack developed sexual organs. Why did they evolve so differently from mammals?
They didn’t. Monotremes may be a remnant of a lineage of mammal species that used to exist on Earth tens and millions of years earlier. Monotremes are one of the few species that have not lost their ability to lay egg. They have adapted it. They retained traits of their reptilian relatives.
Read more– Ancient Species represents bridge between Echidna & Platypus.
Monotremes – What Are They? Monotremes are defined as creatures that have a single opening. This is an euphemistic term for those who defecate and urinate in the same place, lay their eggs, or mate. The cloaca of monotremes is similar to birds’, and they have no digestive or reproductive tracts. Nicolas Roberto Chimento is a mammal paleontologist working at Bernardino Rivadavia Natural Sciences Argentine Museum, Buenos Aires. He says: “That primitive feature”
Although they used to be more widespread, only five species remain on Earth, and all are found in Oceania. These include the platypus, four species of spiny anteaters, as well as the echidna. These five species all lay eggs instead of giving birth to live babies.
How long ago did monotremes evolve?
There is still a lot of fossil evidence to be found for monotremes. The monotremes date at least back to dinosaur times. In fact, the oldest fossil found dates from the Early Cretaceous period about 125,000,000 years ago. Recent fossil discoveries by Roberto Chimento in South America have shown that these animals were not always found in Australia or nearby islands. His team discovered the oldest fossil of a platypus, Patagorhynchus pascuali in Patagonia. It was described in 2023 by Communications Biology () in an article. The
P. pascuali, has now changed the way we think about these animals’ evolution. One of the oldest fossils of a platypus, the Obdurodon dates back to only 30 million years. Combined with Monotrematum ( ) from Argentina which dates back to 60 million years ago, it is possible that the first platypuses evolved in America and then spread into Australia. The fossil record of echidnas is weaker than that for
. Australia has fossil monotremes that are older than the platypus family. They date from the Early Cretaceous – the oldest is Teinolophos, which is about 125,000,000 years old. When did monotremes start laying eggs?
There is a scarcity of fossils for monotremes. The oldest fossils can only be identified by teeth and skull parts. Paleontologists do not have any evidence for ancient monotreme egg. Most paleontologists agree, however
that egg-laying has been present since the dawn of time. Today, mammals are divided into three main groups. Monotremes lay eggs, marsupials give birth relatively undeveloped children, and placentals give birth more developed children. These mammals, along with humans, make up the vast majority of the world’s mammals today.
According to scientists, monotremes split from the common ancestor for all mammals earlier than placentals or marsupials. It would also explain their appearance, which is similar to that of reptiles. This includes their egg-laying habits and more primitive reproductive systems. Platypuses may have inherited their venomous spurs from reptile ancestors. The fossil record of ancient monotremes does not include spurs, in part because the group’s limbs were never found by paleontologists. Roberto Chimento claims that Multituberculatahad spurs. He speculates that it is possible for early mammals, such as Multituberculata, and monotremes to have inherited spurs with venom from their reptilian small ancestors in order to defend themselves against the larger dinosaurs at the time.
Why Did Monotremes Survive?
The question is, if monotremes have never evolved to not lay eggs, like other mammals, then why are they only found in Oceania? Evidence shows they lived once in South America. Why did they not survive? Roberto Chimento,
says it’s likely a matter of lack of competition. Evolution of the placenta has given the biggest group of mammal an evolutionary edge. Many mammals, who spend so much time in their mothers’ bodies before birthing them, are already quite developed. They can see and walk right away. This adaptation gave placental mammal a head start over the competition. It’s also why there are only a few marsupials outside Oceania, who must stay in their mothers’ pouches until they’re fully developed. According to Britannica, monotremes are likely to hatch in eggshells outside of the body, after 10 days incubation. Roberto Chimento claims that fetuses inside the body are better protected than eggs growing outside. They can’t be eaten, trampled on and they have more exposure to cold. There were few placental mammal species in Australia before the arrival of dingos and humans. The only ones that competed with them are rodents. The Australian mammal population was able to survive for tens, if not hundreds of millions, of years with little competition. New Guinea was also relatively late in receiving the majority of placental mammals. Roberto Chimento states that “there wasn’t as much competition in New Guinea” compared to other continents.
Read more– Evolutionary Origins Of The Strange Platypus And Echidna Discovered At Dinosaur Cove.
Sources for Articles
Discovermagazine.com’s writers regularly use high quality sources and peer reviewed studies for their articles. Our editors also review them for accuracy, scientific standards and other editorial standards. Check out the following sources for this article.
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The article cites information published in Communications Biology. First monotreme found from the Late Cretaceous period of South America.
- The following article is a reference to Britannica’s platypus (19659034).
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