Monotremata

## About

Monotremes are an ancient group of mammals confined to the environments of Papua New Guinea, Indonesia and Australia. Together with placental mammals and marsupials, Monotremes make up the three main groups of mammals. Montremata is a very small group only containing five extant members. Monotremes are characterised by their ability to lay eggs, which is in sharp contrast to the placental births of placental mammals, and the pre-mature births of marsupials. Monotreme means 'one-hole' in ancient Greek, this is in reference to Monotremes excreting, urinating and laying eggs out of the same opening known as a cloaca. Cloacas are not exclusive to Monotremes as both birds and reptiles share the same feature however, cloacas are specific to monotremes in mammals, as both placental mammals and marsupials do not have cloacas. The most well known feature of Monotremes is their ability to lay eggs, unlike any other mammals. Monotreme eggs are unlike bird eggs and are closer to that of reptile eggs, being small round and rubbery instead of the hard shelled oval-shaped eggs of birds. The similarities between Monotremes and reptiles does not stop there however, Monotremes actually act very similarly to reptiles in their movements and attributes, even being venomous in the platypus' case. The reason for the Monotreme's egg laying is widely believed to be because they have retained many of the long extinct features of ancient mammals. Monotremes are extraordinarily old mammals first diverging from other mammals earliest in the fossil record around 200 to 145.5mya during the late Triassic-early Jurassic period. This makes Monotremes the oldest mammal order still in existence.

### Historic Classification

Monotremes are an ancient order, and exactly deciphering where they fit into the mammalian lineage is incredibly difficult. None of the theories surrounding Monotreme relationships have been universally accepted according to Britannica. This elusiveness has made Monotremes one of the most interesting groups to those who study mammalian evolution. The most common explanation to Monotremata's lineage is that they evolved from three groups known as Steropodontidae, Teinopholophidae and Kollikodontidae, all originating from the Cretaceous period and also being represented by one sole species for each.

#### Steropodontidae

Steropodontidae is believed to beone of the first families of Monotremes and is represented by the species Stropodon galmani. Steropodon was first discovered at the Girman Creek formation, Lightning Ridge, New South Wales. It is believed that steropodon resembled a round, stout platypus-like creature. Almost as though it was a cross between the body shape of an echidna and the features of a platypus. Steropodon lived on the Australian part of the ancient continent of Gondwanaland. Evidence from the bones of Monotremes during this time suggests that Monotremes were much more widespread in the distant past, inhabiting South America and Antarctica and not just Australia. This evidence may mean that Monotremes were the dominant group on Australia before the marsupials arrived. Most of these primitive Monotremes are thought to have resembled platypuses, sharing their semi-aquatic behaviour. Steropodon is only known from a single isolated jaw bone however, this bone possesses a large hollow groove in it. This groove is also present in modern day platypuses and for this reason it is inferred that the sterapodon had a bill as well. Steropodon would have only grown to the size of a modern platypus, making it one of the largest mammals to coexist with dinosaurs.

#### Teinolophidae

Teinolophidae (pronounced TINE-OLOE-FI-DAY) is a mysterious family of Monotremes that like Staropodontidae and Kollikodontidae only has one extinct member. Teinolophos was tiny mammal that was about five times smaller than the modern day platypus. Teinolophos was a small and primitive Monotreme that probably also resembled Steropodon galmani or the modern day platypus. Teinolophos is extremely divergent from Monotreme evolution and because of this reason was moved from Steropodontidae to its own seperate family.

#### Kollikodontidae

Kollikodontidae is represented by one extinct species known as Kollikodon richiei. Kollikodon is also known from the Girman Creek formation, and is also only known from a jaw bone. This jaw bone possesses large molars which is where the animal gets the name Kollikodon, with 'Kolix' being an ancient Greek word meaning 'bread rolls'. This strange almost joke of a name is because the animal's teeth resemble hot cross buns, this naming convention was almost taken even further when Michael Archer (the founder of the jaw bone) wanted to call the animal the *'Hotcrossbunodon'* This name however, was frowned upon by his colleagues for its obvious comedic undertone. The Kollikodon jaw bone can be found at the Australian museum, Sydney.

## Modern Monotremes

Today only two families of Monotremes remain, being Ornithorhynchidae and Tachyglossidae. Both are small groups only containing five extant members in total, Ornithorhynchidae includes only the duck-billed platypus ***(Ornithorhynchus anatinus)*** whilst Tachyglossidae includes three species of long-beaked echidna and one species of short-beaked echidna. All species of Monotreme currently extant are only known to live on the Islands of Oceania, with platypuses being exclusive to Australia and Tasmania.

#### Ornithorhynchidae

Ornithorynchidae (pronounced ORN-ITH-OR-INK-E-DAY) is a Monotreme family that only includes the platypus as its only extant member. Ornithorhynchidae is also comprised of two other genuses being Monotrematum and Obdurodon. Monotrematum includes one extinct species being the Monotrematum sudamericanum, this platypus-like ancestor would have roamed the plains of Patagonia, Argentina during the Paleocene around 61mya. This discovery stands as the only Monotreme found outside of Oceania. Monotrematum would have resembled a modern day platypus quite closely aside from its large teeth, which were twice the size of even Obdurodon's. Obdurondons were large platypus-like animals that roamed Australia as late as 5-15mya. The genus Obdurodon includes the oldest Ornithorhynchid, Obdurodon insignis which would have lived on modern day Australia 26mya. The largest Ornithorhynchid also belongs to this genus, Obdurodon tharalkooschild would have grown to an astounding 1m (3.3ft) compared to the 50cm (19in) long platypus of modern day.

#### Tachyglossidae

Tachyglossidae (pronounced TAK-E-GLOSS-E-DAE) is a family of Monotremes that includes both extant long and short-beaked echidnas. This family includes four genuses being Tachyglossus, Zaglossus, Megalibgwilia, Murrayglossus. Tachyglossus is a tiny group that is comprised of one extant species being the short-beaked echidna ***(Tachyglossus aculeatus)*** short-beaked echidnas are differenctiated from long-beaked echidnas by their beak length as well as their smaller size when compared to Zaglossus species. Zaglossus is the genus that includes all extant long-beaked echidnas as well as the extinct Zaglossus robustus. Long-beaked echidnas live exclusively on the Island of New Guinea in forested environments most commonly on mountains. Long-beaked echidnas are comparatively much more rare than short-beaked echidnas, and one species the Sir David's long-beaked echidna ***(Zoglussus attenboroughi)*** has not been sighted by Western science since 1961. Megalibgwilia is only known from two extinct species, M. ramsayi and M. robusta. Both these species are known solely from fossil evidence in Australia. Murrayglossus is a genus comprised of one extinct species being the M. hacketti. M. hacketti was previously thought to be an extinct member of Zaglossus but was changed to be in its own family. M. hacketti was discovered in Mammoth cave, Western Australia and is believed to be the largest Monotreme to ever have existed being 1m (3.3ft) long and 30kg (66lbs) in weight. Though its length is identical to Obdurodon tharalkooschild, M. hacketti could also grow to be 0.6m (2.0ft) tall, making it about the size of a small pig. Because of its towering stature (compared to other Monotremes) M. hacketti has also been informally called the giant echidna. M. hacketti would have roamed Australia as recently as the Pleistocene, 3mya and may even have survived on Tasmania until 20,000 years ago.

## Sources

* Monotreme | Wikipedia - https://en.wikipedia.org/wiki/Monotreme
* Steropodon | Wikipedia - https://en.wikipedia.org/wiki/Steropodon
* Kollikodon | Wikipedia - https://en.wikipedia.org/wiki/Kollikodon
* Teinolophos | Wikipedia - https://en.wikipedia.org/wiki/Teinolophos
* Tachyglossus | Wikipedia - https://en.wikipedia.org/wiki/Echidna#Tachyglossus
* Ornithorhynichidae | Wikipedia - https://en.wikipedia.org/wiki/Ornithorhynchidae
* Monotreme | Britannica - https://www.britannica.com/animal/monotreme