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Introduction

The platypus (Ornithorhynchus anatinus) is one of the most unusual of living creatures. It is a mammal which has fur and suckles its young, but it also lays eggs, has webbed feet, a bill that looks like that of a duck, and a tail resembling that of a beaver. Males have a poisonous spur on their hind legs which can cause excruciating pain to humans and kill dogs. So, you can imagine the surprise on the face of Captain John Hunter, the second Governor of New South Wales, when in 1798, he saw and sent a souvenir pelt and sketch of a platypus to Great Britain. [1] It was so odd and so unlike anything else they had ever seen that the British scientists dismissed it as an elaborate hoax, «a high frolic practised on the scientific community by some colonial prankster” [2].

How did the platypus evolve in this unlikely fashion?

The purpose of the study is a summary review of English sources. In the course of studying the sources, tasks aimed at answering the questions posed will be solved.

How closely are they genetically similar to birds and reptiles?

How did the evolution of platypuses proceed?

Material and methods

Foreign sources were the main material for our study. The main method used was the review of the information with its further analysis.

During the study, foreign sources were analyzed, including articles, diagrams and video sources on topics related to the evolution of monotremes, their systematics, genome. The information from various sources gave a complete picture and cause-and-effect relationships.

of the animal.

Results

According to the analyzed data, the clades of platypuses and echidnas dispersed during the Early Cretaceous period and existed on Australian territory even before the arrival of marsupials, which came to Australia from South and North America via Antarctica when the continents were much closer to each other (about 45 Ma).

Teinolophos is one of the oldest representatives of what can be interpreted as one of the earliest platypuses, it lived 123 Ma. [3] It is known from parts of its lower jaw, from which we know that its size was 10 centimeters long, which is 4 times smaller than living platypuses. Unlike modern monotremes, which have suspended ear bones much like placentals and marsupials, Teinolophos still had them connected to the jaw via the Meckel's cartilage. This reinforces the idea that the modern ear condition evolved independently among monotremes and therians.

Obdurodon had more similarities to modern platypuses. It lived from 5 Ma; the later appearance (25 Ma) of Obdurodon probably reflects the sparseness of earlier Tertiary mammal sites in Australia. [4] The diversity of monotremes is reduced contracts to only platypus-like taxa, as their aquatic lifestyle gave them an advantage in competition with marsupials over other monotremes. [5] It was similar in body shape to the modern platypus, but larger in size. The wear pattern on the teeth suggests that it fed on turtles and crustaceans similar to those eaten by modern platypuses. This allows us to conclude that the transition to the proventriculus (organ that combined esophagus, stomach, and small intestine [6]) present in modern monotremes began at that time. This type organ is characteristic of animals whose diet is rich in molluscs. The shells of molluscs have chalky shells that can neutralize pepsin. [7]

In addition to studying the remains of platypus ancestors, it is also possible to study the genome of living platypuses. According to new research, the platypus has genetic similarities to both reptiles and birds.

The platypus genome was compared to the genomes of birds, mice and humans. It was found that the platypus DNA includes a number of genes not peculiar to mammals. For example, the platypus retains a copy of the vitellogenin gene, which encodes a precursor of egg yolk and is characteristic of birds and reptiles. Doubtless, this gene remained in platypuses from distant ancestors, but due to the ability of platypus to produce milk, the number of copies of this gene is less than in the same birds and reptiles, due to unnecessary large amount of yolk. But not all features of the platypus are similar to those of reptiles, suggesting a common ancestor.

For example, the platypus venom from its spurs is similar in composition to reptilian venom, but it has a different transport pathway, which may indicate convergent evolution.

On the other hand, the platypus has a trait common to all mammals - lactation. A gene responsible for milk production has been discovered in the genome. This gene is characteristic of mammals, but the gene itself originated 166 million years ago. This fact indicates that lactation of milk occurred before mammals began to bear offspring. [8]

Conclusion

Based on the results of the analyzed sources, we can conclude that, although the evolution of platypuses began long before the arrival of marsupials in Australia, this is what secured platypuses in the ecological niche in which they are now. They have successfully competed with marsupials and other monotremes in Australia, so they have retained some reptilian traits, such as egg-laying. Despite all the known data, the platypus is still an important object of study for scientists, because on its example it is possible to trace the evolution of mammals in relation to each other and reptiles.

References

[1] https://animalofthings.com/platypus/

[2] https://naturenook.co.uk/2021/05/05/the-puzzling-problem-of-the-platypus/

[3] Rowe, T., Rich, T. H., Vickers-Rich, P., Springer, M., & Woodburne, M. O. (2008). The oldest platypus and its bearing on divergence timing of the platypus and echidna clades. Proceedings of the National Academy of Sciences of the United States of America, 105(4), 1238–1242. https://doi.org/10.1073/pnas.0706385105

[4] Phillips, M. J., Bennett, T. H., & Lee, M. S. (2009). Molecules, morphology, and ecology indicate a recent, amphibious ancestry for echidnas. Proceedings of the National Academy of Sciences of the United States of America, 106(40), 17089–17094. https://doi.org/10.1073/pnas.0904649106

[5] https://www.livescience.com/5746-odd-egg-laying-mammals-exist.html

[6] https://goo.su/jBowWq

[7] https://www.livescience.com/41661-why-platypus-wont-regain-stomach.html

[8] https://goo.su/NpzrB

Потенциальные вопросы и ответы:

1. Why you chose this topic? (I chose this topic because the platypus itself is very interesting to me and it would be nice to know how its evolution took place)

2. How many eggs do they lay? (The female of platypus lays from 1 to 3 eggs (in hole) and the incubation period last 10 days, after which the young 3 month feed on milk)

3. Where did you get the information from? (I took information from foreign sources such as proceeding of the National Academy of scinces of the USA)

4. How do baby platypus feed on milk? (The mother feeds the cubs with milk, which comes out through the enlarged pores on her stomach. Milk flows down the mother's fur, accumulating in special grooves, and the cubs lick it off.)

5. All about poison (The venom is made in venom glands that are connected to hollow spurs on their hind legs; it is primarily made during the mating season. Glands are located in the leg)

To the skull of Obdurodon: Fm (foramen magnum), V2 (foramina for maxillary nerve), P3 (upper thirdpremolar); P4 (upper fourth premolar) - these designations (дизагнэйшнс) are used for comparison (кампарасн) with other animals.

Cretaceous period – this is about 120 Ma

Therians – subclass that contains marsupial and placental mammals

Fig 1. Illustration from the first scientific description in 1799

Fig 2. Emergence of traits along the mammalian lineage and overview of the phylogenetic relationships and divergence times between monotremes, marsupials, and placentals

Fig 3. reconstruction of the mandible of Teinolophos trusleri

Fig 4. Obdurodon skull in lateral (B), dorsal (D), and posterior (F) views.