Everything you always wanted to know about the platypus. "The platypus is a frequent subject of research in evolutionary biology."

As usual I look things up then copy and paste.

The amazing platypus, the egg-laying mammal. The platypus is one of the most fascinating species on our planet. The platypus is one of only five species in the order Monotremata, which differ from all other mammals in that they lay eggs covered by a shell that hatch outside the mother's body.

The platypus, sometimes referred to as the duck-billed platypus, is a semiaquatic egg-laying mammal endemic to eastern Australia, including Tasmania. Wikipedia

Scientific name: Ornithorhynchus anatinus
Conservation status: Least Concern Encyclopedia of Life
Lifespan: 17 years (In captivity)
Trophic level: Carnivorous Encyclopedia of Life
Mass: Male: 2.2 – 5.3 lbs, Female: 1.5 – 3.5 lbs
Did you know: Platypuses have been heard to emit a low growl when disturbed and a range of other vocalisations have been reported in captive specimens.animalcorner.co.uk

What about the evolution of these strange creatures? That's the most interesting thing for me. I will of course look it up. Wikipedia is my friend.

Modern monotremes are the survivors of an early branching of the mammal tree, and a later branching is thought to have led to the marsupial and placental groups. Molecular clock and fossil dating suggest platypuses split from echidnas around 19–48 million years ago.

Because of the early divergence from the therian mammals and the low numbers of extant monotreme species, the platypus is a frequent subject of research in evolutionary biology. In 2004, researchers at the Australian National University discovered the platypus has ten sex chromosomes, compared with two (XY) in most other mammals (for instance, a male platypus is always XYXYXYXYXY), The sex chromosomes of the platypus have been found to have great homology to the bird Z chromosome.

The platypus genome also has both reptilian and mammalian genes associated with egg fertilisation. Though the platypus lacks the mammalian sex-determining gene SRY, a study found that the mechanism of sex determination is the AMH gene on the oldest Y chromosome. A draft version of the platypus genome sequence was published in Nature on 8 May 2008, revealing both reptilian and mammalian elements, as well as two genes found previously only in birds, amphibians, and fish. More than 80% of the platypus' genes are common to the other mammals whose genomes have been sequenced.

https://en.wikipedia.org/wiki/Platypus#Evolution