what prehistoric creature is the platypus related to?

Long answer: "In 2004, researchers at the Australian National University discovered the Platypus has ten sex chromosomes, compared to two (XY) found in most other mammals (for instance, a male platypus is always XYXYXYXYXY). Furthermore, one of the platypus’s Y chromosomes shares genes with the ZZ/ZW sex chromosomes found in birds. This news has further pronounced the individuality of the Platypus amongst the animal kingdom, and a target for further research into evolutionary links between mammals, birds and reptiles." [1]

Short answer - "Nobody knows for sure." [2]

Ornithorhynchus anatius(duck-billed platypus) which belongs to Order:Monotremata of Class:Mammalia,evolved from a reptilian dinosaur-like creature called Therapsid. I only know that Mammals evolved from Therapsids-but there is no direct evidence that supports the evolution of an Avian creature into a Mammalian one.

Evolutionists insist that the duck-billed platypus is an evolutionary link between mammals and birds.

I dimly recall hearing about an extinct giant platypus.

To Charmayne: no evolutionist thinks there is a missing link between birds and mammals. We know they are distantly related.