Platypodes being eaten by Teleosaurus and hunted by Pterodactylus
Strictly speaking, platypodes did not exist in their current form back in the Late Jurassic (the only time Teleosaurus and Pterodactylus co-existed). When this illustration was published, Monotremata were thought to be “sub-mammalian”, and closely related to reptiles, and as such assumed to have split off from reptilian ancestors farther back than other mammalia.
Thanks to genetic analysis, we now know that Monotremata are more closely-related to reptiles than other mammals, but not for the same reasons that were originally assumed. It’s unknown exactly when the order split from its last common ancestor to placental and marsupial mammals, but the oldest fossils we’ve found so far have only been 100 million years old or so. Those fossils were of Obdurodon, which looked a lot like a larger platypus with small shearing incisors, and molars in the back of its mouth.
Aside from the fact that Teleosaurus was almost exclusively an open-ocean crocodilian, who knows? It might have snacked on an Obdurodon or two when it came to land to lay its eggs!
The Book of the Animal Kingdom: Mammals. W. Percival Westall, 1910.