Non-placental mammals are faced with an unique predicament. Possessing epipubic bones, they cannot expand their torsos, and thus cannot give birth to complex, well developed young you take for granted. Marsupials eject joeys, monotremes eject eggs that hatch into puggles — joeys by another name. This is likely the same for all mammals prior to the Cenozoic, from the rapacious Repenomamus to the peaceful Schowalteria.
This means that an aquatic lifestyle is limited. Platypodes, yapoks, stagodontids, Castorocauda and even a few eutriconodonts produced otter-like forms, but never fully aquatic critters on the same level as cetaceans and dugongs.
In another world, a lineage of monotremes found an agreeable solution. The so called Cybelotheres, these animals evolved from an Obdurodon like ancestor that began to forage at sea. For a while these animals spread across the world’s costlines as pinnipede analogues, until one crucial event that took place in the Miocene: the milk egg
Like all monotremes, cybelotheres produce a single egg. When expelled from the body, this egg is cvered in a thick gel, preventing osmosis from obliterating the egg and adding a small layer of insulation. As soon as the egg is laid into the open water, the female wastes no time and glues it against her belly. This tiggers the release of copious amounts of a highly specialised milk that adheres to the gel, soon coating it into a ball up to three times larger than the actual egg.
The milk congeals and forms a short of “extra eggshell” with roughly the same consistency as cheese. The milk near the actual eggshell is still mostly fluid, while the outermost layer is hardened, more so if in contact with salt water. Within its matrix there are several bubbles, allowing heat to be preserved.
The end result is a free-floating egg that functions as an external womb, the abundant nutrional resources pouring into the gradually dissolving eggshell. Soon, they become an extension of the amniotic sack, and the milk crust the replacement shell.
The end result is a rather fast developmental process, that may last as little as a month. Lungs are the very last organs developed, well after the puggle already has functional flippers and even a blubber layer of its own. Upon hatching, the new swimming monotreme can already fully fend for itself, rendering it one of the few mammals that doesn’t need parental care.
Different cybelotheres handle their eggs differently. Some keep them attached to the belly in order to provide bodily warmth and additional milk; this is likely the basal condition, as in many such species the milk shell is continuously added too and not fully formed. In other species the eggs are left in nurseries, gluing to each other like a mammalian milk caviar. Others still are left fully to the void of the sea.
This survival strategy proved to be a success, though the cooling global temperatures of the Pliocene meant that most species rely on nurseries or reverted back to sticking the egg to the belly.