Yes, dolphins are placental mammals/animals.
Dolphins are similar to land mammals and other marine mammals in several ways and share many of the same characteristics.
It includes being warm-blooded animals, requiring oxygen to breathe, going through a pregnancy period, giving birth to their young, and producing milk which they use to nurse and feed their young.
This is very different from non-placental oceanic animals such as fish and sharks (sharks are a variety of fish), which produce eggs, are cold-blooded, extract oxygen from the water through gills, and do not nurse their young or have milk.
While dolphins give birth to offspring, they generally only produce a single offspring once every 2 to 5 years.
On the other hand, fish may produce dozens, hundreds, or even thousands of eggs in a year.
Regarding reproduction, one significant difference between placental animals such as dolphins and egg-producing animals such as fish and sharks is that dolphins not only give birth to their young but also care for them during their first few years of birth.
Most fish and sharks do not care for their young but instead lay hundreds or thousands of eggs to maximize their chances of passing their genes on to the next generation.
Once a fish is born, it is often left to fend for itself; it would be virtually impossible for a female fish to care for all her offspring.
As stated earlier, dolphins produce milk from their memory glands which they use to feed their children.
This feeding and nursing period can last anywhere from six months to two years until the child can hunt and survive on its own and break away from the psychological dependency they’ve formed around its mother.
Eventually, these dolphins will grow into young adults, form relationships with their pods, and bear their offspring.
Most dolphin species reach sexual maturity between 6 – 15 when they begin mating and reproducing.
Once impregnated, the average gestation period can last anywhere from 11 – 17 months, depending on the species.
As stated previously, fish are very different as they can lay thousands of eggs in a year and often do not provide any care for their young.
When a fish lays its eggs, the eggs fall to the bottom of the water, where the child is left to hatch and fend for food and survival on its own.
These baby fish are born with a sack attached to them to aid the child in surviving, providing them with the nutrients they need to stay until they can hunt for their food.
As you can see, dolphins are not only placental animals but are very different from fish and numerous amphibious species.
Not only do they give birth to their young, but they also act as caretakers.
Like many other mammal species, they are often limited to the same physical constraints (such as producing milk and giving birth to a single child) as land and other marine mammals.