Ed Yong in The Atlantic:
The oldest penis ever found is 425 million years old, and belongs to an animal whose scientific name—Colymbosathon ecplecticos—means “astounding swimmer with a large penis.” Large is relative, though. The entire creature is just a fifth of an inch long, but for its size, its penis is still “large and stout,” according to its discoverers.
That’s not unusual for the ostracods—the ancient group of crustaceans to which Colymbosathon belongs. From their origins almost half a billion years ago, these animals have diversified into some 70,000 species. At first glance, they look like little seeds. Look closer, and you’ll see what appear to be distorted shrimps, encased in hard, clam-like shells. Male shells tend to be longer than female ones, because they have to accommodate a pair of large penises, and outrageously big sperm that, when uncoiled, can be six times as long as the ostracod itself. In some species, all of this reproductive gear can take up a third of the male’s shell.
This anatomical extravagance is the result of intense sexual selection, where organisms evolve traits that give them an edge in the competition for mates. That competition leads to excessive body parts, like peacock tails or deer antlers. It leads to colorful plumage and flashy courtship displays. In many groups—ostracods, flies, ducks, dolphins—it sculpts genitals and sperm into an extraordinary diversity of shapes and sizes. And in the ostracods, it can lead to oblivion.