Scientists find two-headed fossil.
While a reptile, Hyphalosaurus was not a dinosaur. Instead, it belonged to a diverse group of primitive aquatic and semi-aquatic creatures called choristoderes. Some choristoderes looked like lizards or crocodiles, while others resembled miniature versions of plesiosaurs, ancient marine reptiles with barrel-shaped bodies, short tails, paddle-like limbs and, in some cases, long serpentine necks — somewhat like the mythical Loch Ness monster.
I swear, I totally made this thing up when I was eight years old.
Being a sea creature, these fossils have been found all over the place. Two-headedness, as the article shows, usually results from early injury to the developing embryo. A German embryologist named William Roux first showed this in 1888 by stabbing a 2-cell stage frog embryo with a hot needle, effectively destroying that totipotent cell. The resulting embryos mostly just died. Other experiments with defects introduced in later divisions showed marvelous effort to field a functioning animal, but resulted in some disturbingly crippled creatures somewhere between functional and lethal. A successive series of defect, recombination and isolation experiments after Roux's work laid the bedrock for what is modern developmental biology.
Keep in mind, most of these ideas and experiments were done without the re-discovery of Mendel's groundbreaking work, and Darwin's grasp of cellular mechanics in Origin of Species was nowhere near even this sophistication. This is causal science at its best.
Being a reptile, the poor guy (guys?) probably had a heck of a time deciding which way to go.
Image credit: Jianjun Li And Eric Buffetaut