Furcacauda heintzae
Name: Furcacauda heintzae [Heintz's fork-tail]
When: Early Devonian Period, 420 - 390 million years ago
Where: MacKenzie Mountains, Northwest Territory, Canada
Claim to fame: Furcacauda Does not look exciting at first. Like many other vertebrates of that time, it lacked jaws or paired fins. It belongs to a larger group called Thelodonti, whose members' bodies are covered by fine scales instead of armor plates (like many of its contemporaries). Most thelodonts are cigar-shaped with slightly flattened undersides, allowing them to lie on the bottom at rest. Furcacauda and its relatives, in contrast are the first known vertebrates with deep laterally compressed bodies, requiring them to float with neutral buoyancy in the water.
That, by itself, makes them trendsetters, but there is more. Furcacauda's gut tube has an expanded chamber at its front end - the first evidence of a vertebrate with a distinct stomach!
But the obvious weirdness is in the forked tail from which it get its name. The lower lobe is the true tail, supported by the creature's spinal column. The upper lobe is actually a dorsal fin that has become large. The tail fin is suspended between these two lobes, supported by six smaller fin rays.