However 'alien' we might envisage extraterrestrial life to be, the real thing will undoubtedly trump our imaginations. We only have to look at how diverse and extraordinarily weird life on this planet can be to see that. So I thought it would be interesting to look at several examples of Terran weirdness ... just to accentuate this point. And I'm starting with that most ancient of fish - the shark.
In recent years, our deep sea exploration cameras have provided startling visuals of some of the planet's odder sharks, like the Goblin Shark and the Frilled Shark. But did you know that even the most common sharks have some really strange traits - such as having two penises and foetal cannibalism?
Sharks have two claspers (pterygopodia) that develop on the anterior region of the ventral flippers. The name is a bit misleading as they are not used for grasping - rather they 'clasp' together to make a pseudo-penis. Each has a cartilaginous support and a groove on their inner edge and, during copulation, the two come together to form a channel through which the sperm is passed. So I suppose you could say that sharks have two half-penises - unlike snakes and lizards that really do have two functional penises.
But if you want real weirdness in sharks, you have to take a trip back into prehistory to look at some of the designs that were tried but didn't make it to the present day - such as Heliocoprion, a shark with an extraordinary lower jaw. Some scientists believe that this was a kind of spiral 'conveyor belt', in which new teeth would advance to replace old ones. But the true arrangement and purpose of the teeth remains a mystery. Some scientists have also suggested that it may have operated like a spiked whip, possibly curled underneath the lower jaw like a weapon-heavy elephant trunk.
And then there was Stethacanthus which, on the whole, looked like a modern shark except for its outrageous dorsal fin. Shaped like an ironing board, the top of this fin was covered in rough, tooth-shaped scales (denticles) that matched a similar patch of skin on the snout. Scientists believe that it was used as part of a courtship display ... or maybe it was supposed to mimic a huge mouth and make the creature appear more frightening?
Just goes to show ... there's nowt so queer as (Earth) folks.
Fossil shark reconstructions by Todd Marshall