I kind of gleaned over the subject in my last blog, so I thought I’d provide some more examples of clear transitional forms in the fossil record (especially since one dude was not satisfied)..
What Is A Transitional Form?
Different forms of life can be objectively defined and distinguished from one another only by grouping them via their shared characteristics. This is how we have terms like bird or dinosaur or mammal or reptile. We know a bird is not a mammal because it has bird characteristics, not mammalian characteristics, and so on.
So we have groups of living things, like canines, felines and so on, groups of closely related species. Different “kinds” of life.
What we find however is that ALL multicellular life fits into bigger and bigger groups.
So a cat and a tiger and a lion and a panther and a leopard are all the same “kind” of animal (felines), but if you zoom out further you find that a dog and a cat are also the same “kind” of animal. They are both mammals, for instance.
Similarly humans are all the same “kind” of organism as each other, and humans and chimpanzees are also the same “kind” of organism because they are both primates, etc, etc.
When you define groups of living things this way by their shared characteristics, you get something like this:
This is just our tiny little branch and there are many, many species in each of these groups.
So getting back to the point, what is a transitional fossil?
It is a fossil of an extinct species at one of those forks in the road, which has the defining characteristics of more than one group, and therefore possibly represents an ancestor species of both groups.
On To The Examples.
Here is a very incomplete list of some of the major transitional forms found to date.
Bats And Turtles.
The oldest bat fossils don’t have the inner ear structures required for sonar, so they couldn’t fly around by echo location like modern bats and had to fly by sight like birds. The oldest turtle fossil ever discovered had no shell.
It was a frigging turtle without a shell! How crazy is that? It had extended ribs on it’s back and armor on it’s belly. And teeth! It had teeth! The second oldest known turtle fossil had a fully formed shell, but it’s head and tail couldn’t retract into it, the other feature most people associate with turtles! Instead it had spikes on it’s tail and looked more dinosaur-like than turtle-like.
Birds And Dinosaurs
It was suspected that birds descended from either dinosaurs or mammals, because they have traits indicative of both groups like laying eggs rather than giving live birth like mammals. Now we know they descended from dinosaurs because we’ve found twenty-four different dinosaur species that had feathers (including the velociraptor), and a few that had wings:
But fossils are old and dusty and boring. Lets bring on the…
Living Transitional “Fossils”.
These are LIVING modern descendants of these ancient species which still retain characteristics diagnostic of more than one group of species.
One of the most obvious examples is snakes which still retain their legs:
They are the living descendants of intermediates between lizards and snakes.
Whales, discussed in my previous blog are another example actually. Modern whales have hands with 5 digits under their fins:
They breathe air, not water, some have hip bones where their legs used to be and various other four-legged-land-dwelling mammalian traits.
But the best, the alltime winner of most freaky weird loner species, has got to be…
Is it a mammal? It produces milk like mammals, but it has no nipples (secretes milk through patches of skin). It lays eggs like reptiles, but the eggs are soft and leathery. And it’s one of only a handful of living mammals which is poisonous. It’s hind legs emit venom which isn’t usually fatal, but can give you a really bad (and painful) day. And it’s aquatic!
It has a “bill” (there is an entire family of billed mammals in the fossil record, and the platypus is the only surviving example of this family). The platypus is often called a “duck billed platypus” but it’s bill is physiologically distinct enough from the bills of birds that it is not likely to have a bird ancestor. The superficial resemblance however is reinforced by the platypus’ webbed feet.
This is the skull of a platypus:
And this is the skull of a duck:
The resemblance is only “skin deep”.
Anyway, hope you enjoyed this nerd rant and the pictures : D
Rec’ if you’re as big a nerd as I am : )