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Transitional Fossil Time!
Mammalodon
An ancient whale that has all the hallmark features of a baleen whale (loosely sutured bones in the snout, a broad and flat roof of the mouth, and an unjointed mandibular symphysis between the two halves of the jawbone)... Except it doesn’t have any baleen.
Transitional Fossil Time!
Pezosiren portelli
Age: 50 million years old
Found in: Jamaica
Transitional form of: Sea cows becoming waterbound.
The earliest known ancestor of sea cows, it had 4 legs it used to walk on land, but it also had heavy ribs that could indicate it lived part time in water (not unlike a hippo).
We know it’s a sea cow despite it being leggy because apart from the legs, its body plan is basically that of a modern sea cow.
Transitional Fossil Time!
Anchiornis
Age: 160 million years
Location: Liaoning, China
Transitional form of: Dinosaurs to birds.
Anchiornis is at the point in the transition where it’s hard to decide whether it falls in the bird or dinosaur side of the fence:
It has wings, but they’re symmetrical and rounded, probably not used for flight but instead are used to keep warm, for mating displays, and gliding.
Its legs are more like that of other-bird-like dinos, but like birds and unlike bird-like dinos, Anchiornis had arms nearly the same length as the hind legs.
Its spine connects at the back of the skull instead of at the base like in birds, but it had flexible wrists, which is more a bird thing.
It had a mostly bony tail, but also had primary and secondary contour feathers on both arms, legs, toes, and wrists.
This is probably why it was originally considered a bird but then got shuffled over to the dino category later.
Transitional Fossil Time!
Puijila
Lived: About 21 to 24 million years ago
Transitional form of: Earliest known form of seals
Discovered: 2009
You might note that this weirdo looks more like the lovechild of an otter and a weasel.
Being a very early form of seal, it has notably very few adaptations for aquatic life.
So how do we know it’s an early seal?
While its body is wildly different from its modern brethren, these cute long bois have very distinct pinniped skulls and teeth, marking them as definitely being part of the seal lineage.
Transitional Fossil Time!
Rodhocetus
Lived circa 45 million years ago.
Discovered: 1994
Location: Pakistan
Transitional form of: Whales becoming waterbound.
We have two partial skeletons which together give us a full picture of this early whale.
As you can see in the above representation by Pavel Riha, its limbs were shortening but its toes were very long, indicating they were probably webbed and paddle-like. The overall look of their limbs is already remarkably similar to that of modern whales’ fins:
But the real key feature of their transitional state I wanna bring to your attention though, is... their nostrils.
Most animals with a muzzle have their nostrils at the front of their muzzle.
The ones that don’t tend to be water-dwellers like crocs, who tend to have their nostrils slightly further up the muzzle so they don’t have to stick their whole head out of the water to breathe to avoid giving away their location.
That nostril further up Rodhocetus’ muzzle was because it was in the process of migrating up to the forehead to become what we now know as blowholes.
Transitional Fossil Time!
Dimetrodon
Year discovered: 1877
Transitional form of: Reptile to mammal
Dimetrodon is probably the most famous of the synapsids (commonly referred to as “mammal-like reptiles”).
It looks an awful lot like a reptile, perhaps even like an early dinosaur, but it was anything but.
Like reptiles, it was cold-blooded, and dependent on an outer heat source, which is where the sail came into play -- when they were cold they could turn their sails to the sun and soak up the heat (they may have also have had secondary uses of helping to scare away predators and/or to attract a mate). They also had no secondary palate, a trait found in modern mammals.
Another primitive trait it had was an awkward-looking sprawling gait (not unlike a croc’s gait on land).
But you know what game-changing trait it did have that’s like a bright neon sign that it was our ancestor?
Multiple kinds of teeth.
That’s a very mammalian trait.
You can thank dimetrodon and its synapsid brethren for why you have the toothy equivalent of a swiss army knife in your mouth.
Transitioning from Fish to Amphibians
Panderichthys
^ Had very flexible and powerful fins. Based on the shape and size of its spiracles, it’s very likely it used its lungs quite a bit for breathing. It also notably had lost the third set of fins common in a lot of fish.
Tiktaalik
^ Had strong fleshy fins that included bony bits that would become elbows in later animals and could be used to flop/drag awkwardly on land not unlike how mudskippers do today.
^ Modern giant mudskipper
Elginerpeton
^ Had shoulderblades and webbed limbs closer to feet than fins.
Acanthostega
First known animal with toes rather than fins.
Ichthyostega
^ Had toes and likely spent much time on land but still had gills and a tail with fin rays (a feature found in fishtails).
Hynerpeton
^ Had toes and shoulders that suggest it was a powerful walker.
Tulerpeton
^ Had paddle-like toes and, most crucially, did not have gills in adulthood, making it the first known true amphibian.
Pederpes
^ Had limbs that ended in feet adapted primarily for walking rather than paddles for combined swimming and walking.
Transitional Fossil Time!
Pikaia
The oldest-known ancestor of all vertebrates, it was a tiny, rather unimpressive-looking lancet-like critter who had what is clearly an early example of what would become notochords in later vertebrates such as ourselves (the notochord being a rod-like structure in between vertebrae that protect the spinal cord in the areas between vertebrae -- where it is most vulnerable -- and give backbones the ability to be flexible-ish).
You are looking at your roughly 257 millionth great-grandparent, give or take.
