Today I learned about the Pezosiren, a “basal sirenian,” which means it’s one of the earliest found ancestors of modern manatees and dugongs. Manatees and dugongs are in the clade Afrotheria, meaning they are most closely (but distantly!) related to elephants, hyraxes and certain species of shrews. I’m fascinated by this sort of transition from sea to land, land to sky, land back to sea, etc., as a beautiful picture of how God has used evolution to provide new options for feeding for entire branches of the Tree of Life.
Wikipedia says of Pezosiren,
“It is the first known quadrupedal sirenian, and is considered a transitional form between land and sea sirenians. P. portelli had four limbs perfectly adapted to walking instead of flippers, but the typical skull, teeth, and ribs of the fully aquatic, ‘normal’ sirenians. Its heavy ribs, providing ballast, indicate it was semiaquatic, perhaps like a hippo.”
Below is an artist’s rendition of Pezosiren based on the nearly complete fossil found in 2001.
It always amazes me that one of the most common arguments used by our young-earth brethren (and sistren) is still the lack of transitional fossils, despite the rich record we now have.
So what’s your favorite transitional fossil? Let’s put a few out for our YEC forum visitors to peruse.
P.S. I submit that the only way you could make Pezosiren cooler is by turning it into an English word instead of a Latin name. What an amazing Scrabble bingo it would make, under the right circumstances…
I am still partial to the tetrapod transitionals, if we are allowed to nominate a group of transitionals. Tiktaalik, Ichthyostega, and Acanthostega are great illustrations of how tetrapod limbs evolved.
The forelimb of Ichthyostega has always been one of my favorites because of how well it is preserved and how it demonstrates such obvious homologies to the limbs that you and I have (in order from shoulder: one bone, two bones, bunch of little bones, finger bones).
Well, if the thread is “What is your favorite fascinating evolutionary factoid that is also a great Scrabble word?” then I present you with “axolotl,” a neotenic salamander, which means that, though it’s an amphibian, it lives its whole life in the water. Get that baby on a triple-word score, and you’ll be sitting pretty at 92 points…
While we’re talking about limb bones, I’ll go ahead and share my other favorite transitional form example I came across today, since everyone else is, um, waiting to go all at once. I didn’t know until today that, in the category of marine mammals, aside from whales/dolphins, seals/walruses, otters, and manatees/dugongs, there used to be AQUATIC SLOTHS! How crazy is that?
These sloths appear to have gone through a series of intermediate forms that show a clearly graded transition from land-dwelling sloths like Choloepus through semi-aquatic varieties (like hippos) down to the most likely fully aquatic T. carolomartini.
The Wikipedia § called “Movement and adaptation to aquatic life” details a number of specific adaptations of Thalassocnus’s body to an aquatic lifestyle, which I won’t quote (though it’s interesting) in order to keep this post a readable size. But the adaptation they specifically tabulate in a beautiful little diagram (below) is bone density, because “heavier bones help lower buoyancy and thus conserve energy.”
The Ultimate Transition Fossil? Ambulocetus of course… half way between Tetrapod and Marine Mammal…
Nostril on the move from tip of the nose to the top of the head… legs on way to getting smaller and more
useless…
The Ultimate Proof that it is frequently impossible to distinguish between so-called Devolution and
Evolution…
I would choose the pterodactyl. Discovered only a few years after “Origin of Species,” it gave the first strong evidence that Darwin was on the right track.
Pterodactyl also figured in one of my favorite Far Side cartoons. Amid a primordial landscape, a pterodactyl is flying headfirst into a tree, a dinosaur has fallen down, and a caveman is tripping over his club. The caption is, “Although it lasted only 2 million years, the Awkward Age was considered dangerous for most species.”
Pterodactyls are great (and I love The Far Side!). Is there anything transitional in the pterodactyl, though? I mean, as you know, they were a completely separate lineage with powered flight that evolved after flying insects and before birds and bats.
Not that everything in this thread has to be about transitional fossils. (e.g., Scrabble )
Because pterosaur anatomy has been so heavily modified for flight, and immediate transitional fossil predecessors have not so far been described, the ancestry of pterosaurs is not fully understood.
There are transitional fossils between the two major groups of pterosaurs, but not for the transition to powered flight in the first place.
So… science isn’t done yet! An interesting hole in the data to be filled in someday…
This recent article was fascinating, perhaps the crocoduck is really a crocoswan or crocogoose:
“When we look beyond fossil dinosaurs, we find most of Halszkaraptor’s unusual features among aquatic reptiles and swimming birds,” concludes lead author Andrea Cau.
The snarky side of me likes this infamous transitional fossil:
Early in the ID/creationist fossil record you find this paragraph in the book “Of Pandas and People”
“The basic metabolic pathways (reaction chains) of nearly all organisms are the same. Is this because of descent from a common ancestor, or because only these pathways (and their variations) can sustain life? Evolutionists think the former is correct, creationists accept the latter view.”
In a later draft you find the transitional fossil:
“The basic metabolic pathways (reaction chains) of nearly all organisms are the same. Is this because of descent from a common ancestor, or because only these pathways (and their variations) can sustain life? Evolutionists think the former is correct, cdesign proponentists accept the latter view.”
I thought…none had ever been found…? That all fossils are independent species, with no transitions…
I was at the debate in Toronto between Stephen Meyer, Lawrence Krauss, and Denis Lamoureux, at which Lamoureux introduced the compelling, although maybe not so dramatic, development of teeth (from scales) in “acanthodians.”
The mutation to intelligent design was already present in the population before the selective pressure of Edwards v. Aguillard was applied. You can see an example in The Natural Limits to Biological Change, an overtly creationist book, in 1984. After Edwards, the “ID” variant greatly increased in frequency. It was a Lederberg experiment of sorts.
I didn’t know until today that, in the category of marine mammals, aside from whales/dolphins, seals/walruses, otters, and manatees/dugongs, there used to be 
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