Avian Crocodylomorph

Question

Today, dinosaurs, alligators, crocodiles, gharials and caimen are all that remains of a special group of reptiles called the archosaurs. Recently, it has been accepted by the public that birds did not evolve from dinosaurs but are themselves dinosaurs.

But what if birds evolved from a different group of archosaurs?

Unsurprisingly, as far as reptiles go, Crocodilia is as bird-like in anatomy and even behavior as we're going to get. In the distant past, there were crocodylomorphs of a great slew of varieties imaginable, so it would make sense, in an alternate Earth where pterosaurs definitely never existed and dinosaurs might not exist, for a crocodylomorph to become Aves, the traditional name for the avian dinosaurs, known vernacularly as "birds".

But giving benefit to the presumption that all crocodylomorphs shared at least one anatomical attribute that separated them from dinosaurs, would an avian crocodylomorph have certain and noticeable anatomical differences from avian dinosaurs that I should watch out for? Or would an avian crocodylomorph look exactly like an avian dinosaur?

Answer 1

Yes an avian Crocodylomorpha would look just like a regular bird, and would have to go through a stage that would just be a dinosaur by a different name. Your question is like asking for an orangutan like creature descended from monkeys but without ape characteristics. Crocodylomorpha is just too close to the early dinosaurs, the path to avian from them would be virtually indistinguishable from dinosaurs. The major differences in pseudosuchia groups(the group that includes dinosaurs and Crocodylomorpha) is ankle structure, that's the feature separating early crocs and early dinos, the shape of the ankle joint, which really is not going to make much difference on that long of an evolutionary line. Keep in mind there is more time between early dinosaurs and the first birds than there is between t-rex and us.

Answer 2

By way of water.

To get a flying tetrapod you need a predecessor that can spare two limbs from full time locomotion such that they can differentiate into wings. The ancestors of birds are thought to have been semibipedal and semiarboreal - they could run on their back legs which opened the possibility of differentiation of the forelimbs into appendages first to aid in jumping and then wings to flap and fly.

The prospect of a crocodylomorph running around on hind legs seems a stretch. But there is another way to free up the 2 forelimbs of a tetrapod.

http://novataxa.blogspot.com/2011/07/2010-geosaurus-dakosaurus.html

Metriorhynchids were a peculiar group of fully marine Mesozoic crocodylomorphs

This body plan is similar to other fully marine reptiles and mammals - a muscular tail in charge of propulsion and diminutive limbs used for steering, if that. Whales have done away completely with their hind limbs but looking over marine reptiles it seems like the forelimbs are what gets small.

In any case - here is a reasonable starting point for evolving a bird - a fully aquatic tail-propelled crocadylomorph which can spare its forelimbs for a new job.

We will accomplish this by way of the flying fish.

The flying fish propels itself via its strong tail and then once out of the water, uses its differentiated forefins to sustain itself. They fly to escape predators but also because it is fast and effectively increases the size of their habitat by allowing them to skip over draggy dangerous water.

Hold that down vote! I am aware that flying fish do not fly, only glide. I assert that they never developed true flight because they are residents of the open ocean. There is no place for them to go except open water and open air. But our croc protobirds do have a place to go. Here is where they live.

https://www.marlimillerphoto.com/swamps.html

The brackish cypress / mangrove swamps are good habitats for small predators. The croc protobirds make their livings here. But larger things live there too - including other crocodilians (as have always been the apex predators in such habitats) and larger croc protobirds. The juvenile croc protobirds can escape large predators, including their hungry great uncles, by putting on a burst of speed and then gliding, just as a flying fish.

Like the flying fish, a phenotype evolved for escaping predators can also be used to enlarge habitat - the croc protobird can skip over shallow water and swampy areas common in this ecosystem. Unlike the flying fish, the juvenile croc protobirds might sometimes find themselves in a tree at the end of their flight. Trees are full of insect life, and crabs are the apex predators here - all delicious meals for a juvenile croc protobird in a flight for its life one minute and in a banquet the next.

Neoteny is a great method for evolution. Neotenous croc protobirds do not get big and waterbound, retaining their access to the safe havens and rich pickings of the mangroves. Initially the croc protobirds would have to get a runup in the water using the tail and then jump out to get into a tree. Once in the tree it will probably eventually have to go back into the water. This is analogous to the terrestrial ancestors of birds, which probably jumped or climbed into trees for safe havens / feeding but had to go back down the earth to get from tree to tree.

A croc protobird which can push off from a tree and use its flying fins to help it through the air to another tree can get access to trees unreachable from the water. I envision the long swimming tail becoming prehensile, in the way of chameleons. This crocdylomorph is on its way to becoming a bird.

Answer 3

From an evolutionary perspective you need some incentive to develop flight, it doesn't happen just because some Crocodilia looks at the sky and wants to be up there.

The way to go (aside from the flying fish theory) is for tree dwellers to have some evolutionary advantage if they could fly. Now, modern crocodiles are not the best at climbing trees, but back in the good old days, some quadrupedal dinosaurs did.

Now whenever a tree dweller is chased by a gigger tree dweller that wants to eat it, it's neccessary to get away as fast and as far as possible. The fasted way to flee is to jump down from the tree, but there are more predetors lurking on the ground. So the safest way to flee is to jump to the next tree. The evolutionary race of arms creates ever better jumpers, until one is born with a mutated gene that gives it big, stretchy skin. Imagine a crocodilian flying squirrel.

The next step is, again, advanced by evolutionary advantages. The flying croc squirrel needs to evolve from a glider to a true flyer and therefore needs bigger wing-spread and better uplift. To get there, you need lots and lots of time for chance mutations and environmental changes that kill the worst flyers. That could bee food scarcity or flying predators.

In the end, the descendants of flying croc squirrel either develop feathers and look quite like birds, or they end up as scaly bats or flying foxes.

Answer 4

Would an avian crocodylomorph have certain and noticeable anatomical differences from avian dinosaurs that I should watch out for? Or would an avian crocodylomorph look exactly like an avian dinosaur?

Well, we don't have an actual example fitting your question, but we have a lot of examples which fit the same concept: would different species achieving the same complex function (flying in this case) have the same form?

This is what we call convergent evolution, and we can see for examples in dolphins or sea urchins. They definitely are not sharks nor descend from sharks, nevertheless they have evolved to have a very close resemblance. Their body is hydrodynamic, they have fins, so they look much more similar to sharks than to other mammals.

But of course if we dive a bit deeper we can see anatomical differences between sharks and water mammals. The breathing system is one, for example.

So, to answer your question, I would say that yes, there would be some differences, but you might have to carefully look for them.

Answer 5

Positing flightless proto-feathers for insulation purposes _{(heck even full blown feathers unless you're going to argue penguins/dodos/emus aren't birds)} and saying any group even non-crocodilia could have them but especially that group. (Ie we have feathered chimps and crocs).

Then the distinctions would almost always be present since the chance of two divergent species converging on similar profiles is small. The source of the distinction would be whatever set of divergent traits between two examples conflicted least with feathers. Which would be very few of them if you're looking at them for insulation and waterproofing. So you probably couldn't have camels with feathers, but I don't see any reason you couldn't have a rat with feathers.

Assuming you're not by circular logic locking your definition of bird down to what already exists as a bird... then maybe what you're really asking is not "How do I tell birds apart from other birds", but "How do I tell flying things with feathers apart from other flying things with feathers" with the precondition that they came from different sources, then the answer is you most likely don't. Within flying birds there's massive variation to the point that pegging down the essence of a bird is pretty hard. You can't ask the usual questions of "Does the skeleton match the generic profile?" because you've thrown out a bunch of those presuppositions by arguing for a different root species.

Taking some of those grounding for-granted things out: there's such a large variety in current birds that even the locked down definition is hyper vague. Four chamber heart? Humans. Endothermic vertebrates? Humans. Feathers? Penguin. Toothless beaked jaws? Platypus. Hard-shelled eggs? Platypus. High metabolic rate? Cheetah. Strong yet lightweight skeleton? Bats, Insects, Fish, Amphibians. The only thing that the definition begs for is feathers. Typically ones for flying but not necessarily. You could even hit all of them with a feathered, light-boned, high-metabolism platypus.

So as previously stated, my answers to the questions "But giving benefit to the presumption that all crocodylomorphs shared at least one anatomical attribute that separated them from dinosaurs, would an avian crocodylomorph have certain and noticeable anatomical differences from avian dinosaurs that I should watch out for? Or would an avian crocodylomorph look exactly like an avian dinosaur?" are:

Yes. No.

But my preferred answer would be the answer to this question: "What about the definition of a bird are you allowed to change and still make it a bird?"