Did Dinosaurs Evolve Into Crocodiles?

Hey all, I’ve been thinking about making this blog post for quite some time, and with a little motivation from one of our members, I decided to do it. I’ll keep updating this as the debate goes on.

So, did dinosaurs evolve into crocodiles?

Not likely.

Crocodiles coexisted with dinosaurs during the Mesozoic. They evolved from the phytosaurs, which evolved from the archosaurs, which, granted, are the same group from which dinosaurs evolved. However, the two families of reptiles evolved side-by-side, and, therefore, you’d think there should be a link between the two long before the spinosauridae family evolved.

You also have to consider location. Spinosaurs typically existed in the eastern hemisphere, in places like Africa and Asia. There have been some found in South America as well. The early crocodiles hail from North America, with a few in Europe and Africa. Fossils of these early crocs have been found in rock as recent as the early Jurassic, so they may have lived a long time.

Now, although spinosaurs and some crocodiles lived in the same place, they are very different. Spinosaurs were typically bipedal, although some scientists believe that they could walk on all fours. Crocs were exclusively quadrupeds, meaning they walked on all fours. If spinosaurs were a variant of crocodile, there would be some sort of evolutionary link that shows the crocodiles evolving the ability to be bipedal. The earliest known spinosaur is Baryonyx (technically there’s one from the late Jurassic, but it’s only known from teeth; actual skeletal fossils have not been found). Baryonyx is a bipedal creature, and although it might have been able to walk on all fours, its hands looked very much different from the hands of the early crocodiles you claim the spinosaurs evolved from. Also, the overall skeletal structure looks much different than the skeletal structure of early crocs. There are a couple similarities, such as the elongated jaw, but they are, for the most part, too different for Baryonyx, the earliest fully classified spinosaur, to be that missing link I mentioned.

And who knows? Maybe that early spinosaur that’s only known from its teeth could be the missing link you’re looking for. But, for now, I’m not convinced. I’m willing to change my opinion if enough evidence surfaces one day, but, until it does, I’m sticking with my original belief. Dinosaurs are more closely related to birds than crocodiles.

Tyrannosaurs and crocodiles share absolutely no anatomical similarities, with the exception of both of them having carnivorous teeth. Beyond that, they’re very different. Crocodiles are well known as cold blooded reptiles. About ten years ago, scientists found a small amount of preserved flesh on a T. rex specimen now famously known as B. rex. This flesh contains what appears to be red blood cells, which, of course, crocodiles don’t have. This is a very strong indicator that tyrannosaurs, or at least Tyrannosaurus, was warm blooded and therefore not crocodilian.

It’s generally accepted that Tyrannosaurus younglings had protofeathers, which crocodiles have never had and will never have. The feathers were used as a way to keep the little creatures warm, and they wouldn’t need this if they were cold blooded because their body temperature would simply adapt to the temperature around it. Yutyrannus, a recently discovered tyrannosaur, was about 25 to 30 feet long, weighed over one and a half tons, and was a feathered dinosaur. It’s the largest feathered dinosaur found so far. And it was found in China. How could the tyrannosaur family be crocodiles if their early members were feathered?

Also, crocodiles were aquatic. Tyrannosaurs are in no way built to be aquatic or even semi-aquatic. The skeletal structure of a typical tyrannosaur is far different from the skeletal structure of a crocodile.

Even if it was just protofeathers, nothing of the sort has been found on crocodiles, ancient or modern. There is a theory out there that crocodiles could have had the genes for protofeathers, but solid evidence to support this theory is not currently present.

Also, on the thread where this discussion originated, I noticed that someone thought that the b. rex find is a hoax. While it certainly is extraordinary (and almost unbelievable) that small bits of tissue were found on a fossil almost seventy million years old, it’s well known that this is real. Smithsonian has an article on it:

http://www.smithsonianmag.com/science-nature/dinosaur- shocker-115306469/

Sure, the article is eight years old, but it includes an actual photo of the material taken from the bone. As for how this flesh was preserved, that’s a bit of a mystery. But all branches of science have their mysteries. Traces of what appear to be red blood cells were found in b. rex, and nothing like that has ever been found in a crocodile. Crocodiles have always been cold blooded, and have therefore never had red blood cells. If Tyrannosaurus was warm blooded, it stands to reason that its early ancestors were as well, and the feathers (or protofeathers, whichever it was) on Yutyrannus also support this. Cold blooded animals need no insulation, but warm blooded animals do. Yutyrannus isn’t the only tyrannosaur that had those feathers; a tyrannosaur called Dilong, also from China, had them too. And again, nothing like that has been found on crocodiles. If it ever is, that will put a big hole in my argument, but for now, it’s fairly definitive.

And just as with the spinosaurs, there’s no direct fossil link between the tyrannosaurs and the crocodiles. If they are one and the same, where along the line did the crocodiles evolve to be bipedal? This link is missing, and until it is found, I’m very much not convinced. Why did the “crocodilian” tyrannosaurs evolve such short arms when the crocs of the day used them? If both tyrannosaurs and spinosaurs are crocodiles, why did one group evolve with long arms and the other with short arms? These are the evolutionary questions that one must consider when trying to make up their mind about this debate.

One of the most recent blog posts on Dinosaur Home is entitled “A ‘Terror of Tyrannosaurs’: The First Trackways Of Tyrannosaurids And Evidence Of Gregariousness and Pathology in Tyrannosauridae”. (link: http://www.dinosaurhome.com/a-terror-of-tyrannosaurs- the-first-trackways-of-tyrannosaurids-and-evidence-of- gregariousness-and-pathology-in-tyrannosauridae- 903.html) In summary, the blog post postulates that tyrannosaurs may have hunted in packs. I even wrote a blog post about this topic a couple years ago, and although at the time I didn’t believe pack behavior among tyrannosaurs was common, I’m starting to go more toward the belief that it was common. Also, in 1910, Barnum Brown discovered almost 30 Albertosaurus, a type of tyrannosaur, together in a place now known as the Dry Island bonebed. Although it’s possible that natural factors (such as adverse weather conditions) drove these animals together, it could also be a strong indicator that this was a large pack of tyrannosaurs. Crocodiles are not known for being group animals. They do sometimes congregate at rivers in large groups, but they rarely ever interact, except in the cases of territory defending and mating. They do not form social groups. The Albertosaurus found by Barnum Brown had a wide age range, from juvenile to adult. Even with a congregation of crocodiles, you rarely (if ever) see age diversity as wide as that. Barnum Brown’s find indicates that tyrannosaurs may have been very social (rather than solitary, as most crocodiles are) animals, and this kind of behavior is a very strong indicator that tyrannosaurs are not crocodiles.

You also have to consider geographical location. Tyrannosaurs have been found as far north as Canada (Albertosaurus was from Canada). Up in those higher latitudes, you have colder temperatures. The colder the temperature gets, the more difficult it is for a cold blooded animal to survive. You can also consider altitude, but that will be more difficult to factor in considering the terrain of the Cretaceous was much different from what it is today. However, plenty of tyrannosaur fossils have been found in the high altitudes of the Rocky Mountains in Canada, so it’s safe to assume that they lived at higher altitudes as well. Sure, the Rockies weren’t as large then as they are today, but they were starting to gain substantial size during the last ten million years or so of the dinosaur age. Cold blooded reptiles are not going to be able to survive temperatures like those, and especially not tyrannosaurs if they were cold blooded crocodiles. The tyrannosaurs of North America were large enough that they could maintain their body heat even in colder temperatures. As a warm blooded creature gets bigger, its ability to retain heat increases due to a decrease in the surface area to volume ratio. If tyrannosaurs were cold blooded, they couldn’t survive in places as far north as Canada, which, even during the Cretaceous, was fairly cold. To my knowledge, no species of crocodile, ancient or modern, have been found as far north as Canada. A few Deinosuchus specimens have been found as far north as Montana, sure, but that’s about the farthest north you’ll get with crocs. Albertosaurus and other Canadian tyrannosaurs (even Tyrannosaurus itself, since it has also been found in Canada) wouldn’t have been able to survive if they were cold blooded.

Image By Zina Deretsky, National Science Foundation (National Science Foundation) [Public domain], via Wikimedia Commons

Top Image from Dougal Dixon “The New Dinosaurs”

So... what do you think? Please leave me a comment.


  • Vale: Great blog! Do scientists have any theories about which prehistoric animal the crocodile did evolve from?
  • Dinosir: Nice post. I assume Coelophysis’ll have a lot to say on this matter.
  • JMD: Too bad coelophysis never posts on blogs...
  • Ramin: Zoologists putting the land vertebrates in 4 classes (amphibians, reptiles, birds and mammals). But I came to the conclusion that their crocodile are a separate class. As birds and mammals, with obtaining, four-chambered heart, have been derived from the class of reptiles and create a separate class. Crocodile also, have made four-chambered hearts and create a new class and has been derived from the reptile class. Mammalian four-chambered heart from a reptile-like three-chambered heart is extension of the septum (the wall dividing the chambers, lost of the right systemic arch and the left systemic arch persists). Vice versa in birds, lost of the left systemic arch and the right systemic arch persist. The lineage leading to crocodiles evolved a four-chambered heart along a different pathway, keeping both systemic arches. We should putting the land vertebrates in 5 classes (amphibians, reptiles, Crocodiles, birds and mammals).
  • Ramin: Crocodiles are not reptiles. Crocodile, just like birds and mammals, are a separate class, that has been derived from reptiles.
    Mammalian four-chambered hearts come from the reptile-like three-chambered heart with an extension of the septum (the wall dividing the chambers) and there is the loss of the right systemic arch while the left systemic arch persists. In birds, there is also the loss of the left systemic arch while the right systemic arch persists.
    A similar lineage lead to crocodiles evolving a four-chambered heart, although they followed a different pathway and kept both systemic arches.
    The earliest crocodiles were small, terrestrial, two-legged sprinters.
    Deinosuchus was one of the largest prehistoric crocodiles that ever lived, as long and 10 times as heavy as the largest crocodiles alive today.
  • Ramin: 180 million years ago some of the small size of reptiles gained 4-chambered hearts, they become bigger and created the Mammals class and all the rest reptiles that were owner of the 3-chambered hearts become small gradually.
  • Ramin: 140 million years ago, some of the smaller-sized reptiles gained 4-chambered hearts. As they continued to evolve, they became bigger and eventually created the Birds class. Meanwhile the reptiles, which had only 3-chambered hearts, become gradually smaller.
  • Random Scientist Inc.: A very interesting theory, Ramin.
    By the way, I’ve updated this blog post with some of my more recent arguments.
  • Rex Fan 684: Good post Random Scientist Inc.
  • ThaAnonymousPerson: well crocs do belong in the family or archosaurs, with birds.

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