Crocodiles are more closely related to birds than lizards.

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Crocodile Family Tree

­How could a delicate robin or sparrow have any connection to a crocodile? It's all about common ancestors. Taxonomists organize the animal kingdom based on where species came from, or who descended from whom. All of the descendents of one common ancestor are referred to collectively as a monophyletic taxon. Those taxons can then be arranged visually into a cladogram, which illustrates how the various taxons are related to one another.

To understand how this works, let's think about corn. Imagine listing out every conceivable corn-based product: cereal, syrup, plastic, biofuels and so forth. The lists would contain wildly different items, but you could link them all with that single commodity. When it comes to crocodiles, birds and dinosaurs, a prehistoric creature called the archosaur serves our umbrella.

Archosaurs, also known as the ruling reptiles, originated about 250 million years ago during the Carboniferous period [source: University of California Museum of Paleontology]. The primary characteristic that unifies archosaurs is two openings on the side of their skulls. These tetrapods, or four-legged vertebrates, splinter into two groups: bird crocodiles (Ornithosuchia) and false crocodiles (Pseudosuchia). In a bizarre twist of taxonomical fate, crocodiles don't belong with the false crocodiles.

So what does all of this classification have to do with whether crocodiles came from dinosaurs? Alongside birds and oth­er flying reptiles, dinosaurs are lumped into the Ornithosuchia branch. Though dinosaurs and crocodiles have the common ancestor with the archosaur, they evolved separately.

Today, habitat destruction threatens the livelihood of some crocodile species. Though they could weather an asteroid blast that altered the physical world, humans might be edging them out. The Chinese alligator and Philippine crocodile are the two most threatened crocodilian species [source: NOVA]. Both are on the International Union for the Conservation of Nature's red list, denoting their highly endangered status. Captive breeding may help the populations rebound before they die out completely.