Did crocodiles descend from dinosaurs?

Crocodiles are built to last. Evolving around 200 million years in the Mesozoic epoch, crocodiles have far outlived the dinosaurs. Scattered across more than 90 countries, 23 species of the Crocodylia order haunt freshwater rivers, streams and marshes. Those 23 crocodilian separate into three families: alligatoridae (alligators and caimans), crocodylidae (crocodiles) and gavialidae (gharials).

Voluble crocodiles exhibit a range of calls signaling anything from courtship to distress [source: Florida Museum of Natural History]. They swim through their habitats chomping on frogs, fish, turtles and other vertebrates. To help them float, many crocodiles will ingest rocks to weigh down their centers of gravity [source: Burton and Burton]. Evolutionary biologists suspect that crocodiles' aquatic nature may have helped them survive the theoretical asteroid impact that led to the extinction of dinosaurs.

But if crocodiles and dinosaurs were living in the same time period, how closely were they related? For starters, dinosaurs and crocodiles are both reptiles. They share cold-blooded circulations that rely on the environment to heat and cool. Physical similarities, such as their rubbery skin and fierce teeth and claws are also apparent. But if you've read How Dinosaurs Work, you know that many scientists agree that birds, not crocodiles, descended from dinosaurs. Yet, we also know that birds and crocodiles are the only extant species that share a common ancestor with dinosaurs.

If we take a closer look at the crocodile's family tree, some surprising animals are hanging out on the various branches. Crocodiles are members of the Reptilia taxonomic class with other creepy crawlers, including snakes and lizards. And according to paleontologists, birds are also lumped in that category. In fact, crocodiles are more closely related to birds than they are to snakes and lizards [source: University of California Museum of Paleontology].

With a little more investigation into this unlikely bond between croc and bird, we can find out where dinosaurs fit into the prehistoric family portrait.

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.

Crocodiles Come From Dinosaurs FAQ

Are dinosaurs closer to birds or reptiles?

Even though dinosaurs were reptiles, many palaeontologists believe that they're closer to birds than they are to reptiles.

Did crocodiles live with dinosaurs?

Crocodiles and dinosaurs lived in the same period millions of years ago. However, unlike dinosaurs, they survived and still exist today all around the world.

What were prehistoric crocodiles called?

Prehistoric crocodiles were called archosaurs. These ruling reptiles originated over 250 million years ago, long before dinosaurs, birds and modern-day crocodiles. Theyâ€™ve since evolved into 23 different species.

How are birds more closely related to dinosaurs than crocodiles?

Birds are more closely related to dinosaurs than crocodiles because they share a common meat-eating ancestor called theropods. The divergence happened a very long time ago, which explains the many notable differences.

Lots More Information

Sources

Animal Diversity Web. "Crocodilia." 2002. (Feb. 27, 2009)http://animaldiversity.ummz.umich.edu/site/accounts/information/Crocodilia.html.
Burton, Maurice and Burton, Robert. "International Wildlife Encyclopedia." Marshall Cavendish. 2002. (Feb. 25, 2009)http://books.google.com/books?id=g2_St32S_5wC
Florida Museum of Natural History. "Crocodile Talk." (Feb. 27, 2009)http://www.flmnh.ufl.edu/cnhc/cbd-faq-q6.htm
Florida Museum of Natural History. "Do crocodiles cry 'crocodile tears'?" (Feb. 27, 2009)http://www.flmnh.ufl.edu/cnhc/cbd-faq-q6.htm
NOVA. "Outlasting the Dinosaurs." Public Broadcasting Systems. December 2003. (Feb. 27, 2009)http://www.pbs.org/wgbh/nova/crocs/whos/
University of California Museum of Paleontology. "Archosauria: Systematics." (Feb. 27, 2009)http://www.ucmp.berkeley.edu/diapsids/archosy.html
University of California Museum of Paleontology. "The Great Archosaur Lineage." (Feb. 27, 2009)http://www.ucmp.berkeley.edu/diapsids/archosy.html

Did Crocodiles Descend From Dinosaurs?