A Curious Case of Evolution Repeating Itself
/The 23 species of crocodilians in the world today enjoy a global distribution, limited only by habitat loss, hunting, and cold weather. The order Crocodilia includes giants like the saltwater crocodile, but it also contains smaller species that average under 5 feet, like Cuvier’s dwarf caiman. Some eat fish, some eat land animals, and many do a bit of both. Though some populations are threatened, it’s hard to deny the success of crocodilians as a group.
This image from our Secrets of Fossil Lake exhibition shows the crocodilian Borealosuchus preying upon a turtle 52 million years ago. Image by Sean Murtha
Crocodilians seem even more successful when you consider their evolutionary history. The first true crocodilians evolved 83.5 million years ago during the Cretaceous Period. Just based on a visual examination, those first crocodilians would have looked very similar to the ones alive today. Their appearance has changed so little in the last 80 million years that crocodilians are often called living fossils. There have been some odd evolutionary dead ends, like terrestrial crocodilians and species with horned crests, but there have been species that looked and acted much like modern crocodiles throughout that long time frame.
It’s remarkable to think that during the time that crocodiles spent looking like crocodiles, mammals evolved from tiny rodent-like creatures to the diversity we know today. For a group to stay relatively unchanged for so long, crocodilians must be doing something right. There is something about being a crocodile that is very evolutionarily advantageous. It’s so advantageous in fact, that animals that look and act like crocodiles have evolved time and time again throughout the evolutionary history of the earth. This phenomenon is called convergent evolution.
In biology, convergent evolution is a term that describes when animals that are not closely related evolve similar traits independently. Wings are a classic example of this. Birds and bats both fly, but they evolved their wings separately. Flight has evolved four times so far: In birds, pterosaurs, insects and bats. The condition of “being like a crocodile” has evolved at least five times, and likely more.
Prionosuchus
There were no dinosaurs in the Permian Period, but there was Prionosuchus. With its long snout, lengthened body, and short legs, Prionosuchus certainly looks like a crocodile. The suffix “-suchus” even means crocodile in reference to this similarity. However, Prionosuchus isn’t even a reptile. Rather, it’s one of a number of giant amphibians that were common during that time.
Image by Smokeybjb
Most Prionosuchus individuals unearthed thus far were of moderate length, at about 8 feet (2.5 m). However, paleontologists discovered one giant skull belonging to an animal closer to 30 feet (9 m) long. This makes it one of the largest amphibians that ever lived and certainly one of the largest predators of the Permian. Adult Prionosuchus individuals were at the top of the food chain, hunting other amphibians, fish, and maybe even land-dwelling organisms that ventured too close to the water. There is little doubt that Prionosuchus filled a very similar niche to modern crocodilians, and it was a force to be reckoned with in Permian waters.
Phytosaurs
Image by Dr. Jeff Martz
Phytosaurs are a highly successful group of reptiles that lived during the Triassic Period. They looked similar enough to modern crocodilians that you might get them confused on safari, but they have several significant differences. The most obvious one is the position of their nostrils. While a crocodile’s nostrils are at the end of its nose, a phytosaur’s are positioned closer to their eyes.
During the Triassic, phytosaurs were widely spread across the supercontinent Pangea. The climate was much warmer back then, allowing ectothermic (cold-blooded) reptiles to thrive far closer to the poles than they do today. Unfortunately, all phytosaurs went extinct at the end of the Triassic. The small land-dwelling ancestors of modern crocodiles survived and would soon evolve to fill the niches left empty by the extinct phytosaurs.
Champsosaurus
Image by Nobu Tamura
The first member of the genus Champsosaurus was discovered in 1876 by legendary paleontologist Edward Drinker Cope. Even though champsosaurs have been known to science for over a hundred years, they remain relatively unknown of outside of scientific circles. Champsosaurus was a reptile and although its evolutionary relationships remain murky, it is not closely related to modern crocodiles. Despite this, it bears a lot of resemblance to modern gharials and false gharials. It looks especially similar in its long thin snout, bedecked in cone-shaped teeth that suggest a diet consisting of fish and other fast-moving aquatic animals.
The first champsosaurs arose in the Cretaceous Period. They managed to survive the k/t extinction that wiped out non-bird dinosaurs, but fell extinct in the Eocene. Eye position is a major difference between Champsosaurus and crocodiles. Champsosaurus has eyes positioned far forward. The bulbous back portion of the skull may have housed powerful jaw muscles.
Ambulocetus
Image by Nobu Tamura
Ambulocetus is about as close as you can get to a mammal crocodile, though the comparison is looser than with the species mentioned previously. Ambulocetus is a transitionary whale, somewhere between land-dwelling ancestors and their limbless descendants. Like a crocodile, it had a long snout and short legs that were held splayed out from the body. The fluked tails of modern whales had not yet evolved and Ambulocetus probably swam using some combination of its tail and flippered feet. It had strongly muscled jaws, leading scientists to believe that it was a crocodile-styled ambush predator as well. It preyed upon fish and other swimming animals, and possibly terrestrial ones as well.
The above list is by no means comprehensive. Being crocodile-like has evolved so many times that it must be assumed that living like a crocodile carries great evolutionary benefit. Crocodile-shaped bodies are perfectly honed to take advantage of shallow water environments. With their legs they can maneuver along the bottom or shore. Their long flat tails make explosive bursts of speed possible for lunging at prey or leaping out of the water. Having eyes and nostrils on top of their heads mean they can float with just a portion of their head showing, leaving little visible for prey to notice. All of these features unite crocodiles with their extinct comrades, like Prionosuchus and Champsosaurus. When you spend a lot of time in the water and survive by ambushing prey, it’s good to look like a crocodile. It’s in the jaws that you find more variability.
The various species of crocodilians alive today all have the same basic body plan, but the shape of their jaws reflects their specific diet. A gharial’s long thin snout encounters little resistance when snapping shut underwater, so is excellent for catching fish. A broad alligator snout is better for catching larger prey. In between, there are all sorts of jaw shapes, each ideal for a certain feeding strategy. You can see these distinctions in extinct crocodile-like animals too. There are many that look like a crocodile in body but have a differently shaped head.
It is only by looking at the full spectrum of life that we can recognize these repeating patterns of convergent evolution, and there are plenty of other comparisons that can be made. Everyone knows what a wolf looks like, and the marsupial thylacine (or Tasmanian tiger) evolved a very wolf-like body plan completely independently. It would seem that looking like a wolf has its advantages as well.
I’m personally fond of using the example of convergent evolution and crocodiles to make a point about life on other worlds. Crocodile-like animals have evolved so many times on Earth that if there is life on another planet, and if that life has creatures that even vaguely resemble our own vertebrates, then you’d be very likely to find an alien crocodile in the mix. Take note, film astrobiologists and science fiction authors. It’s also a fairly safe bet to assume that if all living crocodile species went extinct that sooner or later something very much like a crocodile would evolve once more.
It's all fun and games until the alien crocodile attacks.
This is the moral of the story: Be very careful when standing near bodies of water in warm climates, whether you’re on today’s Earth, took a time machine to the past, or are stranded on an alien planet.
- Kate Dzikiewicz, Paul Griswold Howes Fellow
