Scientists find surprise lurking in crocodilian jaw

"When we discovered that crocs had built this new jaw joint, it made us re-evaluate how mammals actually evolved our jaw joint and reinterpret what we thought we knew about where parts of our jaw joint came from," said Casey Holliday, Ph.D., assistant professor of anatomy at the University of Missouri, who led the research. "It's one of those awesome 'tapestry of life' stories that's given us a new way of looking at 250 million years of evolution for crocs and also 250 million years of independent evolution toward mammals."

Holliday will present the new findings and other highlights of recent research about crocodilian anatomy at the American Association of Anatomists Annual Meeting during Experimental Biology 2016. He is the recipient of the 2016 American Association of Anatomists Morphological Sciences Award.

Crocodilians, which include alligators, crocodiles and caimans, live in tropical and temperate regions worldwide and are top predators in watery environments. With a crushing force of more than 16,000 newtons, they have the strongest bite of any animal on Earth, a distinction scientists believe they have likely maintained going back to the Mesozoic era, when their giant ancestors co-existed with the T-rex and other impressive biters. By comparison, the typical human bite involves around 500 newtons of force.

"Though they have a reputation as 'living fossils' and indeed have patrolled Earth's waterways for millions of years, they aren't just some holdover or relic from the days of the dinosaurs," said Holliday. "In fact, they have continued to evolve and continue to reveal new features that are surprisingly similar in function to those we find in mammals, birds and other animals. Crocodilians are a treasure trove of adaptations that can help us understand the form, function and evolution of many animals."

The research team made the discovery by using a variety of imaging, computational and 3-D modeling tools to investigate the bones, cartilage, and tissues of the alligator head. It appears the second joint helps to distribute the bite force throughout the skull and stabilizes the jaw to prevent it from twisting during feeding. Mammals have only one jaw joint; birds, lizards, snakes and fishes have multiple flexible joints in their heads, though these joints are not all considered jaw joints.

Because the crocodilians' second jaw joint is similar in structure to the temporomandibular joint in people, Holliday said the findings could have relevance for understanding a group of painful jaw-related conditions known as temporomandibular joint dysfunction, TMD or TMJ. The causes of TMJ, which is estimated to affect up to 30 percent of adults, are not well understood.

Holliday also highlighted his team's other active areas in alligator research. One recent study yielded new insights on a pair of nerves that run down each side of the animal's long snout. A series of small holes along the snout allow the nerves to sense pressure and vibration in the environment, akin to invisible whiskers that help the animal locate prey when hunting at night.

Another promising area is the study of crocodilian cartilage. Holliday said a better understanding of how this cartilage develops and functions could help researchers find new ways to counteract one of the downsides of human anatomy -- our relatively thin layers of cartilage that become worn down over time, creating arthritis.

"Over on the other side of the animal tree, we have this whole stock of successful animals that don't have anything like arthritis -- they have these huge caps of cartilage that they maintain throughout life. Our hope is that there might be some way to bioengineer gator cartilage for research or even clinical applications to help us deal with human cartilage problems like arthritis," said Holliday.