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Mammal-like mastication for the dinosaur Leptoceratops


We all chew, but hardly ever think about it. Even a moment's consideration, though, reveals how complex of a process it actually is. Jaws move, teeth gnash, and food gets broken from big bits into smaller bits. Even that is a vast oversimplification, though.

Mammal-like mastication for the dinosaur Leptoceratops
Skeleton of Leptoceratops [Credit: Jaime Headden, CC-BY]
Ask a scientist who studies chewing across the vertebrate kingdom, and they'll tell you about all of the different styles of chewing–teeth translating up and down, side to side, in big circles or little circles in just about every plane of movement imaginable. Different chewing styles characterize different diets as well as different groups of organisms; if you ever get the opportunity, take a look at the fairly simple motions of an iguana chopping up a leaf versus the complex dance of teeth, jaws, and tongue in a cow chewing on grass. More complex motions can allow more thorough processing of food before it enters the stomach, which has all sorts of advantages for digestive efficiency and nutrient extraction.

Why do we care about chewing? From a biological perspective, feeding is an incredibly basic yet often poorly understood aspect of an organisms' life. Everything needs to eat, and it's the style of eating that can make it or break it for a species on geological timescales as well as within an individual organism's lifetime. Different styles of chewing can open up different food sources, and is one part of what helps species compete against each other ecologically. If we understand feeding in extinct organisms, we can better understand how they fit within their immediate environment and their world.

Chewing is relatively easy to observe in today's animals, but rather obviously is impossible to observe directly for extinct organisms. If you want to know how a mammoth or Triceratops chewed (or if they chewed, even), it's time for a close-up look at their teeth.

Mammal-like mastication for the dinosaur Leptoceratops
Schematic showing two stages of jaw movement in Leptoceratops, with the direction 
of microwear produced by each [Credit: Varriale 2016]
The tooth surface is under constant wear from the rigors of feeding. Grit adhering to plants, tooth-on-tooth contact, and even resistant pieces of the plants themselves leave tiny pits, scratches, and divots. By looking at these structures, called dental microwear, under a microscope, you can reconstruct the movements that the jaw went through. The orientation of scratches will tell you which direction the jaw moved, and their curvature will tell you something about how the jaw motion changed through the chewing cycle. Couple this with careful reconstruction of jaw muscle locations, and you can make a relatively confident reconstruction of chewing in a particular animal.

The horned dinosaurs, also known as ceratopsians, are fascinating to paleontologists for their unusual jaws and dentition. Large spaces on the skull for jaw muscles meant a powerful bite. A sharp beak helped to lop off plants (and also shows why even the small and cute species of ceratopsian would have been bad housepets–I wouldn't doubt that a fiesty Yinlong could sever a finger!). Once the plants were in the mouth, they met the teeth–and here is where things get even more interesting. Ceratopsian teeth showed a lot of variation across the group's evolution. The earliest members had simple, oval teeth with a slightly angled cutting surface, probably for simple slicing of plants. Some later species developed a shelf-like surface across the teeth, perhaps better suited for grinding plants into bits. Finally, the largest species (such as Triceratops) had closely-packed teeth that formed a continuous vertical cutting surface along the whole tooth row–similar to a pair of scissors.

Most studies of ceratopsian chewing to date have used basic principles of physics (treating the jaws as a lever system) along with examination of overall tooth and jaw muscle placement to infer chewing styles in ceratopsians. These have provided plenty of solid information on horned dinosaur jaw movements, but were largely limited by independent tests of jaw muscle movement.

Mammal-like mastication for the dinosaur Leptoceratops
Chomping surface of a Leptoceratops tooth — note the curved scratches (one is highlighted in blue) 
[Credit: Varriale 2016, CC-BY]
Paleontologist Frank Varriale of Kings College has been taking a close-up–microscopic, in fact–look at ceratopsian teeth. Traveling the world, he has amassed a library of microwear for representatives of most major groups of ceratopsians as well as their close relatives. This has allowed Dr. Varriale to test more concretely hypotheses about the evolution of teeth and jaw movements in the group.

In the first paper to result from this work, Dr. Varriale focused on Leptoceratops, a small (2-3 meter long) ceratopsian that lived in what is now Montana and Alberta, around 67 million years ago. This small cousin to Triceratops didn't have any horns, and only a short, stubby frill projected back over the neck. Some specimens show a shelf-like chewing surface, and several nicely preserved fossils are known. This made it an interesting and unusual target for study.

When Dr. Varriale took a close-up look at Leptoceratops teeth, the scratches left behind by jaw movements were surprising–rather than the linear scratches typical of many other horned dinosaurs, Leptoceratops teeth were covered in curved scratches. This suggested that the jaws were changing direction mid-bite (technically speaking, during the "power stroke"). If you were to visualize the teeth in side view, they would be moving in little circles against each other, rather than simply up-and-down or back-and-forth.

Mammal-like mastication for the dinosaur Leptoceratops
Tiny scratches on a dinosaur tooth, viewed under a scanning electron microscope. 
Viewed at actual size, the image would only be a millimeter tall! 
[Credit: Varriale 2016]
Leptoceratops wasn't the only animal with this type of chewing, but Varriale had to look far afield to find other examples. Similar types of tooth wear (and presumably similar types of jaw motions) are found in a handful of small living (some rodents) and extinct (multituberculate and haramiyid) mammals. This dinosaur had independently evolved a chewing style that matched that in those mammals.

This unique chewing style (termed "circumpalinal chewing") almost certainly helped Leptoceratops to process the tough plants that composed its diet. Significantly, this is the first time that it has been identified for a dinosaur, and now Varriale is on the look-out to see what other dinosaurs (especially ceratopsians) may have had it. There's lots more to do!

The most important lesson here concerns how we think about non-mammals. Often, dinosaurs (and even modern lizards) are thought of as "primitive" and inferior to mammals. Yet, as we look closer, dinosaurs were just as complex if not more so in many aspects of their biology. Complexity is in the eye of the beholder! This is surely not the last time we'll be surprised by horned dinosaurs.

The findings are published in the journal PeerJ.

Author: Andrew Farke | Source: Public Library of Science [July 08, 2016]
TANN

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