fossils

How duck-billed dinosaurs evolved to have more than 1,000 teeth

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Pete
Buchholz

Senior Writer
Duck-billed dinosaurs were chewing machines. Detailed examination of teeth and jaws shows how they evolved one of the most sophisticated dental batteries in vertebrate history.

Hadrosaurs, or duck-billed dinosaurs, are a hugely successful group of ornithopod dinosaurs that lived in the Late Cretaceous on all northern continents as well as in South America. Besides having toothless duck-like bills, hadrosaurs had large heads with hundreds of teeth packed into what's called a dental battery, large bodies, and the ability to walk on both two legs or on all fours. Additionally, many hadrosaurs had elaborate crests on their heads, and include spike-crested Saurolophus, dinner-plate crested Corythosaurus, and tube-crested Parasaurolophus. Hadrosaurs are part of a group of dinosaurs called the ornithopods that not only included duck-bills, but also spike-thumb Iguanodonts, and superficially kangaroo-shaped hypsilophodonts. The iguanodonts can be considered to be something of an evolutionary transition between hypsilophodonts and hadrosaurs; their bodies were fairly similar-looking to hadrosaurs although they generally lacked crests.


The dental battery of hadrosaurs didn't come out of nowhere – it has a well-documented fossil record. Ornithopods, like most reptiles, continually replaced their teeth throughout their lifetime. Hypsilophodonts, the smaller kangaroo-shaped ornithopods, don't have dental batteries. Instead, their teeth are all somewhat leaf-shaped or triangular, and like most vertebrates the portions of the teeth that are out of the gums are covered in enamel, and the core of the tooth is filled with living pulp tissue. Iguanodont teeth, as you might have guessed, show a transitional condition. Their teeth are very large, shaped like a long diamond with a rounded apex, and closely packed, eliminating the gum tissue between tooth positions. Additionally, the enamel on the teeth is asymmetrically deposited, being thicker on the cheek-side of the upper teeth, and the tongue-side of the lower teeth.


The condition that's seen in hadrosaurs is similar to that seen in iguanodonts, only more extreme. Hadrosaur teeth are still closely packed, eliminating spaces between them, but are substantially smaller in comparison to the jaw size, with enamel solely on one side of each tooth. Each tooth position, or "tooth family" holds as many as five stacked teeth continually growing and ready to erupt. The closely-packed diamond-shaped teeth are locked into a battery, which as a whole, resembles pineapple skin.


New research by Aaron LeBlanc, Robert Reisz, David Evans, and Alida Bailleul published in July, 2016, takes a detailed look into the evolution and structure of hadrosaur teeth and the dental battery. The researchers examined numerous thin sections of teeth and jaws from hadrosaurs and several other vertebrates in order to compare how different tooth tissues change in development as well as across evolutionary time. They found that hadrosaurs had teeth that modified existing tissues to create a continually growing chewing surface that's unique among vertebrates.


Like all vertebrates, hadrosaur teeth are made up of three mineralized tissues: very hard and dense enamel that usually creates the crown of the tooth, calcified cementum (generally a root tissue), and softer dentine found on the interior of the tooth. Hadrosaurs have drastically modified the location of these tissues in comparison to their ancestors. The transition point of the enamel and cementum is no longer at the gum-line, but instead has moved to the apex of the tooth so that enamel is just on one side of each tooth. The upper and lower batteries ground against each other when the hadrosaurs chewed and the three different tissue types allowed hadrosaurs to be among the most efficient and sophisticated herbivorous vertebrates.


Previous researchers had suggested that cementum was also present as a mortar-like tissue locking all the teeth together in a solid unit, making the hundreds of teeth in each jaw into a giant "super-tooth." When examining the teeth and jaws of hadrosaurs under the microscope, the researchers found no evidence of cementum between individual teeth. In fact, none of the teeth were in direct contact and many of the gaps between teeth were filled with silt grains. LeBlanc and colleagues propose instead that the teeth were held in place by periodontal ligaments, a soft but tough tissue that would decompose before fossilization. The periodontal ligaments were strong enough that the dental batteries could still act as a "super-tooth" but also allowed flexibility within the battery.


What's more, by examining the teeth in the same tooth family, you can see how a tooth changes over time. Hadrosaur teeth start out with an extensive pulp cavity filled with living tissue, but the pulp cavity over time gets smaller and replaced with dentine. By the time the teeth erupt out of the gums, they were composed entirely of "dead" tissue, the pulp cavity having been completely eliminated. This allowed hadrosaurs to grind their teeth down to the root, and were the only vertebrates that never shed teeth. Because of this, any isolated hadrosaur teeth found in the fossil record (which number in the many thousands) must have fallen out of the jaws of dead animals.


Finally, they note that two other groups of dinosaurs, the horned dinosaurs and rebbachisaurid sauropods, also evolved sophisticated dental batteries that share many features with hadrosaurs, but differ in key details.


Original findings published on BMC Evolutionary Biology.

Image Credit: Andrey Atuchin

1c7cb27d01c79a07ffe3b6a6c5f95e58

Pete
Buchholz

Senior Writer


http://sulc.us/hj6tu
http://www.eartharchives.org/articles/how-duck-billed-dinosaurs-evolved-to-have-more-than-1-000-teeth/