DINOSAUR FEEDING (123)

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Fossilized stomach contents are rare, so ideas about just what dinosaurs ate are derived from inferences from their teeth and bones.

Dinosaur Teeth

Teeth still provide the best evidence at this time of what dinosaurs ate both because so many have been found and because of the obvious close connection between teeth and food. Both the wear pattern and the tooth shape are important. While there are obvious differences between the sharp, knife or dagger like tearing and ripping teeth of carnivores and the broad, flat teeth of herbivores , additional information can be obtained from close examination of teeth from different types of dinosaurs.

Carnivore Teeth

Carnivore teeth generally show little change in basic design from the early, primitive types through to the superbly designed large theropods such as Tyrannosaurus, with changes mainly in size. Many theropods , such as ceratosaurs and carnosaurs , also showed adaptations in jaw design that allowed partial dislocation so that quite large pieces of meat could be swallowed, in a similar though less pronounced way to that of modern snakes. At its peak development in Tyrannosaurus, bite force reached levels not seen in any other animal so far measured.

Herbivore Teeth

The greatest divergence of tooth type and eating adaptations occurred within the herbivores , both the sauropods and the beaked ornithischians . For example, camarasaurids are well supplied with thick, broad, spoon-shaped teeth that are pitted and have coarse scratch marks suggesting that they ate tough, coarse vegetation. In contrast, diplodocids have slender, peg-like teeth with finer scratches and are not pitted, suggesting that they ate soft, aquatic plants or fern fronds. In other instances, coarse marks on teeth might indicate more grit in the diet and hence that the animal browsed on lower vegetation.

Gizzard Stones

Diplodocids had no molar teeth at all and the question has been asked - how did they manage to eat enough food to sustain them, particularly if they were warm-blooded? The answer would seem to lie in the discovery in a number of sauropod remains of piles of rounded stones found in the area of the stomach in relatively undisturbed skeletons. In keeping with the resemblances between dinosaurs, birds and crocodiles, these are almost certainly gizzard stones (or gastroliths). The existence of a bird-like muscular gizzard with gastroliths would not only allow much greater food consumption than if it all had to be chewed by the teeth, but also allows the processing of quite tough plant material, removing the previously held necessity for sauropods to survive on soft water plants.

Fermentation Vats

It seems quite likely that the huge gut volume of the large herbivores would have allowed the development of enzymic fermentation processes like that of modern ruminants to further process tough plant material and extend the range of available food. The existence of such large intestines is supported by the extra ribs found in many of the beaked dinosaurs - one from each backbone segment from the chest right down to the hips. The beaked dinosaurs also had a modified pubic skeleton . The pubic bone slanted backwards and downwards, so that the lower end finished up well behind the hip socket, greatly expanding the available space in the gut area and allowing a larger digestive system without upsetting the animal's balance. This is the same system used in birds. The ankylosaurs and dome-headed dinosaurs had small, lightly packed teeth not well designed for handling tough plants. However, they compensated by expanding their gut cavity sideways as well as back and down, giving them an enormous relative volume of intestine for fermenting and processing food.

The World's Best Grinder

The late evolving hadrosaurs probably had the most efficient food grinding mechanism ever devised. They had many rows of teeth in both upper and lower jaws. Each tooth had a horny ridge, sharp and deep like a biscuit cutter for cropping tough vegetation, and the top teeth fitted into the lower teeth, providing an incomparable grinding surface, made even more efficient by jaw developments that enabled the bottom jaw to move both sideways and backwards relative to the upper jaw. As the teeth wore down, they were continually replaced by new teeth, so that a single hadrosaur might use as many as 2000 teeth during its lifetime. As a final adaptation, the hadrosaurs were the first dinosaurs to develop cheeks, which prevented food from spilling out of the mouth as it was being chewed. Their snouts covered the full range of shapes and sizes, from broad and flat (for 'mowing' low vegetation) to long and narrow for probing into difficult areas after preferred leaves, fruit etc.

The Ultimate Nipper

The horned dinosaurs ( ceratopians ) carried beak development to its ultimate stage. They had huge pincers, with sharp edges to both upper and lower beak. Their specific development, however, was the bony frill, which is actually an extension of the rear rim of the skull holes where the jaw muscles are attached. As the frill grew up and back, so these muscles grew longer and heavier, resulting in bite power that could have chopped up virtually any plant material of the time. It made them particularly suited to eating plants that were too tough for other dinosaurs to tackle.

The Kitchen Whiz!

Most advanced beaked dinosaurs developed one or another of a range of food processing options. Perhaps the ultimate in feeding development was the parrot dinosaur Psittacosaurus, which showed the entire range of dinosaur adaptations - strong, deep beak; closely packed teeth for grinding: large jaw muscles; gastric mill with stones; very long digestive system.

In living animals, the isotopic ratio of carbon-13 to nitrogen-15 in their bones is linked to diet, and can distinguish marine from land animals, herbivores from carnivores and even browsers from grazers in savannah regions. Some such studies have been done on dinosaur bones, and indicate, for instance, that the hadrosaur Anatosaurus was a grazer. As there was no grass at this early stage of plant evolution, it probably grazed on ferns. In contrast, the carbon-13 values in another Canadian hadrosaur suggest that it probably browsed on shrubs.

Mesozoic Giraffes

The large sauropods and stegosaurs , contrary to a popular view, were not sluggish monsters confined to swamps and lakes and barely able to lift their heads to occasionally gather a mouthful of water-weed. Both of these types of dinosaurs reached their peak during the Jurassic period, and both were high grazers - the giraffes of the Mesozoic . The sauropods came in 2 distinct types - those (like brachiosaurids ) that had very long necks to reach the highest trees (some could reach up to 20 m/65 ft), and those (like diplodocids ) with rear legs considerably longer than their front legs. This type also had extensive modifications to their tails with large vertebral spines at the hips for attachment of large muscles and sled-like lower tail bones for balance and support. Together with massive hind legs and short backs, they could easily pivot up onto their hind legs and tail, thus adding their body length to their long necks for increased reach. The stegosaurs also adopted this tripedal method.

Dinosaurs and Flowers

At the end of the Jurassic , most of these high feeders became extinct, and were replaced by successive waves of beaked dinosaurs specialized for low feeding. No new high feeders evolved. This change in feeding pattern must have had an impact on plant evolution (plants and herbivores are always locked in evolutionary advance and counter-advance), and it has even been suggested that the increased pressure on low growing plants opened the way for a take-over by plants with the ability to grow rapidly and reproduce rapidly. Thus they could colonize a grazed area quickly and produce a new generation before the herbivores returned to crop the area again. One of the plant types best suited for such an environment would have been the angiosperms, or flowering plants. Today these make up most of what we recognize as plants, but in the late Jurassic were a struggling minority. Suddenly in the Cretaceous period and at the same time as the low feeding dinosaurs were taking over as the major herbivores , the angiosperms flourished. This raises the interesting question - did the dinosaurs (indirectly) give us the flowers?

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