A Physical Description of Cretaceous Feathered Dinosaurs
Dinosaurs capture the imaginations of children and adults alike. Why? Because they’re bizarre and wonderful creatures. Now, the only surviving members of the dinosaurs are birds. Although dinosaur fossils can include bones, teeth, tracks, skin impressions and perhaps one day DNA, this evidence can only give us a tiny fraction of what their lives may have been like while they were alive. Dinosaurs varied widely in size, shape, behavior and physiology. Keeping track of all of these differences can get confusing, but paleontologists, yearning for information, do their best to keep it in order.
One order of dinosaurs are known as the Saurischia or “lizard hipped” dinosaurs. The most distinguishing feature of these dinosaurs is their hips which have a forward facing pubis and a backwards facing ischium. The two suborders included in the Saurischia are the Sauropods and Theropods.
The hip structure of a saurischian pelvis. Image by Colbert et al., 1945.
The first group is the Sauropods, which include the iconic “long-neck” dinosaurs that we see in many children’s programs such as Argentinosaurus and Brachiosaurus. These dinosaurs tend to push the limits of land animal body mass, being the largest land animals to ever have lived on earth. Most sauropod dinosaurs have large thick feet resembling those of modern elephants. The thick feet are needed to hold up the massive bodies that Sauropods tend to have. Sauropods are herbivorous dinosaurs with either many small bladed teeth used for stripping leaves from branches, or combs of teeth used for much the same purpose but with slightly different shapes.
Theropod dinosaurs are distinguished by their bipedalism and general carnivorous diets. They can be further classified into other groups based upon distinct skeletal features. Most Theropods have grasping hands with fingers that differ in size. In some species the thumbs are strong and offset from the other fingers; although this is rare when looking at animals such as tyrannosaurs, which have lost most of their fingers through evolutionary selection. Scientists in 1902 once thought Tyrannosaurus rex had 3 fingered hands like Allosaurus, until more specimens were discovered which showed tyrannosaur dinosaurs only had two. The second finger in theropods is longest, and the other fingers become smaller toward the edge of the hand. In a few of these carnivorous dinosaurs, the grasping hand developed a number of adaptations including the capacity of flight. One advanced (further along in evolutionary timeline) feature of these dinosaurs includes the evolution of a 3-toed hind foot. In tyrannosaurs for example, 3 toes are prominent and used for standing on while the 5th and 1st toes are reduced or absent. This toe evolution is similar to that of the horse, who have lost all but their middle toe. In dinosaurs this three toed structure is very similar to that of modern birds. Most birds have four toes, three prominent front toes and one reduced or rear toe.
Albertosaurus liberatus is another theropod quite similar to Tyrannosaurus rex, its close relative, in evolutionary terms. Both dinosaurs have two fingered hands, and a large skull. But, they differ in proportions of the leg bones as well as their overall size. One common feature they share are a unique anatomical feature called gastralia. All tyrannosaur dinosaurs have gastralia, which are small pseudo-ribs that cover the belly region. There are two types of these ribs: medial (middle) gastralia and lateral (side) gastralia. The lateral gastralia of small theropods tend to be much longer than their median counterparts, while the two types of gastralia fuse in larger theropod dinosaurs (Claessens, 2004). Medial gastralia are similar to our sternum in that they lay flat against the animal’s chest. The lateral gastralia extend up from those along the sides of the animal, but are not the ribs.
Highlighted in red are the gastralia ribs. Image by Lamanna MC et al., 2014. Click to see larger image.
Large meat eating dinosaurs of the cretaceous period (e.g., Tyrannosaurus) were probably active predators, although paleontologists often debate this. A helpful factor in determining exactly what their diet consisted of was the scimitar-like serrated teeth set in sledgehammer jaws. A joint in the lower jaw may have helped in absorbing some of the shock generated by struggling prey. The arms of tyrannosaurs are incredibly tiny as compared to the proportions to the rest of the body, and probably did not aid the animal whatsoever in hunting.
The most advanced group of theropod dinosaurs includes Tyrannosaurus rex and Albertosaurus liberatus– There are several major differences between Tyrannosaurus and Allosaurus. In allosaurus, all 3 foot bones are about the diameter, whereas, in tyrannosaurs such as Daspletosaurus, the middle foot bone became reduced and moved near the ankle. In tyrannosaurs the fingers were reduced to only two digits, the third having only a vestigial structure beneath the skin. One feature of many tyrannosaurs was the astounding evolution of feathers; eventually giving us modern day birds.
Literature Cited:
Barsbold, R. & Osmolsaka, H. (1999) The Skull of Velociraptor(Theropoda) From The Late Cretaceous of Mongolia– Acta Paleontologica Polonica 44,2 189-219
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Claessens, L. M. (2004) Dinosaur Gastralia: Origin, Morphology & Function
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Norell, M. A. & Balanoff, A. M..(2012) Osteology of Khaan Mckennai(Oviraptorosauria, Theropoda) Bulletin of the American Museum of Natural History Number 372, 77 pp.
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