The largest of the pachycephalosaurs, the name Pachycephalosaurus translates to "thick-headed lizard." A large orbital socket indicates good vision, and, according to my "Prehistoric Life" book, the small teeth in its mouth indicates either herbivory or omnivory. I have never heard anyone ever say anything about the pachycephalosaurs being omnivorous, so I have doubts about the authenticity of the statement. Jack Horner suggested that Pachycephalosaurus and the other two pachycephalosaurs that lived in North America at the same time, Dracorex and Stygimoloch, were actually the same dinosaur and just represented different ontogenetic stages in their growth development, but that hypothesis has been almost completely refuted. Due to the extremely tough-looking and thick skull, many paleontologists also have suggested that the pachycephalosaurs would clash heads much like the extant bighorn sheep, but studies of the necks of these animals indicate that these animals were likely not able to do this without breaking their necks. Needless to say, this behavior was probably not something that they would attempt to do more than once. Remains of Pachycephalosaurus have been found in the United States in the states of Wyoming, South Dakota, and Montana.
5. Amargasaurus
A fairly small sauropod, Amargasaurus is certainly quite unusual, as you can see in the pictures below! On each of its twelve neck, or cervical vertebrae, a pair of spikes project out. The functions of these spikes are unknown, but many theories have been put forth, from defense, to a sail being attached between each of the spikes to make the animal look bigger, even for heat control. If a sail was not attached to the spikes, it is likely that they were covered in a layer of horn. Whatever was on those spikes, it doesn't look like Amargasaurus had a very wide range of vertical mobility in its neck, so it seems likely that the small sauropod would have been mostly a ground-level feeder. Amargasaurus has been found in Argentina in South America.
4. Utahraptor
While making Jurassic Park, Steven Spielberg decided that the real-life Velociraptor wasn't scary enough for his movie. Not only was Velociraptor actually pretty small (see the size comparison chart below), it was feathered! But while they were making the movie, remains of the creature that would one day be known as Utahraptor were uncovered, and Spielberg had his monster. (If you want to read more about this interesting happenstance, check out the introduction to Robert T. Bakker's FANTASTIC novel, Raptor Red, one of my top five favorite books of all time). Utahraptor remains have been discovered in (can you guess? I hope you can!) Utah.
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