The dinosaurs roamed the good mother earth for over 150 million years. They met their demise some 65 million years ago and yet still they fascinate us today. In a sense, the dinosaurs live on, as we bring them to life with our enthusiastic research and various endeavors to unearth their hidden remains. This article is the first in a series that will take a brief look at the dinosaurs and how and why we study them.

The dinosauria was a term first used by Sir Richard Owen at the end of the 19^th century. He used the term to describe an unusual reptilian fossil that had come into his possession. Since then the term has been shortened to its modern form; dinosaur. The dinosaurs are classified based on the morphology, or shape, of their fossil bones. They are broadly separated into two distinct groups; the Saurischia and the Ornithischia. These groupings are based on the anatomy of the their hips. The Saurischia have a broad pubis that articulates anteriorly, or forwards, at about a 60 degree angle from the ischium. The Ornithischia are different in that the pubis articulates posteriorly, or to the rear, with the ischium. Both the Saurischia and the Ornithischia originate from a common Triassic age ancestor, a reptilian group called the archosaurs. It was from the archosaurs that the dinosaurs evolved.

The dinosaurs first arose in the Triassic. They were so successful in out competing the other animals of the time that by the Jurassic they dominated the earth. In fact, the reign of the dinosaurs spanned the entire Mesozoic era. Their demise is still a point of some debate. Was it an asteroid impact? Was it a fatal new strain of bacteria that spread disease across the Late Cretaceous landscape? Or perhaps it was gradual climate change. A period of cooling temperatures and changing vegetation that the dinosaurs were unable to adapt to, and ultimately fell before. These are some of the big questions that modern vertebrate paleontologists have before them. This series of articles will not so much dwell upon the end of the dinosaurs, as much as what made them so dynamic. For if they were not so successful, they wouldn’t capture our imagination today.

This article is the second installment in a series of articles that focuses on the dynamics and classification of the dinosaurs. The dinosaurs are broadly classified into two divisions based upon the morphology of the hip (the ilium, ischium and pubis). The two groups are; the Saurischians and the Ornithischians. This article is the first of two to explore the dynamics of the group known as the Ornithischians. The Ornithischian hip varies from the Saurischian hip in that the pubis articulates posteriorly with the ischium. The Ornithischians derive their name from the latin "ornithos", which means bird. The name is based upon the similar morphology that their pelvic arrangement shares with that of birds. This was actually an unfortunate name for them, for it was not from this group that birds evolved. Avian evolution actually is traced back to a specific group of theropods, which are Saurischians. However, that is a point of discussion for another day.

The Ornithischians include the broadest and most dynamic groups of dinosaurs. They are separated into seven groups, each with unique features. Each of these has a unique adaptation that allows for either defense from predators or for the more successful attraction of a mate. All of these dinosaurs were herbivores, most were bipedal while others were quadrupeds. The seven groups of Ornithischians are as follow:

1. Ceratopsians
2. Iguanodontids
3. Ankylosaurids
4. Stegosaurids
5. Hadrosaurs
6. Hypsilophodontids
7. Pachycephalosaurs

Of the seven Ornithischian groups, those that have three toes are given adesignation of their own; the Ornithopods. The ornithopods are a general classification for dinosaurs like the hadrosaurs or hypsilophodonts. Three toed ornithopod tracks may be viewed along the coast of Lake Grapevine, TX. in exposures of the Cretaceous age Woodbine Formation.

The Ceratopsians are a classic Ornithischian dinosaur that are most popularly known for the genus; Triceratops. They were the "cows" of the Cretaceous, horned herbivores that roamed the Cretaceous plains in search of their next meal. They had unique shields, or frills, that covered the upper neck and one to three horns. The use of these horns and shields is the source of much paleontological debate. Were they used as defense from predators or were they for sexual display and mating? The answer to that, as with all paleontological debates, depends upon whom you ask.

The Iguanodontids were mistakenly named for modern iguanas based upon a misidentified isolated element. The most popular genus is Iguanodontid . They are most commonly known for the infamous thumb spike that was originally placed on the skull as a snout horn! This mistake was of course corrected, but it still lingers in the literature and in our imaginations.

The Ankylosaurids are commonly referred to as the "Mesozoic Tanks" due to their dense dermal armor and spikes. Of all the Ornithischian dinosaurs they were probably the most heavily armored and carried the best defense. These squat little armored quadrupedal dinosaurs were tough but none the brighter, with small brain cases that could have only supported the smallest of poodle noodles. They are most commonly known by the genus; Ankylosaurus. Ankylosaurs were common during the Cretaceous of North America. They do however merit study due to their unique lifestyle. In fact, the Dallas Museum of Natural History’s new fossil preparator; Heather, did her Masters thesis on an Ankylosaur from the Lance Formation.

The next installment of this series will conclude our look at the Ornithischians in the Ornithischians (part 2) and will set the stage for a discussion of the next group of dinosaurs, the Saurischians (theropods, sauropods, oh my!…).

This article is the third installment of a series of articles that focuses on the dynamics of the dinosaurs. The dinosaurs are broadly classified into two divisions based upon the morphology of the hip; the ilium, ischium and pubis. The two dinosaur groups are; the Saurischians and the Ornithischians. This article is the second of two to explore the dynamics of the group of dinosaurs known as the Ornithischians. The Ornithischian hip varies from the Saurischian hip in that the pubis articulates posteriorly with the ischium. The Ornithischians derive their name from the latin "ornithos", which means bird. They were named such based upon the similar morphology that their pelvic arrangement shares with that of birds. This was actually an unfortunate name for them, for it was not from this group that birds evolved. Avian evolution actually is traced back to a specific group of theropods, which are Saurischians. The Saurischians are the topic of the next article and the mysteries of their classification will then be explored. This time we will further explore the variety of morphological adaptations seen in the Ornithischians.

The last article introduced the seven groups of Ornithischians:

1. Ceratopsians
2. Iguanodontids
3. Ankylosaurids
4. Stegosaurids
5. Hadrosaurs
6. Hypsilophodontids
7. Pachycephalosaurs

Each of these groups displayed unique morphological adaptations that allowed these animals to survive in the harsh Mesozoic landscape. Most paleontologists think that these adaptations were either for defense from predators or for the attraction of a mate. Some adaptations were perhaps for feeding. Previously the Ceratopsians, Iguanodontids and the Ankylosaurids were discussed. This article will pick up that discussion with the Stegosaurids.

Of all the dinosaurs the Stegosaurids stand out in stark contrast to the others. Their large dorsal fins and spiked tails stir the imagination and widen the eyes of every child that beholds one for the first time in a museum or in a dinosaur book. Although fierce in appearance, the Stegosaurids were herbivores that roamed the Earth feeding upon plants from the Late Jurassic until the Early Cretaceous. The reason for their unique dorsal fins and spiked tails is a point of debate among vertebrate paleontologists. Were the fins used as thermal regulation of body temperatures, sexual display, or defense from deadly predators? Without soft part preservation the thermal regulation argument lacks quantitative support. Without a group of live Stegosaurids interacting naturally in their environment, it is difficult to make paleobiological inferences to their behavior. However, inferences are made none the less. I would have to say that adaptations such as seen in the Stegosaurids are purely for defense from predation. Otherwise they would make for mating rituals from hell, talk about a first date gone bad.

The Hadrosaurs were the most diverse, numerous and widespread of the herbivorous dinosaurs. They roamed the Earth throughout the Cretaceous feeding upon the plants that had evolved at that time. An interesting correlation between plant evolution and herbivore diversification can be drawn. As plants diversified across the Cretaceous landscape, so did the dinosaurs that ate them. The Hadrosaurs are a good example of this coevolutionary phenomena. The reason for the Hadrosaurs herbivorous success in the Cretaceous may principally be traced to their teeth. The Hadrosaurs had the most complex dental batteries seen on the planet at that time. They could grind a wide range of vegetation quite efficiently. One of the most popularly known genus is Parasaurolophus. This animal had a unique hollow crest that ran posteriorly along the skull over the neck. Since the crest was hollow, some paleontologists hypothesize that it could have generated low frequency sounds. Again, without living populations of the animal such ideas are hard to quantify, yet they make for interesting discussion. One of the oldest Hadrosaurs was discovered in Texas by Gary Byrd. The dinosaur was studied by Dr. Louis Jacobs student, Jason Head and named Protohadros byrdi after the fossils discoverer.

The Hypsilophodonts were a diverse group of herbivores that were widely spread across the Mesozoic Earth. They are traced back to the Mid- Jurassic and endured until the Late Cretaceous. They are considered ancestral to the Iguanodontids. The most commonly known genus is Hypsilophodon. Two Hypsilophodontids are currently on display at the Dallas Museum of Natural History as part of the Texas Dinosaurs exhibit, the Tenontosaurus and a new, as yet to be named new genus. The new genus comes from a SMU excavation at Proctor Lake, TX. Along the shores of this Texas lake is what may be one of the most prolific sites for Hypsilophodontids in North America. The only mounted skeleton of this exciting new dinosaur is at the DMNH. The mount itself is a marvel that was constructed by the talented Geb Bennett formerly of the DMNH. Both the discovery of this new dinosaur and the mounting of the first specimen are testaments to the talents of the Texas paleontological community.

The final group in our discussion, are the unusual Pachycephalosaurs. These animals had an enlarged cranium that for all intents looks like a bloated blow pop. The enlarged, thickened cranium of these animals has been the point of much debate among vertebrate paleontologists. One of the common ideas for this adaptation is that it was used for battering other animals. It could have been used as a defense against predators or perhaps as a rights of passage for young bucks trying to earn the affections of a mate. Both of these ideas were debunked by Dr. Jack Horner at the annual SVP meeting in 1998. Horner presented a paper on the histological analysis of the thickened craniums. His data showed that the skulls were too thin to withstand any "head on" impacts. If the animals were to ram one another in an attempt to attract a mate, they more than likely would have knocked each other out. Not a good way to impress the ladies.

The Ornithischians were a diverse and fascinating group of dinosaurs. These herbivorous animals developed morphological adaptations that have since disappeared from the story of life. Their unique features capture the minds of the young and old alike. Although these animals are long extinct, they live on in our imaginations. Next we will discuss the other group of dinosaurs, the Saurischians!