Chapter 4: Cerapodans- On the left side of the cladogram - The distinguishing feature of the Cerapodans is a gap between the front and back teeth called a diastema. - There are two major groups of Cerapodans – Ornithopods and Marginocephalians. - Today, we will discuss the Ornithopods. o Ornithopods seem like “duds” when you first see them, but they are actually quite interesting. o Many types of these are called the cows of the Cretaceous Period because they were so common – lots have been found. o The distinguishing feature of the Ornithopods was front teeth that are well-below thecheck teeth. In other words, the gum line for the teeth at the front of the skull is at a different level than for the back teeth.o Heterodontosaurs (This name means different-sized teeth) were one of the earliest types of ornithopods – they were small herbivores, about four feet long. o REMEMBER – ALL TYPES OF DINOSAURS START OFF SMALL AND THEN EVOLVE TO BECOME BIGGER. o These were the first animals to chew things with their teeth like mammals do. o What’s the deal with herbivores having fangs? Fangs on plant-eating mammals are not uncommon – these structures were used for sexual display. In mammals, the male is the bigger sex, but with dinosaurs, NO ONE CAN TELL WHICH SEX IS WHICH. All there is to look at is skeletons, so it’s hard to tell. o But, in the case of dinosaurs like Heterodontosaurus, we have evidence of sexual dimorphism (one sex is bigger than the other). Sexual dimorphism is when there are clear differences between the two sexes. o One of the morphs is much larger than the other and has much bigger forms – we just don’t know which sex it is. o Advanced ornithopods are characterized by – Criss-crossed back and tail tendons that balanced the tail like a teeter-totter asopposed to it being bent. o The first of the advanced ornithopods were the Hypsilophodons. o Unlike most reptiles today, the teeth of these dinosaurs came together and were semi-mobile to allow precise slicing/chopping.o When first described, a research named Abel erred in his conclusion that the first toe of Hypsilophodons was completely reversed. However, this was not true – the bones were just crushed and messed up when fossilized. o REMEMBER, DINOSAURS ARE PREDOMINANTLY LAND ANIMALS. o Extremely advanced ornithopods include the iguanodonts and the hadrosaurs. These fairly large animals were faculatively quadrapedal – two or four wheel drive; that is, they walked on their three-hoofed middle-most toes. Characterized by broad, flared-out snouts with no front teeth. Presumably, these aided in the browsing on low-to-the-ground plants – this makes sense because at the time these animals showed up, plants (flowers) were just showing up as well. o Iguanodonts were large, extremely advanced ornithopods, many forms of which had thumb spikes. They were originally thought to go on the nose. These spikes were probably for defense and same-species fighting. Believe it or not, there are birds called “screamers” that exist today with similar spikes. There is also a type of frog called the gladiator frog that has these spikes. o In 1887, thirty-nine Iguanodons were found in a Belgian coal mine, a former swamp,1,000 feet underground. Bones were mined out for years. Studied by Dollo, the first Paleobiologist. o A “crazy” Iguanadont called the Ouranosaurus from Niger was bizarre, with huge dorsal spines. Some say it was a fat storage hump like a buffalo or a heat regulator. Others say it was just a display fin – this is probably the most-likely explanation. o By the late Cretaceous period, the Iguanodonts were replaced by the hadrosaurs – aka duck-billed dinosaurs. They were also large – one was bigger than the T. rex. They are found everywhere from Alaska to Argentina in all kinds of settings. The cows of the Cretaceous. The characteristic feature of hadrosaurs was an extremely expanded, duck-like bill. They also had dental batteries, stacks of hundreds of teeth, used up one after the other – like sharks use their teeth. - The tooth batteries are interesting in that they were self-sharpening. - The hard enamel was on one side of each tooth and created a wedge-like wear pattern that kept the teeth continually sharp. The teeth were replaced one after another throughout life, as happens in most reptiles. These animals were able to eat a wide variety of substances because of how sharp their teeth were. There were daily forming growth lines in the teeth of these animals. - By counting the total number of daily forming growth lines in the teeth, Dr. Erickson, in his master’s research, determined the formulation rate of each tooth, and by subtracting the number in the next replacement tooth, the replacement rate and wear rates were determined. A tooth was shed from each position every 50 to 80 days. Herds of up to 10,000 individuals have been found all together. This explainswhy we know so much about them. We have also found their nesting grounds, with eggs, with embryos, with babies inside them. Footprints tell of locomotory capabilities, group dynamics. Occasionally, mummified species are found. - Mummies in this case are animals that got dried out. o They tell about skin, ornamentation and can include stomach contents that reveal diet. Bite-marks on the animals show that they were fed on by other, bigger dinosaurs, like the T. rex. We also find tons of coprolites (feces) from these dinosaurs, 10,000 dinosaurs make a lot of dung. There are two types of these duck-billed hadrosaurs. It has been shown that many of the hadrosaurs hastily named by Cope, Marsh, and others were actually juveniles of the same species whose crests had not developed. - We call this problem over-splitting, and it stems from the fact that all baby hadrosaurs had stout skulls with no crests. Later in age, they got longer and ornamented. Because of their broad tails, webbed feet, and duck-like bills, it was assumed that hadrosaurs must be aquatic. - Because of this, crazy theories about the aquatic function of the crestswere posited. o These included use as snorkels, even though they don’t have holes in the top. o Airlocks to keep water out. o And more We now know that these animals were terrestrial because their tails were immobile, and they only ate land plants. So, what were the crests actually for?-
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