The origin of the Dinosauria

No doubt we all think we know what a dinosaur is - just by saying the word "dinosaur" we imagine Tyrannosaurus, Apatosaurus, or Triceratops, or any of the large land-lubbers from the Mesozoic Era. Among the uninitiated, an ichthyosaur, a plesiosaur, or one of the other sea-going reptiles of the Mesozoic bestiary (none of them are dinosaurs) may come to mind. Worse yet, a few people envision as dinosaurs mere youngsters, such as the 100,000-year-old woolly mammoth.

It turns out that the question "Who are the dinosaurs?" is one and the same as "Where did dinosaurs come from?" because evolution is the driving force behind the history of life. Consequently, our goal in this chapter will be to consider precisely how dinosaurs fit into the history of life.

What dinosaurs are and how they came to be are questions pondered since the creation of the name by Sir Richard Owen just over 150 years ago. As we learned earlier in this book, Owen created the "Dinosauria" to encompass a group of exceedingly large, pachyderm-like reptiles from what he referred to as the Secondary Age (the Mesozoic Era).1 Iguanodon (an ornithopod), Megalosaurus (a theropod), and Hylaeosaurus (an anky-losaur) were its first members.

From 1842 onward, membership in Owen's Dinosauria grew by leaps and bounds. Alongside activity in the field and laboratory, paleontologists struggled to figure out their relationships. Much of this was done at

History of the Dinosauria

Figure 5.1. The pelvis of the hadrosaurid Prosaurolophus. A splint of the pubis lies along the base of the ischium, exemplifying the ornithischian condition, (Photograph courtesy of the Royal Ontario Museum.)

the low and uncontroversial level of genera and families. For example, a scientist might ponder: "What is this creature? A new genus? A species of an existing genus? Maybe even a new family?"

Though the discovery and naming of new beasts might be superficially appealing, such activities are intellectually unadventurous. They do not take by the horns more significant questions about patterns of, and processes driving, the great ebb and flow of the biota through time. Two paleontologists who did, however, were Harry Govier Seeley, a vertebrate paleontologist at Cambridge University in England, and Friedrich von Huene, Dean of German dinosaur paleontology at the University of Tübingen, Germany.

Dinosaurs divided In 1887, Seeley first recognized a fundamental division among dinosaurs.

Ornithischia (ornith - bird; ischia - hip) were all those dinosaurs that had a bird-like pelvis, in which at least a part of the pubis runs posteriorly, along the lower rim of the ischium (Figure 5.1). Saurischia (saur -lizard) were those that had a pelvis more like a lizard, in which the pubis is directed anteriorly and slightly downward (Figure 5.2). This

I The term "Secondary" came from a now outdated concept of how rocks were formed. In 1759, G. Arduino, an Italian mining engineer; developed a history of the rocks in northern Italy, He viewed these rocks as having been deposited by a retreating ocean. For him, the oldest rocks were designated Primary, and, as the seas receded, Secondary andTertiary rocks were consecutively deposited. Thus Primary, Secondary, andTertiary strata represented sequentially younger nocks.The scheme was further developed and elaborated by the eminent German naturalist A. G. Werner who in 1787 published a highly influential history of the earth in which these terms were applied, Although, in both schemes, rock types and age relations were embodied in the terms, the term Tertiary (and another Quaternary) remain with us today as age designations (the notion of a characteristic rock type has been abandoned, because most rock types can be produced and deposited at any time).

Figure 5.2. The pelvis of the ornithomimosaur Ornithomimus.The pubis is directed forward only, exemplifying the saurischian condition. (Photograph courtesy of the Royal Ontario Museum.)

pelvic distinction has held sway ever since, and has been bound up in the debate of the origin of birds (see Chapter 13).

That dinosaurs had one or the other land of pelvis was of great importance in understanding the shape of the evolutionary tree of these animals, but in Seeley's hands it went considerably further. For it implied to him that the ancestry of Ornithischia and Saurischia was to be found separately and more deeply embedded within what was then called Thecodontia (a name also coined by Owen in 1859 and one that we met in Chapter 4 and will meet again in Chapter 13), a heterogeneous group of non-dinosaurian animals essentially linked by the fact that they were all considered to be primitive archosaurs. Therefore, Dinosauria to

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