CPSP118G Spring Semester: Earth, Life & Time Colloquium

"Fearfully Great Lizards": Dinosaurs

Thomas R. Holtz, Jr.

Dinosaurs are inherently cool, but how can we reconstruct their biology from their limited fossil remains?

Prior to 1800s, no one knew about dinosaurs. (Fossil marine reptiles and fossil flying reptiles had just been discovered at this point). Fragmentary remains of a giant fossil carnivorous reptile Megalosaurus), a giant fossil herbivorous reptile (Iguanodon), and a giant fossil armored reptile (Hylaeosaurus) indicated presence of large-bodied lizard-like land animals in the Mesozoic Era. In 1842, Sir Richard Owen grouped these three together as the Dinosauria ("fearfully great lizards"). He recognized them as distinct from other groups of reptiles because of their giant size and because their limbs were directly underneath the body (like a mammal or bird), not sprawling to the sides (as in lizards, crocodilians, and turtles).

Later discoveries on all continents revealed more details about dinosaur anatomy, biology, and relationships.

In its modern (cladistic) definition, Dinosauria is the most recent common ancestor of Iguanodon and Megalosaurus and all of its descendants.

Dinosaurs were by no means the first major group of land vertebrates. Like all land vertebrates (tetrapods), dinosaurs were the descendants of fish capable of moving for some period of time on land. In the Devonian Period (416-359 Ma), the first vertebrates adapted to living on land appeared. These diversified in the following Carboniferous Period (359-299 Ma). While these had limbs adapted to crawling around on land, they still had to return to the water to lay their eggs, and their young were fully aquatic. (Modern amphibians have retained this life cycle).

During the Carboniferous, however, some of these land vertebrates evolved the amniotic egg. : eggs with a protective casing and internal structures to allow the embryo to develop on land. The amniotic egg allowed vertebrates to leave the water entirely, and the clade of land-dwelling vertebrates, the Amniota, underwent a major series of adaptive radiations.

During the Permian Period (299-251 Ma) and early part of the Triassic Period (251-199.6 Ma) of the Mesozoic Era the Synapsida were the dominant group. Synapsids include mammals (although true mammals do not appear until later in the Mesozoic Era). Among the primitive varieties of synapsids were small-headed, heavily-built herbivores;semi-aquatic fish-eaters; sail-backed plant and/or mollusk-eaters; and sail-backed carnivores with different-sized teeth in different parts of the jaw. The latter gave rise to more advanced synapsids called therapsids, including various types of predators; large slow-moving herbivores; beaked, two-tusked burrowing omnivores; and some smaller but specialized carnivores and omnivores (that would later give rise to the mammals).

These Permo-Triassic "protomammals", however, were eventually displaced by various groups of reptiles. Most significant of the Triassic radiation of reptiles were the archosaurs: ruling reptiles.

Archosaurs differed from other reptiles in a number of features. Among them:

Among the archosaurs were two main branches. One, the Crurotarsi (or Pseudosuchia) dominated the Triassic, and included

  • rmored spiky herbivores; semi-aquatic predators, convergent on crocodiles in form (okay, actually it is the other way around!); herbivorous quadrupeds; toothless bipeds; numerous terrestrial carnivores; and fast-running quadrupeds that may have been the direct ancetors of the protocrocs of the Early Jurassic and the later crocodilian lineage. At the end of the Triassic, however, there was a mass extinction which wiped out all crurotarsans except for the crocodylomorphs.

    The other major branch of archosaurs were the Ornithodria("bird necks"), which differed from their relatives by having elongated lower legs, suggesting that they were even faster than typical reptiles. All (except perhaps for the most primitive) had cervical vertebrae greatly modified so that they look very different from dorsal vertebrae, but instead could allow the neck to form an S-shaped curve (hence the name of the group).

    One branch of the ornithodirans that appeared in the Late Triassic are the Pterosauria: the first group of powered fliers among the vertebrates. The other major branch of ornithodirans are the dinosauromorphs. The oldest dinosauromorphs are known from the Middle Triassic of Argentina. The taxa modified the hindlimbs further to evolve:

    The combination of these traits allowed the little dinosauromorphs to run and accelerate in a bipedal mode all the time, not just at top speeds like typical diapsids. (It is not yet certain if the earliest dinosauromorphs such as Lagerpeton and Marasuchus were primarily quadrupeds or bipedal.

    The parasagittal gait and hinge-like ankle also allowed dinosauromorphs the ability to move more actively and constantly rather than only in short bursts of speeds; thus, they were striders. Additionally, although early dinosauromorphs were small (~30 cm long for Marasuchus, 1-2 m long for "silesaurs" and basal dinosaurs, etc.), the presence of limbs directly underneath the body meant that this lineage to grow to much larger size than any previous clade while still remaining terrestrial and mobile (sprawlers relagated to a semi-aquatic life if they became too big to support their weight).

    The first true dinosaurs are known from fossils around 228 Ma (the very beginning of the Late Triassic), and so the group as a whole probably first evolved slightly earlier (in the Middle Triassic). Early dinosaurs were all small (1-2 m long), bipedal (walked on their hind legs), and had grasping hands.

    From the first dinosaurs evolved many different branches:

    These branches included:

    Among the diversity of carnivorous dinosaurs were some that evolved a covering of fur-like hollow structures. These protofeathers were probably originally for insulation, but became adapted by more specialized forms into true feathers. There were many lineages of feathered dinosaurs, one of which includes modern birds and their Mesozoic ancestors. Birds are therefore a group of dinosaurs: they are descendants of the most recent common ancestor of Iguanodon and Megalosaurus.

    Dinosaurs hatched from eggs, and even the largest dinosaurs grew from babies that could fit in an egg the size of a soccer ball. Recent studies show that it took big dinosaurs only 7 to 20 years to reach full size! This, along with many other lines of evidence, suggests that dinosaurs had active metabolisms, more like mammals (and modern birds) than like lizards and crocodilians.

    Dinosaur extinction is a topic of major interest. The last dinosaurs other than birds died out 65.5 Ma, at the Cretaceous-Paleogene mass extinction. Many other kinds of organisms-on land and in the seas-died out at this time. The main causal agent seems to be a tremendous asteroid (~10-15 km in diameter) that collided with the Earth at what is now the Yucatan Peninsula of Mexico. The ash and dust produced by this explosion cut off light for weeks or months, shutting down photosynthesis. Only those organisms that could find food during that time could survive. Large, metabolically active creatures like the big dinosaurs did not make it through. Thus ended the world of the dinosaurs.

    For more information about dinosaurs, go to my course website for GEOL 104 Dinosaurs: A Natural History.

    Last modified: 10 January 2008