GEOL 104: Dinosaurs — A Natural History

Key Points

Plate Tectonics and the Earth Engine

The great geological discovery of the 20th Century: Plate Tectonics

Some early indication that continents may have moved in past:

• Matching coastlines between continents or other landmasses
• Distribution of fossil (and modern) plants and animals
• Matching mountain belts in different landmasses
• Evidence of ancient ice ages that only line up if you move the continents around

Alfred Wegener proposed model of continental drift (in 1915):

• All continents unified into a single supercontinent (Pangaea), and all oceans a single ocean (Panthalassa) during the Triassic
• Some force broke up Pangaea into northern (Laurasia) and southern (Gondwana) smaller supercontinents, which drifted away from each other
• Wegener's model suggested that the continents somehow skidded over the oceanic rocks

However, big flaws in continental drift:

• What mechanism drives the drift?
• How can the continents glide over the ocean basins? (Physically impossible)

Idea was considered unlikely by many geologists.

In 1940s through 1960s, new information. Harry Hess and colleagues discovered that fossil, radiometric, and magnetic dates showed that ocean basins were not ancient, but that they got younger closer to mid-ocean ridges, where new rock was forming. Hess called this "sea-floor spreading".

In 1960s, the models of continental drift and sea-floor spreading were combined by John Tuzo Wilson and colleagues to form plate tectonics.

• Earth's surface is comprised of numerous rigid lithospheric plates
• Plates themselves carry thick continental and/or thin oceanic crustal rock
• Energy source: heat from core of Earth forms convection cells in mantle; drag plates along above them
• Since plates are mostly rigid (although they can be broken or crushed or pulled), most motion occurs at the boundaries of plates:
• New material generated at divergent boundaries (mid-ocean ridges in sea, rift valleys on land)
• Plates slide past each other at transform boundaries
• Plates come together at convergent boundaries, whose type depends on whether or not one of the plates is oceanic crust:
• Oceanic crust is lost under other ocean crust or continents at subduction zones (site of deep-sea trenches, earthquakes, volcanoes, etc.)
• Two continental masses meet at collisional boundaries: after the collision, they will fuse ("suture") together

Heat from Earth's core moves plates, forming mountain ranges at subduction and collision boundaries. Weather erodes uplifting mountains, wind and water and ice transports sediment to depositional environments. Over time, material becomes buried.

Plates wander over Earth's surface, so continents move from tropics to poles or back. Also, action of mid-ocean ridges causes sea levels to rise up (flooding continents) or lower (draining continents). (Current situation is very low sea level).

Big change from the 1960s-1970s model: now recognize there are LOTS of little plates (terranes) rather than just a few big plates.

Plate tectonics ultimately drives geology:

• Volcanoes (and thus igneous rocks) form at divergence and convergent boundaries
• Heat from these volcanoes, and from compression at convergent boundaries, form metamorphic rocks
• Uplift from convergent boundaries provides source rock for sedimentary rocks

Here is an overview of the pattern of plate tectonics over the last half-billion years:

Ecology: Energy in Living Systems

First up, some definitions:

• Ecology: the study of the relations and interactions of organisms to each other and to their natural environment. (NOTE: ecology does not mean the same thing as "environmental activism"!!)
• Paleoecology: not surprisingly, the study of the relations and interactions of fossil organisms to each other and to their natural paleoenivornments.
• Autecology: the study of an individual species with relation to its environment. In general, we deal with issues of paleo-autecology in other lectures (the diversity, behavior, biomechanics, etc. lectures). One very important aspect of autecology is the niche: the particular way of life of that species.
• Synecology, also called community ecology: the study of species of organisms interacting in an environment. We are going to focus on paleo-synecology in this lecture.

How does energy go from the non-living environment into the living world? One major way is photosynthesis. Here is the process greatly simplified:

And here is the photosynthesis equation, simplified:

6CO₂ + 6H₂O + Sunlight yields C₆H₁₂O₆ + 6O₂

Or, in other words, photosynthesizers (plants and algae) take in water and carbon dioxide and using sunlight convert these into sugars and waste oxygen gas. Photosynthesizers are the major form of autotroph: organisms that don't eat other organisms.

In contrast, most other life forms (animals, fungi, most "protists", etc.) are heterotrophs: we get nutrients from digesting other life forms, primarily via the aerobic respiration equation:

C₆H₁₂O₆ + 6O₂ yields 6CO₂ + 6H₂O + Energy

(That is, food (glucose) plus oxygen yields waste carbon dioxide and waste water, plus energy).

In this reaction, we take in sugars and oxygen and release waste water and carbon dioxide.

So plants, algae, and other photosynthesizes are basically made up of air (and a little bit of other matter drawn from soil and/or water), while animals and other heterotrophs are made of food (ultimately plants) and air. This most important aspect of ecology is thus trophic relationships: who eats whom. These mark out relationships in a food chain of organisms that eat the organisms below them. However, since in nearly every case any species eats more than one type of food, trophic relationships are better displayed as a food web, comprised of many interlocking food chains.

The trophic level is the position an organism occupies of a food chain. Producers do not consume other animals: in terrestrial ecosystems, these are plants (which "eat" sunlight). Decomposers break down the bodies of any kind of organisms. Consumers are those that eat other organisms: 1st order consumers are herbivores (plant-eaters); 2nd order consumers eat 1st order consumers; 3rd order consumers eat 2nd and 1st order consumers; and so on, up to the apex predator (consumers that eat lower order consumers but are not typically eaten by any species but themselves).

As you move up a food chain, there is less and less energy available. Plants receive much (but by no means all) of the energy of incoming sunlight; some of that energy becomes stored as their tissues, but the rest of it is used for them to run their bodies. Herbivores consume plants, and gain some (but not all) of that stored energy, some of which they used to build their own tissues but the majority being used for their own operation (running, breathing, pumping blood, etc., etc., etc.). Predators eat the herbivores, gaining some (but not all) the stored energy in the tissues of the herbivores, using some to build their own bodies, but the rest for their operations. Thus, the amount of energy available at each trophic level becomes less and less: this is described as the energy pyramid.

Assessing Trophic Relationships

One of the most important aspects of ecology (and paleoecology) is determining who ate whom: that is, reconstructing the food web. Sometimes there is direct trace fossil evidence: for example, this humerus of the Late Cretaceous Asian duckbilled dinosaur Saurolophus: the arrows point to bite marks generated by a tyrannosaurid (almost certainly Tarbosaurus, which is known from the same fauna).

However, sometimes we do not have such direct evidence. In these cases, we can make relatively reasonable assessments. For example, it is unlikely that small theropods habitually killed the largest herbivores in their environment (while the latter were adults, at least: babies would be fair game), and it is unlikely that there were species that were not eaten by SOMETHING.

In the case of the herbivores, however, it becomes somewhat more difficult. For one thing, it is often the case that the conditions that are best to preserve plant fossils (especially of leaves and fruits) are not good for preserving bone, and vice versa. So we often lack good knowledge of the flora that co-existed with any fauna. That said, we have the digested plant remains in coprolites as positive evidence that dinosaurs ate particular plant species (although referring a coprolite to a particular dinosaur species can be difficult.)