What Killed The Dinosaurs?

THE GREAT MYSTERY Surely ever since the first fossils of obviously extinct animals were found, humankind has wondered: "Why did they die?" A poignant question, for it has relevance to us — if extinct animals were wiped out by some catastrophe, couldn't that just as easily happen to us? Could we be found as fossils someday, and would no one know why we died?

History: Until recently, people simply knew that dinosaurs went extinct — their fossils were found throughout the Mesozoic era, but were not located in the rock layers (strata) of the Cenozoic era. So, we knew that dinosaurs went extinct some 64-66 million years ago, but that was all. Many wild ideas about how the dinosaurs were rendered extinct were presented over the years.
1980: Few satisfactory answers to the mystery behind the extinction of dinosaurs were offered until 1980, when a group of scientists at the University of California at Berkeley — Luis and Walter Alvarez, Frank Asaro, and Helen Michel — proposed a stunning and convincing mechanism for the "K-T extinction" (meaning the extinction of dinosaurs at the boundary between the Cretaceous period (K) and the Tertiary period (T)). This hypothesis is discussed later. Since the Alvarez hypothesis was first proposed, the search for the "perpetrator" of the K-T extinction has been a thriving area of scientific research. It incorporates scientists from many different fields including astrophysics, astronomy, geology, paleontology, ecology, geochemistry, and so on. The mystery has drawn extensive media coverage over the last 15 years, as you may know; some paleontologists have since lost interest in the issue, preferring to study how the dinosaurs and their contemporaries lived rather than why they died.
Mass Extinctions: But before we dive into the complex issue of the K-T extinction, we need essential background information to understand the basics of the controversy. The "great dying," as it is sometimes called, is an example of a mass extinction: an episode in evolutionary history where more than 50% of all known species living at that time went extinct in a short period of time (less than 2 million years or so).
Other Mass Extinctions? We know of several mass extinctions in the history of life; the great dying is not nearly the largest! The largest would be the "Permo-Triassic" extinction, between the Permian and Triassic periods, of the Paleozoic and Mesozoic eras. In this obviously catastrophic event, life on Earth nearly was wiped out — an estimated 90% of all species living at that time were extinguished. We are fairly sure that the extinction was due to many changing global conditions at that time, but even that is not solved yet. The issue has not received much press because the dinosaurs were not involved, but another familiar group, the trilobites, were wiped out among others. Who Died? How does the K-T extinction compare to this debacle? Well, about 60% of all species that are present below the K-T boundary are not present above the line that divides the "Age of Dinosaurs" and the "Age of Mammals." In fact, dinosaurs were not among the most numerous of the casualties — the worst hit organisms were those in the oceans. Large groups of organisms, including some members of Foraminifera, Echinodermata, Mollusca, and the marine Diapsida all were devastated by the K-T event. On land, the Dinosauria of course went extinct, along with the Pterosauria. Mammals and most non- dinosaurian reptiles seemed to be relatively unaffected. The terrestrial plants suffered to a large extent, except for the ferns, which show an apparently dramatic increase in diversity at the K-T boundary, a phenomenon known as the fern spike.
Complications
Now we're heading into the tough stuff; the reasons why we have no conclusive answer to the mystery of the K-T event. Several complications that make work hard for the scientist/detectives trying to crack this case:

1. The Fossil Record: It's not perfect, as you may know; that's why paleontologists keep finding new fossils: so much is hidden in the rocks! Most data on the K-T event comes from North America, which is one of the few areas known that has a somewhat continuous fossil record (remember, fossils are only formed under certain rare conditions, and are only found in sedimentary rocks). The infamous Hell Creek locality in Montana is one such continuous site enclosing the K-T boundary. UCMP researchers have led and continue to lead expeditions to Hell Creek, gathering fossils from the rich fossil beds. The secret to the K-T event may lie within our collections; who knows! Anyway, we don't know much about what was occurring in the rest of the world at the time of the K-T event. The marine fossil record gives us great hints about what was occurring within the sea, but how applicable is that to what went on in the terrestrial realm?
2. The Nature of Extinction: Extinction is not a simple event; it is not simply the death of all representatives of a group. It is the cessation of the origination of new species that renders a group extinct; if species are constantly dying off and no new ones originate through the process of evolution, then that group will go extinct over time no matter what happens. New dinosaur species ceased to originate around the K-T boundary; the question is, were they killed off (implying causation, especially a catastrophe), or were they not evolving and simply fading away (perhaps implying gradual environmental change)?
3. Time Resolution: Determining the age of rocks or fossils that are millions of years old is not easy; carbon dating only has a reasonable resolution when used with organic material that is less than about 50,000 years old, so it is useless with the 65 million year old K-T material. Other methods of age determination are often less accurate or less useful in certain situations. So we don't know exactly when the dinosaurs went extinct, and matching events precisely to give a picture of what was happening at a specific moment in the Mesozoic is not easy. Thus, the ultimate question of a gradual decline of dinosaurs vs. a sudden cataclysm is almost intractable without a wealth of good data.
4. Reconstruction: To truly understand the situation of the dinosaurs around the K-T boundary, we need to understand the paleoecology of that time on Earth. Paleoecology is an extension of the discipline of ecology, attempting to understand the interactions of organisms with their environment, using geological (the rocks tell you what the soil was like, and thus tell a lot about the abiotic (non-living) environment) and paleontological (what plants and animals are found as fossils tell you a lot about the biotic (living) environment) evidence. With the problems of the fossil record and time resolution, it is difficult to understand the paleoecology of a region at a specific time in the past.
5. The Signor-Lipps Effect: Proposed by Phil Signor and UCMP's own Jere Lipps, this concept helps us to understand the limitations of the fossil record. The theory states that groups of organisms may seem to go extinct in the fossil record before they actually do; this is an artifact of the fickle nature of the fossil record rather than actual extinction. Thus, it is possible that some groups of organisms did not go extinct at the K-T boundary, and also possible that some organisms that seemed to have gone extinct earlier may have survived up to the boundary, and then gone extinct. This matter further complicates the important issue of the selectivity of the K-T extinction (discussed later).
6. Falsifiability: Sad but true: many hypotheses about dinosaur extinction sound quite convincing and might even be correct, but, as you know, are not really science if they cannot be proven or disproved. Even with the best hypothesis, such as the impact hypothesis, it is very difficult to prove or disprove whether the dinosaurs were rendered extinct by an event that occurred around the K-T boundary, or whether they were just weakened (or unaffected) by the event. This is not to say that all extinction hypotheses are not science; many are excellent examples of good science, but a linkage of direct causation is a problem. "Why" questions, such as "Why did the dinosaurs die out?" or "Why did dinosaurs evolve?" are among the most difficult questions in paleontology. Ultimately, a time machine would be required to see exactly what killed the dinosaurs.