The talk to be presented at the North Dakota State University on January 25 concerns the climate changes that occurred about 65 million years ago and the associated extinction of numerous forms of life. The organization of life into major time units, the Paleozoic, Mesozoic, and Cenozoic eras (old, middle, and new life), was done in 1840. Both the end of the Paleozoic and the end of the Mesozoic were known to represent two of the most dramatic changes in the Earth's biota. Smaller periods of time subdivide geologic time. The Cretaceous, the last time period of the Mesozoic, is followed by the Tertiary, the first period of the Cenozoic. The boundary between the Cretaceous and Tertiary is known as the K/T boundary. The dramatic change in the composition of the fauna and flora at or near the K/T boundary 65 million years ago has been known for over 100 years yet is ever increasing in interest as we understand the biotic change in more detail.
 

The K/T boundary mass extinction event is best known to the public because of widespread knowledge of the demise of the dinosaurs at the end of the Cretaceous and that this event is somehow connected with the impact of an extraterrestrial object. The expressions "dinosaur killer" and "nuclear winter" are identified with the mounting evidence of the consequences of the confrontation of the Earth with a relatively large meteor or other orbiting object. The effects of a large impactor on the global biota are widely acknowledged, if still lacking definitive precision in its repercussions to Earth's climate.
 

The mass extinctions, or loss of many life forms over a geologically brief period of time, at the end of the Paleozoic and at the end of the Mesozoic are associated with significant climate change. As mentioned, the K/T boundary is well known because of a large extraterrestrial impact in Yucatan, Mexico. Less known to the public are other events and conditions that typically have a significant effect on the biota because of changes in both climate and habitat space. One such event begins one to two million years before the end of the Cretaceous and continues into earliest Tertiary time. During this time, an enormous outpouring of ocean floor-type lava (basalt) flowed over a large area of western peninsula India, forming a great thickness of rock known as the Deccan Plateau. It is important to note that a similar (but even larger basaltic extrusion) occurred in what is now Siberia at the end of the Paleozoic and is associated with the largest known mass extinction. Interestingly, there is no evidence of an impact during this earlier mass extinction. The effect of gas emissions from these immense lava flows is thought to be substantial and resulted in an apparently significant climate change beginning just before the end of the Cretaceous.
 

One last contingency that helped foster the loss of many species as a result of climate change and loss of habitat space is the retreat of seas covering the margins and interiors of continents both at the end of the Paleozoic and the end of the Cretaceous. In the past, marine waters have frequently covered substantial portions of all of the Earth's continents (unlike today). Large, relatively shallow marine environments promoted a great diversity of life. When the seas retreated from the continents, the effect on marine life could be harsh, depending on how far the seas retreated and at what rate. Very near the end of the Cretaceous, the seas retreated rapidly. At the end of the Paleozoic, the seas retreated as the continents merged to form the relatively dry, all encompassing supercontinent known as Pangea.
 

Thus at the time of the extinction of dinosaurs and many other components of the biota, the climate of the Earth was changing in response to effects on the Earth's atmospheric conditions and oceanic circulation patterns. A combination of events, from impact collision and large volcanic eruptions to changes in sea level, undoubtedly led to the destruction of ecosystems and dominant forms of life at the end of the Cretaceous.
 
 
 

Extinctions in the marine realm at the Cretaceous-Tertiary (K-T) boundary.

John W. Hoganson

North Dakota Geological Survey
600 East Boulevard Avenue
Bismarck ND 58505-0840
(701)328-8000,  jhoganso@pioneer.state.nd.us

 

A major biological catastrophe occurred on Earth at the end of the Cretaceous Period about 65 million years ago, the Cretaceous-Tertiary(K/T) boundary extinction event.  It has been estimated, by some scientists, that  up to 2/3 of all life forms that inhabited this planet became extinct
at that time.  North Dakota is one of the best places to study this extinction event because of extensive exposures of Late Cretaceous and overlying Tertiary fossil-bearing rocks.  The extinction of dinosaurs, other terrestrial vertebrates and invertebrates, and plants at the end of the Cretaceous has been well documented in North Dakota.  There have been few studies, however, on the effect of this extinction event on marine fish faunas that existed at the end of the Cretaceous.

 For the past several years some of my research has been directed toward addressing the question of the effect of the K/T boundary extinction on marine cartilaginous fish (sharks, rays, and ratfish).  In North Dakota, rocks containing fossils of cartilaginous fish, mostly teeth, were deposited
in shallow marine environments just before and just after the K/T extinction.  Marine cartilaginous fish species represented by fossils in the Late Cretaceous Fox Hills Formation and the Breien Member of the Hell Creek Formation are completely different than the cartilaginous fish species
documented by fossils in the early Tertiary Cannonball Formation.  In fact, none of the cartilaginous fish species from the Late Cretaceous occur, as far as I know, in Tertiary rocks anywhere in the world.  This implies that cartilaginous fish suffered major extinctions at the end of the Cretaceous
and that new cartilaginous fish taxa originated early in the Tertiary.