Rebekah K. Nix, Ph.D.



One Geologist's Perspective

This mini-lesson was presented on April 29, 1998 in partial fulfillment of the requirements for a Marine Science course taken at the University of Texas at Dallas under the direction of Dr. Cynthia Ledbetter. All photographs and samples were collected by the author, Rebekah Nix.

The main purpose of this mini-lesson is to introduce the principle of uniformitarianism, one of the foundations of geology. It is designed to hopefully open at least one door of interest within each individual of a diverse class and emphasize the value of seemingly non-related fields. Just because we may not have a vested personal interest in a subject does not mean that it is not important or that it does not have some relevance.

It is assumed that the students have a basic knowledge of marine plants, animals and processes. If not, this lesson may be revised to serve as an intriguing introduction to the study of the marine environment.

Although it is intended as a general guide, or sample application of one viewpoint, you are certainly welcome to use this online version as is! Note that this approach may be used with any of several specimens and locales. Modify it to meet your needs - and to work with whatever is available to you. (Numerous images of "living fossils" are available via the World Wide Web.)


You'll need a sheet of paper and something to write with...

For this exercise, I'd like for you to take one minute to investigate each sample and write down the following things about each item:

  1. What you think it is.
  2. Where you think it came from.
  3. How you think it got there.

(Four - ideally one per student - brown paper bags, each containing a fossil specimen, were distributed to classmates for independent review.)

Please begin. (Cue students to swap bags every minute.)

Okay. Let's see how your "whats & wheres" compare to the actual descriptions of each item.


Item 1 is a pile of fossilized crinoid stems. Crinoids are brilliantly-colored echinoderms, commonly called "sea lilies". They live attached to the deep floor in tropical seas. They're actually animals with long stem-like bodies and feathery arms that extend upward to form a cup. Thousands of now-extinct species flourished during the Paleozoic era, 230-600 million years ago. Only a few hundred species linger today. I found this sample, along with lots of sponge fossils, in a stream bed in middle Tennessee.

Item 2 is a crocodile tooth that's been fossilized in a finely bedded sandstone. Crocodiles first appeared about 200 million years ago. They quickly diversified into water-dwelling and amphibious forms, living in tropical and sub-tropical areas of the world. I discovered this sample, along with several other fossils fish, while digging in the Warfield Fossil Quarry in Kemmerer, Wyoming.

Item 3 is a fossilized "heart urchin", another echinoderm. Sea urchins are spiny skinned marine animals. They have a water vascular system that uses seawater for respiration, locomotion and reproduction. I picked this one up on our property in Sanger, Texas, about an hour north of the Dallas city limits.

Item 4 is a piece of fossilized tree bark, probably a cycad. Cycads are large, slow-growing, palm-like plants. Only a few species survive in tropical and sub-tropical areas today. They were the dominant plant form during the Age of Dinosaurs, some 200 million years ago in the Jurassic Period. I collected this piece from a road-cut on Raton Pass (elevation 8,000') in the Rocky Mountains, at the New Mexico - Colorado border.

 Crinoids in Tennessee? Crocodiles in Wyoming? Sea urchins in north Texas? Palm trees in Colorado?

What does any of this have to do with Marine Science?

My presentation takes an interdisciplinary approach to the study of Marine Science. I wanted to show you why a geologist, paleontologist, biologist, environmentalist, or any of several other types of "-ists" might be as interested in Marine Science as I am - even though I don't particularly like to mess with wet, wiggly critters.




Imagine yourself standing nearly 9000' above the surrounding terrain, on the very top of El Capitan in the Guadalupe Mountains, near Carlsbad Caverns.


After a grueling climb up through McKittrick Canyon in 100+ degree temperatures you can finally enjoy the cooling air as it sweeps across the Chihuahuan Desert, past you on what is called the "Top of Texas", and swirls on beyond and into the canyons.



You sit down to take a sip of water and, without thinking, pick up a rock to toss about as you daydream.


After a while you look at the sample and are shocked to recognize it from your last trip to the islands. Now you really are afraid the heat's gotten to you It's a piece of coral! In fact, on closer observation, you discover that the entire ridge is one giant limestone reef. The world's largest coral reef is in Texas!

Marine plants and animals have been around for a long, long time - especially in geologic terms. The ocean provides one of the best environments for fossilization and has contributed significantly - almost entirely - to the fossil record. One of the foundations of geology is the "Principle of Uniformitarianism". In 1785, James Hutton, the "father of geology", realized that "the present is the key to the past".

With that understanding, we can start to make sense out of these strange findings. For example, we know that coral reefs grow in a particular, rather specific environment. How would you describe the main ingredients for a healthy reef, say one that could reach the size of El Capitan?

(Solicit responses.)

In order for corals to grow and reproduce, ocean water conditions must fall within certain physical, chemical & biological parameters:

  • Temperature - 75F (23.8C) to 85F (29.4C)
  • Light Penetration - need shallow and sediment/algae-free waters
  • Salinity - 34 to 37 ppt (normal for sea water)
  • Food Supply - Zooplankton, etc., from open ocean & within lagoon
  • Nutrient concentrations (Nitrogen & Phosphate) - must be low
  • Pollutants, Silt & Sediments - must be low

What then can we infer about the climate, topography and general environment in southwest Texas during the Permian period, some 225 million years ago, the estimated age of the Capitan reef system?

Same list!

The Guadalupe Mountains formed as a barrier reef on the edge of an extensive sea that covered the area during the Permian. In fact, the sea transgressed and regressed across Texas many times.

Cambro-Ordovician (500-700 million years ago)

Permian (300 million years ago)

Jurassic (250 million years ago)

Late Cretaceous (100 million years ago)

Images adapted from "Roadside Geology of Texas"

More recent oceanic episodes are clearly evident on a geologic map. Similar reef complexes have formed and fossilized along these ancient shorelines. (Point out banding parallel to Gulf shore.)



Geologic map of Texas adapted from "Geology of Texas", 1992, Bureau of Economic Geology, The University of Texas at Austin

The relatively recent Lower Cretaceous fossil reef trends are clearly evident on the tectonic map. (For example: Stuart City Reef, Cupido Reef, Edwards Reef and Sligo Reef.)

Tectonic map of Texas adapted from "Geology of Texas", 1994, Bureau of Economic Geology, The University of Texas at Austin

So now it hopefully it's a little easier to believe that I did find crinoid stems in middle Tennessee, a crocodile tooth way up in Wyoming, echinoids nearly 1000' above sea level in north Texas and evidence of a tropical swamp on the crest of Raton Pass. We could write geologic histories for each area. They would all need to tie into the big story somehow to create the big picture. Unfortunately, many scenes are missing.

The marine sequences described on similar "paleontological" pages are some of the most complete and more interesting on record.


Carla Nix, Rebekah Nix and Linda Scott prepare to snorkel in the Sea of Cortez, B.C.S.

It's exciting to be able to dive into the depths and see a living crinoid or to snorkel in the warm blue waters of Belize over the second largest living coral reef.

This fossil fish

is basically the same as these modern fish.

This fossil leaf

is basically the same as this modern leaf.

This fossil ammonite

is basically the same as this modern nautilus.

This fossil coral

is basically the same as this modern coral.

We can study "living fossils" in their natural habitat and apply that knowledge and understanding to develop the geologic history recorded in marine sediments!

It's just as wonderful, and somehow a bit more magical, to visualize the same images from the top of a solitary mountain peak. Sometimes, when you close your eyes, if the fresh breeze is just right, imagination will take you back in time and you can hear the waves breaking on the ancient shore beneath your feet.


That's part of the reason I have enjoyed this class so very much. Thanks for your attention.

I hope that you enjoyed this mini-presentation and will be able to use it in your teaching of science. - R. Nix