Fossilization

Introduction

Fossilization is the process by which biological remains or the traces of biological remains are preserved. Fossil formation is an extremely long process; paleontologists define a fossil as the remains or impressions of an organism that are at least 10,000 years old. Fossilization is also a rare occurrence, despite the hundreds of thousands of fossils that have been discovered.

There are two major categories of fossils: those that are the direct remains of organisms, such as bones, feathers, or fur; and those in which the original materials have been replaced with another material, such as minerals or sap, producing a preserved duplicate or impression of the original form. Within these two categories, fossils are further divided into different types, body fossils and trace fossils. Body fossils are those fossils that were once part of the body of an organism, while trace fossils represent the activity of living animals from the past, including footprints and burrows.

The study of fossils, called “paleontology,” was essential to the development of key scientific concepts, including the theory of evolution. By categorizing and dating fossils, paleontologists created a fossil record, providing hard evidence of the history of life in terms of the appearance and disappearance of species. Advancements in the study of fossils include novel techniques that help organize fossils into chronological or evolutionary groups. The ability to determine the age of fossils is a major area of inquiry in paleontology, involving both relative dating techniques and the use of radioisotopes to determine the absolute age of fossils.

Key Terms

Body Fossil: Fossilized remains of the biological components of an organism (bones, teeth, fur) formed through fossilization processes such as permineralization or replacement.

Fossil: The remains of an organism or an impression created by the activity of an organism, and which have been preserved for more than 10,000 years.

Fossilization: The process by which biological remains or traces of biological remains are preserved.

Fossil Record: The collective remains of life on earth, as reflected in the fossilized remains of organisms throughout geologic time.

Permineralization: Fossilization process in which mineral-rich water infiltrates the microscopic structure of tissue and coalesces into a mineralized reproduction of the original object.

Radioisotope Dating: Also called “radiometric dating,” it is a process that uses the decay of radioactive isotopes, contained within a mineral in a rock to approximate the numerical age of the rock.

Relative Dating: Determining the relative age of fossils or other geological objects based on the position at which the object appears in the geologic strata; relative dating is based on the idea that newer layers of sediment are deposited on top of older layers.

Replacement: Fossilization process in which the original material is gradually dissolved by water and replaced by minerals, resulting in the formation of a mineral replacement of the original shape and structure of the object.

Superposition: The deposition of one sedimentary layer onto another, leading to the formation of different layers of sedimentary rock in the earth's crust.

Trace Fossils: The fossilized remains left by the activity of organisms (footprints, burrows, and other impressions).

Key Players

Nicolas Steno: Nicolas Steno, also known as Niels Stensen, was a seventeenth-century Danish anatomist and geologist who put forth the first theory of fossilization. In his 1669 book De solido intra solidum naturaliter contento dissertationis prodromus (Preamble to a dissertation on a solid naturally enclosed within a solid), Steno described the process of replacement in fossil formation and elaborated on the theory of superposition, which is the process by which layers of strata become stacked in the earth's crust. Steno's work was largely based on the fossilized remains of shark's teeth. He was one of the first to suggest that fossils might represent the remains of organisms that predated modern life on earth. (See Thomsen 2009.)

Georges Cuvier: Geologist and zoologist Georges Cuvier is often hailed as the “father of paleontology.” Utilizing evidence from the fossil record, Cuvier helped to show that different layers of strata (layers of sedimentary rock or soil) could be organized and classified based on the types of fossils found within. In two landmark papers on extinct elephants (published in 1796 and 1798), Cuvier presented the best scientific evidence that fossils were the remains of extinct organisms. He also detailed several periods of great “catastrophe,” during which many species went extinct, establishing a convincing case that the earth was far older than believed. (See Rudwick 1998.)

Clair Cameron Patterson: Twentieth-century geochemist Clair Patterson was one of the first scientists to demonstrate that the decay of radioactive isotopes could be used to obtain absolute dates for mineralogical samples. Patterson's landmark paper “Age of Meteorites and the Earth” (1955) laid out the principles of radiometric dating, which were refined by the use of other isotopes to examine samples from other geological periods. (See Patterson et al. 1955.)

Mary H. Schweitzer: In 1993, Schweitzer became the first paleontologist to isolate soft tissue from dinosaur remains. Her resulting publication, Biomolecule Preservation in Tyrannosaurus rex (1993), detailed the process used to find and isolate soft tissue, which included dissolving fragments of fossilized bone in acid. In their 2006 journal article “Soft Tissue Preservation in Vertebrate Skeletal Elements from the Cretaceous to the Present,” Schweitzer and colleagues argue that the accepted concept of preservation and fossilization may need to be amended to understand cases in which conditions allow soft tissue preservation.

William Carlson and Timothy Rowe: Paleontologists Carlson and Rowe were among a group of scientists to pioneer the use of x-ray computed tomography in paleontology. Their 2003 publication, “Applications of High-resolution X-ray Computed Tomography in Petrology, Meteoritics and Palaeontology,” discusses ways that x-ray studies can be used to gain insight into anatomic function and relationships between fossils.

History

The Birth of Paleontology: The naturalists who discovered the first fossils did not suspect that the objects represented the remains of extinct animals. The first scientist to make that intellectual leap was Danish anatomist Nicolas Steno, who developed his theory after comparing shark's teeth to their fossilized remains found in ancient sediment. Steno also developed a rudimentary theory for how mineral replaces organic material to create fossils.

In the 1700s, fossil hunting became a popular pastime in Europe and North America. Geologists, working alongside amateur fossil hunters, developed many of the tools and methods used to uncover and preserve fossils. French naturalist George Cuvier, later called the “father of paleontology,” produced a variety of research showing how fossils could be organized into distinct groups associated with different time periods. ( British civil engineer William Smith, for example, mapped and systematically described the British countryside and associated strata). During this time, the processes of replacement and permineralization were identified as geological determinants of fossil formation.

Paleontologists had been finding dinosaur fossils for decades before they were correctly organized into a group of related organisms. Paleontologist Richard Owen coined the term “dinosaurs” in 1841, and fascination with this new group of animals created an international hunger for new fossils, culminating with the early twentieth-century Bone Wars, a period of intense fossil discovery in North America.

Geological Timescales: Early paleontologists had no way of measuring the exact age of their specimens. Using such estimates as the rate at which soil erodes and the rate at which sediments are deposited in the earth's crust, paleontologists created a system of “relative dating,” in which the age of geological samples was estimated in relation to other samples from the same area.

A major advance in paleontology came with the development of “absolute dating,” a dating system based on the decomposition of radioactive isotopes. Because isotopes decay at a predictable and constant rate, the decomposition of isotopes in minerals in sediment can be used to pinpoint the time at which the sediment was created. Paleontologists can then estimate the time at which fossils were deposited in the surrounding sediment.

The idea that radioisotope dating could be used to estimate the age of the earth came to fruition in the early 1900s. Physicist Ernest Rutherford was one of the first to use uranium-lead dating to estimate the age of geologic samples. Because isotopes decay at different rates, certain isotopes are used for estimating “young” geologic samples; other isotopes are needed for ancient samples. Major advances in the field of radioisotope dating came in the 1950s; geochemist Claire Patterson, a pioneer in the field, was the first to use lead isotopes to accurately estimate the age of the earth in 1956.

Another method, “potassium-argon dating,” was demonstrated by nuclear physicist Wolfgang Gentner and proved to be effective at pinpointing accurate dates for samples from the Mesozoic. By the 1960s, paleontologists were using a combination of absolute and relative dating techniques to organize fossils into a geologic timeline of life on earth.

Current Research and Implications

Paleontology in the Computer Age: In the twenty-first century, computer modeling and computer-aided tomography (CT) have become common tools in paleontological research. While paleontologists have been using x-rays to examine fossil bones since the 1950s, new advances in CT are used to examine the microstructure of fossil specimens, providing new insight into the structure of extinct organisms. Paleontologists William Carlson and Timothy Rowe are two scientists currently using CT technology to gain further insight into fossils. In 2003, Carlson, Rowe, and colleagues reported the results of an effort to use x-ray computed tomography scans to examine fossilized shells.

Modern tomography can also be integrated with digital modeling systems to create three-dimensional models of organisms. This type of research has helped paleontologists to determine the correct placement and function of fossilized fragments, as well as to gain a better understanding of how extinct animals functioned in their environment.

Additional usage of modern technology has led to better understandings of dinosaurs. A 2024 study by UCC paleontologist Dr. Zixiao Yang and Professor Maria McNamara revealed that Psittacosaurus had reptile-like skin in areas where it didn't have feathers, which had previously been unknown.

Reconstructing Extinct Animals: The discovery of soft tissues connected with some fossils led to the idea that it may be possible to obtain deoxyribonucleic acid (DNA) from extinct organisms. Some scientists have speculated that it may be possible to reconstruct extinct species by using DNA to create an embryo and then implant the embryo in a closely related species. Although most paleontologists doubt that it will be possible to find or extract DNA from ancient organisms such as dinosaurs, some paleontologists have made promising strides in finding soft tissue (potentially containing DNA fragments) from animals that died during the last Ice Age.

In 2005, a team led by paleontologist Mary Schweitzer isolated what appeared to be soft tissue from the thigh bone of a Tyrannosaurus rex skeleton that was at least 68 million years old. Schweitzer's team first discovered evidence of preserved soft tissue in 1993, a discovery that caused controversy in the paleontological community. Since that time, Schweitzer has worked with other paleontologists in an effort to discern the conditions needed for the preservation of soft tissue. Schweitzer and other paleontologists believe that preserved soft tissue may be present in additional fossil samples and that new methods are needed for finding and preserving soft tissue in future paleontological studies. (See Schwitzer et al. 2006.)

Bibliography

Benton, Michael J., and David A. T. Harper. Introduction to Paleobiology and the Fossil Record. Wiley-Blackwell, 2009.

Carlson, William D., et al. “Geological Applications of High-Resolution X-Ray Computed Tomography in Petrology, Meteorics and Palaeontology.” Applications of X-Ray Computed Tomography in the Geosciences, edited by F. Mees, et al. Geological Society, 2003.

Fastovsky, David E., and David B. Weishampel. Dinosaurs: A Concise Natural History. Cambridge UP, 2009.

---. Evolution and Extinction of the Dinosaurs. Cambridge UP, 2005.

Foote, Michael J., et al. Principles of Paleontology. 3rd ed., McMillan, 2007.

Martin, Anthony J. Introduction to the Study of Dinosaurs. Wiley-Blackwell, 2006.

Patterson, Claire C., et al. “Age of the Earth.” Science, vol. 121, Jan. 1955, pp. 69–75.

"Researchers Discover Hidden Step in Dinosaur Feather Evolution." ScienceDaily, 21 May 2024. www.sciencedaily.com/releases/2024/05/240521124309.htm. Accessed 29 Sep. 2024.

Rudwick, Martin J. S. Georges Cuvier, Fossil Bones and Geological Catastrophes. U of Chicago P, 1998.

Schweitzer, Mary H., et al. “Soft Tissue and Cellular Preservation in Vetegrate Skeletal Elements from the Cretaceous to the Present.” Proceedings of the Royal Society of Biology, vol. 274, 2006, pp. 183–97.

Thomsen, Elsebeth. “Niels Stensen—Steno, in the World of Collections and Museums.” Geological Society of America, vol. 203, 2009, pp. 75–91.