Conodonts
Conodonts are amber-colored, tooth-like microfossils composed of apatite (calcium phosphate), a composition similar to that of vertebrate teeth. When he first described them over a century ago (1859) from clays near St. Petersburg, Russia, C. H. Pander thought that they were a variety of small fish teeth. For the next 80 years, conodonts were nothing more than a paleontological curiosity and only a few geologists attempted to obtain them by washing marine shales through fine sieves or splitting open layers of black shales. This early work demonstrated that conodonts had been an abundant group throughout most of the Paleozoic and into the early Mesozoic Era. The tremendous morphological diversity of shapes of conodonts is far greater than that typical of fish. Some conodonts are slender conical forms that closely resemble teeth (coniforms). Others possess numerous fine tooth-like denticles with complex three-dimensional shapes (ramiforms). A third major morphological group (platforms, or pectiniforms) includes low flat elements with complex morphological features on their upper surfaces.
Some attempts were made to use conodonts to date Paleozoic rocks, but like most other fossils they were limited by the distribution of shales from which they could be collected. In addition to collecting individual conodont specimens, a few paleontologists discovered natural assemblages of conodonts, where several different morphological classes had been preserved together and clearly belonged to the same animal. However, the discovery of the natural assemblages did not resolve a basic question: to what type of animal do conodonts belong? Only the assemblage was preserved and no outline of the animal _ _s body could be seen.
In the late 1930s, a revolution occurred in the study of conodonts. Paleontologists in the Iowa and Missouri discovered that when limestones are dissolved in weak organic acids, conodonts do not dissolve and are easily separated from the small insoluble residues. This discovery meant that paleontologists could collect every layer of marine limestone from an outcrop and expect to find conodonts in each sample after it was dissolved in weak organic acid. Some of the first work in acid dissolution occurred in Texas. Sam Ellison, who taught at the University of Texas in Austin, and a student (Roy Graves) published one of the first papers in 1941 using the acid dissolution technique, a description of Pennsylvanian conodonts from the Marathon region of west Texas, just north of Big Bend.
During the next twenty years, conodont workers in North America and Europe dissolved vast quantities of Paleozoic limestones in hope of obtaining conodont faunas. Although not every limestone contained conodonts, most did, and in many instances, thousands of conodonts were obtained from a kilogram of rock. Hundreds of new species were described and named. This evidence showed that conodonts had a complex evolutionary history that included many rapid bursts of evolution, where numerous species, especially platform species, appear successively over short intervals of time. Out of this work arose biostratigraphic zonations where the presence of a particular conodont species could be used to recognize a small interval of geologic time. This interval of time could be traced, or correlated, wherever the diagnostic species was found. Within 25 years, conodonts had risen from a paleontological curiosity to the primary means by which Paleozoic time was subdivided.
Conodonts are amber-colored, tooth-like microfossils composed of apatite (calcium phosphate), a composition similar to that of vertebrate teeth. When he first described them over a century ago (1859) from clays near St. Petersburg, Russia, C. H. Pander thought that they were a variety of small fish teeth. For the next 80 years, conodonts were nothing more than a paleontological curiosity and only a few geologists attempted to obtain them by washing marine shales through fine sieves or splitting open layers of black shales. This early work demonstrated that conodonts had been an abundant group throughout most of the Paleozoic and into the early Mesozoic Era. The tremendous morphological diversity of shapes of conodonts is far greater than that typical of fish. Some conodonts are slender conical forms that closely resemble teeth (coniforms). Others possess numerous fine tooth-like denticles with complex three-dimensional shapes (ramiforms). A third major morphological group (platforms, or pectiniforms) includes low flat elements with complex morphological features on their upper surfaces.
Some attempts were made to use conodonts to date Paleozoic rocks, but like most other fossils they were limited by the distribution of shales from which they could be collected. In addition to collecting individual conodont specimens, a few paleontologists discovered natural assemblages of conodonts, where several different morphological classes had been preserved together and clearly belonged to the same animal. However, the discovery of the natural assemblages did not resolve a basic question: to what type of animal do conodonts belong? Only the assemblage was preserved and no outline of the animal _ _s body could be seen.
In the late 1930s, a revolution occurred in the study of conodonts. Paleontologists in the Iowa and Missouri discovered that when limestones are dissolved in weak organic acids, conodonts do not dissolve and are easily separated from the small insoluble residues. This discovery meant that paleontologists could collect every layer of marine limestone from an outcrop and expect to find conodonts in each sample after it was dissolved in weak organic acid. Some of the first work in acid dissolution occurred in Texas. Sam Ellison, who taught at the University of Texas in Austin, and a student (Roy Graves) published one of the first papers in 1941 using the acid dissolution technique, a description of Pennsylvanian conodonts from the Marathon region of west Texas, just north of Big Bend.
During the next twenty years, conodont workers in North America and Europe dissolved vast quantities of Paleozoic limestones in hope of obtaining conodont faunas. Although not every limestone contained conodonts, most did, and in many instances, thousands of conodonts were obtained from a kilogram of rock. Hundreds of new species were described and named. This evidence showed that conodonts had a complex evolutionary history that included many rapid bursts of evolution, where numerous species, especially platform species, appear successively over short intervals of time. Out of this work arose biostratigraphic zonations where the presence of a particular conodont species could be used to recognize a small interval of geologic time. This interval of time could be traced, or correlated, wherever the diagnostic species was found. Within 25 years, conodonts had risen from a paleontological curiosity to the primary means by which Paleozoic time was subdivided.
