Mass Extinctions – Through History and Today
New Paleontological Database Refines Extinction and Recovery Rates
Oct 28, 2008
Information derived from a new Paleontological Database, a compilation of almost 134,000 names of species, genus and higher-order taxa, developed by Dr. John Alroy of the National Center for Ecological Analysis and Synthesis, gives scientists a new look at mass extinctions in the past.
Alroy explains in his August 2008 paper, published in the Proceedings of the National Academy of Sciences (PNAS), that by using this database he can predict extinction and origination rates much more accurately. These rates can help conservation biologists make predictions concerning the current extinction occurring due to human causes.
Background Extinction in the Fossil Record
Paleontologists are in general agreement that extinction occurs on a fairly constant basis of approximately two to five families per million years. These extinctions take place mainly in the marine realm but in the terrestrial as well.
However, standing out amongst this background rate are dozens of well-documented extinction events, says Alroy. Of particular interest to paleontologists are the largest in magnitude, which are referred to as “The Big Five.”
“The Big Five” - Extinctions That Changed Taxonomic Composition
According to Donald R. Prothero in his 1998 text Bringing Fossils to Life, all of "The Big Five” extinctions, with the exception of the Devonian, occur at the boundary of a geologic period. Each is distinct in terms of its timing, duration, magnitude and hypothesized causes. Below they are described in order of time.
- Late Ordovician Extinctions: Occurring approximately 440 million years ago, this event lasted around 2 million years, and about 57 percent of the marine genera were lost.
- Late Devonian Extinctions: Taking place nearly 365 million years ago, this event saw the demise of approximately 50 percent of the marine genera and 75 percent of the marine species dominant at the time.
- Permo-Triassic Extinctions (P/T): Around 250 million years ago, this event was the largest of all of the extinctions, lasting over 8 million years. Ninety-six percent of marine species and 75 percent of land vertebrates went extinct during this time.
- Late Triassic Extinctions: Although the smallest of the five in magnitude, 48 percent of marine genera were lost during this protracted event spanning over 17 million years, taking place around 230 million years ago.
- Cretaceous-Tertiary Extinctions (K/T): The most recent mass extinction, occurring about 65 million years ago, saw the end of the dinosaurs and several other groups such as the ammonites, marine reptiles and many marine invertebrates.
Of these five major events, the P/T ranks as the largest, followed by the K/T and then the late Devonian extinctions. These three are the biggest, or the so-called "Big Three," due to the magnitude of total taxa loss.
Possible Causes of Mass Extinctions
Various hypotheses have been suggested for the mechanisms connected with each mass extinction event, and many have been surrounded by disagreement and controversy. Some scientists have proposed the idea that there may be a common cause for mass extinctions or patterns associated with extinction events.
What is clear is that mass extinctions can be caused by long-term changes, such as climate change, or by sudden catastrophic events such as a bolide or extraterrestrial impact, as proposed for the Cretaceous/Tertiary extinction of the dinosaurs.
The Sixth Extinction - The Current Crisis
Today’s biodiversity crisis, unlike “The Big Five” extinctions, is human induced. Climate change and expansion of human civilization are only two of the determining factors in the surprising loss of species occurring right now.
“I think there’s good agreement amongst conservation biologists that the current mass extinction would rank somewhere amongst 'The Big Three' if we could predict its ultimate intensity precisely,” says Alroy. The recovery rate for an extinction of this magnitude would take tens of millions of years, he says, and “the fact that I can make numerical predictions about recovery rates makes this [PNAS] paper relevant to society in a way that most paleontology isn’t.”
 |
 |
 |
