Steve Dutch
University of Wisconsin-Green Bay
Trying to find out how long taxa of different levels last and how often they go extinct is a surprisingly difficult problem. There is no comprehensive data set of extinct organisms with detailed data on their first appearance and disappearance in the fossil record.
Also, that nice neat kingdom-phylum ... system is a whole lot messier than it first appears. Consider vertebrates. Vertebrates are members of the phylum Chordata, which is defined by having a stiffening rod running along the body. But only some chordates have bones, so we need something for bony organisms, but within that level we have a pretty diverse array of classes: bony fishes, amphibians, reptiles, mammals and birds. So we need something between the two levels, the subphylum Vertebrata. Every taxonomic level has sub- and super- levels.
Also, just what is a species? Mounds and Almond Joy bars are different brands of candy bar made by the same company. They compare well to species. What about the Ford Mustang and the Ford Fiesta? They're made by the same company but the differences are at a whole different level of complexity than whether or not they have almonds. Is the difference between two species of trout comparable to the difference between two species of deer, or rattlesnake, or hawk? Does the similarity of birds represent the same range of complexity as the similarity of mammals?
Most estimates of the average life span of species are around 1-10 million years. The exact number of species on earth is unknown but easily in the millions. Thus the "background" extinction rate of species is around one (maybe several) per year. This is the rate during environmentally stable or slowly changing conditions and does not include mass extinctions or human caused extinctions. However, the vast majority of species on earth would never show up in the fossil record, which is obviously what we have to rely on for estimates spanning millions of years. They either lack hard parts, are rare, or geographically restricted. The losses of Steller's Sea Cow or the Dusky Seaside Sparrow were tragic, but they would be unlikely ever to have been noticed in the fossil record. If one species in a hundred is ever likely to be fossilized, and species go extinct roughly once a year, then species likely to be fossilized would go extinct about once a century.
Examining human lineage to estimate how long genera endure is unproductive for several reasons. First, we haven't been around anything like as long as many other genera. Second, when it comes to human ancestry, the battle between the lumpers and splitters constantly seesaws back and forth, with splitters constantly defining new variants of early humans, and lumpers combining them into larger categories.
Dogs and their relatives, the family Canidae, give us a lot of insight. There are lots of taxa and the group is well studied. There are over 30 extinct genera lasting anywhere from a few to around 20 million years. One of our more dubious accomplishments was driving one genus, Dusicyon, to extinction. This was a small canid, the Falkland Islands Wolf, which was hunted to extinction out of concern that it preyed on sheep. Because the Falkland Islands are such prime agricultural habitat. There are somewhat fewer extinct genera of cat, but they show a similar duration spread.
There are many recently extinct genera, some killed directly by humans, others wiped out by introduced predators like rats.
We can probably make a ballpark estimate that a typical genus lasts 10 million years.
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There are several extinct orders of mammals. Creodonts, extinct carnivores, lived from 55 to about 8 million years ago. Condylarths, unspecialized early mammals, died out around 50 million years ago. Desmostyles, somewhat seal-like mammals, lived from 31 to 7 million years ago. Embrithopods, rhinoceros-like animals, lived between 35 and 25 million years ago.
The most recently extinct bird order is Dinornithiformes, the moas of New Zealand, were hunted to extinction in the 1500's. Aepyornithiformes, huge ostrich-like birds of Madagascar were also probably driven to extinction by humans. They lived until 1000 years ago. Neither extinction, let us note, is attributable to Western culture. Before those, the only other extinct bird orders died out over 70 million years ago.
At least among mammals and birds, it appears that orders last tens of millions of years.
The class Trilobita, the trilobites, went extinct during the Permian mass extinction around 220 million years ago. Among vertebrates, the Placoderms, primitive armored fish, lived in the Silurian and Devonian Periods and went extinct around 300 million years ago. Another class of primitive armored fish, the Ostracoderms, lived from the Ordovician to the late Devonian. Acanthodii, which shared features with both bony and cartilaginous fish, lived about at the same time as the Placoderms.
Now we can see why all the hoopla over the decline of amphibians. Classes endure hundreds of millions of years, and we have not had a class level extinction in over 200 million years. We would like not to have one happen on our watch.
Many biologists flatly deny that any animal phylum has ever gone extinct. These biologists believe that unusual organisms that have been assigned to extinct phyla should actually be classified with existing groups. It is more common to find biologists who place the bizarre fossils in the Burgess Shale of Canada (about 550 million years old) in several extinct phyla.
The equivalent of phyla among plants is often termed a division. Several early divisions of plants went extinct in the middle to late Paleozoic about 300 million years ago. A group of early seed plants called Bennettitales or Cycadeoids lived throughout the Mesozoic, and are called a phylum by some sources. It's a little harder to deny extinctions of plant phyla since complex plants didn't even exist until the early Paleozoic and plant structures are often better preserved than diagnostic features among animals.
In any case, there has not been a phylum level extinction since the Mesozoic, and before that, for hundreds of millions of years.
Once upon a time, the upper end of biological classification seemed static, even trivial. There were animals and plants, and hardly anyone was likely to mistake a cow for a geranium. Surprisingly, a lot of the most exciting developments in taxonomy are now happening at this upper end. There had long been a proposal to place single celled organisms in a separate kingdom since many shared characteristics of both animals and plants. As long as we're thinking along those lines, why did we consider fungi plants? Fungi had been considered plants only because they didn't move around and capture prey - mostly - except one group, called the slime molds, does. In all other respects, fungi are so completely different from plants that they were finally given a kingdom of their own.
Animals, plants, fungi and protists (single celled organisms) all share one feature in common: they have a cell nucleus. Bacteria are different in an even more fundamental way: their DNA is not enclosed in a nucleus. We need a level higher than the kingdom. "Empire" would be a great label but biologists prefer "domain" or "superkingdom." We now know of three domains: Eukarya (organisms with cell nuclei), bacteria and archaea, single celled organisms that include many extremophiles, or organisms that can tolerate extreme temperatures and chemical conditions.
So why did it take over 200 years after Linnaeus to figure out there were levels above the kingdom? Because it takes electron microscopes and molecular biology to see the differences. Early in Earth's history there could very well have been other kingdoms and domains of micro-organisms that went extinct, but our likelihood of ever knowing anything about them is very slim. Today, every organism we can see with the unaided eye is in the domain Eukarya, and we don't know of any extinct kingdoms or domains.
The one outside possibility is that some of the organisms that lived around 600 million years ago, the Ediacaran biota might have been so radically different from everything else as to deserve a separate kingdom. At least one paleontologist has made this suggestion. I second it.
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Created 3 Feb 2006, Last Update 15 January 2020