Scientists think that amphibians are rapidly disappearing around the world because of their sensitivity to environmental changes. So how did frogs outlive the dinosaurs?
You talk about an asteroid causing the extinction of the dinosaurs, but what about frogs? Frogs are a very delicate species, greatly affected by changes in their climate.
The fossil record shows that — 65 million years ago — perhaps half of all species living on Earth became extinct. But frogs probably fared well during this time. In fact, a zoologist collecting live samples before 65 million years ago might have found the same frog families then as we find on Earth today.
No one knows for certain why the dinosaurs disappeared. You might think that in order for such big animals to become extinct, there must have been a catastrophic, global event. But if you look at a modern ecosystem, it’s always the largest animals that are the worst hit by environmental changes. That’s because they need more resources — more food and more space.
A popular theory suggests that dinosaurs became extinct after an asteroid struck Earth. But at least one part of this theory doesn’t seem possible — the idea that sulfuric acid rained down on Earth for years after the impact. Frogs are very sensitive to acid in their environment, and yet they survived whatever happened at that time.
Today, populations of frogs and other amphibians are declining around the world. Scientists think that it’s due at least in part to their sensitivity to change — such as increases in ultraviolet radiation, pollution, and destruction of habitat. But this has left researchers with a puzzle — how could such sensitive animals have survived whatever caused the dinosaurs to become extinct?
The fossil record of amphibians is problematic for a couple of reasons. Because they are soft-bodied, very little survives to be fossilized. Also, the areas that form the best fossils are not the environments in which amphibians commonly lived.
From an interview with Dr. Farrish Jenkins (9/96):
* What other creatures (besides frogs) seemed to escape the mass extinctions relatively intact (crocodiles?)?
Turtles, lizards, snakes (which had already evolved by then and did well during the KT extinctions) were all affected but not greatly by the KT event. These creatures suffered only small numbers of extinctions of certain genera. The fossil record for dinosaurs is for 50 million years — but vertebrate fossils go back to the Triassic.
One of the subjects of study recently being pursued is to establish more precise stratagraphic information — to determine the age range of certain animals — we want to know if the time range of these animals extended up to 65 millions years ago (the KT boundary), or if they were disappearing already at 66, or 67 million years ago — perhaps they went extinct earlier than the KT event.
* Besides the fossil record showing that many dinosaurs became extinct in a short period of time, what environmental evidence do we have for that time period? (geological evidence, climate, vegetation, atmospheric composition, water quality, …)
At that time (of the KT event), flowering (broad leaf) plants were still evolving. They evolved in the early Cretaceous to become much like broad leaf plants of today — like maples or magnolia. Oddly enough, conifers (like modern cypress and redwood) seem to have been unaffected at all by the KT event. Ferns and cycads weren’t greatly affected (they had already declined by the end of the Cretaceous — they were being squeezed out by broad leafed, flowering plants).
Land dwelling snails didn’t change much then, but marine snails, clams, and oysters, did. Amonites, which resemble modern nautilus, and all other nautalites (nautilus relatives) all became extinct. Only one group, the modern nautilus, survived. It’s really a throwback to an earlier time.
Marine water creatures changed a lot following the KT event, but fresh water creatures didn’t seem greatly affected.
We lost all swimming reptiles in the KT event. The Plesiosaurs and mososaurs (powerful swimming animals which looked like dolphins) became extinct. There were ancestors of the modern monitor lizard that lived in the marine water, at lengths of 40 or 50 feet with huge powerful jaws. These became extinct. Monitor lizards, land dwellers, did live at that time too, but did survive the KT event.
In the microscopic world, there is little fossil evidence from that time and before. There were some creatures like modern amoebas, called foraminiferas — one celled animals that grew shells like snails do. They became very large — the shells have been found up to 1/2 inch across. They were very abundant in the Cretaceous, but in the Tertiary, they almost totally disappeared. Ecologically, the disappearance of foraminiferas may have had a more profound effect than the disappearance of big land dwellers, the dinosaurs that died. The biomass of these ocean dwellers was much larger than the carnivores and other dinosaurs that became extinct — several magnitudes larger.
* Why are frogs and other amphibians generally considered very sensitive indicators of environmental health? Why are frogs declining today, when they seemed oblivious to a changing environment then?
Is the current decline of frogs and other amphibians around the world foretelling more massive ecological problems? Directly, amphibian extinction wouldn’t have a big impact today, but it may signal a much more severe problem on the way.
Some paleontologists think that we’re in a massive extinction period right now. They think many rain forest species that have never been discovered will become extinct before they can be identified.
From an interview with Dr. Andrew Blaustein:
* Does the theory that plants went first, then the larger creatures went next go against the idea that amphibians (herbivores) thrived then?
It probably did have a big effect on them, but not big enough to wipe them out.
* How have frogs done under other circumstances between KT and now (eg. ice ages)?
Amphibians have done well compared to other groups of animals — they came through better than the dinosaurs, but that doesn’t mean that they can survive the changes that are going on today.
Amphibians are soft-bodied and not well preserved as fossils, so the fossil record is a bit obscure. But, paleontologists feel that amphibians did fairly well during the KT event.
Frogs and other amphibians had the right mechanisms to survive then, but they may not have the mechanisms needed to persist in today’s environment.
* What is the state of the environment today?
Now there’s something wrong, but scientists don’t know all of the reasons for the decline in frog and amphibian populations today.
They may make it through now, they did so in the past .. but then again, we don’t know.
Frog and amphibian populations seem to be in decline today and their ranges are decreasing. This is due partly to humans destroying their natural habitats — and this is much more drastic than any other changes they could be subjected to.
But, in some areas of the world — humans aren’t destroying habitat at all — for example, forests of the Pacific Northwest, and parts of South America and Australia.
There is a very good theory that the eggs of amphibians are being exposed to increased u.v. radiation which is causing a decline. There are also pollutants put into the air and water by humans.
Some other “natural” reasons for decline might be bacterial and fungul infections that have been identified in some amphibians.
* Is there anything that we humans can do to help amphibian populations?
We don’t know all of the problems yet, but there are some changes that we can make now that might help … it’s just that it’s a very controversial issue for politicians and scientists alike. But, so far, there hasn’t been much doubt cast on the u.v. radiation theory.
Some people are arguing that there is not really a decline in these populations today — some of them say we haven’t studied the population long enough to decide if it’s a permanent decline or just natural fluctuations in populations. One thing against that idea is that some amphibians have been missing from their ranges for 30 or 40 years.
In the past year — at Savannah River (Carolina) — a new study has found another example of declining amphibian populations.
Basically, all of the new evidence of the past year has served to corroborate the idea that frog/amphibian populations are declining around the world.
From an interview with Ann Weil:
* Why are amphibians so sensitive to changes in their environment? And what does this tell us about what could or couldn’t have happened at the KT boundary?
They’re sensitive to certain things — some are very sensitive to UV radiation; they’re very sensitive to acidification of their environments. One result of an asteroid impact would be huge amounts of acid rain falling on N. America. But we don’t see fish, frogs, or salamanders being affected at all by this. So, this is an argument against there being acid rain falling on Earth for 10 years, as is predicted by the asteroid theorists. There was a paper published that calculated that 10 to the 13th to 10 to the 14th moles of sulfuric acid rained down on Earth over ten years following the KT asteroid. But this can not be true — this isn’t supported by the fossil record, which shows that frogs and salamanders fared well then.
We’re guessing what would happen chemically if an asteroid hit Earth, but we don’t have any real evidence. We haven’t been hit by anything so large in recent years to see what effect it has on life.
One other thing that has been hypothesized about the KT asteroid impact that hit the Yucatan, which may not be true either is that immediately after, North America was scorched by heat — wild fires raged. But this can’t be refuted by the fossil evidence, because most frogs and salamanders burrow in the ground, and so they can withstand fires. They probably could have survived this. They also burrow during droughts, surviving in dried mud for a long time.
Recent fires in the Oakland and Berkeley hills of California haven’t affected salamander populations — 1990 or 91 (?). So you can’t use these animals to completely disprove the impact theory.
* Scientists theorize that an asteroid caused the extinction of the dinosaurs. What about the frogs? Frogs are a very delicate species, greatly affected by changes in their climate. Yet frogs were not greatly effected by this event according to Bob Bakker, a noted paleontologist.
No one can tell you definitely why the dinosaurs went extinct — if they tell you they know why, then they’re making it up. We know that whatever killed off the dinosaurs could be withstood by frogs and salamanders. This puts a limit on what the causes could be. This gives us a good set of parameters in searching for the causes of extinctions.
It’s common for physical scientists studying the KT boundary to say — “these big animals went extinct, so it must have been a catastrophic, global event.” But if you look at a modern ecosystem, it’s the large ones that are always the worst hit by environmental changes — they need more resources, more food, more space, etc.
The thing that is hard with studying dinosaurs — is that we think we know a lot, but we really don’t know anything about their life histories, their physiological tolerances, etc. We can only guess at their characteristics from birds and croccodilian descendants that did survive the extinctions. Even among today’s large mammals, there is a lot of variety.
* Compare the amphibians from just before to just after the KT event. How did they change?
There wasn’t a large change at all. During the Cretaceous, the “microvertebrates” — animals smaller than the dinosaurs — would look very familiar to us. There were possums, snakes, fish (like bowfin, gar, and sturgeon). These along with frogs and salamanders looked then as they do now. In the Triassic (before the KT boundary) — we see the first frogs in the fossil record, and they look similar to those of today.
When scientists do molecular and genetic studies on the DNA of frogs, they think that the major families of frogs we see today had already diverged by the late Cretaceous. They aren’t very common in these faunas — we don’t find many of them in the fossil record — so it’s hard to tell from the fossil evidence how accurate this is. To determine when the various families should have diverged, they look at the differences between the various modern frogs and estimate how long it would have taken for those differences to develop assuming a constant rate of mutation of genes. They estimate that they had differentiated into different families before the KT boundary.
So, the modern frog families might have been recognizable if you were walking around in Cretaceous collecting frog samples.
* Are the current declines we are witnessing in frog/amphibian populations worse than we suspect happened during the KT event?
There’s an argument in the scientific community over what is causing today’s extinctions — is it just one or many factors? Ideas have been suggested, from UV light, to human habitat reduction.
You can look at a wide variety of environments today and amphibians live in all of them. But in contrast to today’s biological studies, paleontological sampling is poor. What we’re stuck with in paleontology is a record from only a couple of ecosystems — one example is the lowlands that filled up with sediments — this only preserves the creatures that live in one type of ecosystem — fresh water, primarily aquatic creatures. This is what’s called a “raparian” environment. Frogs and salamanders were probably very common, but we only have records from a few places. It’s one of the weaknesses of working with fossils — some environments just aren’t preserved at all.
Small animals are hard to work with too. I specialize in microvertebrates. And what you typically find are many creatures at once, all jumbled together. A paleontologist is looking in river bottom sediments in which fossils of many things have been mixed. They become disarticulated — the bones fall apart — limbs separate from the body, jaw bones disconnect from the skull; pieces of dinosaurs are mixed with bits of birds, amphibians, etc.
So you do a count of the “minimum number of individuals” in the area. To do this, you count how many bones you have of a unique type of bone that each creature only has one of. If you find 50 of this particular type of bone, then you say that you have a minimum of 50 individuals in the area.
You find the fossils in “lag deposits” — for example in Waller creek in Austin. You generally find them in small pockets of bones — bars on the river. Some of these pockets are big enough to work with a bulldozer, pulling tons of material out.
* What will be done in the future to improve our data from this time?
Paleontology is a low-tech field. There won’t be these big advances, like you see in the movies. Paleontologists won’t be out in the field using sonar to locate skeletons.
The data set will improve as more areas are explored.
The area in which most terrestrial generalizations are made are from Eastern Montana — it has been sampled for over 20 years. We have good stratagraphic evidence for that area too.
Extinction is exciting, dinosaurs are big with big teeth. It’s what you hear about in the newspaper. But fundamentally, the ones that go extinct, are those we know little about… It’s the ones that survive that we can really study. We know more about survivors — frogs and salamanders — we can study them in the wild and in the laboratory. People just aren’t as interested in that.
Physical scientists and biological scientists need to get together more to look at the KT boundary. They have worked closely together to determine what happened in the Pleistocene (ice age).
Frogs and salamanders had a range shift then, as did lots of other animals… one thing that was true of this time, it wasn’t so much that it was colder, as that there was less seasonality. What really limits a specie’s range is the extremes in temperature. In North America, there were “disharmonious fauna” — creatures that don’t live together today — living together. The ranges of these animals overlapped in different ways than today. For amphibians, the ranges shifted greatly during the Pleistocene.
Amphibians require a certain dampness in the soil. In Texas, during the late Pleistocene and throughout the Holocene, precipitation and moisture changed, causing amphibian ranges to move where there was more moisture at the ground.
If extinctions are caused because of perturbed environments, whatever wiped out dinosaurs wasn’t something that all animals were sensitive to.
The UC Berkeley Museum of Paleontology has the largest paleontological collection west of the Mississippi. It’s a “library of things”, with little exhibit space.
Check out the Virtual Museum here.