To survive, dig in

kw: book reviews, nonfiction, science, paleontology, zoology, burrowing, mass extinctions

Shortly after we moved to our house 22 years ago we bought some flat stepping stones for high-traffic areas in our yard, such as the path through a "gate" in a hedge. I dug these in to be an inch or so above ground level, a little lower than the mower blade at its lowest setting. Now, nearly all of them have sunk to ground level or below. Two examples are shown here. Is this just soil compaction from the stones being walked on? Not entirely. Wherever I dig in my yard, I encounter several earthworms in every shovelful.

Charles Darwin spent about 20 years studying earthworms, and using "worm stones" plus an ingenious measuring device attached to bedrock beneath, determined that bioturbation (the modern term) of the subsoil by earthworms caused the stones to sink by an average of 2.2 mm/year. Darwin's earthworms must have been very energetic. The "sink rate" for my stepping stones is closer to 1.0-1.5 mm/year.

One of Darwin's worm stones is pictured in The Evolution Underground: Burrows, Bunkers, and the Marvelous Subterranean World Beneath Our Feet by Anthony J. Martin. Dr. Martin's thesis is simple: burrowing and other means of living below ground at least part of the time is so beneficial that many animals are burrowers. I don't know if you could say "most animals", but that might be true (he doesn't say). Also, burrowers provide homes for other species that share their spaces. The author makes a good case, with numerous examples, that living at least part time underground enabled many animal species to survive the various nastinesses we call "mass extinctions".

The "big five" mass extinctions had such profound effects on both biology and geology that they mark geological boundaries (the abbreviation "mya" means "million years ago"):

• Ordovician-Silurian boundary, 429 mya. About half of species vanished, and about 85% of all animals died.
• Late Devonian, 364 mya. About 75% of species became extinct.
• Permian-Triassic boundary, 251 mya. The baddest of the bad, this one drove 96% of species extinct. All living things today are descended from the remaining 4%.
• Triassic-Jurassic series, between 214 and 199 mya. By the end of this 15-million-year period, more than half of species had been eliminated.
• End-Cretaceous, 65 mya. This is the best known, because it centers on an asteroid impact and led to the demise of the dinosaurs…or, at least, the non-avian dinosaurs. It is now known that birds are dinosaurs, or, if you prefer, birds are descended from theropod dinosaurs. 76% of species went extinct.

Many cases show that animals that were underground during the big smash, or whatever happened, were the most likely to survive in numbers sufficient to restore their populations afterward and become the ancestors of modern life. But before the first of the mass extinctions, there were big changes as animal life arose and developed, including the development of the first burrowing creatures. An odd group of animal species called the Ediacara Fauna did just a little burrowing, but were followed by the "Small Shelly Fauna" that burrowed more and deeper, and then the proliferation of hard shells that marks the beginning of the Cambrian period also marks the beginning of rather thorough bioturbation of ocean floor sediments.

The author shows the history of animal life from the perspective of an Ichnologist, a scientist who studies trace fossils. This picture, a 6"x8" section of a rock about 15" square, shows trace fossils on a rock I picked up from a sandstone bed near the base of the Morrison Formation in South Dakota, so it is about 150 million years old. This is a bottom cast; we are "looking up" at sediment that settled into tracks and shallow burrows in the late Jurassic sea bed.

Somewhat visible are ripples crossing from top right towards bottom left, showing that this was in rather shallow water. At least three kinds of tracks are visible, though I don't know what animal made any of them. Other dug-in structures are seen, or rather, their casts. Dr. Martin and his colleagues are experts in discerning the meaning of such traces.

Before digging into his subject, however, the author discusses "A brief history of humans underground." If you've heard of Cappadocia, you may know of the underground homes dug into the soft sandstone. That has been going on for several thousand years! Long before that, humans utilized natural caves, not only for shelter and burials but even for their art (think of the amazing art in the caves at Altamira and Lascaux).

While we tend to denigrate "cave men", thinking only Neanderthals lived in caves, the "art gallery" caves were painted by our species. When there were only a few humans worldwide, it makes sense to consider that many or most of them used caves and sometimes stayed in them for extended periods, not just during bad weather or extreme seasons. A cave is easier to defend from predators. And just as the burrows of gopher tortoises permit them to thrive in areas with tough winters, so caves shield those who dwell in them from climatic extremes. Indian Echo Caverns, in Pennsylvania about two hours from where I live, was the home of William Wilson from 1802-1821. The "Pennsylvania Hermit" stayed pretty well wrapped up most of the time, because the cave stays a nice, chilly 54°F (12°C) all the time.

There just aren't enough caves to go around, so now we build artificial caves we call "houses". One of the professors at South Dakota Tech had an "underground house" when I was there in the 1980's. It was technically a house built into a tight place between two rock outcrops. An underground house is nearly free to heat or cool, if it is in the "temperate band" across the world where average temperatures are between about 60°F and 75°F (16°C-24°C). The below-ground temperature near Rapid City, SD is closer to 47°F (8°C), so my professor had to insulate the excavation, pour concrete for the dwelling, and insulate more. South of Oklahoma in the U.S.A. an underground house would not need heating or cooling (just moisture control, perhaps!); in Europe, think Spain, Italy, Greece and Turkey, including Cappadocia.

This may become more pertinent in another generation, if the climate continues to warm. I will be even more pertinent when the "Holocene warming" that began about 12,000 years ago comes to an end and another 100,000-year Ice Age begins! Today's "global warming" caused by "carbon pollution" (an oxymoron; we are made of carbon and its oxy- and hydro-derivatives!) may actually delay an ice age by a century or so.

The most ubiquitous burrowers and tunnelers, humans aside, are invertebrates. Earthworms don't leave open tunnels; their burrows fill in behind them with the excreted feces from which they've digested key organic materials. But ants and termites produce long-lasting tunnels. Some of these have been studied by pouring in plaster or even molten aluminum. This cast of an ant nest is from leaf-cutter ants of Central America.

There is a surprising array of vertebrate burrowers, however. We are familiar with gophers and voles, perhaps, but certain birds burrow, such as kiwis, bee-eaters, and some penguins. The gopher tortoise, as its name suggests, is quite a digger, and its burrows shelter at least 400 species that are enabled to live in otherwise inhospitable places because of a tortoise's "hospitality".

The author also discusses the most amazing tunneler of all prehistory, the giant ground sloth. You might not think of an animal the size of a 4-door sedan as a burrower, but in southernmost Brazil there are hundreds, perhaps thousands, of burrows you could literally drive a truck through! The tunnels are 4-4.5 m wide (13-15 ft) and 2-2.5 m high (6.5-8 ft).

The last Brazilian ground sloths died (probably eaten by early Brazilians) about 12,000 years ago. They had used their strong claws to dig though soft, semi-cemented sandstone. The various species of giant sloth lived through numerous ice ages, having evolved about 23 million years ago, or perhaps earlier. Great bulk is itself helpful for surviving great cold, but burrowing confers an added advantage.

Biologists and paleontologists in general pay most of their attention to animals that lived above ground. True, finding and recognizing the fossil of an animal that died underground is more difficult. But there is so much going on beneath our feet, and so much of prehistory that took place underground, that we must realize that the livability of our environment is largely a result of these hidden lives. Scientists of all stripes would do well to take note.

Are we the cause of a great extinction being called, by some, the Anthropocene? If we are, it is mainly affecting the critters above ground. If we should extinct ourselves at some point, the "rulers of the underworld" will remain, and may hardly notice much difference. They will continue their ecosystem services as before, keeping a significant percentage of the subsurface a nice place to make a home.