Digging In

 kw: book reviews, nonfiction, biology, paleontology, burrowing, trace fossils

Are we still cavemen, somewhere deep inside us? Some folks are. The people who live in a certain part of Cappadocia certainly are, if not cave dwellers, certainly burrow dwellers. The soft volcanic stone in the area is easily dug. Several thousand people live in underground, or within-rock, dwellings. Some of these unique burrow-houses, along with churches and other public places, were carved in the rock as long ago as 300 AD.

Does this make humans the largest burrowing animals? Actually, that distinction belongs to grizzly bears, as we read in The Evolution Underground: Burrows, Bunkers, and the Marvelous Subterranean World Beneath Our Feet, by Anthony J. Martin.

Dr. Martin is an ichnologist, a scientist who studies trace fossils: fossilized tracks, trails, and burrows made by animals. His book shows how knowledge of the ways animals have trod on, dug into, and tunneled underground have created the natural environment. For example, a motto in his field is, "Without animals that tunnel and poop, there would be no mud." Geological forces tend to mix clay and silt and organic sludge into larger-grained sediments. Animals that tunnel within those sediments are frequently like earthworms and marine worms, that ingest the "dirt", digest the organic part, and defecate pellets of the remaining mineral bits mixed with mucus, typically onto the surface (look for little piles of pellets near wormholes after a rainstorm). These pellets glomp together into "mud". (Yes, Virginia, dirt is mostly silt and clay mixed with animal poop and poop eaten and re-pooped. Now, don't you want to wear gloves when you garden?)

It is likely that ants plus termites make up more than half of the total mass of all animals. And they are nearly all inveterate tunnellers. How far back did "bioturbation" (the stirring of the sediment by animals) begin? The book has a tentative answer: Around 541 million years ago, or a little before, during the transition from the Ediacaran Period to the Cambrian Period.

The Ediacaran Period, from 635 to 541 million years ago (mya) is named for a region in Australia where these unusual soft-bodies fossils were first found. In this image the scale bars are either 1/2cm (black) or 1cm (white). The best analysis of the environment of these animals, or proto-animals, is of quiet seabeds with a surface composed of bacterial mats, which sometimes humped up into stromatolites, which originated about two billion years earlier. None of these critters had shells or teeth, and they seem to have fed on the waste products of the bacteria and perhaps a little bit on the bacteria themselves. It seems they did not feed on each other; there were no predators yet.

They apparently did not have the wherewithal to dig into or under the bacterial mats. At the very end of this period, transitional animals called the Small, Shelly Fauna (SSF) appeared, and they did begin to dig in. They also seem to have fed on the soft-bodies feast around them, because the "softies" soon vanished.

The SSF quickly gave way to the animals of the Cambrian Period, from 541 to about 485 mya, which were shelled creatures such as the beloved trilobites, but included all modern phyla plus a number of phyla that have gone extinct. Here we see a trilobite and a blastozoan (distant relative of sea stars).

The book has quite a chapter on the trackways left by trilobites, and the confusion that sometimes results when other many-legged creatures leave tracks that look similar at first glance.

Cambrian animals didn't just leave tracks in the bottom. Burrowing as a lifestyle seems to have begun among nearly all phyla during the Cambrian Period.

Why burrow? For some, food is found there (ask any mole or earthworm). Protection and privacy: it is easier to defend eggs and babies when they are in tunnels or bunkers or burrows. Making babies is safer in a burrow also; the blissful couple is less likely to be interrupted. There are actually birds that tunnel to protect their eggs and young.

The creatures that survived the "big five" extinctions were mostly burrowers. This is seen on a small scale in a description that begins Chapter 9, "Viva La Evolución: Change Comes from Within". Pocket gophers that happened to be in their tunnels during the Mount St. Helens eruption of May 18, 1980, found their tunnel mouths buried under loose ash, through which they had to tunnel upwards to attain the new surface of the ground. They did so, in large numbers, all over the area that was devastated and incinerated by the nuée ardent ("glowing cloud") of superheated gas and melted glass that roared off the mountain. 57 humans that were within around a 10 mile radius of the volcano died. Thousands of pocket gophers, including some very much closer to the mountain, were safe in their dens and emerged to repopulate the area with their own mini-population explosion.

Chapter 8, "Rulers of the Underworld", surveys the breadth of kinds of animals that live literally underfoot, from ants to armadillos, and some that are (or were) a bit too big to be literally underfoot, such as the giant ground sloths that left tunnels you can almost drive a car through in parts of South America.

A major theme of the book, found in most chapters, is that the diggers all around us are ecosystem engineers. The gopher tortoise is a superstar of ecosystem engineering. These middlin-sized tortoises tunnel industriously, making spaces not only for themselves, but for about 400 species of animals that get the opportunity to dwell in those spaces, or in side tunnels off of them. A foot-long tortoise makes a one-entrance tunnel 5-15 meters long, going as deep as 3 meters, to an enlarged den. If you were to excavate a well-used tunnel, however, you would find numerous side tunnels made by mice and toads, also by dung beetles and other insects. The tortoises move tons of earth about, and areas with many burrowing animals in general are well-aerated because they are to well-perforated! The constant digging and mixing means we live amidst an extensively re-worked landscape…or, at least, those of us who live outside cities.

Even in my suburban area, a typical shovelful of garden soil contains one or two dozen earthworms (multiply by the thousands of square feet in my yard). There are also the burrows and tunnels of mice, voles, camel crickets, and a dozen species of ant.

It's good to be reminded, or enlightened, regarding the many uses of the underground and the wildlife that inhabits and creates it. A thoroughly enjoyable book.

Pardon me for continuing with a criticism or two; you can stop reading here if you prefer. The points below don't diminish the value or enjoyment of the book.

More and more I find myself wishing authors and publishing houses would make more and better use of copy editors and proofreaders. A spell-checker is only 10% of the task. Some examples:

• On page 142 I found this in a description of the impact of the asteroid that wiped out the dinosaurs 65 million years ago: "The impact…instantly converted its potential energy into kinetic energy…". Hardly! The rock was moving about 30 km/s, and that's all kinetic energy. It was converted, first to thermal energy (melting and evaporating rock and ocean water), and then to more kinetic energy of the "splash stuff", molten rock lobbed halfway around the planet. The copy editor needs to know some physics.
• The word "had" was omitted from a phrase that should have read, "…cobbles of sandstone that had fallen off the slope…" Page 148.
• Faulty math: the statement that ants probably outweigh humans (true), is followed up by "one million ants per person". Hmm. I weigh just under 100 kg, or 100,000 grams. One millionth of my weight is 100 mg. I suspect a 100 mg ant would be a fearsome critter! Large (12mm) carpenter ants weigh 20-30 mg; maybe the colony's queen approaches 50 mg. The average worker ant of all species weighs about 2 mg, so it would take 50 million ants to balance me on the scales. Page 226.
• The author in one place states that a hectare is 100 square meters, but a hectare is actually 100 meters squared, or 10,000 square meters (107,639 sq ft). That is 2.471 acres. I found a few places (p. 235 is one), where the ratio is reversed, indicating that the author (or someone he quoted) calculated 2.5 hectares per acre. That's quite different from both 100 sq m and 10,000 sq m.

To be honest, these complaints total half a page; out of a 400-page book, that isn't bad. I like Dr. Martin's writing.

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.