Life of the Past

Ancient Sea Life of North America

John Foster

• Publisher: Indiana University Press
• Year: 2014
• ISBN:
  9780253011886
• Topic: Ciencias físicas
• Subtopic: Geología y ciencias de la Tierra

This volume, aimed at the general reader, presents life and times of the amazing animals that inhabited Earth more than 500 million years ago. The Cambrian Period was a critical time in Earth's history. During this immense span of time nearly every modern group of animals appeared. Although life had been around for more than 2 million millennia, Cambrian rocks preserve the record of the first appearance of complex animals with eyes, protective skeletons, antennae, and complex ecologies. Grazing, predation, and multi-tiered ecosystems with animals living in, on, or above the sea floor became common. The cascade of interaction led to an ever-increasing diversification of animal body types. By the end of the period, the ancestors of sponges, corals, jellyfish, worms, mollusks, brachiopods, arthropods, echinoderms, and vertebrates were all in place. The evidence of this Cambrian "explosion" is preserved in rocks all over the world, including North America, where the seemingly strange animals of the period are preserved in exquisite detail in deposits such as the Burgess Shale in British Columbia. Cambrian Ocean World tells the story of what is, for us, the most important period in our planet's long history.

1.1. Adam Sedgwick, the man who named the Cambrian period, in an 1867 photograph by William Farren. Sedgwick was 82 at the time and had named the Cambrian 32 years earlier.

Courtesy of the Sedgwick Museum of Earth Sciences, University of Cambridge. Reproduced with permission.

1

Natural Mystic:
An Introduction to the Cambrian

IMAGINE A TROPICAL MORNING ON THE OCEAN. THE AIR IS comfortably warm and moist, but it is not muggy or hot; scattered clouds are slightly pink with the last colors of dawn as the sun glares orange, low on the eastern horizon. The ocean on which you are floating is deep blue, and the surface waves are only a few feet high. From all appearances, it could be offshore Hawaii. Pitching lightly on a large, inflatable dinghy, we prepare technical diving gear and extra tanks and notice that there is no land in sight; we are probably at least several tens of miles from the nearest land beyond the horizon to the south, but how far we can’t tell. We are going deep, and this type of diving requires special training, equipment, and experience. As the sun climbs in the sky we notice that the sea surface color transitions to a lighter blue away to the south and west.

Ducking under the waves in our diving gear we see a world of medium blue all around and below us. There are no fish in sight. We aim toward the deep and begin kicking our way down into the azure world below. As the pressure increases to nearly 100 pounds per square inch on our bodies and puts pressure on our lungs and ribs, the water around us is turning darker and darker blue. The sea around us now is midnight blue and we can barely see; we have reached the edge of available light at nearly 91 m (300 ft.) down. Knowing that we are near the bottom of the ocean in this area we pull out our dive lamp and turn it on. The bottom soon comes into view as a flat, muddy plain, and suddenly we see movement as indistinct animals shrink into burrows in the bottom sediments. Hovering 2 m (6 ft.) above the seafloor and watching the scene below us, we notice an inch-long, segmented arthropod moving slowly like a sowbug across the bottom muds. There is a scattering of tube- and cup-shaped red and purple sponges; a few tufts of green algae; and one or two short, orange, stalked organisms that look a little like spindly flowers. Suddenly a few feet below us a silvery shape flutters into our lamplight, startling us and reminding us that this ocean is an exotically distant one. The animal is about 45 cm (1.5 ft.) long and is shaped like a segmented halibut. A rounded head contains stalked eyes, followed posteriorly by multiple flap-edged segments that ripple consecutively in waves along each side as on a hovering cuttlefish, and the tail consists of pairs of elongate blades reaching up and out from a central segment. The animal moves along smoothly, thanks to the waving of the lateral flaps. Most strangely, the animal has two spined appendages protruding down from under the head. This odd inhabitant of the deep disappears out of view of our light and we are left looking at each other is amazement. What was that? Moving slowly over the bottom we also notice in the water column, just below us, small shrimp-like arthropods with nearly translucent double-valved shells over their backs. Eventually our dive watches indicate our bottom time is almost over and it is time to begin the slow process of staged ascent to the surface.

During our dive, our ocean world consisted of animals familiar and foreign, but there was a very good reason that we saw no fish beneath the waves nor birds above them. As much as we can see strange things while diving the deep ocean even today, our bizarre destination on this trip was that of the Cambrian ocean world of offshore Utah about 505 million years ago. We saw an area that was probably 113 km (70 mi.) offshore at the time, in 91 m (300 ft.) of water, and whose bottom muds became a relatively thin unit of shale exposed now in the desert of the Great Basin, one that yields fossils of trilobites and other animals by the thousands. The world we visited in our imaginations is not only based on real evidence, but it is also important to remember that this ocean world was a very real place, one that really existed at a particular time and in a particular place. Although we can see that world only in mental imaginings now, its tactile reality was once every bit as solid as the fossils and rocks that record its existence today. It was the Earth’s reality at a time that proved to be critical to our planet’s evolutionary history. The Cambrian ocean world was one of the most fascinating in Earth history, and it has a tremendous story to tell. In many ways, it made the world of today.

Indeed, one might argue that the modern biological world began nearly 540 million years ago. The story we are beginning here paints a picture of the birth of that world in a journey through the 54 million years of the Cambrian period, illuminating the creatures and environments that populated such a wondrous time and set the stage for most of the lineages of modern biology. One could argue any number of beginnings of our current biota, from just a few tens of thousands of years ago back to the time before the Cambrian – it all depends on one’s definition of “modern.” Some animals around during the Cambrian resemble modern species, but many do not, and certainly by my claim in the first sentence of this paragraph I would have to argue not only that today’s biological world began during the Cambrian but by extension so did that of the Carboniferous or Triassic periods, for example. I do not claim that species and ecosystems were the same – only that nearly all the major animal groups we know today (and which were present for much of relatively recent Earth history) originated or diversified into recognizable forms during the Cambrian period. The Cambrian set the cast of characters in place – act one, you might say. This appearance of animals, particularly its rapidity, is the phenomenon you have likely heard of as the Cambrian “explosion.” It is the nature of that “explosion” that has intrigued, mystified, and lured paleontologists for generations.

Life was not new at this time. It had been around for a while, and so had the Earth. What was new was that suddenly, just before the Cambrian, and after untold millennia of life on the planet consisting of microorganisms, algae, and not much more, animals appeared and during the Cambrian diversified in form and function, setting the stage for the modern biological world and leaving the previous six-sevenths of Earth history in the metaphorical dust.

The Cambrian period (and also to some degree – as we have been discovering in recent decades – the period right before it, the Ediacaran) was a time of outright biological revolution, the likes of which had never been seen before and haven’t been since (or at least it hadn’t been seen before except perhaps since the origin of life itself). No extinction and recovery event since the Cambrian has resulted in the kind of biotic expansion and ecological and morphological invention that occurred during the Ediacaran–Cambrian times – not the recovery from the end-Cretaceous extinction, not that after the Permian–Triassic boundary. The origins of animal groups during the Ediacaran and Cambrian and the explosive diversification of these groups throughout the Cambrian turned the Precambrian world on its head and set the stage for everything we know today. But as an introduction, let’s take a look now just at the Cambrian world itself – and what a world it was!

Next Stop, Mojave

Nelson Horatio Darton started walking from the Santa Fe train tracks at Siam Siding in the hot and very dry Mojave Desert of California; he had nearly a mile of flat alluvial sand to cross, and wound his way through creosote bushes the entire time. What lay ahead of him was a ridge several hundred feet high known at the time as Iron Mountain. He found there rock types that indicated they had been laid down in shallow seas, but – better yet – he found fossils that could indicate the rocks’ age.

Darton was the son of a civil engineer who had helped construct the Civil War ironclad Monitor. Because Darton rather enthusiastically took to the math and science his father taught him beyond his class work, he quickly tired of school and by the age of 13 had become a chemical apprentice. Through collecting minerals he became interested in geology and eventually ended up as an essentially self-taught geologist working for the United States Geological Survey, along with the likes of John Wesley Powell and Charles D. Walcott. Darton was an excellent mapper of geologic formations and structures, and he spent years in the field mapping and naming rocks all over the United States – but particularly in the Rocky Mountains.

Darton was in his early 40s when he came through the Mojave Desert of southeastern California on the train, riding a line that cut across the heart of the desert between Needles and Barstow. He studied the rocks at Iron Mountain, measured their thickness, and collected some fossils – but had, as he said, limited opportunities for a more detailed study of the site. Darton showed the fossils to Walcott, the director of the U.S. Geological Survey, who identified them as probably Middle Cambrian in age. Darton had discovered a previously unrecognized outcrop of Cambrian rocks in the Mojave and published a short note on the site in 1907.¹

What Darton probably could not have predicted was that he had first recognized Cambrian outcrops that would ultimately yield thousands and thousands of fossils of Cambrian animals of various kinds and that the locality, now known as the Marble Mountains, would eventually be known as one of a handful of sites found to preserve the less well known elements of Cambrian faunas, species without hard skeletons. Sites that preserve rare species, or soft tissues, or animals in great abundance are particularly prized in paleontology. Although he didn’t realize it, Darton had found in these Cambrian rocks a special type of fossil locality known as a lagerstätte, and his site would be worked by paleontologists, geologists, amateur collectors, students, and folks on vacation for now a hundred years running.

Of course, Darton’s Marble Mountains site was not the first identification of Cambrian rocks in the region, as he himself acknowledged in his paper. And Darton’s site was neither the first nor nearly the best of the Cambrian lagerstätten that have been found. Walcott and others had worked in the area earlier and found Cambrian rocks to the north near Death Valley, near Las Vegas (one can only imagine what Las Vegas was like in 1907), and of course along the length of the Grand Canyon. But Darton had shown that Cambrian rocks and important fossils could be found even in a tiny range of hills in the middle of the forgotten center of the dry Mojave, where most of the surrounding ranges consist of mangled igneous rocks of much younger age and where the intervening valleys are nothing but sand and gravel. Darton’s little piece of the Cambrian is not a Rosetta Stone of Cambrian paleontology, but it is yet another piece of the puzzle that many paleontologists have been working on putting together for a long time. And still we struggle.

First Steps: The Early Stratigraphers

So, how did we end up with the Cambrian as a period and a concept in the first place? The story begins more than 300 years ago with early “geologists” (most were hobbyists – it wasn’t really a job in the eighteenth century) trying to make sense of the rock record they saw around them. No one could fail to notice the layering of the sedimentary rocks seen in various countries, but most believed in the permanence of rocks. They had always been there in the shapes and form we now see. It was a Danish physician and priest named Niels Stensen (sometimes spelled Steensen, also known by the Latinized “Steno”) who realized that the presence of fossils in rocks suggested that the particles of such rocks had once been soft and had hardened around the fossil. The origin of sedimentary rocks included a loose, soft stage of deposition and a later hardening of the matrix. With this came the realization that tilted sedimentary rocks had not only been hardened but also had been bent upward out of their original orientation. Based on these observations Stensen elucidated three absolutely cornerstone concepts for the understanding of sedimentary rocks (and of extrusive igneous rocks): (1) such rocks were originally laid down as flat-lying, horizontal beds; (2) the beds were originally continuous up to the point that they were bound by a valley or basin wall, for example (i.e., discontinuities in a bed not caused by basin edges were likely due to subsequent erosion); and (3) unless overturned or otherwise deformed by faults or folding, overlying beds are younger than underlying beds (i.e., beds pile up one at a time starting at the bottom). Thanks to Stensen’s observations, all who work in sedimentary rocks since his time instinctively think in order from bottom to top. With these three concepts in mind one can begin to understand the sedimentary rocks in the landscape.

One of the first of the early hobby geologists to recognize the significance of the structure of the layering and some of its complexities was James Hutton, an English farmer with lots of time for walking the hills and wondering about the rocks he saw. What he did see helped him realize that the only way to understand the past processes, including the formation of rocks, was to assume that processes we observe today also operated in ancient times and likely factored in to the origins of what he was seeing. This is the concept of uniformitarianism, and Hutton was among the first to use it to interpret rock formation. Previously, those interested in geology had mostly assumed that most rocks were in fact made by unusual processes such as large floods and sea level fluctuations. Among the phenomena Hutton observed that suggested to him the cyclicity of rock formation and thus the antiquity of the Earth was the angular unconformity. In such a situation one sees underlying beds standing on end and cut off on a horizontal surface, on which is lying another set of overlying beds. The angle is that between the sets of beds and the unconformity is the separating surface between them, which often represents a significant period of time. Using Stensen’s three laws, Hutton was able to determine that at these angular unconformities he saw ancient sediments that had piled up in layers, been solidified into rock, uplifted to a high angle, and eroded off, at which point sediments piled up again in layer after layer and solidified into rock also. In order for Hutton to see the angular unconformity at Siccar Point on the Scottish coast, then, the entire package had to again be uplifted and eroded a second time. What Hutton eventually wrote about – and what it took a long time for his contemporaries to accept – was that the erosion of rock, formation into transported sediments, their deposition, their solidification into rock, their uplift, and their erosion, is a cycle that has repeated itself over and over throughout all of Earth history, and that understanding this is a key to both the processes of sedimentary geology and the antiquity of the Earth.

It is the fossils, as we will see, that are the key to defining all geologic periods, including the Cambrian, and the geologist who laid the foundation for the modern understanding of biostratigraphy was a man who actually made his living working on rocks. Biostratigraphy involves using the fossils in rocks to tie together in time, or correlate, layers of rock that are separated by some distance, and William Smith was the engineer/surveyor who first convinced the English geological community that each layer of rock had its own distinctive group of fossils that allowed him to do just that. Robert Hooke had suggested that this might be possible years earlier, but Hooke studied fossils in detail, often with a microscope, and didn’t have Smith’s depth of practical field experience, so it was not until Smith put the idea into practice that Hooke was proven correct. Smith worked on many construction projects across the countryside that involved digging into fresh rock, and he noticed that the groups of animals found in the lowest layers were quite different from those of the upper layers. Stensen’s law told Smith that this was not random. The rocks were ordered oldest to youngest from bottom to top. Unlike the rock types, which often repeated themselves in a stack of rocks, the fossil fa...

Table of contents

• Cover
• Half title
• Title
• Coyright
• Dedication
• Contents
• · Acknowledgments
• · List Of Abbreviations
• 1 Natural Mystic: An Introduction to the Cambrian
• 2 Into the Heart: Cambrian Geology
• 3 A Long Strange Trip: The First 4000 Million Years of Earth History
• 4 Welcome to the Boomtown: The Early Cambrian Seas
• 5 On Top of the World: The Middle Cambrian Begins
• 6 Magical Mystery Tour: The Biological Psychedelia of the Burgess Shale
• 7 Glory Days: The Later Middle Cambrian
• 8 Taking Off: The Late Cambrian
• 9 Home by the Sea: A Closer Look
• 10 On and On: Legacy of the Explosion
• · Glossary
• · Notes
• · References
• · Index