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An early restoration of Archaeopteryx, from the book 'Darwinism Illustrated'

Now that it is 2010 and the “Darwin Year” is over, we can expect the tide of evolution-themed documentaries and books to ebb. A notable exception, however (if I do say so myself), is my forthcoming book about evolution and the fossil record called Written in Stone. After years of hard work it will finally hit shelves on November 1, 2010. But before I get too carried away with self-promotion, I have to address a question that I have been asked many times during the writing process: “Do we really need another book about paleontology?”

The answer is an emphatic “Yes!” Even though numerous summaries of the overwhelming evidence for evolution have been published in the past year, the discipline of paleontology has often taken a backseat to genetics and microbiology. Fossils might demonstrate the fact of evolution, some authors imply, but to really understand how evolution works we need to look at living systems that we can observe right now. As Richard Dawkins wrote in The Ancestor’s Tale, even if there were no fossil record at all, living organisms still would provide abundant evidence for evolution. Hence, in Dawkins’ view, the fossil record is a non-essential “bonus” that is not truly necessary to understand how life has changed.

But I am not convinced by Dawkins’ argument. During the 19th century, speculations about evolution (or a “natural law” that regulated the origin of species, as it was often referred to at the time) were heavily influenced by fossils. Not only had fossils confirmed that species could become extinct, but they also illustrated that life on Earth was composed of a shifting cast of actors that appeared and disappeared over time. This was particularly significant to Charles Darwin, who began the introduction to On the Origin of Species by explaining how the relationship between the living and fossil mammals of South America got him thinking about the “mystery of mysteries,” evolution.

Even though Darwin deduced the mechanisms of natural and sexual selection from what could be observed among living organisms, the true power of his view of evolution was that all living things were linked together by common ancestry through a natural process that had operated over millions of years. What was observed in the present could be used to make predictions about the shape of the tree of life, and so the findings of paleontology have acted as a test of Darwin’s ideas. At first many paleontologists felt that the fossil record did not accord well with evolution by natural selection, but during the past sixty years scientists have confirmed that Darwin was right about the pattern of life.

This is where many accounts stop, with paleontology acting as a sort of handmaiden to other evolutionary sciences, but there is much more to the story. Geology and comparative anatomy still make up the core of paleontology, but many researchers have been combining these more traditional aspects of the discipline with methods from genetics, microbiology, embryology and other sciences. Paleontologists are no longer just documenting transitions in the fossil record. They are using new approaches to explain how those transitions could have occurred.

Research published within the subdiscipline of dinosaur paleontology during the past year alone illustrates the spread of this synthetic approach. Last June paleontologists published a description of Limusaurus, a strange theropod dinosaur that may help explain a developmental shift in how the hands of dinosaurs and their bird descendants formed. Another team found degraded soft tissue material inside the bones of a different dinosaur, Brachylophosaurus, and the research drew upon microbiology and genetics as much as on traditional paleontology. And, since we know that birds are living dinosaurs, some paleontologists have even considered trying to reverse-engineer a chicken into something dinosaur-like by fiddling with genes the birds still possess. There are many more studies that could be mentioned, but this handful of examples illustrates how paleontology is becoming more of an interdisciplinary science that can provide new insights into how life evolved.

So while I will not dispute Dawkins’ point that we could learn a lot about evolution by studying living creatures alone, I cannot so easily write off the fossil record as just a collection of curiosities. An understanding of the history of life on Earth has always been important to considerations about evolution. The interdisciplinary nature of many new studies has made paleontology more important than ever. No understanding of evolution is complete without a healthy appreciation for the fossil record, and I hope that my forthcoming book will help explain why so much of what we understand about evolution is written in stone.

A restoration of the skull of Shaochilong by paleoartist Brett Booth. The areas in white represent known parts of the skull. From the Zootaxa paper.

Several months ago paleontologists Stephen Brusatte, Roger Benson, Dan Chure, Xu Xing, Corwin Sullivan, and David Hone described the dinosaur Shaochilong, the first representative of the group of large predatory dinosaurs called carcharodontosaurids to be definitively identified from Asia. Now members of the same team have elaborated on their discovery in the journal Zootaxa with a full description of the known bones of the dinosaur. While closely related to some of the giant predators of prehistoric South America, such as Giganotosaurus, Shaochilong was a bit different.

The most obvious difference between Shaochilong and its carcharodontosaurid relatives was that it was much smaller in size. While the bones typically used to estimate body size, such as the femur, have not yet been found for Shaochilong, the authors were able to compare the length of the tooth row in the upper jaw to the same part of the skull in related dinosaurs known from more complete remains. What they found was that Shaochilong, while still a large theropod by any standard, was substantially smaller than Allosaurus and even more diminutive when compared to closer relatives such as Carcharodontosaurus. Shaochilong probably measured only between 15 and 20 feet long, though as the authors point out Shaochilong was still larger than all the tyrannosaurs of its time.

But the greatest significance of Shaochilong is that it represents what the authors call a “dark period in large theropod history.” Between 140 and 120 million years ago, large relatives of Allosaurus were the dominant large predators in the northern hemisphere, but by 83 million years ago they had been supplanted by the tyrannosaurs. The 40 million years between the dominance of the allosauroids and the rise of the tyrannosaurs is still incompletely known, but the presence of Shaochilong and its close relative Chilantaisaurus (which it lived alongside) in Asia about 92 million years ago suggests that the allosauroids remained dominant for longer than has been previously understood. This might be important to understanding the evolution of the tyrannosaurs, a group that evolved much earlier (around 170 million years ago) but stayed small until the late Cretaceous. Might the dominance of the allosauroids have kept the tyrannosaurs small? At present it is impossible to know, but future discoveries of more theropods from the “dark period” might help explain one of the most dramatic turnovers in dinosaur history.

STEPHEN L. BRUSATTE, DANIEL J. CHURE, ROGER B. J. BENSON, XING XU (2010). The osteology of Shaochilong maortuensis, a carcharodontosaurid (Dinosauria: Theropoda) from the Late Cretaceous of Asia Zootaxa, 2334, 1-46

A CT scan of the respiratory pathways inside an alligator. From the Science paper.

On the surface, a pigeon and an alligator could hardly seem more different. While the pigeon is a flying, feather-covered creature that pecks its food with a toothless beak, an alligator is an amphibious, armored predator that crushes its prey in jaws studded with conical teeth. Despite the disparate forms, however, they are joined together by a common ancestry. Both the pigeon and the alligator are archosaurs, the group of “ruling reptiles” that also includes pterosaurs, non-avian dinosaurs, and a host of related forms that died out tens of millions of years ago. The archosaurs that exist today are only a fraction of the different forms that once existed, but a new paper in the journal Science underscores that they share some deep similarities beneath their skin.

As I breathe in and out while writing this post air enters my body via my nose, travels down into the “dead end” of my lungs, and then is exhaled after the oxygen has been absorbed. Not so in birds. Birds have a more efficient respiratory system adapted to unidirectional air flow, or, in other words, air moves in one direction through the bird’s system in a kind of circuit. And, as it turns out, alligators can breathe the same way.

Scientists have known for some time that birds breathe in a different way than mammals do. What has been more difficult to determine is how alligators breathe. There had been some suggestion that alligators might breathe via unidirectional air flow, too, but no one had conclusively illustrated that this was the case. To test the hypothesis, C. G. Farmer and Kent Sanders placed sensors in two parts of the respiratory passageways of four dead alligators, artificially ventilated the lungs, and watched to see how the air moved.

The results suggested that the airflow inside the alligator’s body was capable of moving in a unidirectional manner along a circuit of pathways, but would they actually breathe in such a way while alive? To find out the scientists placed airflow measurement devices in six living alligators. They found that the airflow in the alligator’s respiratory system continued through the transition between inspiration (new air coming in) and expiration (old air going out). Air kept on moving through the system during both phases, again suggesting that the alligators were breathing through unidirectional airflow.

Just how the alligators are able to do this is not yet understood, but the discovery that they can breathe like birds might provide clues to some major events in the history of life on earth. If living alligators and the living theropod dinosaurs we call birds both share this physiological mechanism, then it is possible that the last common ancestor of dinosaurs and alligators was a unidirectional breather, too. If this is correct unidirectional breathing evolved in the first archosaurs over 230 million years ago in the wake of the worst mass extinction in the history of this planet.

The close of the Permian period 251 million years ago was marked by the extinction of up to 96 percent of the animals known to have lived in the seas and over 70 percent of those that lived on land. Among the survivors were the earliest archosaurs (or their close ancestors), and if they had unidirectional breathing it might have given them an advantage. Unidirectional breathing is a more efficient way of obtaining oxygen from the air than the way we breathe, and if the Permian mass extinction was triggered by major changes in the atmosphere as scientists suspect, such as the depletion of oxygen, the archosaurs may have been better able to survive than the early relatives of mammals they lived alongside. This hypothesis requires further evidence, but if correct the beginnings of the archosaur rise to dominance could all have come down to a difference in breathing.

Farmer, C., & Sanders, K. (2010). Unidirectional Airflow in the Lungs of Alligators Science, 327 (5963), 338-340 DOI: 10.1126/science.1180219

The entrance to Dinosaur National Monument. From Flickr user jdurchen.

When I visited Utah’s Dinosaur National Monument this past summer the main visitor center (containing the famous rock wall speckled with fossils) was closed, but when it reopens in a few years it will feature a brand new piece of art. Over the course of three months, scientific illustrator Liz Bradford dug into the prehistory of the monument to produce a restoration of what the area might have looked like when Allosaurus and Stegosaurus roamed the area about 150 million years ago. Even better, Bradford took a photo of her work in progress every hour and strung all the photos together to show how the mural came to life. You can see that video on the Dinosaur National Monument website.

The fossil hunter Charles H. Sternberg. From Wikimedia Commons.

A Story of the Past, or A Romance of Science is a very unusual book. In it readers will find frequent references to Jesus, the American West, fossil mammals, and extinct marine reptiles, often all in the same poem. Who else but one of the greatest fossil hunters who ever lived, Charles H. Sternberg, could have written it?

Sternberg was more of a collector than a scientist, a consummate “bone sharp” whose finds remain museum centerpieces to this day, but in his personal life he was also a deeply religious man. This shines through in A Story of the Past. The collection of poems is dotted with religious odes such as “One Hundred and Seventh Psalm” and “Calvary,” but Sternberg’s epic-length paleo poems were what most immediately grabbed my attention.

While I have seen no indication that Sternberg was a young earth creationist, he did see the work of God in the fossils he collected. They were testaments to divine power, and Sternberg saw it as his job to catalog the part of creation documented only by fossils. In the opening poem, “A Story of the Past,” Sternberg wrote:

I’ve found the crust of our old earth
A mighty funeral urn
Where countless forms of life had birth;
Then others took their turn

And left in sepulchers of stone
The dead He buried there.
But they are not dry bones alone;
I see them as they were

Indeed, Sternberg is at his best when he envisions ancient landscapes in which fossil bones come to life. In his opening poem Sternberg focuses on the plesiosaurs and mosasaurs that haunted the Western Interior Seaway in the Cretaceous of North America, while he describes extinct mammals as he sails down the “tide of time” in his work “The Permian Beds of Texas.” One of my favorite passages, though, comes from “In the Laramie,” in which Sternberg describes the discovery of the famous “Trachodon mummy” he made with his sons in verse:

The glory of this specimen—
He lies there as he floated in
With bloated body on the wave.
The gas escapes he found his grave,
As he sinks to his long rest,
Skin clinging fast to bone and breast.

Sure, it’s not Shakespeare, but Sternberg’s poetry does have a quaint charm about it. Nor was he the only paleontologist to describe his thoughts about the fossil record in verse. Almost a century earlier the Amherst geologist and theologian Edward Hitchcock penned an ode to the “sandstone birds” represented by the tracks he found around the Connecticut Valley (which turned out to have been made by dinosaurs). I wonder how many other naturalists wrote paleo poetry.

[Hat-tip to Andy Farke for bringing this book to my attention.]