The vast majority of vertebrate fossils found by paleontologists have been of the hardest parts of animals—bone, teeth, horn. Impressions in rock have been found of muscle, skin, and internal organs. The preservation has sometimes been remarkable.
One example was reported in 1998 in
Nature
. This came after Mary’s work on the apparent red blood cells in MOR 555, but it is worth noting because of how remarkable the preservation was. This was a small theropod dinosaur, found in lower Cretaceous sediments in southern Italy. It was, in fact, reported in 1993 as the first dinosaur ever found in Italy.
The skeleton, classified as
Scipionyx samniticus,
is less than ten inches long from the tip of its nose to the end of its tail, or in scientific language, “from the tip of premaxilla to the last (ninth) preserved caudal vertebra.” The skeleton seems almost complete and the internal organs, in particular the intestines, are completely visible. Cristiano Dal Sasso and Marco Signore, who described the intestine and what may have been a liver in their 1998 report in
Nature
, concluded that periods of low oxygen in a limestone deposit in a lagoon resulted in the preservation of the internal organs.
The image of this fossil, in the dry environment of a scientific journal, is still quite moving, perhaps because of its small size, or perhaps because of how complete and exquisitely detailed the preservation is. Sometimes it is tempting, given the traditional museum reconstructions, to think of dinosaurs as skeletons, not full-fleshed creatures. But this fossil skeleton, of a creature the size of a small lizard but so clearly a dinosaur, with its internal organs still visible, is a vivid, whole creature. Not living, certainly, but so fully present that it is hard to grasp how many tens of millions of years it had been in limestone before it was recovered.
Of course, there are many kinds of fossils. There are impressions in rock of plants, which, are, of course, soft. And there are microfossils, traces of microscopic life in rock. There are claims of fossil evidence of life dating back to three and a half billion years ago, although they are not completely accepted. These are impressions in rock, not the tissue of the cells.
And there are coprolites. One intriguing study was led by Karen Chin, of the University of Colorado. This also came after Mary’s work on the red blood cells, and in fact Mary did some of the identification of the fossil tissue. Karen was working on a coprolite—a chunk of fossilized dung—apparently from a tyrannosaur. Coprolites are not common. And this one was unusual because it appeared to contain undigested muscle tissue from whatever the carnivorous dinosaur had been eating.
That was a true rarity. In the introduction to her paper on the coprolites, Chin referred to just over a dozen examples of fossils of muscle or skin or other tissue preserved well enough that its microscopic structure could be clearly seen. As for muscle tissue in coprolites, before Chin’s discovery only two early papers, in 1903 and 1935, had reported such a find, without photographs to document it.
The coprolite was found in rock dating to the Cretaceous in Alberta, Canada. It was lying on the surface, and had been for some time, in its fossilized, rock form, since some lichen had begun to grow on it. But it was immediately recognizable. It was about two feet long and six inches wide—about a gallon and a half of dinosaur dung. And judging from an uneven surface on the underside, it appeared to have been “deposited in viscous state on uneven terrain.” Usually, the language of scientific papers is so abstract that you have no idea what the authors are talking about. This description was a vivid exception.
As I said, this work came after Mary’s report on the fossilized red-blood-cell remnants. They were not part of the context in which she was working, but they suggest the predominance of bone fossils, and the excitement about finding something else. Before her work, apparent fossilized red blood cells had also been reported, but rarely. One find was in two-thousand-year-old human bone. In 1939 evidence of red blood cell remnants was reported in a lizard tens of millions of years old. If anyone had found remnants of red blood cells in a dinosaur bone, I didn’t know about it then, and still don’t. I should point out, also, that calling the fossils red blood cells is a shorthand way of speaking that may be a bit misleading, just like calling a fossilized dinosaur femur a bone. It is not a bone in the sense that a femur of a recently dead cow is a bone. Minerals in the dinosaur bones have been replaced, chemical changes have occurred. What Mary was seeing, would, if they were real, be remnants of red blood cells with some of the original chemicals remaining, and some of the structure, but with other parts changed forever. Unlike sea monkeys, they could not be reconstituted by adding water.
This does not lessen how extraordinary it is to find any preservation of red blood cells. There are, after all, obvious reasons why, in animals, the hard parts are the ones that survive. Flesh rots. It is eaten by animals large and small. Any creature left on the surface is quickly dressed down to a skeleton. Even when a dead animal is buried, insects, worms, microbes, and chemical disintegration usually leave nothing but bone. Despite occasional reports of cell-like structures in some fossils, the idea of doing a dissertation announcing the finding of red blood cells was shocking.
In the paper based on her dissertation, which Mary published with me as one of the coauthors (a common role for dissertation advisors or supervising scientists), Mary eventually claimed only that there were heme compounds in the fossils, parts of hemoglobin molecules, indicating that the full hemoglobin molecules had been, or were still, in the bone. Hemoglobin is a protein that enables red blood cells to bring oxygen to muscles. More hemoglobin is what bicyclists competing in the Tour de France are after when they use illegal so-called blood doping techniques.
Part of the reason for concentrating on hemoglobin and byproducts is that the presence of chemicals can be tested with established procedures. It would support the idea that what we were seeing were remnants of red blood cells, but did not require determining how much of cell structure had survived and what the degree of fossilization was. Fragments of hemoglobin molecules had been found before in bone a few thousand years old, and blood residues on stone tools up to a hundred thousand years old.
CHEMICAL TRACES
There were other discoveries that made it seem reasonable to look for preserved protein molecules, like hemoglobin, in truly old bone. Since the 1970s sequences of amino acids—which are strung together to make a protein molecule—had been found in mollusk shells that were 80 million years old, and in dinosaur fossils from 150 million years ago. Recent work on the biochemistry of fossils had led to discoveries of a bone protein, osteocalcin, in dinosaur bone.
It was necessary to marshal a variety of techniques that had seen little use in paleontology, such as liquid chromatography and nuclear magnetic resonance, to test for hemoglobin. After finding the chemical and physical signatures of parts of hemoglobin in the bone, but not in the sandstone surrounding it, Mary sought biological evidence and sent another lab extracts of fossil bone material. Rats injected with the extract made antibodies—against avian hemoglobin. That is to say, their immune systems recognized something foreign and brought forth weapons—antibodies—specifically tailored to meet and disable the new invader. Using antibodies for testing is a common practice, so it was possible to determine that these antibodies would also work against hemoglobin from birds—not mammals or reptiles, but birds. This was completely consistent with the dinosaur bone’s containing elements of hemoglobin from dinosaurs, and ruled out contamination by humans or other mammals either in the lab or before the bone was collected.
So it seemed that hemoglobin or products from the breakdown of hemoglobin were still present after sixty-eight million years.
The evidence pointed to ancient molecules from red blood cells that had been preserved. Were the structures that she had observed fossilized red blood cells? This was the best explanation we could arrive at. But it was presented as a tentative conclusion, open to any challenge that other scientists could think of. Essentially, there was a lot of evidence that was consistent with the idea that the structures observed under the microscope were red blood cells, but not enough for a definitive assertion that these structures were fossilized cells.
“I couldn’t disprove it,” Mary said. “I couldn’t prove it. At this point in time I still don’t know what those things are, and might never know. But the research did get me into the mind-set of thinking of fossils as something other than fossils. I don’t treat fossils like fossils; I treat them as I would modern bone.”
The reaction was quite strong. Unfortunately, the biggest outpouring of interest was from creationists. They absolutely loved the idea that the bones had some remnant of red blood cells.
They argued that since we had thought that such things couldn’t be preserved and now had found they were preserved to some extent, that meant our dating was wrong. They ignored the accumulated evidence of geology, radiometric dating, and numerous other facts that made clear that what we were wrong about was not how old the fossils were, but the possibilities for preserving soft tissue.
In retrospect, Mary, who took the brunt of the attack, points out that science was in part to blame for its acceptance of the conventional wisdom that no biological materials like hemoglobin or red blood cells could survive as fossils. We didn’t know what we thought we knew. That’s common enough in science, which is not a collection of answers but a process of posing questions and then coming up with more questions. Knowledge is always provisional. It is not that previous answers are overturned so much as that they prove to be incomplete or not so widely applicable as they might have seemed.
The knowledge about how flesh and bone disintegrate was, indeed, gathered by observation and experiment. We all know what happens to an animal body left out in a field or on the road. We can see dried-out skulls in the desert, crumbling deer bone in the forest. Everything dies and everything falls apart and decomposes. Scientists have tried to quantify this process by setting out the body of a dead animal and carefully tracking its decomposition.
“We’ve got body farms,” says Schweitzer, “where we know how tissues degrade. We know how long it takes under different environmental conditions.” Laboratory research has tracked the same processes at the cellular level. “We know how long it takes for membranes to break down. We know how long it takes for the nucleus to go away. We even know the cellular kinetics. We know the enzymes involved. We know how they interact with one another. We know how cells degrade. We know how proteins degrade. We know how tissues degrade. We know it. Well, they know it, I’m not that smart.”
But these studies are all done on muscle, skin, and other soft organs. Not bone. “Nobody in their right mind works with bone because bone sucks. It’s really hard to work with.” So, says Schweitzer, the models of how things fall apart, from the large scale to the small, are not based on bone. “They are not based on the microenvironments inside bone. And when you put a cell or a tissue inside a mineral, you change everything. You change the ability of enzymes to attack, you change the ability of microbes to get in and eat. . . . And nobody looks at it because bone is the pits to work with. But since bone is all we have from dinosaurs, I look.”
The gender question would be the research Mary and Jen undertook first. What Mary saw was a specific kind of bone that is known to be created in birds as they are producing calcium-rich eggshell. Because of the close relationship between birds and dinosaurs, paleontologists had predicted that this tissue would be found in dinosaurs as well, but no one had yet seen it. Mary and Jen compared the B. rex bone to ostrich bone, again using the scanning electron microscope as well as the light microscope. The bone was clearly the same and the conclusion was clear.
The microstructure of the fossil was the same as that of medullary bone, which is very rapidly deposited. “It has tons and tons of blood vessels,” Schweitzer said. “One of the things that’s standard for modern bone studies, when you want to get at the architecture, the microstructure of the bone at the level of the protein, you want to remove the mineral. So I told Jen, ‘We want to etch the bone, but don’t leave it in there very long.’ Like everyone else, I thought that if you take away all the mineral from dinosaur bone, you would have nothing left, because of course the organic molecules don’t preserve.” Etching means to put it in an acid bath. So the bone only needed to take a brief dip to clean it up.
SPROING!
“When she went to stop the etch, she went to pick up the bone and put it in the water, it went
sproing!
” This was not a large piece of bone. Jen was picking a small piece out of the acid bath with very fine tweezers under a dissecting microscope. Schweitzer went to see for herself.
“It bent, it twisted, it folded,” she said. “It was the most bizarre thing I’d ever seen. I said, ‘I don’t believe this is happening—do it again, please.’ She did it on the second piece and it went
sproing
.” What the material seemed like was collagen, but to identify a specific protein like collagen required gas chromatography, mass spectrometry, and biological assays as well. That was a research project in its own right. So she set that suspicion aside for the moment and looked to see what she could find in another piece of bone, not the reproductive tissue that made B. rex unique, but the cortical bone all dinosaurs and all four-limbed vertebrates have. Jen set up demineralization baths for the new samples and checked their progress.
Under the dissecting microscope the demineralized material, washed in distilled water, had what looked like fragments of tubing, very, very small tubing. And if she picked up a piece of this material she could see it move back and forth under the microscope. It was flexible, some kind of flexible, transparent tubing from sixty-eight million years ago.