Dinosaurs Rediscovered
Page 13
It has become obvious, after all this effort, that DNA is not a tough molecule. Indeed, chemists identify a spectrum of the toughness of organic molecules, a scale from those that can stand a great deal of pressure and heat through to those that break down at the slightest challenge. During fossilization, most biological tissues break down soon after death; they decay in air or soil or water, and animals may scavenge the flesh, and bacteria break it down further. Only in rare cases is a skeleton not quickly stripped of skin, muscle, and internal organs, and this usually involves covering by water and sediment and the exclusion of oxygen. In such cases, the biomolecules may be buried, but then they have to survive high pressures and temperatures, and most will disappear, or become so modified that they cannot be recognized.
Molecules that survive include lignin, chitin, and melanin. Lignin is a structural molecule that makes up the wood inside trees, and it can survive for hundreds of millions of years. So too can chitin, a carbohydrate that makes up the tough cuticle of arthropods – think of the hard wing casings of a beetle. Finally, as we have seen in Chapter 4, melanin is a pigment that generally gives a black or dark brown colour, and it is found in feathers and hairs (and in dark skin and freckles), as well as in the retina of the eye, the ink of squid, around the liver and spleen, and in the membranes of the brain. It’s thanks to these refractory properties of melanin that the colour of feathers of fossil birds and of dinosaurs could be determined, as we have seen.
Experiments on chitin and melanin have shown how the complex molecular structure changes as pressure and temperature are increased (see overleaf), and indeed the ancient fossil forms show the same detailed chemical characteristics of these experimentally produced samples. As expected, when similar experiments were done with biomolecules such as DNA, it simply broke up completely and nothing recognizable survived the fossilization experiment. Therefore, any organic chemist could have said that if you want to find ancient organic molecules, look for examples of lignin, chitin, and melanin, and not ancient DNA. Yet, there have been persistent reports of the long-term survival of dinosaur blood. In a delicious twist, work published just as this book was going to press showed that my scepticism was partly right, but also partly wrong.
Experiments show how melanin decays, very slowly, under high temperature and pressure; the experimental values are close to those in the fossils.
Can we identify dinosaur soft tissues and blood?
It was a huge disappointment to realize that DNA can’t survive for more than a few thousand years. So, all those reports of DNA from insects, plants, and bacteria that were millions of years old have been rejected. But what if other kinds of proteins might survive in dinosaur fossils, perhaps specific proteins within the bones? Fresh hope came in 1997 with the report of traces of dinosaur blood. A team of researchers from Montana State University, led by Mary Schweitzer, reported that they had extracted proteins and blood compounds from unusually well-preserved bones of Tyrannosaurus rex. If this were true, it would bring us close to the physiology of the dinosaurs – the structure of their haemoglobin might give clues about its oxygen-carrying abilities, and hence whether dinosaurs were warm-blooded or not.
Mary Schweitzer was inspired in her quest for ancient proteins by an exceptionally well-preserved tyrannosaur skeleton. ‘In parts it was almost the same as modern bone, with no mineral infilling,’ she said. A dense outer layer of bone seemed to have stopped water washing in, and so the interior bone was apparently as good as new. Schweitzer identified proteins and possible DNA in these inner zones. She reported the buzz of excitement at the time:
The lab filled with murmurs of amazement, for I had focused on something inside the vessels that none of us had ever noticed before: tiny round objects, translucent red with a dark center. Then a colleague took one look at them and shouted, ‘You’ve got red blood cells. You’ve got red blood cells!’ It was exactly like looking at a slice of modern bone. But, of course, I couldn’t believe it. I said to the lab technician: ‘The bones, after all, are 65 million years old. How could blood cells survive that long?’
The bones with possible blood cells were tested. The bones indeed seemed to contain haem, the oxygen-carrying part of the haemoglobin molecule of the blood. Haem is red, giving blood its red colour, because it is rich in iron, and the red colour appears when it combines with oxygen, a little like the colour change of iron when it rusts. Many other scientists queried these reports, however, and suggested that the iron-rich traces in the bone were nothing to do with blood or blood products, and might just have been iron minerals that grew in the bones long after their burial.
After much criticism, some fair and some probably unfair, Mary Schweitzer and her team published a follow-up paper in Science in 2005, entitled ‘Soft-tissue vessels and cellular preservation in Tyrannosaurus rex’. Her team dissolved away the calcium phosphate of the hard portions of some limb bones, and were left with a residue consisting of narrow vessels which contained round bodies that could be squeezed out. The demineralized bone matrix was fibrous and retained some of its original elasticity – pretty amazing for a 70-million-year-old fossil. In a later study of the same materials, Schweitzer and her colleagues carried out a battery of biochemical tests to show that the elastic fibrous strands were composed of collagen, as in the original bones.
Bones are typically composed of two main materials: mineralized needles of apatite, a form of calcium phosphate, which are embedded in the fibrous protein collagen. It’s this combination of elastic protein and hard mineral that gives live bones their interesting properties of being able to bend (to some extent) and then to snap in a brittle manner. Where the apatite crystals are absent, the collagen forms cartilage, the flexible material that stiffens our ears and nose, and that forms the bulk of the skeleton of a shark.
Soon after, in 2008, Thomas Kaye and colleagues reinterpreted all these fossils as artefacts. They said that the supposed blood vessels were probably bacterial films, and the possible red blood cells were crystals of pyrite, the mineral form of iron sulphide. Mary Schweitzer rejected these criticisms, and her work was seemingly confirmed by further reports by another team in 2015 of collagen and red blood cells from eight Cretaceous dinosaur bones.
However, in a paper published in 2017, Manchester-based Michael Buckley and colleagues showed that the T. rex collagen comprised mainly laboratory contaminants, soil bacteria, and bird-like haemoglobin and collagen. In particular, they found that the supposed dinosaur proteins matched modern ostrich sequences, an easy mistake to make when such modern samples might have been in the same laboratory that worked on the fossil materials. Then came some clarity. In a 2018 paper, Jasmina Wiemann, a PhD student at Yale, led a group that looked again at the blood vessels and other pieces of brownish material left after fossil bones had been processed to remove all mineral components. She applied sophisticated tests and found that the vessels and tissues were real, but not made of original proteins, except perhaps collagen. The others had decayed to alternate forms called N-heterocyclic polymers – so in fact Mary Schweitzer was right that she had found blood vessels, skin cells, and portions of nerve endings, but their proteins had been substantially converted during fossilization.
Collagen might also be preserved, although care must be taken to be sure it is original and not contaminated. Another bone protein, osteocalcin, was reported in 1992 from the bones of two Cretaceous dinosaurs by the Dutch researcher Gerard Muyzer. Osteocalcin is found in the bones of all vertebrates, and it functions like a hormone in stimulating the repair of bones, as well as other physiological functions. Osteocalcin is a tough protein that is bound into the bone minerals very securely, and it’s that relationship that seems to protect it from decay. It is also a relatively small protein, consisting of about fifty component amino acids. Complete osteocalcin molecules from a 55,000-year-old fossil bison were sequenced in 2002. Maybe we can hope that dinosaur osteocalcin will be sequenced some day.
Can we identify the sex of a dinosau
r?
Palaeobiologists have long suggested that some dinosaurs, at least, were sexually dimorphic, meaning that males and females had a different appearance, as we saw in Chapter 4. In the early days, this was suggested for the horned ceratopsians and the crested hadrosaurs. In the Late Cretaceous, these were the dominant herbivores, and in each case, the skeleton is nearly identical among all the species, but the headgear differs. In one famous case, it later turned out that all the males lived in one spot at one time, and all the females, with some differences in their skulls, happened to live at another place and at another time. Collapse of hypothesis!
Sexual dimorphism in dinosaurs has come back to the fore, however, now that we can identify in some detail the colours and patterns of their plumage. It is now accepted that the function of the feathers of many dinosaurs was probably display, and the stripes and crests suggest pre-mating displays by males, as among birds, and hence a key role for sexual selection in the evolution of dinosaurs, as we saw in Chapter 4.
Amazingly, it is possible to sex some dinosaurs based on unequivocal evidence. Most female birds show a specialized kind of bone called medullary bone, which is spongy bone that fills up the medullary cavity – the core – of certain limb bones. In modern birds, it was first noted in 1934 in pigeons, and then observed in sparrows, ducks, and chickens. The medullary bone can be laid down very quickly, and recycled very quickly, acting as a store for calcium, which can be mobilized rapidly when needed to form an eggshell. Later observations have shown that this occurs in all modern birds. Physiological experiments show that it builds up in the core of many bones throughout the skeleton just as the female bird begins to lay down yolk, and then it diminishes as calcium passes into the developing eggshell. Medullary bone development and transfer are cyclical according to the seasons of the year, and they are controlled by oestrogen and other hormones relating to the breeding cycle.
The most amazing fossil: two Confuciusornis on one slab, one female and one male (with long, banner-like tail feathers).
Medullary bone was first noted outside modern birds in Tyrannosaurus by Mary Schweitzer in 2005. Since then, it has been reported from other theropod dinosaurs, the ornithischians Tenontosaurus (see overleaf) and Dysalotosaurus, and the extinct birds Confuciusornis (see overleaf) and Pinguinis. The report on Confuciusornis by Anusuya Chinsamy-Turan and colleagues from the South African Museum in Cape Town was especially telling, as it proved that the medullary bone identified in the fossils occurred in female specimens (see pl. xiii). Among the thousands of specimens of the crow-sized Confuciusornis in Chinese museums, two sexual forms had been identified. One classic specimen shows a male and a female bird on the same slab – the supposed male has single long, banner-like tail feathers, while the supposed female does not. So, as with modern birds, the male has the ridiculous adornments, to show off to the more sensible, drab female what a tough character he is, and so what a good father he will make. Chinsamy-Turan and colleagues identified medullary bone in the inner cavity of a microscopic thin section, spongy bone tissue quite distinct from the regular, more compact bone. The medullary bone was only ever found in females, never in males – although not in all females, since they were not all in reproductive mode when they were killed.
Other cases, though, have been disputed, including the reports of medullary bone in larger dinosaurs, including Tyrannosaurus and Allosaurus. Another explanation of the spongy bone in these large dinosaurs is that it might have been associated with growth spurts. Some of the larger dinosaurs grew really quite fast, and put on hundreds of kilograms of weight in a few months, as we shall see in Chapter 6, and so they needed to capture and mobilize calcium quickly for that purpose. There is no doubt about the reproductive purpose of medullary bone in living birds, and probably also in fossil birds, but maybe it was found only in smaller dinosaurs for which egg-laying was a big effort, as in birds today.
Delving into dinosaur bones to understand their physiology and sexual habits is one thing. How about the opening theme of this chapter – could we ever engineer a living dinosaur?
Genus:
Tenontosaurus
Species:
tilletti
Named by:
John Ostrom, 1970
Age:
Early Cretaceous, 115–108 million years ago
Fossil location:
United States
Classification:
Dinosauria: Ornithischia: Ornithopoda: Iguanodontia
Length:
6.5–8 m (21–26 ft)
Weight:
0.8–1 tonne (1,764–2,205 lbs)
Little-known fact:
The first fossils were found in 1903, but they were not fully understood until complete skeletons were excavated in the 1960s.
Genus:
Confuciusornis
Species:
sanctus
Named by:
Lianhai Hou and colleagues, 1995
Age:
Early Cretaceous, 125 million years ago
Fossil location:
China
Classification:
Dinosauria: Saurischia: Theropoda: Maniraptora: Avialae (birds)
Length:
0.5 m (1¾ ft)
Weight:
0.5 kg (1 lbs 2 oz)
Little-known fact:
This is the best-known fossil bird of all time, with thousands of specimens in museums.
Could we bring dinosaurs back to life by genetic engineering?
Perhaps we will never recover any dinosaur DNA, because this biomolecule is well known to decay rapidly. But what about cloning? We’ve all heard about Dolly the cloned sheep, and there are constant suggestions that scientists could apply such techniques to bring mammoths back to life. Could this work?
Dolly the sheep became a scientific sensation when her birth was announced in 1997. In 1995, a group of scientists at the Roslin Institute, near Edinburgh, were looking for a way to genetically modify farm animals. They cloned two sheep, Megan and Morag, from embryo cells grown for several weeks in the laboratory, but Megan and Morag did not develop very far and they could not be brought to birth. Dolly was born on 5 July 1996, although the world didn’t find out about her until early the following year. She was the first mammal cloned from an adult, rather than embryonic, cell, and her birth – splashed over newspaper front pages – brought the issues surrounding cloning to breakfast tables around the world.
Sadly, Dolly died in February 2003. Did she die young because she was a clone? Was Dolly unnatural, a Frankenstein monster created by crazy scientists in the laboratory? There are many debates about the ethics of cloning. Some people object to the whole idea of genetic engineering, for either religious or political reasons, while others say that scientists should be free to carry out experiments that push back the frontiers of human knowledge. In food production, genetic engineering of plants has been a regular part of agriculture now for decades, and most sweetcorn, and many other grains and pulses you consume, have been genetically engineered to improve the crop yield or nutritional content.
Cloning means literally ‘making a copy’, and the idea of cloning is to find a way to make an egg develop into an adult plant or animal, using its DNA, but without the normal process of the male fertilizing the egg in the female. The steps in cloning in the laboratory are: (1) remove some complete DNA from the cells of the animal or plant you want to clone; (2) remove the nucleus from an egg of the host animal; (3) inject the DNA into the emptied host egg; and (4) grow this cell in the womb of a living mother animal, probably a very close relative. This is how Dolly the sheep was cloned, and nurtured through to adulthood.
As a first step towards cloning an extinct species, biotechnologists have focused on cloning at-risk species. One example is the gaur, a large wild ox species that lives in India and southeast Asia. It is huge, about 2 metres (6½ feet) tall at the shoulder, and weighs not much less than a tonne (2,205 pounds). It used to be common, but its population size has been
reduced to some 36,000 by hunting. So, a biotech company, Advanced Cell Technology (ACT) in Massachusetts, USA, decided to try the experiment of cloning a gaur. They wanted to do something a little different from the case of Dolly the sheep, and to use another species to act as mother. Although they could have transplanted the gaur eggs into a female gaur, they wanted to use this as a test of concept for bringing extinct species back to life. If a species is extinct, there is no living mother, and a female from a related species must be used.
In 2001, the scientists announced the successful birth – and death – of the first endangered animal clone. The baby bull gaur, Noah, died within forty-eight hours. Researchers from ACT said the problem was unlikely to be related to the cloning procedure itself. The clone had been carried by a domestic cow called Bessie. Noah was produced in a cross-species cloning procedure. The genetic material taken from the skin cells of a male gaur that had died eight years previously was fused with the emptied egg cells of common cows. From a total of 692 eggs used in the experiment, only one live clone was produced – Noah. Noah died of a common illness (dysentery) that probably had nothing to do with the fact he was a clone. Bessie, the surrogate, remained fit and well.
The first attempt to bring an extinct species back to life was with the Pyrenean ibex. This mountain goat lived in the Pyrenees until it was hunted to extinction. The last Pyrenean ibex alive was a female named Celia, and she was found dead in the year 2000. Before she died, Spanish biologists captured her and took a tissue sample from her ear. ACT, the cloning company that had produced Noah, then announced that the Spanish government had commissioned them to clone the Pyrenean ibex, bringing it back from extinction.