At Argentina’s Sanagasta Geological Park, there is a volcanic nursery for giants. It’s a site that is strewn with the fossilised eggs of giant dinosaurs – sauropods. Each of their 80 or so egg clusters sits next to a geyser, a hot vent or other volcanically heated sites. This is no coincidence – eggs need moisture and heat to incubate properly and big eggs are particularly demanding. These dinosaurs were using the planet to keep their babies warm.
Argentina is a haven for any palaeontologist looking for dinosaur eggs. Different provinces have yielded several large nesting sites. Most belonged to the giant sauropods and some even contain eggs with fossilised embryos inside. The sites have told us much about how dinosaurs looked after their young and even what ate baby dinosaurs but until now, scientists have largely ignored the question of why these particular sites were such inviting locations for expectant dinosaurs.
Even extinction and the passing of millennia are no barriers to clever geneticists. In the past few years, scientists have managed to sequence the complete genome of a prehistoric human and produced “first drafts” of the mammoth and Neanderthal genomes. More controversially, some groups have even recovered DNA from dinosaurs. Now, a variety of extinct birds join the ancient DNA club including the largest that ever lived – Aepyornis, the elephant bird.
In a first for palaeontology, Charlotte Oskam from Murdoch University, Perth, extracted DNA from 18 fossil eggshells, either directly excavated or taken from museum collections. Some came from long-deceased members of living species including the emu, an owl and a duck. Others belonged to extinct species including Madagascar’s 3-metre tall elephant bird and the giants moas of New Zealand. A few of these specimens are just a few centuries old, but the oldest came from an emu that lived 19,000 years ago.
It turns out that bird eggshells are an excellent source of ancient DNA. They’re made of a protein matrix that is loaded with DNA and surrounded by crystals of calcium carbonate. The structure shelters the DNA and acts as a barrier to oxygen and water, two of the major contributors to DNA damage. Eggshells also stop microbes from growing and it seems that ancient ones still do the same. Oskam found that the fossil shells had around 125 times less bacterial DNA than bones of the same species did.
This is important – bacteria are a major problem for attempts to extract ancient DNA and they force scientists to search for uncontaminated sources, like frozen hair. Eggshells, it seems, provide similarly bacteria-free samples. Still, Oskam’s team took every precaution to prevent contamination. They used clean rooms and many control samples. Many of their sequences, like those of Aepyornis, were checked by two independent laboratories.
The Aepyornis sequences are particularly encouraging because many scientists have previously tried to extract DNA from the bones of this giant and failed. Eggshells seem like a more promising source and it certainly helps that the eggs of many of these giant species were massive and thick. But Oskam did also recover DNA from a fossil duck egg, which suggests that it should be possible to sequence the genes of even small extinct birds, like the dodo.
This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.
Getting excited when fish produce sperm would usually get you strange looks. But for Tomoyuki Okutsu and colleagues at the Tokyo University of Marine Science and Technology, it’s all part of a day’s work. They are trying to use one species of fish as surrogate parents for another, a technique that could help to preserve species that are headed for extinction.
Okutsu works on salmonids, a group of fish that includes salmon and trout. Many members of this tasty clan have suffered greatly from over-fishing in the last few decades, and their populations are dwindling their way to extinction.
If stocks fall below a critical level, they may need a jump-start. One strategy is to freeze some eggs to be fertilised artificially, in the way that many human eggs are in fertility clinics. But it’s much harder for fish eggs – they are large and have lots of fat, which makes them difficult to freeze effectively.
Okutsu’s group have hit on a more effective solution. They use transplanted sexual stem cells to turn another species of fish into surrogate parents for the endangered ones.
For humans, sex is a simple matter of chromosomes: two Xs and we become female; one X and a Y and we develop into males. But things aren’t so straightforward for many lizards – many studies have found that the temperature of the nest also has a say, even overriding the influence of the chromosomes. But the full story of how the lizard got its sex is even more complicated. For at least one species, the size of its egg also plays a role, with larger eggs producing females, and smaller ones yielding males.
The discovery comes from Richard Shine’s group at the University of Sydney. In earlier work, they showed that if the Eastern three-lined skink (Bassiana duperreyi) incubates its eggs at low nest temperatures, XX carriers develop into males regardless of their chromosomes.
Now, Rajkumar Radder, a former member of Shine’s team, has shown that the amount of yolk also determines the sex of a skink, but only at low temperatures. By deliberately adding and removing yolk from eggs using a syringe, he managed to alter the sex of the hatchlings. This degree of complexity is totally unprecedented – it means that for the skink, sex is a question of its chromosomes, the temperature it was reared under and the amount of yolk it had.
In 1995, a palaeontologist called Mark Norrell reported an amazing discovery – the fossilised remains of a dinosaur called Troodon, sitting on top of a large clutch of eggs. The fossil was so well-preserved and its posture so unmistakeable that it provided strong proof that some dinosaurs incubated their eggs just as modern birds do. And since then, two other small predatory species – Oviraptor and Citipati – have been found in brooding positions on top of egg clutches.
But a subtler look at these fossils reveal much more about dinosaur parenting than the simple fact that it existed. To David Varricchio from Montana State University, they also tell us which parent took more responsibility for the young. Based on the size of the egg clutches and the bones of the parent, Varricchio thinks that it was the males that cared for the babies. And given that small, predatory dinosaurs were the ancestors of modern birds, fatherly care was probably also the norm for the earliest members of our feathered friends.
Of all the back-boned animal groups, none show a greater equality of parental care that the birds. Among mammals, the next generation is mainly the mother’s responsibility and fathers help out in less than 5% of species. By comparison, male birds help to care for eggs and chicks in over 90% of living species. But Varricchio (together with Norrell and others) argues that this joint parenting is not how the dynasty started off.
The team noted that the clutches so delicately incubated by Troodon, Oviraptor and Citipati contained a substantial number of eggs, about 22 to 30 eggs apiece. Compared to most of the 433 living birds and crocodilians whose clutch sizes have been studied, the dinosaurs were sitting on far more eggs than animals of their size normally do. The team found that species where both parents chip in, or where mum takes the lead, usually settle for smaller clutches. Only those where dad does almost all of the work tend to rear such large broods.