Editorial: 鈥�Dinosaurs help dispel male existential fears鈥�
GIGANTIC dinosaurs may have disappeared 65 million years ago, but their sex chromosomes live on in birds 鈥� providing the best evidence yet that the famously small Y chromosome carried by men is not doomed to disappear within a few million years.
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The rosy long-term prognosis for the Y chromosome was one of the more unusual studies presented at the Society of Vertebrate Paleontology鈥檚 annual meeting in Raleigh, North Carolina, last week. The meeting also offered a new look into life on four wings (see 鈥�Four wings good, two wings better鈥�, below) and discussed how best to tuck into a Triceratops (see 鈥�Tuck into a Triceratops鈥�, below).
Concern about the fate of the human Y chromosome stems from its small size and the fact that it contains few functional genes.
Early studies of chromosome evolution indicated that the Y chromosome had lost all but a handful of its original genes and was on the verge of vanishing. Now an evolutionary study comparing sex chromosomes and sex determination across a range of land animals, from amphibians to mammals, indicates there could be at least another 100 million years in the Y.
Humans and most mammals have XX females and XY males. It鈥檚 a different story in modern birds, though: males have a matched pair of sex chromosomes 鈥� ZZ 鈥� and the females have a single Z and a shorter W. In crocodiles and some other reptiles, meanwhile, sex depends on gestation temperature 鈥� or sometimes involves both temperature and genes.
Chris Organ at the University of Utahin Salt Lake City looked at sexual determination in 165 living vertebrates. By comparing the mutational differences between sex chromosomes among the species, and with data from the fossil record on how long ago they last shared common ancestors, Organ could work out when the ZW combination first appeared. He says there is a 90 per cent chance that, like female birds, the first female dinosaurs were ZW, 230 million years ago.
鈥淭hese rates of change suggest that the W chromosome has been around for at least 200 million years 鈥� that鈥檚 evidence against the gene-degeneration hypothesis,鈥� says Organ. He speculates that sex chromosomes may override their natural decay by adding in genes as others disappear.
Earlier this year, Jennifer Hughes of the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, showed that the human Y chromosome has lost no genes since our ancestors diverged from the rhesus macaque 25 million years ago (麻豆传媒, 25 February, p 10). She is looking forward to seeing Organ鈥檚 work published.
In 2008, Jennifer Graves at the University of Canberra in Australia noticed similarities between the sex chromosomes of birds and primitive mammals called monotremes. That suggests the ZW combination may have an even longer history, stretching back to the last common ancestor of the two groups, . 鈥淚 love the idea that dinosaurs had a bird-like ZW,鈥� she says.
鈥淚f you are flying in a forest, it behoves you not to hit the trees鈥�
Four wings good, two wings better
Almost a decade ago, fossil-hunters in China unearthed something remarkable: a feathered dinosaur with four wings. Microraptor was no one-off: other four-winged fossils have come to light since. How did these beasts get around, and why did four wings lose out to two?
The key to understanding four-winged flight is to realise that flying is about more than getting off the ground. 鈥淚f you are flying in a forest, it behoves you not to hit the trees,鈥� says Michael Habib at the University of Southern California in Los Angeles.
Habib and his student, Justin Hall, used a mathematical model of Microraptor to show it spread its rear wings to improve manoeuvrability. 鈥�Microraptor is initiating turns with the front wing, and as it begins to roll it orients the outside hind wing to kick out and add to the roll,鈥� says Habib.
He claims that new fossil studies support his theory by showing that the feathers on Microraptor鈥榮 hind wings created a large square surface. That shape made the hind wings aerodynamically inefficient, but that is less important if wings are used primarily for turning.
鈥淚 think they are on the right track,鈥� says Hans Larsson of McGill University in Montreal, Canada. He points out that Microraptor lacked the powerful pectoral muscles that birds use to control the shape of their wings, so it could not have relied on precision turning using its front wings.
That may have led to the downfall of the four-wingers. Once airborne, any excess weight is a hindrance, says Habib 鈥� two wings are simply more efficient. Birds may have lost their teeth for a similar reason.
Tuck into a Triceratops
Tyrannosaurus rex enjoyed nothing more than tucking into a tasty Triceratops (see image). But getting around the horned herbivore鈥檚 formidable head-frill to reach the rich meat below must have presented a challenge even to the famous carnivore. How was the job done?
Denver Fowler of the Museum of the Rockies in Bozeman, Montana, thinks he has worked it out. With colleagues, he examined the mangled remains of 100 Triceratops fossils from the 66-million-year-old Hell Creek formation in Montana. The iconic three-horned skull was usually found separated from the rest of the skeleton. Scratches on 16 of them matched the bite marks of T. rex 鈥� the only big carnivore that lived in the region at the time.
鈥淲e noticed tooth marks on consistent places on the skulls,鈥� says Fowler.
Scratches on the front of the skull suggest T. rex nibbled nostril-region flesh from the bone. But otherwise the front of a Triceratops offered slim pickings, so the meat-eater grabbed its prey鈥檚 frill with its teeth to rip it away from the body.
鈥�T. rex had broad teeth for resisting lateral stress, which you can use for shaking prey,鈥� Fowler says. Tooth marks on the occipital bone at the base of the skull show that Tyrannosaurus pulled the head off the carcass, flipped it over and scraped flesh off the base of the massive skull.
The decapitated body of Triceratops probably proved less of a challenge 鈥� except perhaps to T. rex鈥榮 appetite. 鈥淭here was more meat in one Triceratops than any one T. rex could eat in one sitting,鈥� says Fowler.
There is one question the as-yet-unpublished research cannot answer: did T. rex actively seek out and attack Triceratops, or were all close encounters of the scavenging kind?
Most evidence of active predation involves larger animals eating smaller animals, says Peter Dodson of the University of Pennsylvania in Philadelphia, who wasn鈥檛 involved in the study.
鈥淲hen animals are equal size or the prey is larger, there isn鈥檛 much evidence [of predation],鈥� he says.
An adult healthy Triceratops was a dangerous proposition, even for a T. rex. Dobson thinks the predator would have preferred to attack duck-billed dinosaurs called hadrosaurs, which lacked horns. T. rex might have tackled Triceratops when times were tough 鈥� but more fossils will be needed to work out how it went about dishing out the killer blow.
