Did climate change turn all dinosaurs male?

by Gareth Barton and Jacqui Hayes
Cosmos Online

SYDNEY: Why the dinosaurs died out 65 million years ago may come down to how the sex of their offspring was determined, said American palaeontologists and microbiologists.
To determine the sex of offspring, not all animals use genetics the way humans do (with an XX chromosome pair resulting in a female and XY a male offspring).
Instead, many groups of animals use the temperature of eggs during incubation. For example, Australian salt water crocodiles hatch as females if the average temperature of the mound nest is below 29°C or above 34°C, but male if the average temperature is in between.
Accidental all-male populations?
Until now, it was thought that 65 million years ago non-avian dinosaurs and other species with temperature-dependent sex determination went extinct following a volcanic eruption or enormous meteorite impact because such an event resulted in a global climate change that completely skewed the sex ratios of offspring.
But this hypothesis was discounted in a study published this week in Biology Letters.
A team of palaeontologists and microbiologists looked at the fossil record of 62 species in the Hell Creek and Tullock Formation in Montana to see if they survived the transition from the Cretaceous to Palaeogene 65 million years ago. The species were from groups such as salamanders, frogs, toads, turtles, lizards, marsupials, birds, crocodiles and alligators.
Species that survived 65 mya
"We were able to determine the SDMs [sex-determining mechanisms] of 62 species; 46 had genotypic sex determination (GSD) and 16 had TSD [temperature-dependant sex determination]," the researchers, from the U.S. and Mongolia, wrote in their paper.
"Most surprisingly, of the 16 species with TSD [temperature-dependent sex determination], 14 of them survived into the Early Palaeocene. In contrast, 61% of species with GSD went extinct."
"Our results were definitely unexpected," said Jonathan Geisler, anatomist and palaeontologist at the New York College of Osteopathic Medicine, USA.
"We do not have a compelling explanation" for why groups of animals that used temperature instead of genetics fared better 65 million years ago, the researchers wrote.
"We do not have a compelling explanation"
One possible explanation is that the event that caused the mass extinction did not result in global temperature changes after all, and perhaps Earth's climate is more stable than climate scientists have suggested.
Another possible explanation is that species affected by temperature are able to rapidly adapt, by changing the temperature at which the different sexes develop, altering their nest sites or moving.
In both cases, it may have been another factor - such as being cold-blooded, which is often found in animals that use temperature-dependent sex determination - that helped those species to survive.
Climate change less dire for some animals
"Clearly more work needs to be done to figure out which explanation makes the most sense. We view our study as discovering a problem that now needs to be solved," said Geisler.
The study also means that species most vulnerable to climate change today may fare a better chance than most ecologists believe.
It is thought that almost all dinosaurs used temperature to determine sex, much like today's crocodiles.
Did dinosaurs use temperature?
However, birds - close relatives of dinosaurs - use sex chromosomes, except an bird with a ZZ pair will develop into a male and ZW will develop into a female, which is opposite to humans.
According to the researchers, in the future, it might be possible to apply 'molecular clocks' to find out when sex chromosomes in birds evolved. Molecular clocks use changes in DNA to estimate when species diverged in the geologic past. If such an application is successful, it might be possible to determine if the dinosaur ancestors of birds had sex chromosomes instead of using temperature dependent sex determination.
"It's interesting," said Frank Seebacher, an evolutionary physiologist, at The University of Sydney, Australia, who was not involved in the study. "What killed the dinosaurs is one of the big questions in science. The study tests a hypothesis and discounts it."
However "it is very hard to deduct a strong analyses from very few samples [62, in this study]" because it is unclear whether the fossils represent averages or outliers. But this is a problem for all science that relies on fossil records, he said.
COSMOSmagazine.com