February 18, 2011
What caused the end-Cretaceous mass extinction is one of the greatest mysteries of all time. Paleontologists have racked up a long list of victims—including the non-avian dinosaurs—and geologists have confirmed that a massive asteroid that struck the earth near the modern-day Yucatan peninsula was probably the extinction trigger, but just how that impact translated into a global extinction crisis is still being figured out. Of course, dinosaurs were the most charismatic creatures to perish during the event, and for years Sherman Silber has been forwarding his own peculiar proposal.
An infertility specialist at St. Luke’s Hospital in St. Louis, Missouri, Silber believes that dinosaurs died out because there were not enough females. In a 2004 paper written with David Miller and Jonathan Summers, Silber suggested that dinosaurs had a crocodile-like reproductive strategy in which temperature determined the sex of developing embryos inside their eggs. In American alligators, for instance, lower nest temperatures produce more females, whereas nests with higher temperatures produce more males. If dinosaurs development was also temperature-dependent, Silber and co-authors proposed, then the climatic changes created in the wake of the asteroid strike may have caused many more male dinosaurs to be born than females. Incapable of finding enough mates to ensure their survival, the dinosaurs were wiped out.
Of course, this idea was entirely based on inference. Most birds—the direct descendants of small, feathered dinosaurs—have their sex determined genetically, and there is no clear indication that the sex of developing dinosaurs was determined by variations in temperature. (In fact, the recent discovery that some sauropod dinosaurs repeatedly laid eggs in nurseries heated by geothermal activity would appear to argue against this point.) Furthermore, this hypothesis did not make sense of the fact that turtles and crocodylians—groups in which the sex of offspring is dependent on temperature—survived the extinction and all the subsequent temperature fluctuations the planet has undergone over the past 65 million years. Paleontologists panned Silber’s hypothesis, particularly since it had been previously proposed by other scientists and set aside, but news services ate up the story. “Fried eggs may have wiped out dinosaurs” announced Discovery News, and the BBC reported “Fewer females wiped out dinosaurs.”
Not to be discouraged, Silber has an article in-press with the Middle East Fertility Society Journal which retreads the argument he made in 2004. In it, Silber does not provide details about what scientists have learned about the reproductive habits of dinosaurs, what might have happened after the asteroid impact, or even the range of animals wiped out in the extinction event. Instead, Silber assumes that the sex of dinosaurs was temperature-dependent and that some sort of climate shift would have resulted in an over-abundance of males. As before, he provides no satisfying explanation as to why species with known temperature-dependent sexes survived, merely stating “Crocodilians and other TSD species (but not dinosaurs) survived because they could adapt successfully to the changing environment.” This statement tells us nothing about why these animals did not follow Silber’s predictions.
In fact, Silber was a co-author on another analysis which directly contradicted what he expected. Printed last year in Biology Letters with co-authors Jonathan Geisler and Minjin Bolortsetseg, the study looked at the survival of vertebrates found in Montana’s Hell Creek Formation, representing the very end of the Cretaceous just before the mass extinction, and those in the Tullock Formation, representing environments just after the catastrophe. Since sex determination in dinosaurs is unknown, they were left out, but the scientists found that the performance of Silber’s hypothesis among the other animals was “dismal.”
Of 32 extinctions and 30 survivals, the overwhelming majority of cases were inconsistent with what was expected under Silber’s hypothesis. In fact, most of the species with temperature-determined sex survived whereas 61 percent of species with genetically-determined sex went extinct, with small mammals being the hardest hit. The authors could not explain why this was the case—why turtles and crocodiles survived unscathed while so many other species perished remains an open question—but it was clear that Silber’s hypothesis failed. Being that he was listed as the first author on this paper, it is strange that he has returned to his original hypothesis in the forthcoming Middle East Fertility Society Journal paper.
Whether the sex of dinosaurs was determined by temperature or genetics remains unknown, but we cannot assume that they were just like alligators and crocodiles. After all, birds are modern dinosaurs and most have genetically determined sexes, and recent discoveries have shown dinosaurs to be extraordinarily bird-like. Perhaps dinosaurs were also like birds in having genetically determined sexes. Further research is required to figure this out.
Even then, though, reproduction is just one small part of the global extinction puzzle. Following the intense volcanic eruptions of India’s Deccan Traps, the asteroid impact 65.5 million years ago initiated intense ecological changes on a global scale. Scientists are still trying to find ways to detect how this major event forever changed life on earth.
Clark, J.M., Norell, M.A., & Chiappe, L.M. (1999). An oviraptorid skeleton from the Late Cretaceous of Ukhaa Tolgod, Mongolia, preserved in an avianlike brooding position over an oviraptorid nest American Museum Novitates, 3265, 1-36
Miller, D. (2004). Environmental versus genetic sex determination: a possible factor in dinosaur extinction? Fertility and Sterility, 81 (4), 954-964 DOI: 10.1016/j.fertnstert.2003.09.051
Silber, S. (2011). Human male infertility, the Y chromosome, and dinosaur extinction Middle East Fertility Society Journal DOI: 10.1016/j.mefs.2011.01.001
Silber, S., Geisler, J., & Bolortsetseg, M. (2010). Unexpected resilience of species with temperature-dependent sex determination at the Cretaceous-Palaeogene boundary Biology Letters DOI: 10.1098/rsbl.2010.0882
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