[thedoc (OP)]

Carol Brom  asked the Naked Scientists:
   
Chris,

If crocodiles only produce males or females according to the Earth's temperature, i.e. if high then it is only males (or the other way round?); and if low then only females (or the other way round?). Then maybe that is what happened to the dinosaurs, as the Earth warmed up or cooled down for an extended time period. So only one sex was born and they couldn't reproduce?

Carol Brom.

What do you think?

[Don_1]

Yes, you are right in saying that the Crocodilians are subject to temperature-dependent sex determination (TSD), as are other reptiles such as Gecko’s, Bearded Dragons all species of Tortoises and most turtles.

In the case of the Gecko and Bearded Dragon, lower temperatures will result in females and higher temperatures, males. Oddly though, in the case of the Tortoises, it’s the other way around. For the Tortoises, an incubation temperature range of 27^oC to 31.5^oC is liable to result in both male and female, while the Gecko and Bearded dragon’s mid range at which both sexes may result is around 30^oC to 34^oC.

The Crocodilians, broadly, follow the same pattern as the Geckos and Bearded Dragons, but there appears to be an anomaly with some Alligators, where males will be the result of both high and low temperatures, with females resulting from the mid range.

Some other reptiles, including as some Turtles and Lizards, such as the Green Iguana, are subject to genotypic sex determination (GSD). That is, that just like us and other mammals, their sex will be determined by the X & Y chromosomes. Others in this GSD group, such as all snakes, have Z & W chromosomes, which do the same job. The difference between the ZW GSD reptiles and the XY reptiles and us mammals, is that in our case, it is the female which is homogametic, having two X chromosomes and the male which is  heterogametic, having one X and one Y chromosome. In the ZW GSD reptiles it is the female which is heterogametic, having one W and one Z chromosome, while the male is homogametic, having two Z chromosomes.

Now, why did I go through all of that? Well, it show’s that not all reptiles are subject to TSD. It also rather neatly explains the system of sex determination of the dinosaurs nearest living descendants, the birds. The birds evolved from a group of dinosaurs called the Theropds, and bird sex is determined by the Z and W chromosomes in the GSD system.

Since bird sex is determined in this way, it might be safe to assume that all dinosaurs were subject to the same system.  Of course, I could be wrong. Maybe only the Theropods were lucky enough to have the Z W chromosomes. Maybe all other dinosaurs were TSD, like the Tortoises.

At the beginning of the cretaceous period, there was a prolonged global cooling and the dinosaurs were already in decline. The KT Event was their final undoing and after the KT boundary, global temperatures begin to rise. So on the face of it, it would seem that your hypothesis could be on the right road.

But herein lies a problem. If the demise of the dinosaurs were due to homofication (is there such a word?) or all offspring being hatched the same sex due to the temperatures remaining too stable and at one end of the scale, how can we explain the survival of the Tortoises? They not only survived the KT Boundary, but the Triassic mass extinction before and their early ancestors survived the Permian mass extinction before that. Crocodilians were also around in the cretaceous period and their ancestors date back to the Triassic, yet they too survive.

[Geezer]

Yes, you are right in saying that the Crocodilians are subject to temperature-dependent sex

They are not the only ones. That's all I'm saying.

[Don_1]

Yes, you are right in saying that the Crocodilians are subject to temperature-dependent sex

They are not the only ones. That's all I'm saying.

Ah! The old 'cold weather' problem eh!

I have the same problem; Now where the hell has that gone, it was here a while ago, come out come out wherever you are....

Dag nab it.... pass the magnifying glass will you.