Just to clarify this post’s title: I have not taken up an extremely dangerous but exciting new hobby. The sea monsters from the title have been extinct for a long time. But that doesn’t stop scientists from figuring out how sexual characteristics developed in these Jurassic reptiles.
In humans and in other mammals, the X and Y chromosomes are responsible for determining the sex of a fertilized egg. Females carry two X chromosomes in their cells (XX), whereas males have an X and an Y chromosome (XY). There are lots of species that have arranged things differently than us: in some reptiles and in all birds, sexes are determined in a mechanism that is similar to the one in mammals, except that the situation is reversed: males carry a double Z chromosome (ZZ), while females carry a Z and W chromosome (ZW). Since both the XX/XY and ZZ/ZW system rely on the presence of specific genes, they are known as genotypic-dependent sex determination (GSD) systems. In most reptiles however, sex is determined by temperature. For example, male turtles will hatch from turtle eggs if the temperature is low, whereas females hatch if the temperature is higher. This mechanism is known as temperature-dependent sex determination (TSD).
A baby turtle hatches. Most turtles have TSD. Picture courtesy of Mayer Richard, Wikipedia.
An interesting question is, what role does sex determination play in evolution? You could imagine that different environments require different modes of reproduction. This seems to be the case if we look at amniote species (mammals, birds and reptiles are amniotes) living in the oceans. The only amniotes that currently have a complete marine lifestyle are sea snakes, sea cows and whales & dolphins. All of these species determine sex via GSD. This seems logical, as the temperature in oceans is more constant than on the land, TSD would be a real disadvantage in the open sea. Interestingly, all of these species are ‘viviparous’ as well, which means that they give birth to live offspring (yes, even the sea snake!). This is in contrast to 80% of reptiles, who are ‘oviparous’, which means that they lay eggs. If all current living marine amniotes have GSD and are viviparous, would the same hold for now extinct amniotes?
This is the question Chris Organ & colleagues answered in a recent Nature paper. They were interested in three extinct amniote species, which were true reptilian sea monsters in the Jurassic and Cretaceous era: Ichthyosaurs, Mosasaurs and Sauropterygians, which all gave live birth. Monsters they were: some Ichtyosaurs could become more than 20 metres long. They competed with sharks and between one another for the top of the food chain. Since sex determination mechanisms don’t fossilize, Organ & colleageus developed a clever model to study the reproductive mechanisms in these species (based on Markov Chain Monte Carlo methods). Their model is based on the known family tree of amniotes and information on the occurrence of vivi/oviparity and TSD/GSD in these species. With this model, they were able to draw some cool conclusions and make interesting predictions.
Fossil of an Ichthyosaur giving live birth tail-first, which prevents the young from drowning. This is similar to birth in dolphins, sea cows and whales. Copyright The Natural History Museum, London.
The research team shows that viviparity (live birth) depends on the prior evolution of GSD. The probability that a species reverts from GSD to TSD is also really low, which is in concordance with Dollo’s Law. On the other hand, oviparity seems not to be particularly dependent on either TSD or GSD. Using their model, they correctly predicted the sex determination mechanism and reproductive mode in species that they had not included in the original selection.
Even cooler, they predicted that GSD was present in all extinct reptilian marine species. Organ & colleagues suggest that GSD and viviparity provided the ancestors of these marine predators with great advantages: they no longer needed to return ashore for laying eggs (like sea turtles do) and were free to develop a completely marine lifestyle. In other words, GSD and viviparity are the key traits that paved the way for their evolution from relatively small terrestrial reptiles into the marine monsters that dominated the oceans for millions of years. The authors are the first to point out the coevolutionary relationship between sex determination mechanism and whether a species bears live young or lays eggs.
And so, we know a little bit more about these enigmatic sea reptiles than we did before. We’ve come a long way since 1764, when the first Mosasaurus fossil was uncovered near the Dutch city of Maastricht.
Organ, C., Janes, D., Meade, A., & Pagel, M. (2009). Genotypic sex determination enabled adaptive radiations of extinct marine reptiles Nature, 461 (7262), 389-392 DOI: 10.1038/nature08350
You might also like:
Great article, thank you for writing about sea monsters!
That fossil is utterly, totally, outstanding. and i’d never thought about it before but TSG in the sea really would be a bad idea, one cold current and suddenly you’d have a population consisting entirely of bored males with nothing to do.