Friday, 14 February 2014

Baby fish-lizards

A quite amazing fossil of an early ichthyosaur that died in the process of giving birth has been unearthed by palaeontologists in China. The fossil shows an adult specimen of the genus Chaosaurus, a baby that has just been born, one still in the body, and another that appears to have died midway through the birthing process. The palaeontologist examining the find, Ryosuke Motani, believes that the first baby may have been stillborn, with the mother then dying from complications in the second birth. Although naturally we cannot be certain, this is thought to be the best explanation for the positions of the bodies.

While we tend to consider reptiles as egg-laying creatures, viviparous reptiles are not unknown. The common lizard, Zootoca vivipara, is found all over Europe, even in the chilly climes of southern England, and is well known to be viviparous. It is the most northerly found reptile, and its viviparity is considered to be an adaptation to the colder weather; more southerly populations remain oviparous. Viviparous reptiles are well known in the fossil record, particularly among aquatic species. An egg-laying reptile would have to come to shore to produce young, in the manner of today's sea turtles, exposing herself and her young to considerable risk.

What's interesting about the Chaosaurus specimen is that it is seen to be giving birth to its young head first. This is common among terrestrial species, but not in aquatic forms. It is also the earliest example found of viviparity in a reptile, being a good ten years younger than the previous youngest, another ichthyosaur fossil. The most well-known such fossil, an impressive specimen of Stenopterygius, shows a live birth interrupted. Hundreds of Stenopterygius fossils have been found in Europe, in various stages of maturity, giving a great deal of insight into the ichthyosaurian life cycle. The latest know ichthyosaur fossils, found in North America, also show some evidence of live birth, and further adaptations to make the process more efficient. All of these later forms show the mother giving birth to its baby tail first.

A tail first birth is likely an adaptation that helps prevent the baby drowning while partway out of the birth canal. It's clear evidence that the birth process of the ichthyosauria evolved over time into one more suitable for marine life. It could even show that the viviparity originally developed in the ichthyosaurs' terrestrial ancestors, with their head-first approach being retained to begin with and later adapting to a safer, tail-first approach. This is uncertain, of course, but the evidence is suggestive. Chaosaurus is so primitive that it is sometimes not even considered a true ichthyosaur, but rather just outside the ichthyosauria in the basal ichthyopterygia, the ancestral grouping. It was a particularly small example, only a metre to 170 cm long, in comparison to the other genera that got considerably larger, including the only slightly younger Shonisaurus, which reached at least 21 metres.

Viviparity is also known in other marine reptile groups, including the plesiosaurs and the huge marine lizards, the mosasaurs. It does not appear to have developed in the crocodilia and their relatives. It clearly developed several times in unrelated lineages, an example of parallel evolution in animals adapted to the same environment. It is even possible, although not evidenced, that the more advanced ichthyosaurs could have developed placantae; the Zootoca lizard is known to have one in its viviparous populations, albeit very basic in type.

As an aside, the correct term for a baby ichthyosaur is apparently a pup. Which is nowhere near as good as a baby pterosaur, which is called a flapling.

Further reading:

Viviparity in Ichthyosaurs, Map of Life
Sex Determination in Sea Monsters, Thoughtomics
Full article on the find, Tech Times

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