Monday, 23 May 2016

How Evolution Works - Evolving Ichthyosaurs

A large aberrant stem ichthyosauriform indicating early rise and demise of ichthyosauromorphs in the wake of the end-Permian extinction : Scientific Reports

A nice example this week of how a major, possibly the main, driver of evolution is not as creationists pretend science says, random mutation, but environmental change. It also seems to go some way to solving something of a mystery in marine evolution during the period following the end-Permian mass extinction.

The Permian Era came to an abrupt end about 250 million years ago when increased volcanic activity and climate change and led to rising sea levels causing the extinction of ninty-six percent of all marine life. Unlike terrestrial metazoans which radiated rapidly to fill the newly-vacated ecological niches this mass extinction provided, the marine reptiles were thought to have evolved much more slowly.

This new fossil, named Sclerocormus parviceps, suggests however that marine reptile evolution may have been quite rapid too, based on the fact that the specimen is aberrant compared to other known marine reptiles, and shows extreme adaptation.

Details were published Open Access in Scientific Reports by an internation team of paleontologists from Peking University, University of California, Davis, the Anhui Geological Museum, the Università degli Studi di Milano, The Field Museum, National Museums Scotland, the Chinese Academy of Sciences, and the Smithsonian's National Museum of Natural History.

Abstract
Contrary to the fast radiation of most metazoans after the end-Permian mass extinction, it is believed that early marine reptiles evolved slowly during the same time interval. However, emerging discoveries of Early Triassic marine reptiles are questioning this traditional view. Here we present an aberrant basal ichthyosauriform with a hitherto unknown body design that suggests a fast radiation of early marine reptiles. The new species is larger than coeval marine reptiles and has an extremely small head and a long tail without a fluke. Its heavily-built body bears flattened and overlapping gastral elements reminiscent of hupehsuchians. A phylogenetic analysis places the new species at the base of ichthyosauriforms, as the sister taxon of Cartorhynchus with which it shares a short snout with rostrally extended nasals. It now appears that ichthyosauriforms evolved rapidly within the first one million years of their evolution, in the Spathian (Early Triassic), and their true diversity has yet to be fully uncovered. Early ichthyosauromorphs quickly became extinct near the Early-Middle Triassic boundary, during the last large environmental perturbation after the end-Permian extinction involving redox fluctuations, sea level changes and volcanism. Marine reptile faunas shifted from ichthyosauromorph-dominated to sauropterygian-dominated composition after the perturbation.

Introduction
Ichthyopterygia comprise a group of Mesozoic marine reptiles that are commonly referred to as ichthyosaurs. It is best known for the evolution of fish-shaped body profiles among its derived members that are represented by abundant well-preserved fossils. In contrast, the earliest evolution of the group still remains elusive, although recent studies recognize that they form the clade Ichthyosauromorpha with Hupehsuchia, a group of Early Triassic marine reptiles that inhabited a part of the South China block that later became western Hubei Province, China. However, there is a wide anatomical gap between Hupehsuchia and Ichthyopterygia and only one species is so-far known to fill the gap, namely the basal ichthyosauriform Cartorhynchus lenticarpus. Although the discovery of Cartorhynchus largely advanced our understanding of the early evolution of ichthyosaurs, additional information is clearly missing. Here we report a second species of basal ichthyosauriform that differs significantly from Cartorhynchus in its anatomical features.


Copyright © The authors/2016 Macmillan Publishers Limited reprinted under Creative Commons Attribution 4.0 International License (CC BY 4)

The ichthyosaurs were a very large group of marine reptiles that coexisted with early dinosaurs. They are often regarded as an example of convergent evolution, looking superficially like mammalian dolphins complete with beak-like snouts and tail fins. Sclerocormus parviceps however has a short snout (its species name even means "small skull"), and had a long thin tail without tail-fins. Most ichthyosaurs had conical teeth for catching prey but Sclerocormus was toothless and probably used its short snout to suck up food. In other words it had diverged a long way from the stem ichthyosaurs - something which would not be expected if evolution really had been slow.

Apart from suggesting that there was nothing preventing the expected rapid radiation following a mass-extinction amongst the marine reptiles, the interesting thing about this is the fact that it was the freeing up of evolutionary opportunity itself which drove evolution and that this is an expected outcome of basic Darwinian evolution. The reason the apparent slowness of the evolution of the marine reptiles was a mystery was that this was not what was expected from the theory. Nor was it what was seen in the terrestrial metazoans.

A basic understanding of Darwinian evolution shows that mutations are often neither objectively beneficial nor objectively deleterious but that this is determined by the environment. When there are lots of ways in which a mutation can be beneficial, for example, when there are lots of available niches, then there are more ways in which species can evolve and more directions for them to evolve in. The entire thing is determined not by the randomness of mutation but by the determinism of the environment.

If any creationist can explain why this is not only simple to understand and entirely predictable and why instead it is impossible due to some physical or chemical constraint then please do so. Why would a selective environment not favour alleles which are more beneficial in it than those not so beneficial, and why would more ways in which alleles can be beneficial not result in more rapid radiation and diversification?

For a creationist who understands evolution, if any exist, this should be simplicity itself to explain. Any takers?

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3 comments :

  1. Umm...."evolution is evil"? Help me here!

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    1. Actually, let's say a creationist had the wits to read the facts and assimilate them, and accept that there was explosive evolution of species after the Permian Extinction Event. This creationist could then tell the rest - not a scientist, *the other creationists* - that this was proof that "even science" accepted that "very rapid change into kinds" could happen in a short time. And then he'd say "Hallelujah, even science now admits the story of Noah's Ark!"

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