er, what's an Anomalocaridid?

Discussion in 'Cephalopod Fossils' started by tonmo, Oct 4, 2003.

  1. tonmo

    tonmo Titanites Staff Member Webmaster Moderator

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    Phil, saw this in your poll, and noticed no takers... then did a Board search and couldn't come up with any content about it! What is it? :?:
     
  2. Phil

    Phil Colossal Squid Supporter Registered

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    Sorry Tony, thanks for asking though!

    That was a bit obscure; I should have called the animal 'anomalocaris' in the poll. Then you certainly would have had some hits. I have banged on about this bizarre animal before; probably too many times in a cephalopod web-site.

    Anomalocaris was one of the animals that belong to the anomalocaridids, a diverse group of top predators that inhabited the Cambrian seas. This group were the world's first large predators, at this date the molluscs were confined to small shells and were barely significant in the fossil record. The arthropods dominated the seas in the Cambrian, and Anomalocaris was the most fearsome of the whole group.

    Anomalocaris canadensis from the 525mya Burgess Shale is the best known animal from the group and is usually reconstructed as a gliding manta-like animal with a circular crushing mouth and two large grasping claws on the head. Fragments of these animals have been found in Australia that imply this arthropod could have reached a length of 2 metres or more; the animal was clearly a very impressive animal. Elrathia trilobites have been found in Utah that display semi-circular bite marks that were probably wounds inflicted by this animal.

    Anomalocaris had been assumed to be three separate animals until the early eighties as it was thought that the mouth parts were remains of shrimp-type animals, the mouth being a peculiar form of jellyfish and the body some form of sponge. The chance discovery of a complete specimen at the Burgess site led to a reappraisal of all the old specimens that have been collected and a much more interesting animal was the result.

    Since the eighties, members of the anomalocaridids have been found in the earlier Sirius Passet fauna at Greenland and in the Chenjiang fauna from China, both of which display creatures that were quite diverse; some of the Greenland examples display lobopod-style legs. It is interesting to note that the earliest nautiloids that rose to the position of top predator and diversified massively in the subsequent Ordovician period appeared after the extinction of the anomalocarids. One could almost argue that the molluscs had been held in check by the arthropods in the Cambrian, and it was only after a mass extinction that the molluscs were able to exploit the niches largely vacated by the arthropod predators.

    Interesting stuff, and a fascinating animal! This is the ultimate place to go for details on the anomalocaridids, and it is a truly brilliant website:

    Anomalocaris Homepage

    And (hope you don't mind) are a few clay models of a few of the anomalocaridids I made a couple of years ago. Don't be too harsh, I only made them to keep myself amused, normally they would not see the light of day!

    Anomalocaris

    [​IMG]

    Laggania

    [​IMG]

    Pambdelurion

    [​IMG]
     
  3. tonmo

    tonmo Titanites Staff Member Webmaster Moderator

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    Ok, now I want to change my poll answer! :)

    Very very cool. Yes, I've seen that link before, so I must have come across one of your posts on this before...
    Fascinating. BTW, do we know what caused their extinction?

    Great clay work... For some reason I found myself musing how good those beasts probably were to eat... a little butter, some white wine...
     
  4. Phil

    Phil Colossal Squid Supporter Registered

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    I'll have a look at this tomorrow, Tony. Methinks I've probably rambled enough on the site today for you all.

    As for what those things tasted like, I imagine something like this!

    Cambrian Casserole
     
  5. Phil

    Phil Colossal Squid Supporter Registered

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    Tony,

    It is very difficult to say exactly what caused the extinction of the anomalocaridids. However, it is widely accepted that predators occupying specialised ecological niches are always the first to become extinct following environmental changes. Predators are always, by definition, far fewer in number than the prey species they feed on. It is always the generalists which will tend to survive ecological catastrophies, allowing them to diversify afterwards and fill the ecological niches that were left vacant by those animals that were made extinct. Let's face it, if a new ice age started tomorrow who would our money be on to survive, rats or tigers?

    As an example of this, just think what happened following the extinction of the dinosaurs 65 million years ago. The mammals quickly radiated and came to dominate many more niches than they had occupied previously. Perhaps a similar model could be applied to the Cambrian arthropods and the molluscs, there was indeed a mass extinction at the end of the Cambrian. Some researchers attribute this to to massive glaciation, as evidence of glacial deposits at the Cambrian-Ordovician boundary have been unearthed in South America. With a reduction in sea level caused by mass freezing and the blocking out of sunlight and a drop in temperature, life would have suffered dramatically on the continental shelves.

    The point here is that with the decimation of life in the shallow waters, marine life would have suffered, and especially the specialised predators such as the anomalocaridids. Some groups of trilobites made it through, but it must have been the removal of the larger predatory arthropods that allowed the molluscs to radiate and adapt very quickly; late Cambrian shelled bottom dwelling creeping proto-cephalopods such as Plectronoceras quickly adapted and became large swimming nautiloid top predators in the early Ordovician.

    As a caveat to the above, there is a twist to the tale here. A mysterious fossil was announced at the 1997 annual meeting of the Palaeontological Association at Cardiff. It was found in the Ordovician Soom Shale from South Africa and opinions were invited from the audience as to what this thing was. The common concesus was that this specimen was in fact some form of anomalocarid though it lacked the characteristic front appendages. This thing is problematic, and as far as I know has not been worked on, but for all intents and purposes appears that the situation is not quite cut and dried. (Is it ever?)

    If anyone has any details about this problematic Ordovician fossil I'd love to hear from you!
     

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