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[cnedin@geology.adelaide.edu.au: Re: [bheman@axess.net: anomolocaris]]



Of all the people I know, Chris Nedin looks like the one most likely
to swim after a specimen of  _Anomalocaris_, so I forwarded the
message to him.  Below is his response:

-----------------------
Date: Tue, 17 Feb 1998 17:13:41 +1030
To: "Mickey P. Rowe" <mrowe@indiana.edu>
From: Chris Nedin <cnedin@geology.adelaide.edu.au>
Subject: Re: [bheman@axess.net: anomolocaris]

>Looks like there could be times when it would be good to have you
>around.  But of course you're not here when needed.  I'd always heard
>you weren't to be trusted!

Ha! you invertebrately challenged bunch of vertebratocentrics!
Kindly pass this on to the list ye follower of an insignificant subphylum.

Chris
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Adam Heman wrote:

>I was wondering if anyone has any thoughts on how fast anomolocaris was
>able to swim.
>I know this is a dinosaur list server, but does anyone have any Cambrian
>background?
>
>Adam Heman
>a 9 year old paleontologist

Oh! Oh! Me! Me!

It is difficult to judge how fast an organism could move without being able
to actually go out and measure its speed.  A way around this would be to
measure the distance between footprints and relate that the length of the
leg to come up with an approximation (a la McNeill Alexander), or compare
the organism with a similarly built, modern organism (a la Bakker).
However, both of these methods are not much use when dealing with
_Anomalocaris_ which didn't leave footmarks or have legs, not is it similar
to any organism alive today.

Clues to its speed may found in looking at the size and shape of the
organism together with what and how it would have hunted.

Now while _Anomalocaris_ does not look like any animal alive today, it
nevertheless had to go through the same processes, such as movement, eating
etc, as modern organisms.  _Anomalocaris_  posessed two large, spiney
appendages on its head.  Despite original suggestions as to the function of
these appendages, we are on fairly solid biomechanical ground when we
assume that they were not used to help the animal move (unless
_Anomalocaris_ walked around doing head stands!).  The position of the
appendages, around the mouth, and their flexibility in moving forwards and
back - but not side to side - suggest that they were involved in catching
food.  _Anomalocaris_ appears to have been a swimmer by the up and down
movement of flaps along the side of the body.  Each flat appears to have
been aligned on top of the flap in front of it and underneath the flap
behind.  Swimming by the use of these flaps is surprisingly easy, as an
experiment using a scale model in a swimming pool showed.  Not only could
the model swim forwards, but by reversing the motion of the flaps, it could
hover, and swim backwards.  Clearly _Anomalocaris_ was highly manoeuvrable,
but how fast was it?

Another clue may be found in what _Anomalocaris_ hunted.  We have a lot of
evidence now for anomalocarid bite marks on trilobites, and it seems that
they, and similar but soft bodied organisms were a large part of its prey.
Most trilobites in the Cambrian were rather large, many segmented, spiney
critters, probably as fast as modern crabs and lobsters.

Given that _Anomalocaris_ was manoeuvrable and that it had big, bug eyes,
we think that it cruised some distance above the sea bottom, looking for
trilobites and then nose-dived, making a mad dash to try and catch one.

So we can say that _Anomalocaris_ was probably faster than trilobites (=
modern crabs) - at least over short distances - but slower than modern
squid, who use jet propulsion and are thus considered to be cheating
anyway. :-)

Chris

cnedin@geology.adelaide.edu.au                  nedin@ediacara.org
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Many say it was a mistake to come down from the trees, some say
the move out of the oceans was a bad idea. Me, I say the stiffening
of the notochord in the Cambrian was where it all went wrong,
it was all downhill from there.