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Re: Anomalocaris



Bill wrote:

>Mickey kindly forwarded Chris Nedin's assessment of Anomalocaris's swimming
>ability.
>
>Adam Heman had asked:
>>>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?
>[snip]
>>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.
>
>Hang on...  I remember vividly the 1996 Royal Institution Christmas
>lectures.  These are traditionally given by a scientist and popular science
>writer to an audience of children (plus those few people who want to watch
>something good on television).  If I may digress a little, Richard Dawkins
>was extremely good a few years back, better than his university lectures
>which are themselves of a high standard.  But he was unrecognisable under
>the makeup.
>
>In 1996 the lecturer was a geologist whose name I can't recall.

Simon Conway Morris.

>But while
>talking about the Burgess Shale (he was naturally not very specific about
>the etymology of Hallucigenia) he displayed a life size model of the
>Anomalocaris we all knew and loved.  This was interesting, but the really
>exciting bit was when he added the legs.  These had apparently been
>recently discovered, and IIRC were uniramous and rather diplopod-like.
>(Millipede-like, for vert types.)

Some of the Chinese specimens from Chengjiang have been found with legs,
but by no means all.  Some of the specimens from the Burgess Shale
possessed weakly preserved bars along the underside of the body segments
which may have been legs, but if so they do not appear to have been robust.
The main problem with the having Anomalocaris running around on foot, so to
speak, is that this would tend to negate its primary hunting weapon, the
frontal appendages.  The legs that have been found do not appear to be
large, or long enough to allow sufficient appendage-room at the front to
allow the appendages free movement forward and back, under the head.
[Movement of anomalocaridid frontal appendages was restricted to movement
in an arc from curled up under the mouth to fully extended in front of the
head - in the dorso-ventral plane, i.e. a plane at 90 degrees to the
lateral plane of the animal (a plane running through the animal from one
side to side), and not from side to side -  similar to movement of an
elephant's trunk (which can move in an arc from underneath the head to on
top of the head, but not side to side - unless the whole head turns from
side to side.  Or, if you are still confused, hold your arms straight down
in front of you and curl your hands so that the fingers are pointing at
your abdomen.  Now, keeping you arm movements at 90 degrees to a line drawn
across your shoulders, raise your arms so that your knuckles are pointing
towards the ceiling.  Repeat a few times.  That plane is the only one in
which the appendages could operate.  If you orientate your body to be
parallel to the ground, as in Anomalocaris, it becomes apparent that you
are going to need some room under your body to allow the free movement of
your arms).  Fossil specimens in which the appendages appear curled
parallel to the lateral plane of the body are due to compaction of
appendages lying under the body.]
It is probably significant that the species with the best preserved legs
(Parapeytoia yunnanensis - neatly described by Hou et al. 1995) also has
one of the shortest (in terms of number of segments) frontal appendages (5
compared with 10+ in other species).
Given the lack of robust structures attributable to limbs, the size and
length of the frontal appendages and the size of the flaps on the body on
most forms, it is most likely that the vast majority of anomalocaridids
swam as the primary attack mode.

>If the lecturer was not mistaken, this removes Anomalocaris from the
>shrinking list of Burgess weirdos in no modern phylum, and adds it to the
>growing list of Burgess weirdos in the Arthropoda (which, incidentally, I
>am sure is monophyletic).

Although Hou et al. 1995 did classify anomalocaridids as aschelminths.

>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. :-)

[deleted]

>And as for trilobites, IMO lobsters would be more appropriate.  Crabs
>scuttle sideways, taking long strides without tripping over their long
>legs.  Trilobites weren't built for that.

Lobsters or crabs will do, provided it is remembered that crabs and
lobsters are able to have their knees up above the level of the body,
giving them long legs, and fast speeds.  Trilobites could not do this.

Chris

Hou X-G, Bergstrom, J. & Ahlberg, P. 1995 Anomalocaris and other large
animals in the Lower Cambrian Chengjiang fauna of southwest China.
Geologiska Foreningen i Stockholm Forhandlinger, 177: 163-183

cnedin@geology.adelaide.edu.au                  nedin@ediacara.org
-------------------------------------------------------------------
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.