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Re: the evolution of disparity in body plans (was Re: synapomorphies)



Nick pharris wrote:

>I see the "Ur-arthropod" as a many-segmented, untagmosed animal with
>unspecialized, biramous limbs.  Does anyone have a problem with this
>interpretation?  I see the Cambrian arthropods as a carpet of independent
>specializations from that primitive state, of which only four survived
>the Cambrian:  trilobites, chelicerates, crustaceans, and members of the
>myriapod-insect line.

Using "Ur-arthropod" is dodgy now since Derek Briggs has/is using it. I
can't remember off hand what he uses it to denote, all arthropods, derived
only, or stem groups only. I will ask him.

Also "untagmatosed" is a bit dodgy since it was probable that tagmatization
was well underway in the early annelids, so a basal form which already had
some degree of tagmatization can't be ruled out.

>> "you mean _Anomalocaris_ didn't make it? Get out of here!"
>
>I have a hard time believing that _Anomalocaris_ belongs to a modern
>phylum.

Sveral species from the Chengjiang Fauna in China have now been found with
segmented legs, biramous in some instances. See: Hou, X-G et al. (1995)
_Anomalocaris_ and other large animals in the Lower Cambrian Chengjiang
fauna of southwest China. Geologiska Foreningen i Stockholm Forhandlinger,
117: 163-183. (This actually has quite a good circulation - commonly called
"GFF").
They actually suggest that _Anomalocaris_ belongs with the aschelminth
worms because of the radial arrangement of the plates.

Or _Obapinia_.

Arthropod. Although _Opabinia_ is the last true oddball, I have specimens
from my field area (Lower Cambrian) which appear to be early opabinids.
These show a primative growth pattern of simply adding on segments, with no
standard segment size for the adult form. Plus they have appendages which
are similar to other arthropods. The eyes are weird, but the overal body
plan appears to be arthropodic.
Don't forget that the arthropod basic body play is *very* flexible. In fact
it is like playing 'Mr. Potato Head' - same body plan, just mix and match
at the front! This flexibility is why the arthropods are the single most
successful metazoan group on earth.

Or _Odontogriphus_,

Still unknown, simply because there are so few detailed specimens. Appears
to be an abherent, derived arthropod.

_Wiwaxia_ (I am unconvinced
>by the sea mouse comparison),

SEM work showed that the spine ultrastructure was identical to that of some
annelids. So it may be an annelid or a mollusc. Not a real problem.

_Amiskwia_,

Again, there are so few specimens to make an accurate diagnosis. We simply
do not know enough about it

or _Nectocaris_ (my personal
>fave), for that matter.

Described from only one specimen. My guess is that it is a phyllocarid of
some sort. The so-called chordate structures may well be a preservational
artifact (i.e. unusual orientation). OTOH it may well be a chordate - there
is at least one in the Burgess Shale, and now one from Chengjiang - which
is even older.

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.