[Taxacom] Heterastridium

[Thomas Yancey]

Fred,

I'd like to offer an alternative view on these. Although in 
paleontology we cannot accept the maxim that "if it seems too 
bizarre, it can not be true" (when looking at the fossil record, 
there really are some wildly bizarre organisms that turn out to be 
very real), in this case the size consideration is so extreme that 
the original premise is probably not accurate. Heterastridium is 
mostly in the grape size to golf ball size range, not basketball size.

But the assumption that it is a planktic life form is suspect. The 
organism secreted a calcareous skeleton with radial elements and the 
skeletal mass is so great that it is really unlikely to be planktic. 
Regardless of written statements, it seems improbable as a planktic 
form.

The other thing to consider is that this type of structure is known 
for some Silurian-Devonian fossils, also of the same (smaller) size 
range. The Paleozoic fossils are species of the genus Hindia and 
relatives and are satisfactorily identified as globular sponges. I 
think that is the proper ecologic analog for Heterastridium. 
Heterastridium could be a homeomorph of globular sponges or it might 
even be a globular sponge, improperly labelled as a hydrozoan colony. 
A careful study of well preserved material is needed to evaluate that 
possibility.

So, I'm suggesting that the characterization that you stumbled across 
is misleading and that not enough is known of the late Triassic 
Heterastridium to be able to interpret life habit.

Tom Yancey



>Taxacombers,
>
>I've recently stumbled across references to Heterastridium, and nothing
>I've found in a casual search explains how a globally-distributed
>basketball-size spherical planktonic life-form should arise and
>disappear in the Upper Triassic without apparent ancestors or
>descendants. Is there any contemporary thought on this that's more
>ecological and/or phylogenetic than simple incredulity?
>
>thanks,
>
>fred.
>------------------------------------------------------------
>              Bishops Mills Natural History Centre
>            Frederick W. Schueler & Aleta Karstad
>         RR#2 Bishops Mills, Ontario, Canada K0G 1T0
>      on the Smiths Falls Limestone Plain 44* 52'N 75* 42'W
>        (613)258-3107 <bckcdb at istar.ca> http://pinicola.ca
>------------------------------------------------------------