Paleonet: results: prodisso vs. protoconch

Nikolaus Malchus n.malchus at
Wed Mar 7 12:08:16 GMT 2007

Dear colleagues,        

         Some time ago I asked for opinions on whether larval shell 
terminology in molluscs (protoconch vs. prodissoconch, etc.) with gast. and 
biv. in mind should be simplified. I got as many opinions as answers 
ranging from         

-         don’t fix if it is not broken        

-         homology is not a necessary prerequisite to use the same 

-         yes, reductions desirable        

         Although without any statistical significance, the tendency to 
reduce terminology appears to win. In that case, which terms to use? First 
introduction of a term appears not an argument as prodissoconch refers to 
the bivalved condition, the more general term is protoconch. - 
Nevertheless, I would like to learn who introduced the term, in which 
context (gastropods, cephalopods, ammonites, nautilids (ammonitella), other 
mollusks?) and when?        

         At present it appears best to accept the authors’ choice 
and see which comes out as the winner on the long run (my guess is that 
protoconch would win only if more gastropod people publish on bivalves, 

         The question of homology appears an entirely separate issue (and 
goes much deeper). I was cautioned that “at least the protoconch 2 
(P2) is not homologous. In gastropods it is formed in planktotrophic larvae 
only (veligers), not present in Archaeogastropoda (Patellomorpha, 
Vetigastropoda, etc) and possibly diphyletic (Neritopsina, Caenogastropoda 
and more advanced groups) (Bandel 1982, etc. (summarised, e.g.,  in 
Haszprunar et al. 1995, Acta Zoologica, Stockholm, 76: 141-154)” see 
also v. S.-Plawen, Iberus, Vol. 9 (1-2): 1-33 (1990/1991).
Based on my humble knowledge, I would say this depends on whether we 
understand homology as a synapomorphy or symplesiomorphy or is it truly 
convergent (parallel)?, and on when the mantle margin takes over to produce 
shell. Does the latter happen before metamorphosis or afterwards? If there 
were a common stem group for bivs. and gastr. and this had a 
non-planktotrophic development without mantle margin producing 
prodissoconch then the P2 is a convergent/parallel development.        

Within bivalves, Philobryidae (long-term internal brooders, hatching 
juveniles) appear to lack a P2, but a closer look shows that much of the 
larval shell has commarginal growth increments, id est, in my view an 
indication that it was actually produced by the mantle margin (long before 
metamorphosis), so it has a P2. Can we exclude a similar case for 
“archaeogastropods”, for example? Interestingly, palaeotaxodont 
protoconchs appear to really lack such a P2 phase. And apparently also 
ammonitellas. Any thoughts on this?         

         Although the following paper does not answer any of the above 
questions it is still worth reading (thanks to Alan Kabat pointing this out 
to me):         

R. D. Turner, J. A. Pechenik, and C. B. Calloway, 1986. "The language of
 benthic marine invertebrate development patterns: problems and needs." 
 Pages 227-235, in (Mary-Frances Thompson, et al., eds.), Biology of 
 Marine Organisms: Techniques and Methods as Applied to the Indian  Ocean. 
 Bombay: Oxford and IBH Publishing.        

         Thanks to all contributers,        

         Best wishes,        

Nikolaus Malchus
PhD (Geology-Paleontology)
Dept. de Geologia/Unitat Paleontologia, Universitat
Autònoma de Barcelona, Campus, Edifici Cs
08193 Bellaterra (Cerdanyola del Vallès), Catalonia,
SPAIN; Tel x34-93-581-1464 / Fax x34-93-581-1263
International Congress on Bivalvia, July 2006
published abstracts, field guides and posters

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