The Orthocerid Animal

Discussion in 'Cephalopod Fossils' started by Nauti-guy, Sep 27, 2008.

  1. Nauti-guy

    Nauti-guy Cuttlefish Registered

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    According the the abstract of a paper by Johannes Mehl (1984): Radula und Fangarme bei Michelinoceras sp. aus dem Silur von Bolivien, Palaeontologische Zeitschrift, Band 58, Heft 3/4 . p. 211-229, which some of you may have seen already, Sulurian Michelelinoceras found in Bolivia had seven radular teeth per row, as common in Ammonoids and Coleoids, not the nine found in Nautilus, and ten arms of which two were long tentacles. This seems to corroborate the conclusion by Flower in a 1955 paper in the Journal of Paleontology that certain imprints in the Upper Ordovician Corryville beds of Cincinnati were produced by the tentacles of Orthonybyoceras (=Treptoceras), later determined to be an orthocerid. In his paper, Flower speculated that Orthonybyoceras may have had ten tentacles (not differentiating between them and arms).

    This certaintly indicates that the orthocerid, e.g. Michelinoceras, animal was rather a neocephalopod, as is shown in some more recent taxonomies, in spite of having a basically nautiloid shell. Flower also wondered (I'm recalling this from memory) which was the more primitive or ancestral, many arms or tentacles as in Nautilus or few (8 or 10) as in
    modern coleoids.
     
  2. monty

    monty Colossal Squid Staff Member Supporter

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    Link to abstract:

    http://www.schweizerbart.de/pubs/journals/0031-0220/paper/58/211

    This also raises interesting questions about 8 arms + 2 tentacles vs 10 identical arms. The belemnite hook impressions seem to pretty definitively show that their 10 arms were very similar, and there weren't feeding tentacles, so if much older forms had feeding tentacles, that would seem to either suggest that belemnites were a reversion to an ancestral form, or a separate clade, or a separate branch. All of those seem sort of unlikely.

    I think the nautilus evo-devo paper from a few years back makes it pretty clear that the large numbers of tentacles are derived from 10 arm buds, so the 10-limbed state is almost certainly the ancestral form... but when the feeding tentacles arose is, of course, very hard to pin down with so little soft tissue preservation. Still, the "common wisdom" is that the belemnites represent a connection to the ancestral coleoid, with 10 identical arms, bolstered by Vampyroteuthis having a different pair of specialized appendages than the decabranchian cephs... just using arm arrangements to categorize the clades seemed to work OK, but this early animal with feeding tentacles would seem to throw a wrench into that. (Of course, I've been suspicious that spirula is a potential problem for that whole approach for a while, anyway, since linking it to cuttlefish, while justified in some ways, is a bit of a stretch in others, suggesting that things may be more complicated.)

    Google scholar says this paper is referred to in Cephalopods Present and Past (Neil H Landman, R A Davis, Royal H Mapes 2007), which I own. Hallucigenia has at the moment, but is out of town so I can't get it back for a bit :boohoo: More on that later, or if someone else has the book handy, please feel encouraged to post the details there...
     
  3. monty

    monty Colossal Squid Staff Member Supporter

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  4. Sordes

    Sordes Wonderpus Registered

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    Well, you should not forget that at least male belemnites had this monstrous onychites at two of their tentacles which had no other hooks, so I would not say their 10 arms were very similar. How knows, perhaps feeding tentacles evolved from such arms which were once used during mating?
    BTW, here is a new and very nice reconstruction of a Kosmocerate with two long tentacles: http://kosmoceraten.de/Neuigkeiten/body_ammonitenmodell.html
     

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