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Unlike the other animals I'll be covering as part of Scleritome Week, the Palaeoscolecida were actually known as entire animals long before their dermal armation was described, but they still meet the Scleritome Week qualifications because said armation was described as isolated problematic fossils before a connection was made between the animal and its armour (Ivantsov & Wrona, 2004). The photo above (from here) shows one of the isolated sclerites, originally described under the name Hadimopanella. Palaeoscolecid sclerites are round and button-like, with a central array of nodules that vary in different species from low and rounded to higher and pointed. Opinions on the nature of these microfossils (to appreciate how small they are, the scale bar on the photo above represents 0.03 mm) varied from some sort of dermal armour to the remains of reproductive cysts (Repetski, 1981). The dermal armour theory, of course, won out when the connection was made between the isolated sclerites and ornamentation on the compressed body fossils almost simultaneously by different authors in 1989 (Ivantsov & Wrona, 2004).
Palaeoscolecidans were a successful group of burrowing worms in the early Palaeozoic, when they were probably even more significant than the annelids (the image above of the holotype of Tabelliscolex hexagonus comes from Han et al., 2007). Originally interpreted as annelids, the segmented appearance is apparently only superficial, and results from alternating bands of larger and smaller plates (Ivantsov & Wrona, 2004). Well-preserved specimens from the Chengjiang Fauna possess an anterior spiny proboscis like that of the modern priapulids, and palaeoscolecidans have most often been regarded as priapozoans*. Other authors have suggested relationships with the modern nematomorphs, or as stem-panarthropods (Han et al., 2007). At the very least, a position within the Ecdysozoa, the clade uniting these three groups, seems well-established.
Tomorrow, I'll move on to chancelloriids.
*Bring on the nomenclatorial quibble. Most authors supporting this affinity have simply referred to palaeoscolecidans as "priapulids". The modern priapulids are a small, well-defined group of worms, while the various Palaeozoic taxa regarded as stem-priapulids show a much higher diversity of body plans (many of them, for instance, were far more elongate than any living priapulid, while no living priapulid possesses a dermal armour like that of palaeoscolecidans). Personally, I'd prefer to only refer to the crown group as priapulids, and use the name Priapozoa to cover the larger group including the stem forms.
REFERENCES
Han, J., J. Liu, Z. Zhang, X. Zhang & D. Shu. 2007. Trunk ornament on the palaeoscolecid worms Cricocosmia and Tabelliscolex from the Early Cambrian Chengjiang deposits of China. Acta Palaeontologica Polonica 52 (2): 423-431.
Ivantsov, A. Y., & R. Wrona. 2004. Articulated palaeoscolecid sclerite arrays from the Lower Cambrian of eastern Siberia. Acta Geologica Polonica 54 (1): 1-22.
Repetski, J. E. 1981. An Ordovician occurrence of Utahphospha Müller & Miller. Journal of Paleontology 55 (2): 395-400.
Personally, I'd prefer to only refer to the crown group as priapulids, and use the name Priapozoa to cover the larger group including the stem forms.
ReplyDeleteHear, hear.
Under the current draft of the PhyloCode, the total group could be called Pan-Priapulida. That, or Priapozoa could be defined as a total group with "pan-Priapulida" as an informal synonym.
Personally, I absolutely loathe the concept of "Pan-" names - I think it's a terrible idea. No technical proper name should include a hyphen or other such punctuation. Those are the first things to get left out if a tyupo occurs.
ReplyDeleteThose are the first things to get left out if a tyupo occurs.
ReplyDeleteLike that one.
I think there's something to be said for them. You see Pan-Gooblahoy and you instantly know: 1) Gooblahoy is a crown group, 2) Pan-Gooblahoy is a total group, and 3) stem group members can be referred to as "stem-gooblahoys". (That said, I think it was a good move to make panclade names non-mandatory in the latest draft--and not just because I helped come up with the solution [informal panclade names].)
ReplyDeleteAnyway, you simply couldn't have them without something like a hyphen, given preexisting names like Panarthropoda (when there is an Arthropoda), Panoplosaurus (when there is an Oplosaurus), Panthera (when there is ... well, there's no "Thera", but there is a Theria), etc. (Not to mention our close cousins, Pan!)
Hyphens also come in awfully handy for converting the names of subgenera and intercode synonyms (e.g., Rana-Rana, Zoo-Gastonia). The alternatives are: 1) the Herculean task of renaming most monophyletic subgenera and all monophyletic junior intercode synonyms (which seems like an enormous waste of time), 2) using something else, like apostrophes (which boils down to your taste in punctuation, I guess), or 3) abandoning the entire concept of "one name, one clade".
Unless I'm missing something, of course.
The Botanical Code still allows hyphens in names, but I think that the Zoological Code definitely took a step in the right direction when it disallowed hyphenation of proper names - I couldn't begin to count the number of times I've seen penna-marina turned into penna marina, which of course looks like a trinomial to the unwary.
ReplyDeleteI think that Pan-Priapulida is a different thing to pan-Priapulida. Pan- is fine as an informal descriptive prefix, just like stem-Priapulida or crown-Priapulida, but it's an entirely different matter when it becomes an integral part of a formal name. Of course, there's nothing to stop an author naming "Panpriapulida".
Priapozoa was named for the group of Loricifera and Priapulida, exclusive of Kinorhyncha, so it's a little different from the total group of Priapulida.
Interesting point about penna-marina. I have to admit I hadn't thought much about that.
ReplyDeleteBut if you lose the hyphen in Pan-Priapulida, you're left with "Pan Priapulida", which isn't terribly confusing (unless the text is somehow about chimpanzees and penis worms).
It wouldn't be a problem with converted subgenera, either. Rana-Rana (PhyloCode) vs. Rana Rana (typo) vs. Rana (Rana) (ICZN) -- meaning's pretty clear in any event. There could be confusion if the genus, type subgenus, and type species epithet were all spelled the same, though -- but, honestly, is that any different with the rank-based codes?
I don't see names like Apo-Tetrapoda or Zoo-Prunella causing any problems if the hyphen were accidentlaly omitted.
The only case where I can really see panclade names getting ugly is with preexisting names starting with "Pan". Panarthropoda would be different from Pan-Arthropoda -- not good. And things could get confusing in Hominoidea with names like Pan-Gorilla or Pan-Pan. (Although fortunately the genus Pan requires no subgenera, since it only has two known species.) I have to admit the nomenclature here presents a special quandary.
Priapozoa was named for the group of Loricifera and Priapulida, exclusive of Kinorhyncha, so it's a little different from the total group of Priapulida.
Ah, well, what else would the total group be named, then? (Not to mention Pan-Priapozoa....)
I suppose you could make the case that all panclade names should be informal. I don't know, though, is that really terribly different?
I don't see names like Apo-Tetrapoda or Zoo-Prunella causing any problems if the hyphen were accidentlaly omitted.
ReplyDeleteOnly if there were two different names differentiated only by the presence of the hyphen. I'm guessing that Apo-Gastropoda would be the clade defined by the apomorphies of Gastropoda? That's a quite different clade from the already existing Apogastropoda.
I suppose you could make the case that all panclade names should be informal.
That is exactly the position I'm arguing for, the same as stem-[clade] or crown-[clade].
I don't know, though, is that really terribly different?
I would say so. As soon as questions of priority become involved, it makes a huge difference whether a name is formal or informal - formal names affect priority, informal names don't. You might argue that the rules could state that such hyphenated names can't take priority, but that would just cause confusion in cases like the aforementioned Panarthropoda or Apogastropoda.
Usage of the "pan-" prefix would also be up to the author's personal preference. A priapulid worker might be quite happy talking about pan-priapulids (no chance of confusion), while an arthropod worker would probably avoid pan-arthropods. Of course, one could always just refer to "the total group of Arthropoda".
I'm guessing that Apo-Gastropoda would be the clade defined by the apomorphies of Gastropoda?
ReplyDeleteOnly if 1) Gastropoda is defined as a crown group (Art. 10.4), and 2) the word "Gastropoda" refers etymologically to some apomorphy that evolved within the corresponding stem group (Art. 10.7). (I have to admit I have no idea whether either is the case, although I'd suspect that #1 could be true.)
Unlike panclade names, "Apo-" names need to be formal, because they need definitions that spell the apomorphies out in detail. With panclade names, the definition is mandated (Pan-X := everything sharing closer ancestry with X than with any extant non-X).
That is exactly the position I'm arguing for, the same as stem-[clade] or crown-[clade].
Not going to this year's ISPN meeting, by any chance, are you?
A priapulid worker might be quite happy talking about pan-priapulids (no chance of confusion), while an arthropod worker would probably avoid pan-arthropods. Of course, one could always just refer to "the total group of Arthropoda".
Well, as it is, panclade names are optional (Art. 10.6), so priapulid workers could have Pan-Priapulida and arthropod workers could come up with some other name for the arthropod total group (although it would have to be preexisting, per Art. 10.3), or leave it unnamed. (The term "pan-arthropods" would always be available informally, although I doubt anyone would use it.)
I'm still not convinced that the status quo is all that bad, though. Why not let vertebrate paleontologists have a formal Pan-Aves just because primatologists don't want a formal "Pan-Gorilla"? (I presume -- maybe they'd be fine with it.)
You do make a good case for a new rule, though, that a hyphenated name should not be coined if it consists of the same letter sequence as an established clade name. Rec. 10B comes close to saying this, but a rule (perhaps in Art. 13) might be a better idea. This would override rules like 10.3, 10.7, etc.