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Re: Comments on BIG DEAD FISH etc
> >>Maybe *Dunkleosteus* had lungs, too... *Bothriolepis* had lungs, and
> some new trees get Placodermi and Osteichthyes as sister groups.
>
> I have a few problems with this. I'm not a great subscriber to the
> "Extant Pphylogenetic Bracket" line of reasoning,
Not "extant" this time. :-)
Just to clarify: All I knew about lungs in any placoderm before your post is
the following quote
"The most successful placoderm of all was *Bothriolepis* (Figure 3.13(b)),
known from more than 100 species found in Middle and Upper Devonian rocks of
all parts of the world. [...] One specimen shows evidence for lungs
preserved inside the dermal armour. *Bothriolepis* probably grubbed in the
mud for organic detritus, and it may have been able to survive in stagnant
ponds by breathing air."
on p. 58 of
Michael J. Benton: Vertebrate Palaeontology, Chapman & Hall 1997
Fig. 3.13b is the only one in the whole book that shows *B.*, and it is a
restoration that doesn't show any internal anatomy. There's not even a ref
for the lungs.
> especially at this taxonomic level.
What is a level? :-)
> It's problematic enough when used it the level of
> the Order [...],
We aren't back to the question what an order is, are we?
> never mind when used at the
> level of the Class ("_Dunkleosteus_ might have had had lungs because some
> bony fish have them"?! - perhaps I gaven't understood what you meant).
Here's what I mean: *D.* should be expected to have had lungs if
- *D.* and *B.* are both placoderms, respectively Placodermi is monophyletic
- *B.* indeed had lungs that look enough like those of Osteichthyes to be
possibly homologous
- all basal Osteichthyes that are known well enough have/had lungs
- Placodermi and Osteichthyes are rather closely related, without a taxon in
between that certainly never had anything of that sort
because it's most parsimonious that lungs only evolved once if the criteria
above hold.
> As far as lungs in _Bothriolepis_ go, I recall that the evidence for this
> wasn't that robust (interpretation of some ambiguous stains on a well
> preserved specimen),
I see. So the second criterion is shaky.
> Anyway, even if _Bothriolepis_ did have an air breathing
> organ, that says little about _Dunkleosteus_. _Bothriolepis_ was an
> antiarch, and was a very different animal to the dinichthyid (spelling?)
> arthrodires.
When all four criteria hold, then it does say something about all
Placodermi -- that lungs should be assumed for all of them in the absence of
evidence to the contrary.
> I do seem to recall some discussion about the small arthrodire
> _Groenlandaspis_ having lungs, but I can't confirm this 'cos I don't have
> the reference to hand.
Interesting.
> In any case, there is much more to air breathing in fish than the
> vagarities of inheritance.
> There are many linneages of air-breathing fish. Air
> breathing teleosts include species of catfish, (_Clarias_), climbing perch
> (_Anabas_), tarpon (_Megalops_), swamp eels (_Synbrachus_), mudskippers
> (_Periophthalmus_) - more 'primitive' air breathing fishes include the
> birchir (_Polypterus_), bowfins (_Amia_), and garpike (_Lepisosteus_) as
> well as the true lungfishes. Thus air breathing in fish is not an rare
> habit. As each of these examples have probably evolved air breathing
> independently, they haven't acquired it through a common inheritance.
Some apparently have. First, let's assume that lungs and swim bladders are
homologous. Then let's assume that lungs occur primitively in all
Sarcopterygii (now the basalmost living sarcopterygian, *Latimeria*, doesn't
have lungs, but from what I've read the fat-filled sacs it has don't look
too much like normal swim bladders... and where it lives it can't have lungs
a priori). Next, let's exclude *Clarias* and all others (which ones are
that?) that don't use these organs but the skin or whatever. So if we take 2
cladograms from the book above (pp. 171 and 179) and map those
actinopterygians that I've read have lungs on it (capital letters), we get
(all extinct groups marked + ; AFAIK fossil lungs are known)...
Actinopterygii
|--Cheirolepididae +
`--+--POLYPTERIDAE
`--Actinopteri
|--Mimiidae +
`--[...]
--+--+--Saurichthyiformes +
| `--Chondrostei sensu stricto
| |--Chondrosteidae +
| |--Acipenseridae
| `--Polyodontidae (does *Polyodon* have lungs?)
`--+--Palaeonisciformes +
`--[...]
--Neopterygii
|--LEPISOSTEIDAE
`--+--Semionotidae & Dapedidae +
`--[...]
|--Pycnodontiformes +
`--Halecostomi
|--+--AMIIDAE
| `--Parasemionotidae +
`--Teleostei
|--Pachycormidae (incl.
*Leedsichthys*) +
`--[...]
|--Ichthyodectidae +
`--+--OSTEOGLOSSOMORPHA
`--Elopocephala (incl. all
living teleosts not mentioned above)
If the osteoglossomorph *Arapaima gigas* does indeed use lungs (a book says
"big swim bladder connected to the oesophagus") and not something else to
breathe air (*Clarias* has very special things on its gills...), and if (see
above) the sister group of Actinopterygii, Sarcopterygii, had lungs from the
beginning, then it is most parsimoniously to assume that lungs are at least
a synapomorphy of Osteichthyes, and that lungs evolved into swim bladders
twice: once in Chondrostei and once in Elopocephala. If *Arapaima* doesn't
count, then the second event shifts from Elopocephala to Teleostei. In both
cases we should expect lots of fossil actinopterygians to have had lungs.
(Whether *Saurichthys* had lungs or a swim bladder cannot be resolved this
way, if the cladogram is correct.) Only then we could argue e. g. that
*Leedsichthys* certainly didn't need lungs, but could have made good use of
a swim bladder.
Is there any evidence that *Arapaima* secondarily changed its swim
bladder back into a lung?
> They do, however, have much in common ecologically, [...]
> all environments where dissolved oxygen can become very rare.
>
> Perhaps _Bothriolepis_ and _Groenlandaspis_ might have benefitted from an
> auxillary breathing system, as their fossils are often preserved in
> freshwater or estuarine sediments. But _Dunkleosteus_? To say, by
> extension, that it could breathe air is as erroneous as suggesting that
> coral trout can breathe air, just because climbing perch can.
There is the suggestion (in Carl Zimmer: At the Water's Edge. Macroevolution
and the Transformation of Life) that lungs evolved first in fully marine
high-speed chasers to supply oxygen to the heart. (No idea if *Dunkleosteus*
had such a lifestyle.) The normal condition for vertebrates is that the
heart pumps all blood to the gills, from there to the rest of the body
including the big swimming muscles that take all the oxygen out, and then,
at last, back to the heart which ends up with no oxygen in extreme exercise.
When a lung is present the blood from there gets straight to the heart -- a
definite advantage. Afterwards vertebrates with lungs were able to colonize
oxygen-poor water as you describe.
> The arthrodirous placoderms _Homosteus_, _Heterosteus_, and
> _Titanichthys_ have long been suggested as filter-feeders, and they
> equalled or exceeded _Dunkleosteus_ in size.
Interesting. I only knew the name and supposed length of *Titanichthys*.
> Regarding _Dunkleosteus_ and the other dinichthyids (_Dinichthys_,
> _Gorgonichthys_),
Aren't the first two synonymous? Never heard of *G.*.
> >>>Mosasaurus hoffmanni is documented at about 17 m.... really huge in my
> book, although more of a big snake in body plan... Hainosaurus, a
> johnny-come-lately tylosaurine reached 15 m during the Maastrichtian
> .....
Wow.