Guiyu oneiros

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A fish is a fish, right? They're just a blur of aquatic beasties that most people distinguish by flavor, rather than morphology or descent. But fish are incredibly diverse, far more diverse than terrestrial vertebrates, and there are significant divisions within the group. Most people know of one big distinction, between the Chondrichthyes (fish with cartilaginous skeletons, like sharks and rays) and the Osteichthyes (fish with bony skeletons), but there's another particularly interesting split within the Osteichthyes: the distinction between Sarcopterygians (the word means "fleshy fins", and we call them lobe-finned fishes colloquially) and the Actinopterygians, the ray-finned fishes. The lobe-finned fishes most distinctive feature is the muscular and bony central core of their fins — extant forms are the coelacanth and lungfish. It is this lineage that led to us terrestrial tetrapods, but other than that successful invasion of the land, the sarcops were something of an aquatic failure, with only a few genera surviving. The ray-finned fishes, on the other hand, are a major success story, with more than 28,000 species today. To put that in context, there are only about 5,500 species of mammals.

The Sarcopterygii and the Actinopterygii must have begun diverging a long time ago, and a couple of questions of interest are a) when did the last common ancestor of both groups live, and b) what did it look like? We don't have a good and specific answer yet, because Osteichthyes origins are lost far, far back in time, over 400 million years ago, but every new discovery edges us a little closer. What we now have is a well-preserved fossil of a fish that has been determined to be an early sarcopterygian, and it tells us that a) the last common ancestor had to have lived over 419 million years ago, the age of this fossil, and the divergence probably occurred deep in the Silurian, and b) this animal has a mosaic of primitive Osteichthyan features, which tells us that that last common ancestor may well have shared some of these elements. It is another transitional fossil that reveals much about the gradual separation of two great vertebrate groups.

And here it is:

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(Click for larger image)

a, b, A near-complete fish in part and counterpart. c, Close-up view of the anterior portion of the trunk shield in dorsal view, showing MD1 and MD2 flanked by rhomboid scales. d, Close-up view of the dorsal fin spine. MD1, first median dorsal plate; MD2, second median dorsal plate. Scale bar, 1 cm.

That may be a bit disappointing at first — it looks like Silurian road-kill — but really, that's a remarkable good and useful specimen. The animal was covered with thick bony scales, and the skull was built of thick bony plates, and so while it was squashed flat by pitiless geology, the pieces are all there, and it can be reassembled into a much more fishy state. This drawing may be more satisfying:

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(Click for larger image)

a, Restoration of the entire fish in lateral view. b, Interpretive drawing of the holotype V15541. Areas shaded in grey are unknown, and are reconstructed from other early osteichthyans. ano, anterior nostril; br, branchiostegal ray; cla, clavicle; cle, cleithrum; drs, dorsal ridge scale; dsp, dorsal fin spine; et, extratemporal; eta, accessory extratemporal; f.add, adductor fossa; f.gl, glenoid fossa; gu, gular; ju, jugal; l.ext, lateral extrascapular; lj, lower jaw; m.ext, median extrascapular; mx, maxillary; n.sp., spiracular notch; op, opercular; pa, parietal shield; pcl, postcleithrum; pop, preopercular; ppa, postparietal shield; psc, presupracleithrum; pt, post-temporal; scl, supracleithrum; sop, subopercular; sp., pectoral spine; tr, lepidotrichia; vrs, ventral ridge scale.

Now it looks like a kind of armored, spiky salmon with a thick muscular body (and yes, I too wonder about flavor, and would like to taste a slab of that). It's definitely not a salmon, though — the bony structure is a curious set of compromises where some features are distinctly sarcopterygian, some look like they belong on a primitive actinopterygian, and others are unique or show affinities to characters of ancient extinct fishes, like rhipidistians. This is very cool. What we see here are relics of an ancient common osteichthyan ancestor, which are being honed into the specific characteristics of the Sarcopterygii. The analysis of the totality of the animal's features, though, place it more in the lobe-finned than the ray-finned clade. That places it on a branch of the line leading to us…a very, very old branch, making this your many-times-great grand uncle, or cousin only a few million times removed. Now my curiosity about a taste-test is making me feel mildly cannibalistic.

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(Click for larger image)

The topology is the most parsimonious tree arising from a matrix of 23 taxa coded for 153 morphological characters (tree length = 292, consistency index = 0.572, retention index = 0.737, rescaled consistency index = 0.421). The numbers at nodes indicate bootstrap support (where the value is greater than 50%) and Bremer decay index (bottom and top numbers, respectively). Eif., Eifelian; Ems., Emsian; Fam., Famennian; Fras., Frasnian; Giv., Givetian; Gor., Gorstian; Loch., Lochkovian; Lud., Ludfordian; Prag., Pragian.

When you look at that diagram, what should jump out at you is all the diversity in the Devonian, the so-called Age of Fishes, and the paucity of representative fossils from the Silurian…which is exactly where all the interesting branch points in the fish family tree are located. Once again, paleontology is a predictive science, and this tells us where to look for the next batch of exciting and informative fossils.


Zhu M, Zhao W, Jia L, Lu J, Qiao T, Qu Q (2009) The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature 458:469-474.

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We've been working our way across through the tree of life in the past few editions of Phylogeny Friday. Last week we took a look at the evolutionary relationships of the animals, and we realized that many of the branching orders are extremely difficult to resolve. Today we're going to zoom in on…
In addition to AIG, Reasons to Believe and all the other DI folks falling all over themselves to say nonsensical things about Tiktaalik roseae, Casey Luskin has now jumped into the fray with this silly post at the DI's blog. His argument can be summed up thusly: even though this find fills in a gap…
Tiktaalik: music to my ears. Tiktaalik is the lilting name of a newly discovered fossil fish with fingers. It lived 380 million years ago in the northern reaches of Canada, back when the northern reaches of Canada were tropical coastal wetlands not far from the equator. Tiktaalik's discoverers (Ted…
The paddlefish is a surreal giant, with a spatula-shaped nose that some scientists believe it uses to sense the electric fields of its prey, which it sucks up like a whale. You might not think of it as an animal that has much to offer in our quest to understand ourselves. But in fact, underneath…

Just more irreducible complexity, don't you know.

That's the beauty of ID, every prediction of non-telic evolution that is found becomes a demand to explain every last change, or poof is the only explanation.

Aside from that, what a cool fishy, and cool (basically Darwinian in pattern) diversity of fishies.

Glen D
http://tinyurl.com/6mb592

Nice April fools prank. But, I ain't buying it.

I have nothing to say on the subject, I am merely a lay person. But this is one of the reasons why I come here, PZ has a very clear writing style when he writes about peer reviewed papers.

By Janine, Insult… (not verified) on 01 Apr 2009 #permalink

400 Million years ago??? You mean 6000 years ago, right?

Interesting article! Many thanks, as always, for this GREAT blog!

Very cool, very cool indeed. The gaps just keep getting smaller, except for the gaps in creationist grey matter.

...and so while it was squashed flat by pitiless geology...

That's right!

And so, cue creotard blithering* in 3, 2...

*"SEE!!!! That reconstruction doesn't look anything like the real fossil! See how darwinists twist the truth, any truth, to fit their a priori assumptions that there is no god and that evolution is true?"

Is it wrong to want a "you are here" label on that tree?

By Thomas Winwood (not verified) on 01 Apr 2009 #permalink

After whining on the other thread about why I was "here," figure I'd better comment on a nice biopost. Cool cool fossil; Devonian vertebrates are very cool but Silurian is even better. Why no extant or more recent taxa in the phylogeny?

By Sven DiMilo (not verified) on 01 Apr 2009 #permalink

Fins and a head ehh? Just a fish then, how you explain this stasis?

oooooh: #7 -> #8 by pure synblogicity.

By Sven DiMilo (not verified) on 01 Apr 2009 #permalink

...but too bad no tail or paired fins.

By Sven DiMilo (not verified) on 01 Apr 2009 #permalink

"...Silurian road-kill." :-)

For some reason, I get all tingly inside when I read your fossil related posts. They are so concise and clear that even my ignorant ass can understand...

Thanks

Most people know of one big distinction, between the Chondrichthyes (fish with cartilaginous skeletons, like sharks and rays) and the Osteichthyes (fish with bony skeletons),

...for wildly optimistic values of "most". A pretty good chunk of the population not only doesn't get that distinction, they probably toss cetaceans in there, too.

They're facing the wrong way!

why do scientists keep going out and finding these specimens? don't they know that for every specimen they find, they create 2 more gaps in the fossil record? at this rate, the density of fossils in the record will diminish to zero while the number of gaps diverges and we will have no fossils at all, just gaps.

looking at a display of fossil record gaps at the Smithsonian will be very boring. they should stop now while we still have cool fossils to look at.

:)

[to the tune of "Mr. Ed"]

A fish is a fish?
You wish, you wish.
For not every fish makes a tasy dish.
Unless you opt
For an actinopt,
You'll gnaw on lobey fins!

By Not Cuttlefish… (not verified) on 01 Apr 2009 #permalink

Dreamy.

I must admit, I was expecting, because of the date, the analysis of the fleshy-lobed fish line leading eventually to the Darwin Fish, and the bony-finned fish line to the Jebus Fish.

Each new fossil does not produce two additional gaps, but one. You fill in one gap and there is a gap on either side, but one of them was there in the first place. So the number of gaps is always increased by one.

By Lee Picton (not verified) on 01 Apr 2009 #permalink

it looks like Silurian road-kill

I suspect more like a Silurian mud slide kill. Still, very interesting find, and well explained as to its importance.

By Nerd of Redhead, OM (not verified) on 01 Apr 2009 #permalink

Haven't finished reading the article yet. Isn't it more correct to measure the effectiveness of a species by population biomass rather than number of living species? In that light I bet the lobed fin fishes(and terrestrial verts) seem a little better off than they appear by simply comparing the number of species. Am I correct anyone who can validate or invalidate my intuition?

Lets see, fins, scales, lived in water; looks exactly like any other fish. More proof that evolution is a lie! Or darwinism is a lie. Or both. I forget...

John: While you're correct that there are various measures of animal-group "effectiveness" or "success," in this case the biomass, number of individuals, and species diversity of ray-finned fishes vastly exceeds the terrestrial vertebrates and especially the rare and depauperate lobe-fins, and probably always has.

By Sven DiMilo (not verified) on 01 Apr 2009 #permalink

Something about fossils gives me a warm fuzzy feeling inside, especially ones as cool as this. Or it did before today, but now I've found the LORD, hallelujah! and you're all a bunch of sinners.
This post makes me want to go and finish reading that book I got on ancient marine reptiles. But of course, I'll go read the Bible instead. The Bible's got cattle. Lots and lots of cattle. Which is so much more fun than incredibly cool fish fossils...

The chinese fossils are amazing, from ur-bilaterians to ur-birds, everything seems to be found there.

Yay. A science post! Nice fish ya got there.

the bony structure is a curious set of compromises

Nit to pick: "Compromise" implies an evolutionary goal, or a Platonic ideal of some sort, whereas each organism is just what it happens to be at the time. While everyone here presumably gets that, it invites creotard quotemining IMHO.

(Everybody's a critic, huh?)

...and so while it was squashed flat by pitiless geology...

Don't mess with geology. Don't make me use the hammer. [/threatening glare]

Also, I suppose the lemon and butter comment has already been made.

By jimmiraybob (not verified) on 01 Apr 2009 #permalink

What needs to be pointed out in all this talk about the success of lobe-finned vs. bony fishes is that we (and every other tetrapod species) are lobe-finned fishes.

Lorkas: Correct; that puts the "you are here" sign down among the puke-green branches labeled "crown sarcopterygians." Just waiting for David Marjanović to drop by and tell us exaclty where.

By Sven DiMilo (not verified) on 01 Apr 2009 #permalink

(Creationist rant) But this isn't a different Kind of animal from the other fishes! And besides, that was clearly weighted down and crushed flat by a mudslide, and everyone knows mudslides happen in Floods! See, it only proves that the devil put that there to test your faith! (/Creationist Rant)

addendumb: besides, how else can you explain pygmies and Dwarfs !!*(!!^!!)

Great post - so much fishy goodness (yum), and about the only thing I've read today that doesn't sound like April foolishness.

I've just read Don Prothero’s "Evolution: What the Fossils Say and Why It Matters" and have started Neil Shubin's "Your Inner Fish" (Jerry Coyne's "Why Evolution is True" is cued up next), so I'm hot on paleontology at the moment. I was very keen on hunting and finding fossils when I was a kid; makes me wish I had gone in that direction for a career, now.

And with such wonderfully written and accessible books as these available now, how can the IDiots possibly excuse their willful ignorance of what evolution is actually about.

Andy

Thanks, PZ... very interesting indeed.
I was talking to my mother in law yesterday evening, about Tiktaalik, and how that if we traced the lineage of mammals, reptiles, amphibians and birds, far enough back, we'd find lobed fin fish that developed lungs and moved onto land.
She started to read my copy of The God Delusion, but it was a bit too heady for her. She's interested in these topics, but doesn't have foundation to really grasp it. Growing up in the stifling environment of a strict Catholic household didn't help.
This makes me want to pick up Neil Shubin's, Your Inner Fish, all the more, which I'll likely do tomorrow.

"Click for Larger Image" worked once, then jumped back on its own to the small size after about 10 seconds and now does not work at all. Is it just me? I am missing out on the big pictures.
Stephanurus

By Stephanurus (not verified) on 01 Apr 2009 #permalink

Damn, you are good. Conveying the fun of all this and evoking the pleasure you find in it in your predominantly civilian readers—that's no mean feat. AFAIK you're also not glossing over anything to "simplify" it for us.

Thanks, and Happy St. Stupid's Day to you and TW!

Stephanurus, click to open in a new window, or click the scroll wheel to open in a new tab, and then ctrl+scroll to resize.

The ray-finned fishes, on the other hand, are a major success story, with more than 28,000 species today. To put that in context, there are only about 5,500 species of mammals.

But shouldn't the proper comparison be with the number of species of Sarcopterygians? I seem to recall 9500 birds, 8200 reptiles, and 6400 amphibians. That's 29,600 species, plus whatever other Sarcopterygians there are.

So the Osteichthyes forced our ancestors out of the ocean onto the land. I'll remember that the next time I have a piece of cod.

By 'Tis Himself (not verified) on 01 Apr 2009 #permalink

Now, if they had also found a plate and a knife in the same matrix I would think the creationists might have a point.

What Ron said at #35...
My brain hurt a little (its been an increasingly-stereotypically fucked-up day over here on the Dartmoor-area community nursing front) but I persevered and re-read and kept going. Excellent stuff.

By Thunderbird5 (not verified) on 01 Apr 2009 #permalink

The topology is the most parsimonious tree arising from a matrix of 23 taxa coded for 153 morphological characters (tree length = 292, consistency index = 0.572, retention index = 0.737, rescaled consistency index = 0.421). The numbers at nodes indicate bootstrap support (where the value is greater than 50%) and Bremer decay index (bottom and top numbers, respectively).

If you ever find time to do some more of those nifty "Basics of Biology" post (like the Sonic Hedgehog &c ones), would you perhaps consider doing a brief introduction to cladistics? (That is what those numbers are about, right?)

What's the odd fishie with the anvil on it's head on the top right?

Sometimes I do wonder if all of the best tasting animals are extinct.

By TheNaturalist (not verified) on 01 Apr 2009 #permalink

Surely, the best-tasting animal of all must be alive today, for among its many truly bodacious attributes, it must have the attribute of existence.

</ontological-proof-of-bacon>

Surely, the best-tasting animal of all must be alive today, for among its many truly bodacious attributes, it must have the attribute of existence.

[TAFB1]

(1) If reason exists then God exists.
(2) Reason exists.
(3) Mmmmm, bacon.
(4) Therefore, God exists.

1 Trancendental Argument From Bacon

By heliobates (not verified) on 01 Apr 2009 #permalink

So, which day of creation was this created?

/wink

So, which day of creation was this created?

What, bacon?

Sixth Day. Cherry on top of Creation.

Nah, seventh.

God rested not because he saw that all was good, but because he'd had a full English.

Which was good.

Between Guiyu in the Silurian and the new phylogenies showing both placoderms and acanthodians as paraphyletic, I had to totally revamp my Early and Mid Paleozoic marine life lectures for Historical Geology this semester. Harumph!!

(And next semester I have to work heterodontosaurid protofeathers in to Dinosaurs. Ah, Nature: why do you keep revealing interesting surprises?!?)

Thanks for the post PZ! We actually just finished the chapter about chordate taxonomy in my survey of evolution course, so that was a lovely supplement to the material. You are awesome!! :)

#37: To your 29 000 or so species of tetrapods we can add 3 species of lungfish and 2(?) species of Coelacanth, and I think that's about it. So the estimate doesn't change that much.

#38: Yep. Our ancestors were (probably) driven out of shallow seas into deep waters by arthropods (anomalocaris and sea scorpions), then chased out of the ocean and into fresh water by the placoderms. Then out of the water and onto the land by actinopterygians (and a few fellow sarcopterygians), then off the shoreline and into the dry interiors by amphibians, then out of daylight and into the night and underground by thecodonts, then into the trees by various diapsids, and then out of the trees and onto the savannah by the ancestors of chimpanzees.

We're descended from a long and distinguished line of losers. (Resourceful, adaptable, and lucky losers).

#42: Anvilhead is a primitive shark.

Amphiox, running from danger has worked for Rincewind.

By Janine, Insult… (not verified) on 01 Apr 2009 #permalink

running from danger has worked for Rincewind.

isn't it the case that running from one danger, he typically smacks blindly right into a much larger one, though?

PZ wrote: "...making this your many-times-great grand uncle, or cousin only a few million times removed."

At 400 million years ago, our distant ancestor is more likely a few hundred million times removed....right?

Quibble about this:

"...the bony structure is a curious set of compromises where some features are distinctly sarcopterygian, some look like they belong on a primitive actinopterygian, and others are unique or show affinities to characters of ancient extinct fishes, like rhipidistians."

About the last word, really. "Rhipidistian" is an obsolete term for a paraphyletic group of sarcopterygians, everything that has traditionally been considered close to tetrapods, including osteolepiforms and porolepiforms. So, two complaints: 1) the group doesn't exist and 2) if it did, it would be clearly nested deep within Sarcopterygia, while your sentence seems to claim some other location on the tree.

(Though I see that some are now using "Rhipidistia" to mean "crown Sarpterygii.)

I see I'm going to have to pay more attention to fish phylogeny. Placoderms and acanthodians are both paraphyletic, and at least some of the latter are sister to sharks? By the way, my first thought on seeing that fish was that the head shield looks like a placoderm's, but if the figure is right there's no homology.

By John Harshman (not verified) on 01 Apr 2009 #permalink

Posted by: Ichthyic | April 1, 2009

running from danger has worked for Rincewind.

isn't it the case that running from one danger, he typically smacks blindly right into a much larger one, though?

That's true. And does this not fit into what amphiox was saying?

By Janine, Insult… (not verified) on 01 Apr 2009 #permalink

That's true. And does this not fit into what amphiox was saying?

so it really is turtles, all the way down?

:P

Another instance of Cath's Law! My self-named law, that I have formulated in a pathetic bid for internet immortality, is that every discussion of a new animal species eventually turns to speculation about eating them.

I'm still trying to formulate this correctly. "Animal" is too broad - it doesn't usually apply to insects. Is there a single clade for which the law holds? Vertebrata? Chordates? Further investigation is required. I strongly suspect that it's not mono-phyletic.

@#41 10CH
Nooooo, twas all compliments to Ron on his comment. Mine then below on my probs being a thicky were re the subject of the original post, not on his response.

Nurse, the screens....

By Thunderbird5 (not verified) on 01 Apr 2009 #permalink

@51: I believe that the 'Australian lungfish' is a complex of up to seven species.

So, did it taste like salmon or samlon? Inquiring SF minds want to know. :D

OT With the tireless effort of 80,000 volunteers producing more than 6 million sandbags in ten days, the good citizens of Fargo have managed to thwart gods will. The floodwaters of the Red River of the North are receding.

I did a little Web search on Silurian fossils and found that there are some up the road in Coberg, some down the road near Middleport NY in the Rochester shales, and some in Maine. And thats just for starters. I expect everyone will be out with a geologist's hammer next week....

Sometimes I do wonder if all of the best tasting animals are extinct.
Probably, and I'm sure at least a few were our fault. Steller's Sea Cow sounded delicious from contemporary accounts...

Note:

Anomalocaris - extinct
Brontoscorpio - extinct
Eurypterids - extinct
Orthocones - extinct
Meganeura - extinct
Dimetrodon - extinct
Gorgonopsians - extinct
Postosuchus - extinct
Liopleurodon - extinct
Allosaurus - extinct
Utahraptor - extinct
Mosasaurs - extinct
Tyrannosaurus - extinct
Diatryma - extinct
Andrewsarchus - extinct
Megalodon - extinct
Smilodon - extinct
Megalania - extinct
Short-faced bear - extinct

It seems the meek really do inherit the earth. (Yes, I know there is huge selection bias in my list. There are lots of insignificant little rat-like thingies that went extinct too.)

But it gives some pause in considering the ultimate fate of Homo sapiens, ultrapedator, eh?

I'm curious how one would go about the process of deriving the fishy shape from the squashed shape(less). I can imagine that some of it will be experience derived from other fossils and knowledge of fishy physiology but, damn, that's one squished fish.

And really, has nobody else noticed that the fish came from the crisper zone? Crunchy beer batter anyone?

By tim Rowledge (not verified) on 01 Apr 2009 #permalink

No data at hand, but I speculate that total biomass of clupeid fishes (shad, herring, menhaden, etc)alone, is similar to, or greater than, biomass of terrestral vertebrates, and maybe comparable to total terrestral animal biomass.

By Jim Thomerson (not verified) on 01 Apr 2009 #permalink

#7 - Is it wrong to want a "you are here" label on that tree?

People who work on Paleozoic vertebrates have this annoying habit of leaving the 'You Are Here' labels off (whether that be 'Tetrapoda', 'Amniota', 'Mammalia' or whatever). In this case, the closest relative to Tetrapoda shown on the tree is Kenichthys.

#60 - I believe that the 'Australian lungfish' is a complex of up to seven species.

Actually, no. Australia used to have a bunch of different ceratodont lungfish, but by the Holocene there was just one (Neoceratodus forsteri), present in one river system (the Mary) in south-east Queensland. It's since been translocated to other rivers including the Brisbane, and is also being bred in captivity by two groups, one of scientists and one purely commercial (now a popular aquarium fish in Japan, apparently; not sure how they rate as sashimi). Fossils referable to N. forsteri (i.e. toothplates not distinguishable from the modern form) are known from the mid-Tertiary at Riversleigh, and also from the Albian (late Early Cretaceous, ~110 Ma) of Lightning Ridge, making this a pretty venerable species of huge conservation value. OTOH, African lungfish include multiple (7 or so?) extant species, and there may also be more than one in the South American genus.

By John Scanlon FCD (not verified) on 01 Apr 2009 #permalink

I'd also like it to be known I enjoy and always read such pieces several times, PZ, but of course find no reason to comment, save expressing gratitude, trying not to say dumber-than-usual layperson things, etc.

***

"It's STILL just a FISH!" - A. X. Tionblokead

By Sioux Laris (not verified) on 01 Apr 2009 #permalink

lol PZ, Dickonosaurus? You're joshing me right?

I think raw species count by itself isn't a very good measure of diversity. Yes, there are less species of land vertebrates, but some of them are things like birds, bats and snakes. And I've heard rumors that a small fraction of one particular species has even gained intelligence. On the other hand, I will concede that the aquatic world has wonderful diversity to offer too. I love flatfish, and not just on my plate.

By Anonymous Coward (not verified) on 02 Apr 2009 #permalink

This is why I come here.

By Magnifico Giganticus (not verified) on 02 Apr 2009 #permalink

"aquatic beasties that most people distinguish by flavor, rather than morphology or descent."

And so began a strange argument between the two friends, since they were both familiar with fish but in entirely different ways. . . . ."Third," Conceil continued, "The subbrachians. Their pelvic fins are beneath the pectorals and attached to the shoulder bone. There are four families. . . . ."Superb! Superb!" cried the harpooner, still classifying fish according to their culinary value.

pgs 96-100

Translation Walter James Miller, 1993. United States Naval Academy. ISBN 1-55750-877-1

By Matterhorncyn (not verified) on 02 Apr 2009 #permalink

forgot to mention Jules Verne's Twenty Thousand Leagues Under the Sea

By Matterhorncyn (not verified) on 02 Apr 2009 #permalink

I am curious about the middle point in the image of the fishes' tail fin in the gray area - I assume this is a guess as to what that part actually looked like. Why is it there?
Will. M

Is it wrong to want a "you are here" label on that tree?

Kenichthys.

It's more closely related to us than to the lungfish (which are closely related to Diabolepis, Youngolepis, and the porolepiforms, of which Powichthys IIRC is one). It's right at the base of Tetrapodomorpha; animals like Eusthenopteron are more closely related to us than Kenichthys is.

Why no extant or more recent taxa in the phylogeny?

Weren't necessary for the phylogenetic analysis.

"...Silurian road-kill." :-)

There's a very complete Stegosaurus specimen out there which is called the Roadkill Specimen.

Each new fossil does not produce two additional gaps, but one. You fill in one gap and there is a gap on either side, but one of them was there in the first place. So the number of gaps is always increased by one.

No, because it's very, very rare that at least a vertebrate fossil is a direct ancestor of anything known. So, each new fossil adds a branch to the tree. This new branch is one new gap. Its addition furthermore cuts the old gap in half. One gap is eliminated and three are created, makes two new gaps overall.

The chinese fossils are amazing, from ur-bilaterians to ur-birds, everything seems to be found there.

"China is very big."
-- Mandarin textbook sentence for remembering the four tones.

But shouldn't the proper comparison be with the number of species of Sarcopterygians? I seem to recall 9500 birds, 8200 reptiles, and 6400 amphibians. That's 29,600 species, plus whatever other Sarcopterygians there are.

About ten: two species of coelacanth (the Comoros one, Latimeria chalumnae, and the Indonesian one, L. menadoensis), one of Australian lungfish (Neoceratodus forsteri), so far one of South American lungfish (Lepidosiren paradoxa – but see comment 67), and something like five of African lungfish (Protopterus) – I'm too lazy to look up how many exactly there are.

It's of course easily possible that additional Latimeria species await discovery. But not hundreds.

If you ever find time to do some more of those nifty "Basics of Biology" post (like the Sonic Hedgehog &c ones), would you perhaps consider doing a brief introduction to cladistics?

"REALLY good introduction into cladistics!!"
-- Thomas R. Holtz, Jr.

(That is what those numbers are about, right?)

Yep.

What's the odd fishie with the anvil on it's head on the top right?

An ordinary Carboniferous chondrichtyan. You should see the weird Carboniferous chondrichthyans, har har har.

Our ancestors were (probably) driven out of shallow seas into deep waters by arthropods (anomalocaris and sea scorpions),

What, when, what "deep waters"?

then chased out of the ocean and into fresh water by the placoderms.

All nonsense. Tulerpeton lived in the sea, and many coal swamps appear to have been mangroves. For a long time, people assumed that amphibians couldn't tolerate saltwater simply because most lissamphibians can't; both the reasoning and the conclusion were wrong.

Then out of the water and onto the land by actinopterygians (and a few fellow sarcopterygians),

Ever since the origin of tetrapods there have been obligatorily aquatic tetrapods – both before and after the megalichthyids (those of the self-explaining name) died out.

then off the shoreline and into the dry interiors by amphibians,

Unlikely, considering the Early Permian fauna of Fort Sill (fissure fills from an upland, containing amniotes, temnospondyls, "microsaurs", and an aïstopod).

then out of daylight and into the night and underground by thecodonts

Archosaurs (for the triply paraphyletic stem group of which the name Thecodontia was once used) could be ancestrally nocturnal or crepuscular. Among mammals, drastic adaptations to a nocturnal life are only seen in monotremes and placentals... And burrowing is at least a general cynodont trait, or was it cynodont + therocephalian, I forgot.

"Rhipidistian" is an obsolete term for a paraphyletic group of sarcopterygians, everything that has traditionally been considered close to tetrapods, including osteolepiforms and porolepiforms. So, two complaints: 1) the group doesn't exist and 2) if it did, it would be clearly nested deep within Sarcopterygia, while your sentence seems to claim some other location on the tree.

Even worse: Osteolepiformes doesn't exist either. It's a Hennig comb with us and the equally paraphyletic panderichthyidans inside.

(Though I see that some are now using "Rhipidistia" to mean "crown Sarpterygii.)

Which is very, very strange, because the lungfishes have never before been considered rhipidistians or rhipidistian descendants.

I see I'm going to have to pay more attention to fish phylogeny. Placoderms and acanthodians are both paraphyletic, and at least some of the latter are sister to sharks?

Yes. Nature, January 15th.

By the way, my first thought on seeing that fish was that the head shield looks like a placoderm's, but if the figure is right there's no homology.

The paper does homologize the two plates behind the back and an extra bone in the front part of the palate with bones otherwise known only from placoderms.

Also, that nobody has yet managed to homologize most "placoderm" skull bones with osteichthyan ones doesn't necessarily mean it can't be done...

now a popular aquarium fish in Japan, apparently; not sure how they rate as sashimi

I'm told lungfish tastes more like beef than like teleost.

I am curious about the middle point in the image of the fishes' tail fin in the gray area - I assume this is a guess as to what that part actually looked like. Why is it there?

Probably a guess based on Dialipina, Onychodus and the coelacanths, and maybe Eusthenopteron and a few others.

By David Marjanović, OM (not verified) on 03 Apr 2009 #permalink