Reconstruction of Priscomyzon in dorsal (top) and left lateral (bottom) views. b, Macropthalmia stage of Lampetra showing anterior location of orbit and smaller oral disc, both positioned in front of the branchial region. The total length of the specimen is 116 mm. Drawings in a and b are scaled to show equivalent head lengths: from anterior limit of the oral disc to rear of the branchial region. Horizontal bars indicate the anterior–posterior span of the oral disc in each species.
The life of a parasite must be a good one, and often successful; the creature at the top of the drawing above is a primitive lamprey from the Devonian, 360 million years ago, and the similarities with the modern lamprey (at the bottom) are amazing. It's less eel-like and more tadpole-like than modern forms, but it has the same disc-shaped mouth specialized for latching on to the flank of its host, it has similar circumoral teeth for rasping through scales and skin for its blood meal, the same pharyngeal adaptations for a life spent clamped to a fish.
I've put a photo of the fossil and a cladogram below the fold.
This 360-million-year-old lamprey is the earliest example known in the fossil record, showing most of the specialized feeding structures present in modern forms. a, b, Part (a) and counterpart (b) of holotype AM5750. The total length of the specimen is 42 mm. c, Interpretive drawing of the holotype. ac, annular cartilage; blb, bi-lobed structure; bra, branchial arch; brb, branchial basket; cm, circular mouth; ct, circumoral teeth; df, dorsal fin; hyb, hypobranchial bar; h/eb, hypotrematic/epitrematic bar; oc, otic capsule; od, oral disc; ol, outer lip; or, orbital region; sc, styliform cartilage; 1–7, positions of gill pouches.
It's not a very exciting animal, but put it in context. This creature was swimming about in the late Devonian, in the waning years of the great agnathan radiation. It shared the seas with those exotic armored jawless fishes that are all gone now, with just the hagfish and lampreys remaining. If you'd been alive in those days, would you have picked the lineage of those annoying soft and slimy parasites as the only members of that diverse group that would survive the next few hundred million years? A mere degenerate ostracoderm, thin-skinned and leech-like, is the line with the greatest longevity…perhaps because it was suitably adapted to take advantage of the gnathostome radiation, rather than compete with it.
(click for larger image)
Lampreys and hagfishes are the only two living groups of jawless vertebrate. The 360-million-year-old lamprey Priscomyzon (green) discovered by Gess et al. is very similar to modern lampreys, even though it dates from the twilight age (grey area) of the armoured jawless vertebrates (known as ostracoderms, in red) that were once considered to be ancestors of hagfishes and lampreys. The evolutionary tree now proposed by Gess et al. (simplified version shown here; crosses indicate extinction dates) agrees with the current consensus that ostracoderms are more closely related to jawed vertebrates than to lampreys or hagfishes. This suggests that living jawless vertebrates and their forerunners never developed an extensive bony skeleton, and that their origin must lie among early Palaeozoic jawless vertebrates that lacked scales and bone, such as Euphanerops (blue).
Gess RW, Coates MI, Rubidge BS (2006) A lamprey from the Devonian period of South Africa. Nature 443(7114):981-984.
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Is there a possibility that 'degenerate ostracoderm(s)' transformed or evolved from Anomalocaris?
Is there a possibility that 'degenerate ostracoderm(s)' transformed or evolved from Anomalocaris?
Doug asked:
"Is there a possibility that 'degenerate ostracoderm(s)' transformed or evolved from Anomalocaris?"
No possibility at all: Anomalocaris (see http://en.wikipedia.org/wiki/Anomalocaris) was an arthropod (and therefore a protostome), whereas Priscomyzon was clearly a chordate, and therefore a deuterostome (you can't get much more unrelated than that and still be in the kingdom Animalia). Perhaps Doug's confusion comes from the BBC program "Walking With Monsters" (see http://www.bbc.co.uk/sn/prehistoric_life/galleries/wwmonsters/index.sht…), in which a school of Haikouichthys ercaicunensis is shown attacking an injured Anomalocaris. Haikouichthys, like Priscomyzon, was an early chordate (see http://en.wikipedia.org/wiki/Haikouichthys).
Sorry about that: the web links in the previous post will work, but only if you copy and paste them and erase the final "close parenthesis" mark.
To quote No possibility at all... So what you are saying is that P=0 then and that the hypothesis isn't subject to any possible revision?
Scientism, in the strong sense, is the self-annihilating view that only scientific claims are meaningful, which is not a scientific claim and hence, if true, not meaningful. Thus, scientism is either false or meaningless.
Sorry Pepsi=yummy you know not of what you speak. Paleontology is an historical science and adheres to the same criteria of hypothesis testing and falsification as any other science. All evidence points to the fact that the same processes and relationships that we observe in today's world operated in the past (this is called uniformitarianism and is the foundation of all the historical sciences, including geology and cosmology). If not we would know nothing of our origins from the big bang to the rise of modern humans. Your position is no different from that of any ignorant creationist's.
What the heck? It isn't 'scientism' to explain that there is negligible likelihood within the framework of a coherent theory for some event to occur: that's what theories are for, is to explain constraints and possibilities in the evidence. A theory that allowed all possible results from any set of conditions is no theory at all!
Suggesting that a lamprey descended from an arthropod is like saying that humans evolved from ducks. Nope. Doesn't work.
Doug, although both lampreys and Anomalocaris both had circular mouths, the former are jawless fish, and the latter are near-arthropods.
The former's mouth functions not unlike a barbed suction cup or a plunger with teeth, so as to facilitate ease of the rasping of the flesh of its prey (I like to think of lampreys as predators, given as how it requires several victims over its lifetime, and each one dies soon after being attacked).
The latter's mouth functions sort of like a, uh, the grinding components of a garbage-disposal unit, except, if you adjusted the components to stab more than grind. I think.
Plus, lampreys have gill slits like many other agnathan groups, especially like their closest relatives, the anaspids, and anomalocarids had gill-tufts on their swimmerettes.
It isn't that Pepsi=yummy is ignorant, it is that he is dishonest in taking the colloquial mode of expression common in blog conversation as if it were all edited for acceptability in a peer-reviewed scientific publication.
Plus, what in the name of the 4th circle of Hell is "scientism"?
When I hear the word "scientism", I reach for my gun...
"Scientism" is a fake word, like "Darwinism" coined by those who think (Think?!) that science is a religion, and as uninformed and irrational as their religion(s)
PZ is correct, you can get P = 99.999...99 (to as many places as you like) in a scientific expanation/reason, but never actually 1.
You can get P = 0.
Why, because we KNOW it is WRONG.
Actually, this is one of scince's (the scientific method's) great powers - it eliminates wrong answers...
Like Phlogiston, or Lamarckism, or the Ptolemaic system.
Now that we've settled that, could we get back to the topic at hand, namely, Priscomyzon, the evolutionary history of lampreys, and the swapping of lamprey recipes?
As someone who studied the history of evolutionary science I find the term "Darwinism" useful as it describes the theory published in the "Origin of Species". SImilarly Lamarckism describes the theories of Lamarck (more than just inheritance of acquired characteristics- it involves continuous abiogenesis and a denial of extinction). In the historical context its useful.
Sadly I only see it outside of textbooks when used by creationists. Pity really.
Oh yes back on topic- sweet fossil. More interesting is where the Conodonts are on that cladogram. I'm more used to seeing them as the outgroup, not the hagfish. I wonder if a different cladogram results if the alternative hypothesis () is used.
Not sure what happenned there, the link goes to the Palaeos site about their model of conodonts where the eyes are part of the mouth apparatus, and the eyes are much maller structures.
I think what they're saying about conodonts at Palaeos, is that the first reconstructions of the whole animal, after the first fossils of whole conodonts were found, were wrongly interpretated, so that they were thought to be akin to hagfish or lancelets with (relatively) tremendous and hydrodynamically unsound eyes. Now that there have been newer fossils with the craniums intact found, scientists have begun to realize that the conodont eyes were nowhere near as bulging as originally interpreted, AND that the conodont cranium places them and not the lampreys as the sister group of the craniates (including the ostracoderms and higher vertebrates).
...
So much for my pet theory that conodonts were eyed hagfish.
Hagfish have eyes...
Can you point them out to me, then?
FYI (stanton)-"Scientism" is, in fact, a word. (1875-80)
1. the style, assumptions, techniques, practices, etc., typifying or regarded as typifying scientists.
2. the belief that the assumptions, methods of research, etc., of the physical and biological sciences are equally appropriate and essential to all other disciplines, including the humanities and the social sciences.
3. scienfitic or pseudoscientific language.
Related: "scientistic"
1. characterized by or having an exaggerated belief in the principles and methods of science.
2. of, pertaining to, or characterized by scientism.
So, anyway, I think pespi=yummy is just engaging in a little bit of sophistry, etc. Clearly the second definition for the noun above is what's being referred to by pespi=yummy. It's just the basic idea that scientific methods are useless if you're deciding things like "who is the world's greatest grandpa?" or "what your favorite music video?" or "what's the best poem of all time?"
(I once lived with a guy who insisted he could objectively say band A was better than band B. Of course my totally subjective position was that the appropriate criterion was "I like band B better, so it's better." We had a nice and long, and pointless, argument, and it neatly demonstrated the futility of a king of scientistic point of view to something totally subjective.)
OK I'm done now.
Interesting, Stanton, though I've not seen any news about new conodont body fossils, and this model seems to be unique to Palaeos. If you know better I'd be very interested to see any references.
Saying that Lampreys group with the bony agnathans as craniates in most phylogenies I've seen- often near the anaspids. Hagfish are outside of this, and conodonts usually slot in between the two- or sometimes as the outgroup.
In fact, if you can get probability 0 you can also get probablity 1 (for the contradictory claim). But neither is important here. There is, I think, a technical point pepsi has in mind: Once a sentence is assigned 1 or 0, we can't conditionalize to a lower (or higher) probability for that sentence. So it's unrevisable, if you are a strict probabalist who thinks all revision has to be conditionalization. (So many assumptions here...) The obvious answer is to use common sense-- our present views do rule out the relationship proposed. They could, in principle, be revised-- in the same way that any belief we hold could be revised. But the circumstances in which such revision would be warranted are so extreme and bizarre that we typically, and justifiably, ignore the possibility altogether.
I thoght the prevailing hypothesis was that lampreys are decended from Asaspids?
Can you point them out to me, then?
Posted by: Stanton | October 30, 2006 11:08 PM
Hags have eyes, they are just in a degenerative form and generally do not break the surface skin. This is rather like what we see with ammocoetes, the larval state of the Petromyzontidae, as in the eyes are mostly undeveloped until metamorphosis. There are numerous aspects that point toward hags as being very degenerate in nature, and that they were probably not originally strictly marine in origin, as in chloride cells that are used to osmoregulate in other fish, which are not used in hags, who are isoconformers, as well as a glomerular style kidney that is usually more selective for fresh water fish, which of course, hags are not...
In short, using hags current state as a way to place them in the chordate-vertebrate tree, is risky at its' very best...