…and this is a 
My previous repost was made to give the background on a recent discovery of Jurassic ceratosaur, Limusaurus inextricabilis, and what it tells us about digit evolution. Here's Limusaurus—beautiful little beastie, isn't it?
(Click for larger image)
Photograph (a) and line drawing (b) of IVPP V 15923. Arrows in a point to a nearly complete and fully articulated basal crocodyliform skeleton preserved next to IVPP V 15923 (scale bar, 5 cm). c, Histological section from the fibular shaft of Limusaurus inextricabilis (IVPP V 15924) under polarized light. Arrows denote growth lines used to age the specimen; HC refers to round haversian canals and EB to layers of endosteal bone. The specimen is inferred to represent a five-year-old individual and to be at a young adult ontogenetic stage, based on a combination of histological features including narrower outermost zones, dense haversian bone, extensive and multiple endosteal bone depositional events and absence of an external fundamental system. d, Close up of the gastroliths (scale bar, 2 cm). Abbreviations: cav, caudal vertebrae; cv, cervical vertebrae; dr, dorsal ribs; ga, gastroliths; lf, left femur; lfl, left forelimb; li, left ilium; lis, left ischium; lp, left pes; lpu, left pubis; lsc, left scapulocoracoid; lt, left tibiotarsus; md, mandible; rfl, right forelimb; ri, right ilium; rp, right pes; sk, skull.
What's especially interesting about it is that it catches an evolutionary hypothesis in the act, and is another genuine transitional fossil. The hypothesis is about how fingers were modified over time to produce the patterns we see in dinosaurs and birds.
Birds have greatly reduced digits, but when we examine them embryologically, we can see precisely what has happened: they've lost the outermost digits, the thumb (I) and pinky (V), and retain the forefinger, middle finger, and ring finger (II-IV), which have been reduced and fused together. This is called Bilateral Digit Reduction, BDR, because they've lost digits from the medial and lateral sides, leaving the middle set intact.
Dinosaurs, when examined anatomically, seem to have a different pattern: they have a thumb (I), forefinger (II) and middle finger (III), and have lost the lateral two digits, the ring and pinky finger (IV-V). This arrangement has been advanced as evidence that birds did not evolve from dinosaurs, since they have different bones in their hands, and getting from one pattern to the other is complicated and difficult and very unlikely.
The alternative hypothesis is that there is no conflict, and that dinosaurs actually underwent BDR and their digits are II-III-IV…but that what has also happened is a frame shift in digit identities. So dinosaurs actually have three digits, which are the index, middle, and ring finger, but they've undergone a subtle shift in morphology so that their forefinger develops as a thumb, and so forth.
Now we could resolve all this easily if only the physicists would get to work and build that time machine so we could go back to the Mesozoic and study dinosaur embryology, but they're too busy playing with strings and quanta and dark matter to do the important experiments, so we've got to settle for another plan: find intermediate forms in the fossil record. That's where Limusaurus steps in.
Limusaurus has a thumb, a tiny vestigial nubbin, and has lost its pinky completely. This is a (I)-II-III-IV pattern, and is evidence of bilateral digit reduction in a basal ceratosaur. In addition, the forefinger has become very robust, and while still distinctly a digit II, has been caught in the early stages of a transformation into a saurian first digit. It's evidence in support of the dinosaurian II-III-IV hypothesis and the frameshift in digit identity! It's almost as good as having a time machine.
Want to learn more? Carl Zimmer has a summary of the digit changes, while one of the authors of the paper, David Hone, also discusses the digits (the story is a little more complicated than I've laid out), and also has more on the rest of the animal—it's a herbivorous ceratosaur, which is interesting in itself.
Xu X, Clark JM, Mo J, Choiniere J, Forster CA, Erickson GM, Hone DWE, Sullivan C, Eberth DA, Nesbitt S, Zhao Q, Hernandez R, Jia C-k, Han F-l, Guo Y (2009) A Jurassic ceratosaur from China helps clarify avian digit homologies. Nature 459(18):940-944.
Comments
Posted by: MPG | June 18, 2009 11:11 AM
Excellent. This also further clarifies the previous article (which was already pretty well explained anyway).
Posted by: Tommy Traddles | June 18, 2009 11:12 AM
Humans More Closely Related to Orangutans, Not Chimps
Molecular studies challenged
"Schwartz and Grehan contend in the Journal of Biogeography that the clear physical similarities between humans and orangutans have long been overshadowed by molecular analyses that link humans to chimpanzees, but that those molecular comparisons are often flawed: There is no theory holding that molecular similarity necessarily implies an evolutionary relationship; molecular studies often exclude orangutans and focus on a limited selection of primates without an adequate "outgroup" for comparison; and molecular data that contradict the idea that genetic similarity denotes relation are often dismissed.
"They criticize molecular data where criticism is due," said Malte Ebach, a researcher at Arizona State University's International Institute for Species Exploration who also was not involved in the project but is familiar with it.
"Palaeoanthropology is based solely on morphology, and there is no scientific justification to favor DNA over morphological data. Yet the human-chimp relationship, generated by molecular data, has been accepted without any scrutiny. Grehan and Schwartz are not just suggesting an orangutan–human relationship—they're reaffirming an established scientific practice of questioning data."
Contact: Morgan Kelly
mekelly@pitt.edu
412-624-4356
University of Pittsburgh
Posted by: Ten Bears | June 18, 2009 11:14 AM
Dawg did it
Posted by: Francis | June 18, 2009 11:15 AM
Who does the amazing pen-and-ink representations?
Posted by: Rorschach | June 18, 2009 11:15 AM
Gee,PZ is piling it on today !
What is this,"I can haz evolutionary biology" day?
Great stuff !
Posted by: gg | June 18, 2009 11:20 AM
"Now we could resolve all this easily if only the physicists would get to work and build that time machine so we could go back to the Mesozoic and study dinosaur embryology, but they're too busy playing with strings and quanta and dark matter to do the important experiments..."
Actually, we already built that time machine decades ago, but decided not to share it with biologists out of disciplinary rivalry. Of course, all we use it for is to go back and fake all those UFO sightings in the 1940s-1960s.
Oh, and occasionally we visit primitive tribes and pass ourselves off as gods. Good times.
Posted by: Guy G | June 18, 2009 11:22 AM
Very sneaky. At the end of the last post, I thought "Well that's all very well and good, but I'd like to see some evidence - it sounds like it's just a hypothesis". And here it is. What a lot I've learned today. :)
Posted by: Rorschach | June 18, 2009 11:22 AM
Tommy TraddlesDimmy Dickhead,Please stop crossposting your crap all over the blog.
Posted by: Felix | June 18, 2009 11:22 AM
You sure it's all right? Its neck position doesn't look healthy.
Posted by: Robert Woerheide | June 18, 2009 11:23 AM
Fascinating. Thanks for the explanation.
Posted by: rob | June 18, 2009 11:25 AM
i did develop a time machine. i could only go back 6000-10,000 years for some reason. while i was preparing to return to modern times i accidentally stepped on a butterfly. when i got back, casey luskin was not in charge of the Pharyngula blog!!! some biologist name PZ Meyers was!! i thought PZ was the head of the DI? weird.
i sure am not going to mess with the time space continuum anymore!
Posted by: Glen Davidson | June 18, 2009 11:27 AM
Wow, cool.
Does this have any bearing on the claims that bird femurs make dinosaurs an unlikely candidate for avian ancestors? If a divergent, but closely related to dinosaurs, branch gave rise to birds, would this homology make sense, or would it then be a kind of coincidence?
Glen D
http://tinyurl.com/6mb592
Posted by: DaveG | June 18, 2009 11:28 AM
Awesome! But will Casey Luskin wristulate the science?
http://www.evolutionnews.org/2008/07/tiktaalik_roseae_wheres_the_wr.html
P.S. Someone teach me to embed links?
Posted by: Rorschach | June 18, 2009 11:29 AM
Never heard of the fellow.
Spelling,you fail.
Posted by: Felix | June 18, 2009 11:29 AM
rob #11,
what? you're not the pope? when did that happen?
Posted by: PZ Myers
|
June 18, 2009 11:31 AM
It's just restin'. Beautiful plumage!
Posted by: rob | June 18, 2009 11:38 AM
Rorschach, you're right! i didn't even notice the spelling is diferent here too. PZ Myers blogs this version of Pharyngula. pretty weird. i have even noticed there is an extra letter on the keyboards here too, the "j". and how come rush limbaugh isn't president?
Posted by: Rorschach | June 18, 2009 11:44 AM
rob,
ok.
Posted by: Benny the Icepick | June 18, 2009 11:49 AM
In the words of Randall Munroe,
"Science: It works, bitches!"
http://xkcd.com/54/
Posted by: Zar | June 18, 2009 11:52 AM
That's all very well and good, PZ, but you have obviously forgotten that the fossil record was planted by God as a prank.
Checkmate, evilutionists!
Posted by: Mystyk | June 18, 2009 11:53 AM
This was a particularly enlightening combination of posts. Short (sort of), sweet, and to the point. It also had a particularly important meaning to me.
My sister has for some time been an on-the-streets drug addict. Now, she made her bed and has to lie in it (to a degree - we still try to help her out in ways designed to minimize reinforcement of her problems), but when she brought her second daughter into the world she left that daughter with a host of lifelong problems.
You see, my youngest niece was born without thumbs. There was always a circular spot of skin where a thumb should have been, but not the thumb itself. We were told that such digit loss is not an uncommon result of methamphetamine use during certain stages of development, but we never really understood the mechanism.
Sure, the suppression itself was clear, but why that digit when the rest were fine? The developmental sequence mentioned in your post makes the concept clear with regard to the way species can remove the formation of certain digits. The fact that C-IV is the first to develop and C-I the last explains why that was the only affected digit.
As a side note, when she was about 12-18 months she had surgery on her hands to re-shape the bones and re-attach the muscles on her index finger (which I now know is C-II) so that it can function in an opposable manner as we expect of a thumb. She is doing well, but as she nears an increasingly judgemental realm called "teenager" we know that the problems have just begun. (Also, shout-out to the Shriner's Hospital for Children for their fabulous surgical skills.)
Posted by: dNorrisM | June 18, 2009 12:06 PM
Ham is right:
T-REX really did eat coconuts!
Posted by: David Marjanović, OM | June 18, 2009 12:09 PM
Stop rrrrrrrright here.
In his latest paper, Martin Kundrát (I'll post the full citation later) finds six things in the embryonic ostrich wing and offers two possibilities: they could be digital rays I, II, III, IV, V, and the pisiform (a bone you have in your wrist), or the prepollex* and digital rays I, II, III, IV, and V. He then immediately dismisses the latter idea (which has been proposed in a paper… I'll dig up that citation later, too) because a prepollex has never been found in alligator embryos. But what if the prepollex grows to visible sizes in bird embryos as a byproduct of the fact that birds (and dinosaurs more generally) emphasize the front (thumb) margin of the hand?
Also, no Mesozoic dinosaur is known to possess a pisiform…
And what if the reduction of the thumb is a derived, unique feature of the ceratosaurs, which have markedly reduced arms and hands generally (Limusaurus** taking this to one extreme, Aucasaurus to a similar one)? Why should this be considered ancestral to the tetanuran condition?
Conclusion: The interpretation offered by the paper is possible, but it's not the only option. The situation is horribly complicated.
* A digit-like thing that grows as an extension of the radius-radiale-centrale 1 axis. Present in some salamanders and many frogs in the adult, and present in at least some embryonic mammals like the mouse lemur Microcebus.
** Which should actually have been Limosaurus, but it's too late to fix that. :-(
Posted by: Dr.Woody
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June 18, 2009 12:09 PM
So dinosaurs and birds count in base three (or six, given two arms with 'hands' on 'em)?
Kewl...
Posted by: SteveM
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June 18, 2009 12:17 PM
So dinosaurs and birds count in base three (or six, given two arms with 'hands' on 'em)?
Um yeah, that's why we count in base 5. ;-)
Posted by: Jim | June 18, 2009 12:21 PM
"...it's a herbivorous ceratosaur, which is interesting in itself"
Of course it's herbivorous, all dinosaurs were before Adam chomped the apple and fell from grace. Logical, isn't it?
Posted by: marcus
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June 18, 2009 12:26 PM
What is all this damn science on the Scienceblog. I want the fuzzy stuff; sex, politics, dismembering IDology, bacon. This stuff is soooooo educational. It makes my head hurt! (But in a good way.)Awwwww.(looks dejectedly at toes)I guess I can learn sumthin'if I gots to.
Posted by: David Marjanović, OM | June 18, 2009 12:27 PM
%lt;a href="complete URL here, http and all">link text</a>
Posted by: David Marjanović, OM | June 18, 2009 12:31 PM
ARGH! That's of course supposed to be "<", not "%lt;".
Posted by: cicely
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June 18, 2009 12:54 PM
Mate, this bird wouldn't "voom" if you put four million volts through it! 'E's bleedin' demised!
Posted by: ALex Deam | June 18, 2009 1:08 PM
If time travel into the past is possible, you're going to need to persuade your governments to hand over an immense amount of energy and money to do so, since that is what it would require to build a wormhole. So you see, the strings and the quanta are just part of our plan to let taxpayers get used to funding big projects like the LHC, then WHAM!... we get funding for a time machine. Hee hee hee! It's so simple!
So if you really want a time machine, please help us fund ITER:
http://news.bbc.co.uk/1/hi/sci/tech/8103557.stm
Posted by: Nomen Nescio | June 18, 2009 1:16 PM
"inextricabilis"? there was some difficulty getting the fossil out of the ground?
Posted by: elty | June 18, 2009 1:22 PM
Beautiful. I love these lessons, P.Z. Also the articulate reasons about why I am RIGHT about religion.
Posted by: jeffS | June 18, 2009 1:26 PM
Wow, PZ,
I admire your clarity and your patience in constantly taking on these bogus creationist claims, but your argument about digits in the previous re-post (riposte ?) seemed to go on a bit long. If it were me, I'd just offer the simple evidence of a fully extended DIII.
Posted by: qbsmd | June 18, 2009 1:33 PM
Is there a hypothesis for why natural selection would favor losing a thumb, while simultaneously making a new one out of the index finger? It just seems like whatever pressures are making CII more robust should have prevented the thumb from becoming vestigial.
Posted by: Lynna | June 18, 2009 1:37 PM
Interesting post, PZ. I'm going to track this one for awhile. It seems like lots of people will want to weigh in. The fossil raises questions as well as answering questions.
Posted by: Minnow | June 18, 2009 1:40 PM
@qbsmd I'm pretty ignorant about these things, but is it possible that the frameshift occured first, leading to a more robust CII before the loss of the thumb? Then NS maybe working on a robust CII/weak thumb leading to the loss of the thumb?
Posted by: James Sweet | June 18, 2009 1:47 PM
Hey PZ, this is the second time in two days that you have been picking on physicists! Is there some kind of simmering grudge here?
Listen, just because the Creationists fallaciously use the Second Law of Thermodynamics to argue against evolution, that doesn't mean that all of physics is bad!
Posted by: Jojo | June 18, 2009 1:55 PM
One of the reasons I love Pharyngula is that it's a great escape from my dinosaur obsessed 3 year old. Not so much today though. Did the little punk bribe you to post about this?
Seriously, great posts today. Not only have I been entertained, I've learned a few things too.
Posted by: varlo | June 18, 2009 2:15 PM
Damn ... Two more gaps in the record to defend.
Posted by: jj | June 18, 2009 2:20 PM
Great Set of Posts!
I find it interesting, and slightly counter-intuitive (hey, not everything has to be intuitive in science at first), that Digits had to evolve up to 5 in tetrapods (not too sure if this is where we see the similarities arise) and back down in others. Or that cold just be a result of the language begin used here of "looses" a digit. I guess it could be more "Never fully formed"? Anyway, great description.
Posted by: Luke | June 18, 2009 2:29 PM
Great posting PZ, an awesome one-two punch in the Creationists' collective face.
Posted by: CatBallou
|
June 18, 2009 2:36 PM
I find it mildly funny to talk about "ring fingers" on birds and dinosaurs. By all means, let's put rings on them!
And Felix (#9), I'm pretty sure it's doing the death scene from "Swan Lake." Odette extends her graceful neck and flutters slowly to the floor before going completey "toes up."
Posted by: David Marjanović, OM | June 18, 2009 2:50 PM
There is no escape.
What do you mean by "up to"? All the way to sturgeons, if not sharks, digit homologues ("radials") are high in number.
Posted by: Raymond Freeman-Lynde | June 18, 2009 3:05 PM
Nomen Nescio @ #32
From the Nature paper:
inextricabilis, Latin for impossible to extricate. This
name is in reference to the specimens’ inferred death in a mire.
Raymond Freeman-Lynde
Department of Geology
University of Georgia
Posted by: SASnSA | June 18, 2009 3:29 PM
Well now that they've counted their chickens' fingers before they hatched, the trick's going to be getting them to acknowledge this evidence that the initial fingering on the dinosaurs was wrong.
Posted by: Tim H | June 18, 2009 3:55 PM
That's a silly use for a time machine. The proper use would involve taking advantage of reversed inflation to buy beer more cheaply. This is why physicists don't rule the world. They may be smart, but they have no common sense. (Plus, they evidently don't want to rule the world.)Posted by: Dale Husband | June 18, 2009 4:15 PM
We ASSUME that Limusaurus inextricabilis was a plant eater, but how do we really know? I'm thinking that most dinosaurs in those early days were omnivorous, just as most mammals soon after the extinction of the dinosaurs were probably omnivores and only later became specialized as herbivores or carnivores.
Posted by: Peter Ashby | June 18, 2009 4:15 PM
@Francis #4
How do they do the line drawings? I can't speak for this example but traditionally you use a camera lucida which is a set of mirrors that you attack to a microscope or just look through for larger specimens so that an image of your hand holding a pencil is superimposed on specimen, then you simply trace it. These days there are digital equivalents using graphic tablets etc that make cleaning up the lines easier but basically, they are traced.
Posted by: Nerd of Redhead, OM
|
June 18, 2009 5:21 PM
Didn't get a chance to read these posts until now. Very interesting, and tie together nicely. I find it fascinating how much evolution is really tied up with developmental genes rather than the ones encoding proteins.
Posted by: BAllanJ | June 18, 2009 5:26 PM
The problem with the trip I took in my time machine is that the earth keeps moving... I went back a few million years and appeared in deep space somewhere... the earth ... hell, the whole galaxy moved!... barely had time to get back before my air ran out. Set it to automatic for the next trip and sent it on its own back a fraction of a second... if anyone wants it... they can try to dig it out of my basement floor... I've had enough.
Posted by: Paper Hand | June 18, 2009 5:32 PM
@JJ #41
The earliest tetrapods had more than five digits, as many as eight in some of the earliest. It appears that, for whatever reason, three of those digits were lost (I wonder if the outer digits might've been primarily the outer digits that were lost, which might explain why digit IV is formed first rather than, as one would've thought, the central digit III - namely, that in the original tetrapods, what is now digit IV would've been central, with three digits on one side and four on the other)
@Tim #47
The problem with using time travel to take advantage of inflation is that you have to be very careful to avoid bringing currency with you that has a date later than the time you're going to. In the US, a modern $5, for example, would immediately draw suspicion prior to a few years ago. $1s and coins you might be better off with (assuming you avoid bringing any of the state quarters, or the new nickels), but even there you run the risk of someone taking a look at the coins (and if you go back before 1965, dimes and quarters will immediately draw suspicion as counterfeit because they were silver before that date; the current copper-nickle coins will stand out) and seeing a future date. You should be okay with small purchases using $1 bills, though, after some time in the 1930s when the present size was adopted.
Other countries change their money more frequently, making it even more difficult to find appropriate currency. And buying old currency off a collector would defeat the purpose.
Posted by: Zach Miller | June 18, 2009 5:53 PM
*raises hand*
Keep in mind, folks, that ceratosaurs in general (by which I mean Ceratosaurus + Abelisauridae, not "coelophysoids" or Dilophosaurus) have very stumpy arms and reduced fingers already. Those animals with well-developed fingers retain four digits. I'm not convinced that what we're seeing here is a real frame shift--it could just be an aupomorphy of the genus, and a trend in the larger ceratosaur group. More fossils will obviously illuminate this problem further, but I think it's a little too early to jump on the frame shift train.
Posted by: Darren S. A. George | June 18, 2009 6:11 PM
Tim #42
All you have to do is buy one dollar bill from the 1920s, then run off lots of copies on your laser printer. It takes a bit of photoshop to get rid of the creases and dirt, but the quality is actually higher than the official banknotes from that time- and once it's been gotten dirty, and been carried around in your wallet a few weeks, it passes without a second glance.
Unlike regular counterfeiters, you're not making something that passes for legal tender here and now, so it could be argued that you're not breaking the law. And if people start to catch on where you buy your beer, are they going to send cops to the twenty-first century to catch you in your operation?
Posted by: Dave Godfrey | June 18, 2009 7:07 PM
Dale Husband
Limusaurus shares similar features seen in the other herbivorous (and omnivorous) theropods- loss of teeth and evolution of a beak, so while it may not have been a committed herbivore, it certainly wasn't anything like as carnivorous as its immediate relatives.
While Limusaurus appears to be a primitive ceratosaur, it isn't the earliest known, and all the others are clearly carnivorous. While the hands appear to be primitive the herbivorous adaptations probably aren't- they are absent in its close relatives. (With the possible exception of Elaphrosaurus, but that doesn't have a skull yet).
There are several more primitive theropods known, and some dinosaurs that seem to predate the theropod/sauropodomorph split- AFAIR they all show strong adaptations to carnivory. Herbivory in tetrapods generally seems to be a secondary adaptation- which is not surprising really, meat is easily digested, but plant matter is very hard to break down.
Posted by: David Marjanović, OM | June 18, 2009 7:49 PM
It's got a beak, the shape of the beak fits, it lacks dangerous hand claws, and it's got all those stones in its stomach.
This doesn't exclude all degrees of omnivory, but the carnivory end of the spectrum doesn't fit the evidence.
How does that follow? "Early days"? The Oxfordian was about 40 million years after the Triassic-Jurassic mass extinction, and even before that, very dedicated carnivores and (perhaps to a slightly lesser degree) herbivores already existed among dinosaurs.
In particular, this is a theropod, and all theropods older than this one were very obvious carnivores.
Without exception, those genes do code for proteins: cell-to-cell signals, transcription factors, and the like.
Posted by: amphiox | June 18, 2009 7:58 PM
#52: If I remember correctly, many of the earliest tetrapods had more than 5 fingers, but all of them had only 5 types of fingers. They had duplicate fingers. It seems over time the developmental pathways were tightened such that only one of each type of digit was produced?
Posted by: Zach Miller | June 18, 2009 8:06 PM
David, with it's mild degree of heterodonty, couldn't Eoraptor have been mildly omniovorous? Just a thought.
Posted by: Autumn
|
June 19, 2009 1:21 AM
Awsome post. I love being able to learn a little bit about the interesting debates on the forefront of science, and doing so requires a skilled communicator to bring it down to a level the layman can understand.
Thank you, PZ, and never underestimate the importance of your hobby here.
Beating down an anti-scientific ass-hat while bringing interesting knowledge to the masses:
Priceless.
Posted by: DDeden | June 19, 2009 3:11 AM
@ 23 - Does the prepollex match up with the crab claw?
@ 41 "Digits had to evolve up to 5 in tetrapods" Nope.
@ 52 "The earliest tetrapods had more than five digits, as many as eight in some of the earliest". Nope.
@ 57 "They had duplicate fingers". Yup, that's why ray finned fish (trout) followed lobe fin fish.
The first multi-cellular life forms had 'buckyball egg' pentagonal/pentadactyl architecture, all later animals/plants) with more digits (sturgeon) or less (horse) have derived from the original 5, this preceded bilateral symmetry (starfish).
Posted by: Niket | June 19, 2009 3:54 AM
I hope I don't sound like a creationist when I ask the following.
A cousin of mine has four toes. The fifth toe (smallest one) is small enough to only have the nail. Clearly, this is not "evolution" but caused by some genetic/developmental reason. How do we know that it is a transitional form in case of "Limusaurus"?
Three years back, when I was asked a similar question, I replied saying "biologists/paleontologists know what they are doing". I am guessing its based on structure of bones etc. Still, I would like to know a better answer myself.
Posted by: Owlmirror | June 19, 2009 4:36 AM
I am looking at The pectoral fin of Tiktaalik roseae and
the origin of the tetrapod limb by Shubin, Daeschler, Farish (2006), and it quite clearly shows that Acanthostega had 8 digits, and Tulerpeton had 6.
Wait.... what?
Posted by: Knockgoats | June 19, 2009 8:48 AM
Excellent pair of posts - and thanks to David M. for showing us once again that "There is no problem, however complicated, which, which when you look at it the right way does not become still more complicated". (Anyone know who first said that? One googled page attributes it to "a physicist called Anderson" - presumably Philip Warren Anderson, but I can't find a definite attribution.)
Posted by: Dave Godfrey | June 19, 2009 10:18 AM
Zach Miller:
Quite possibly- some people have suggested its closer to the sauropodomorphs rather than being the basalmost theropod.
Niket:
Generally its more parsimonious to assume that the individual is normal- at least when you have only one specimen. The chances of a deformed animal like this surviving to adulthood is small, additionally two specimens are known (though I've not read the paper yet and it doesn't say if the hands were found in both specimens), which makes it even more unlikely that the features aren't normal for this animal.
DDeden:
Multicellularity evolved independently in plants, animals, fungi, brown algae and the several groups of unrelated "slime moulds". There's nothing pentagonal about what's going on in those groups, and echinoderms have a radial symmetry imposed on an earlier bilateral pattern. How this has anything to do with digits, which are exclusive to tetrapods and their immediate lobe-fin relatives is beyond me. Would you care to explain further?
Posted by: raven | June 19, 2009 11:07 AM
If you look at enough humans, you can find innumerable such differences. Like people with greatly reduced arms. But how common are 4 1/2 toed humans? Or 4 legged ducks, or two headed sheep and snakes, or horses with multiple toes?
Not very. Fossilization is a rare event. There are also innumerable extinct species we will never know about because not one of the individuals fossilized.
So what is the probability that your cousin will:
1. Fossilize
2. Be dug up 10 million years from now by some future, possibly nonhuman paleontologist
3. Be the only large, big brained specimen found?
It is a probabilistic consideration. Paleontologists might be fooled occasionally but it will be rare enough to not be worth worrying about. For example, we have 11 specimens of Archaeopteryx, multiple T. rexs, and so on.
Posted by: Sleeper | June 19, 2009 11:54 AM
You know, thumbs are so useful I've often thought it would be cool to have two on each hand. The extra one next to the small finger, mirroring the existing one. It'd make hands more symmetrical.
Posted by: Anonymous Coward | June 19, 2009 9:24 PM
The limosaurus link is broken, the middle of the --- should not be a hyphen, it should be a dash.
Posted by: DDeden | June 20, 2009 5:31 PM
@ 62 - The earliest tetrapods (in the conventional sense, eg. tiktaalik) had 5 digits per limb. Acanthostega gained more digits (finger duplication) as it re-aquaticized, improving hydrodynamic speed, like ray finned fish with multiple "finger" bones. "Wait.. what?" Buckyball C60 or Volvox algae, has truncated icosahedral (soccerball) symmetry with 12 pentagonal faces, any spherical egg has this pattern, this is the most primitive reproductive form in life. The 5 digits are the joints or lines of the pentagon (later duplicated during bilateral symmetry from incomplete subdivision, then making 10 of 2 hands, then 20 digits of 4), interdigital webbing and fingerprint whorls are remnants of this former condition. Faster locomotion on land loses digits, in water gains digits but may lose limbs (dolphin, snake).
Posted by: DDeden | June 20, 2009 7:58 PM
@ 64 - "digits, which are exclusive to tetrapods" Nope. Crabs (with knuckled digits like our fingers) and squid are also bilateral pentadactyles, where the "thumb" is an extended grasper claw, special tentacle or oral hinged trap door. In fish/tetrapods, the grasper is the mandible pair which fuse embryologically. The crabs mobile toothed claw and your lower jaw are coded the same originally, both have derived since. Thus the "missing" thumb bone.
"Asymmetric regulation of Hox gene expression pre-dates the appearance of tetrapod digits and was co-opted in the development of 'thumbness'." GP Wagner & AO Vargas 2008 Genome Biol.9:213-4 On the nature of thumbs
Echinoderms have pentameric radial symmetry.
http://www.physorg.com/news146406170.html
http://www.ncbi.nlm.nih.gov/pubmed/1363084
BTW, the name Limusaurus is great, because of "limb" in English, in Malay, lima means 5 and in Hawaiian lima means hand and in Inuit talima means 5. Coolies.
Posted by: Dave Godfrey | June 21, 2009 3:31 AM
Are you seriously suggesting that arthropod limbs evolved into vertebrate ones? Because that's utterly wrong. The two groups are totally separate on every decent cladgram I've ever seen, and all the groups in between are limbless.
Additionally crustaceans also have two pairs of antennae- which are also homologous with limbs, and previously had- but have lost, limbs on the abdomenal segments which now only bear gills. The ancestral arthropod was much more trilobite-like, with a series of paired biramous limbs (a limb with two branches, a walking leg and a gill). Very, very different from the bodyplan of vertebrates.
And in Latin Limos means swamp. Which is what the animal was actually named for.
Posted by: DDeden | June 21, 2009 5:57 PM
@ 70 - "Because that's utterly wrong." Of course it is, I didn't say it. I said +/- arthropod digits (popularly called limbs) and squid tentacles = tetrapod digits, originally quantity 5. Segment duplication may result in numerous segments with digit/gill/nerve pairs, these are not original, like nautilus multiple tentacles they are derived.
The antennae and extra maxillae all associate with the number one digit, like the human jaw bone. The "thumb" originally acted as a sort of trap door in filter feeding, in an extremely primitive marine animal (before trilobites), the other digits acted as sort of antennae/tentacle/cilia. Look at the Drosophilia appendages, revealing. Primitive vertebrates were inverted invertebrates +/-. Limos ~ swamp, yes, it looks as if Limusaurus may have been a deep wading dino, probably foraging in wetlands, possibly derived from a fly-gliding archosaur ancestor. I left a note at Darren's blog: comment 68 http://scienceblogs.com/tetrapodzoology/2009/06/limusaurus_is_awesome.php#comments
Posted by: Dave Godfrey | June 22, 2009 11:14 AM
That doesn't work either. The genes referred to in your reference are present in vertebrates, and responsible for the formation of the distal end of the limbs, but they aren't present in arthropods. Drosophila appendages tell us nothing about the modifications of the bits at the end of vertebrate limbs. There are similarities, but humans have 4 times as many HOX genes, and there's a considerable degree of differentiation. I don't know much about development in squid, but cephalopod arms are derived from the molluscan foot, again a different structure. Modern coleoids (and their immediate relatives) originally had 10, but there's no evidence I'm aware of about how many the earlier nautiloids had. It could have been significantly more.
There is evidence that vertebrates are inverted invertebrates, but the hypothetical ancestor would be something wormlike- an organism where there may not have been much differentiation between top and bottom. Certainly nothing involving digits, jaws or tentacles of any kind is involved.
The idea that vertebrate jaws and arthropod limbs are homologous is also very much in error. As I said before this hasn't been taken seriously by scientists since William Patten in the 1920s.
And there's no evidence that dinosaurs are derived from arboreal ancestors. The recent threads in Tet Zoo knock those on the head.
Posted by: DDeden | June 22, 2009 5:08 PM
@ 72 - (response per sentence number) 1- Of course it does. 2- I'm not certain which specific genes, but the original condition was pentadactyle, where the 5 digits surrounded the oral cavity of the (then short) GI tube, before segmentation (worm), before chelation (crab), before bilateralization (squid), and before extension of limbs (tetrapod). 3- It shows the 3 typical forms, antennae-ped, winglet, tentacle. (unless I'm thinking of another one). 4- Flyers lose excess genes, humans don't. (See Ed Yong's post on bird/dino DNA, but note that both derive from "dive bombing" arboreal archosaurs). 5- "Foot" is oral associated and is derived. 6/7- Nautiluses are mobile, they've increased their digits, as is typical. If they were aquatic, mobile and REDUCED their digits, that would be unusual and worth arguing. 8/9- Worms are multiply (adj.) segmented, thus derived, they've 'lost' parts, like snakes, due to burrowing. 10- crab claws and tetrapod jaws are both homologous and analogous, functionally, morphologically and genetically, in the evolutionary sense. (It is like 3 King Kong movies, all based on the same original book but different actors, time periods, etc.). 11- I don't know Patten, just St. Hillaire and Cuvier a bit. 12/13- Theropods & Aves both derive from arboreal archosaurs, which ate seeds, insects and ambushed waterside ground prey, +/- (can't be exact). Haven't seen anything at TetZoo that changes that. I'm not arguing for BCF, just arborial archosaur dive bombers approximately, parallel to sabercats and some owls/raptors.
Posted by: DDeden | June 22, 2009 5:15 PM
@ 72 - (response per sentence number) 1- Of course it does. 2- I'm not certain which specific genes, but the original condition was pentadactyle, where the 5 digits surrounded the oral cavity of the (then short) GI tube, before segmentation (worm), before chelation (crab), before bilateralization (squid), and before extension of limbs (tetrapod). 3- It shows the 3 typical forms, antennae-ped, winglet, tentacle. (unless I'm thinking of another one). 4- Flyers lose excess genes, humans don't. (See Ed Yong's post on bird/dino DNA, but note that both derive from "dive bombing" arboreal archosaurs). 5- "Foot" is oral associated and is derived. 6/7- Nautiluses are mobile, they've increased their digits, as is typical. If they were aquatic, mobile and REDUCED their digits, that would be unusual and worth arguing. 8/9- Worms are multiply (adj.) segmented, thus derived, they've 'lost' parts, like snakes, due to burrowing. 10- crab claws and tetrapod jaws are both homologous and analogous, functionally, morphologically and genetically, in the evolutionary sense. (It is like 3 King Kong movies, all based on the same original book but different actors, time periods, etc.). 11- I don't know Patten, just St. Hillaire and Cuvier a bit. 12/13- Theropods & Aves both derive from arboreal archosaurs, which ate seeds, insects and ambushed waterside ground prey, +/- (can't be exact). Haven't seen anything at TetZoo that changes that. I'm not arguing for BCF, just arborial archosaur dive bombers approximately, parallel to sabercats and some owls/raptors. [Here's Ed's post I referred to above]: http://scienceblogs.com/notrocketscience/2009/06/dinosaurs_provide_clues_about_the_shrunken_genomes_of_birds.php
Posted by: Dave Godfrey | June 22, 2009 8:26 PM
William Patten believed that vertebrates evolved from arthropods- specifically eurypterids, and believed that he had found evidence for limbs in ostracoderms. (Which don't even have jaws). Much of his evidence came from a misinterpretation of embryology and the superficial similarity between eurypterid and ostracoderm headshields. He then derived most other groups as being "degenerate" forms off the "main line" of evolution. He had very similar ideas to you about jaw and limb homology.
But unless you have any actual evidence for this- and you haven't actually presented any for arboreal archosaur divebombers, digit/tentacle/arthropod limb homology what you've said will not be taken seriously.
Posted by: DDeden | June 22, 2009 11:24 PM
@ 75 - 1- Vertebrates evolved from extinct arthropods and extinct molluscs. 2- Jaws are derived. 3- Degenerate? As in 'loss of genetic material', as in flyers that lose extra genetic baggage? 4- I said "crab claws and tetrapod jaws are homologous", you said "jaw and limb homology". Oops, not the same thing. Didn't I clarify that already? Crab digits = human digits, even the knuckles and apical tufts bear strong resemblance, hardly superficial. 5- The best evidence is in your hands, literally. Pentagonal->pentameric->pentadactyle. Seriously. But yes, I agree, more evidence is always good.
Posted by: Owlmirror | June 23, 2009 2:49 AM
Quintessential pentacularianism = .55555555555555̅ Tc
Posted by: Dave Godfrey | June 23, 2009 3:14 AM
"Degenerate" as in became greatly simplified- similar to the manner in which species change as they adapt to parasitic lifestyles. It was a popular idea when evolution was seen as largely a straight line leading to humans.
Any evidence would be a start. So far you have none. Your idea is not parsimonious. There is no evidence of jaws or limbs in the earliest vertebrates, hagfish, lanclets, and sea-squirts. Similarly crab claws and cephalopod arms are derived from different tissues, and not present in anything closer to the root of the tree- most other groups of crustaceans don't have claws.
Posted by: DDeden | June 23, 2009 9:14 PM
@ 77 - Wait...what?
@ 78 - ""Degenerate" as in became greatly simplified- similar to the manner in which species change as they adapt to parasitic lifestyles". To flying lifestyles as well, as noted previously, excess baggage gets lost when disadvantageous.
Hagfish, lancelets, snakes, eels and worms are multiply (adj.) segmented, thus derived, they've 'lost' parts, due to burrowing in soft substrate, tubeworms are also likely derived. As I said, jaws and crab claws are derived (starfish lack them).
"Any evidence would be a start." Once again, the best evidence is in your hands, literally.
Posted by: DDeden | June 24, 2009 8:48 PM
Question: Sauropods have large pneumantic vertebrae, manatees have big super dense ribs, bony fish have many thin bones; so did early vertebrates (way before dinos) swim upside down with dense vertebrae as part-ossified ballast (like early turtle plastron), making them inverted invertebrates, then after developing an air sac, flip back right side up, then sauropods further lightened the vertebrae due to occasional (upright) shallow water wading and swimming? (I know 'semi-aquatic dinos' are old school, but I figure more primitive dinos were more water dependent, later ones much less so.)
IOW was the backbone & bony rib cage initially a ballast & armor trait? Is a notocord lighter than a vertebral column?
Do sharks have a (buoyant) myelin segmented sheath around the spine, while bony fish and tetrapods have a mineralised segmented sheath around the spine? (comment posted to tetzoo, pharyngula, SVPOW, AAT) Anyone?
Posted by: Dave Godfrey | June 25, 2009 12:24 AM
Manatees have heavy ribs to counteract the air in their lungs. Fishes use their swim bladder for the opposite reason, to stop themselves sinking. Sharks have a very large liver for buoyancy. Early vertebrates swam the same way up as modern fishes- the most primitive ones with bone present have armour plates on both the top and bottom, and heavy bony scales. They may well have been negatively buoyant, but most were bottom-living filtering through sediment. The vertebral column was unossified, and made of cartilage. This also occurs in sharks and placoderms (which retained the external bony armour). The vertebrate/invertebrate inversion happened much, much earlier.
You "figure more primitive dinos were more water dependent, later ones much less so", but you haven't actually presented any evidence that they are. Just your hunches.
Are you suggesting that arthropods are closer to vertebrates than echinoderms? The evidence to the contrary is overwhelming. There is more evidence that annelids and arthropods are close relatives- and even that view has fallen out of favour. The major split within the bilateria is between the protostomes ("worms" arthropods, brachiopods, molluscs etc) and deuterostomes (acorn worms, echinoderms sea-squirts and chordates). This is not going to be overturned by your insistence that digits are
Posted by: Dave Godfrey | June 25, 2009 1:09 AM
... the same as arthropod limbs, and vertebrate jaws and crab claws are the same either. The genes mentioned in the paper you quoted earlier aren't present in Drosophila, and aren't active in jaws.
(and I should learn to finish writing a post before I go back to edit bits.)
Posted by: KP | July 10, 2009 2:22 AM
The LYING LIARS at ICR have weighed in on this.
http://www.icr.org/article/4773/
Posted by: Owlmirror
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October 6, 2009 7:37 PM
Over at the TetZoo, Alexander Vargas posted a link to a brief (3-page) interpretation of the digits of Limasaurus from his perspective (which is probably more in line with that of other developmental biologists), which I will also drop here, for the interested:
Limusaurus and bird digit identity
http://precedings.nature.com/documents/3828/version/1