<i>Darwinopterus</i> and mosaic, modular evolution

Posted 15 October 2009 by PZ Myers

It's yet another transitional fossil! Are you tired of them yet?

Darwinopterus modularis is a very pretty fossil of a Jurassic pterosaur, which also reveals some interesting modes of evolution; modes that I daresay are indicative of significant processes in development, although this work is not a developmental study (I wish…having some pterosaur embryos would be exciting). Here it is, one gorgeous animal.

(Click for larger image)
Figure 2. Holotype ZMNH M8782 (a,b,e) and referred specimen YH-2000 ( f ) of D. modularis gen. et sp. nov.: (a) cranium and mandibles in the right lateral view, cervicals 1-4 in the dorsal view, scale bar 5cm; (b) details of the dentition in the anterior tip of the rostrum, scale bar 2cm; (c) restoration of the skull, scale bar 5cm; (d) restoration of the right pes in the anterior view, scale bar 2 cm; (e) details of the seventh to ninth caudal vertebrae and bony rods that enclose them, scale bar 0.5 cm; ( f ) complete skeleton seen in the ventral aspect, except for skull which is in the right lateral view, scale bar 5 cm. Abbreviations: a, articular; cr, cranial crest; d, dentary; f, frontal; j, jugal; l, lacrimal; ldt, lateral distal tarsal; m, maxilla; mdt, medial distal tarsal; met, metatarsal; n, nasal; naof, nasoantorbital fenestra; p, parietal; pd, pedal digit; pf, prefrontal; pm, premaxilla; po, postorbital; q, quadrate; qj, quadratojugal; sq, squamosal; ti, tibia.

One important general fact you need to understand to grasp the significance of this specimen: Mesozoic flying reptiles are not all alike! There are two broad groups that can be distinguished by some consistent morphological characters.

The pterosaurs are the older of the two groups, appearing in the late Triassic. They tend to have relatively short skulls with several distinct openings, long cervical (neck) ribs, a short metacarpus (like the palm or sole of the foot), a long tail (with some exceptions), and an expanded flight membrane suspended between the hind limbs, called the cruropatagium. They tend to be small to medium-sized.

The pterodactyls are a more derived group that appear in the late Jurassic. Their skulls are long and low, and have a single large opening in front of the eyes, instead of two. Those neck ribs are gone or reduced, they have a long metacarpus and short tails, and they've greatly reduced the cruropatagium. Some of the pterodactyls grew to a huge size.

Here's a snapshot of their distribution in time and phylogenetic relationships. The pterosaurs are in red, and the pterodactyls are in blue.

(Click for larger image)
Time-calibrated phylogeny showing the temporal range of the main pterosaur clades; basal clades in red, pterodactyloids in blue; known ranges of clades indicated by solid bar, inferred 'ghost' range by coloured line; footprint symbols indicate approximate age of principal pterosaur track sites based on Lockley et al. (2008); stratigraphic units and age in millions of years based on Gradstein et al. (2005). 1, Preondactylus; 2, Dimorphodontidae; 3, Anurognathidae; 4, Campylognathoididae; 5, Scaphognathinae; 6, Rham- phorhynchinae; 7, Darwinopterus; 8, Boreopterus; 9, Istiodactylidae; 10, Ornithocheiridae; 11, Pteranodon; 12, Nyctosauridae; 13, Pterodactylus; 14, Cycnorhamphus; 15, Ctenochasmatinae; 16, Gnathosaurinae; 17, Germanodactylus; 18, Dsungaripteridae; 19, Lonchodectes; 20, Tapejaridae; 21, Chaoyangopteridae; 22, Thalassodromidae; 23, Azhdarchidae. Abbreviations: M, Mono- fenestrata; P, Pterodactyloidea; T, Pterosauria; ca, caudal vertebral series; cv, cervical vertebral series; mc, metacarpus; na, nasoantorbital fenestra; r, rib; sk, skull; v, fifth pedal digit.

Darwinopterus is in there, too—it's the small purple box numbered "7". You can see from this diagram that it is a pterosaur in a very interesting position, just off the branch that gave rise to the pterodactyls. How it got there is interesting, too: it's basically a pterosaur body with the head of a pterodactyl. Literally. The authors of this work carried out multiple phylogenetic analyses, and if they left the head out of the data, the computer would spit out the conclusion that this was a pterosaur; if they left the body out and just analyzed the skull, the computer would declare it a pterodactyl.

What does this tell us about evolution in general? That it can be modular. The transitional form between two species isn't necessarily a simple intermediate between the two in all characters, but may be a mosaic: the anatomy may be a mix of pieces that resemble one species more than the other. In this case, what happened in the evolution of the pterodactyls was that first a pterodactyl-like skull evolved in a pterosaur lineage, and that was successful; later, the proto-pterodactyls added the post-cranial specializations. Not everything happened all at once, but stepwise.

(Click for larger image)
Schematic restorations of a basal pterosaur (above), Darwinopterus (middle) and a pterodactyloid (below) standardized to the length of the DSV, the arrow indicates direction of evolutionary transformations; modules: skull (red), neck (yellow), body and limbs (monochrome), tail (blue); I, transition phase one; II, transition phase two.

This should be a familiar concept. In pterodactyls, skulls evolved a specialized morphology first, and the body was shaped by evolutionary processes later. We can see a similar principle in operation in the hominid lineage, too, but switched around. We evolved bipedalism first, in species like Ardipithecus and Australopithecus, and the specializations of our skull (to contain that big brain of which we are so proud) came along later.

As I mentioned at the beginning, this is an example of development and evolution in congruence. We do find modularity in developmental process — we have genetic circuits that are expressed in tissue- and region-specific ways in development. We can talk about patterns of gene expression that follow independent programs to build regions of the body, under the control of regional patterning genes like the Hox complex. In that sense, what we see in Darwinopterus is completely unsurprising.

What is interesting, though, is that these modules, which we're used to seeing within the finer-grained process of development, also retain enough coherence and autonomy to be visible at the level of macroevolutionary change. It caters to my biases that we shouldn't just pretend that all the details of development are plastic enough to be averaged out, or that the underlying ontogenetic processes will be overwhelmed by the exigencies of environmental factors, like selection. Development matters — it shapes the direction evolution can take.

Lü J, Unwin DM, Jin X, Liu Y, Ji Q (2009) Evidence for modular evolution in a long-tailed pterosaur with a pterodactyloid skull. Proc. R. Soc. B published online 14 October 2009 doi: 10.1098/rspb.2009.1603

I should have mentioned that Darren Naish has a very thorough write-up on Darwinopterus!

42 Comments

wile coyote · 15 October 2009

It's yet another transitional fossil! Are you tired of them yet?

No, but if you'd said "missing link" I would have called in an airstrike. However, nobody here would use that term except as a gag. Any distinction between "modular" and "mosaic" evolution? The "modular" bizness is new to me.

Novparl · 15 October 2009

Tired. Yes.

wile coyote · 15 October 2009

Oh dear, the Grinch is back.

Novparl · 15 October 2009

Oh dear indeed.

Over on the 1st Ardi thread, I win again. The threatened Matt Young has had to close it to comments. Complaining I never ask any interesting questions. As a physicist he has no interest in what there was before the big bang. Also, why we have mirror image limbs etc. or how the brain evolved. Isn't intellectual curiosity sooo boring?

DS - if you're allowed to read this - thanks for the number. I'll crunch it later. I often think of the Eloi & the Morlocks too. Hope it's not verboten to say that.

wile coyote · 15 October 2009

WILL the Grinch succeed in his EVIL PLAN to derail yet another thread with his malevolent musings? WILL the loyal (if somewhat overly earnest) resident posters be able to resist the urge to reply to his entirely contrived and often incoherent goadings?

Of COURSE they won't. But I will not say more in this particular discussion, having better things to do myself. So go ahead, Grinch, take yer best shot, it's free.

Matt Young · 15 October 2009

I win again.

That about sums it up, doesn't it? The Novparl troll has no interest in having its questions debated, only in being a nuisance. I know it is tempting to think it can learn, but it has no interest in doing so. Please do not feed it any more.

DS · 15 October 2009

Far be it from me to feed the troll (any more). However, I should point out a few things:

1) He didn't "win" on the other thread. All he did was ask irrelevant and ridiculous questions that everyone ignored. For example, anyone who understands how developmental genetics works would not seriously ask how bilateral symmetry could evolve. Anyone who knows anything about limb development would not seriously suggest that limbs could not evolve. Anyone who knew anythng about brain development would not ask how millions of connections in the brain could evolve. Any one who knew anything about evolution would not seriously demand to know exactly how humans are going to evolve.

2) He never answered any of my questions or read the references I provided. As you say, he is only an irritant with no hope of ever learning anything.

3) This guy has never made an on-topic post. Until he posts something relevant to pterosaurs on this thread he should be ignored. Assuming of course that PZ is not going to continuously police the thread and immediately send all off-topic posts to the bathroom wall where they belong.

fnxtr · 15 October 2009

Cool.

The authors of this work carried out multiple phylogenetic analyses, and if they left the head out of the data, the computer would spit out the conclusion that this was a pterosaur; if they left the body out and just analyzed the skull, the computer would declare it a pterodactyl.

I guess it's too early to tell for sure if Darwinopterus belongs on a side branch of monofenestrata or really is a forerunner of the pterodactyla? Or is there other morphology that places it squarely off the lineage to the "P" branch?

FastEddie · 15 October 2009

Wow, another brilliant fossil find! Surely scientists or funding from the Discovery Institute were involved in this discovery.

Vince · 15 October 2009

Great find, but I'm a bit perplexed. Based solely upon the branching diagram given wouldn't this critter rightly fall out as sharing a common ancestor with the Pterodacytls and not the Pterosaurs? Lumping it with the latter would give a paraphyletic group. What am I missing here?

ben · 15 October 2009

I win again

This from a guy who thinks "winning" a debate involves maneuvers like referring to a "scientific theory" of ID, being unable to explain what the theory is or where anyone else describes it, then dismissing his failure by saying "I told you you wouldn't like it." Loser.

PZ Myers · 15 October 2009

Novparl will not be posting on this thread anymore -- any of his comments will be deleted as I discover them. So please ignore him.

I think he just lost.

Frank J · 15 October 2009

Sorry, PZ, but pigeons always win at chess.

wile coyote · 15 October 2009

EXPELLED: No Grinches Allowed!

stevaroni · 15 October 2009

Novparl will not be posting on this thread anymore – any of his comments will be deleted as I discover them.

No, PZ, don't do that. Disruptive as it is to have to tediously point out over, and over, and over again that the emperor still has no clothes, it's even worse to let them have the slightest taste of censored martyrdom.

KP · 15 October 2009

stevaroni said: No, PZ, don't do that. Disruptive as it is to have to tediously point out over, and over, and over again that the emperor still has no clothes, it's even worse to let them have the slightest taste of censored martyrdom.

I'm with steve, I think there is some value in having novparl's flailing on display for lurkers who are on the site to learn. One of the things this site has to teach is how vacuous and irrelevant the creationist position is with respect to the science. I didn't see the other thread, but asking about the Big Bang in a thread about Ardipithecus should to any reasonable observer, look as ludicrous as questions about Obama's birth certificate, for example.

Stanton · 15 October 2009

stevaroni said:
Novparl will not be posting on this thread anymore – any of his comments will be deleted as I discover them.
No, PZ, don't do that. Disruptive as it is to have to tediously point out over, and over, and over again that the emperor still has no clothes, it's even worse to let them have the slightest taste of censored martyrdom.

There is a fine but profound difference between martyring someone who asks pointed questions, and censoring someone who takes pride in not knowing even the most rudimentary etiquette skills. It's fine if someone wants to point out that the emperor has no clothing, provided they do it in a civilized, if not polite manner. But if they're going point out that while jumping, screaming and stripping off all their clothing, I say boot them out of the building and never let them back in.

wile coyote · 15 October 2009

I'm with Stanton. Some of the visitors here are annoying but they have a point to make. The Grinch made no pretense of doing anything but trying to make noise and be a disruptive nuisance.

So he claims he's a martyr. He's a nobody. Nobody is going to care.

DS · 15 October 2009

I don't see a problem with sending off-topic posts to the bathroom wall. That cannot reasonably be considered censorship. Of course it will necessarily eliminate our resident trolls since they never post anything on-topic.

I don't see the problem with letting novparl ignore all attempts to educate him on the bathroom wall. Remember, this is the guy who screamed abuse because I asked him to read a scientific reference! Now if he should post something on-topic we might have a problem because if his posts are not removed and people respond he will of course immediately take that as an invitation to derail the thread.

wile coyote · 15 October 2009

Don't have a problem with the BW, that works fine for me.

DS · 15 October 2009

Vince wrote:

"Great find, but I’m a bit perplexed. Based solely upon the branching diagram given wouldn’t this critter rightly fall out as sharing a common ancestor with the Pterodacytls and not the Pterosaurs? Lumping it with the latter would give a paraphyletic group. What am I missing here?"

Well, if the Pterodacytls are considered a separate group then Pterosaurs are apparently already paraphyletic. If however, Pterodacytls are already considered to be a subgroup of Pterosaurs then the placement of Darwinopterus is not problematic.

eric · 15 October 2009

stevaroni said:
Novparl will not be posting on this thread anymore
No, PZ, don't do that.

And they call us a conspiracy. What self-respecting conspiracy argues in public about whether to allow argument in public?

Henry J · 15 October 2009

Great find, but I’m a bit perplexed. Based solely upon the branching diagram given wouldn’t this critter rightly fall out as sharing a common ancestor with the Pterodacytls and not the Pterosaurs? Lumping it with the latter would give a paraphyletic group. What am I missing here?

Going by that diagram, the pterodactyls are a side branch of the pterosaurs anyway, so all of them have a common ancestor with the pterosaurs. I think it's interesting that the intermediate between them somehow wound up with the name Darwinopterus. Henry

Vince · 15 October 2009

DS said: Vince wrote: "Great find, but I’m a bit perplexed. Based solely upon the branching diagram given wouldn’t this critter rightly fall out as sharing a common ancestor with the Pterodacytls and not the Pterosaurs? Lumping it with the latter would give a paraphyletic group. What am I missing here?" Well, if the Pterodacytls are considered a separate group then Pterosaurs are apparently already paraphyletic. If however, Pterodacytls are already considered to be a subgroup of Pterosaurs then the placement of Darwinopterus is not problematic.

Agreed - But have the authors made the jump and claimed that the Pterodactyls are a subgroup of the Pterosaurs? It would make sense to me, but I guess I'll just have to do what I keep telling my students - read the dang paper! Vince

John Harshman · 15 October 2009

Vince said: Great find, but I'm a bit perplexed. Based solely upon the branching diagram given wouldn't this critter rightly fall out as sharing a common ancestor with the Pterodacytls and not the Pterosaurs? Lumping it with the latter would give a paraphyletic group. What am I missing here?

Nothing. Paraphyletic groups abound in the press coverage of this fossil, including, unfortunately, PZ's. First, "pterosaur" is a term that encompasses all these fossils, not just the "primitive" ones. It's a general term for all these reptiles with a membranous wing supported by one finger. So the new fossil is a pterosaur. Pterodactuls are pterosaurs too. All members of Pterosauria are pterosaurs. And yes, the new fossil is closer to pterodactyls than to any other pterosaurs. That's what makes it so cool: it falls right in a morphological gap between pterodactyls and the more primitive pterosaurs (that used to go by the name Rhamphorhynchoidea before we stopped liking giving names to paraphyletic groups). One more thing. Only two groups of Mesozoic flying reptiles? Well, no. There are many more than that if you want to count pterodactyls, there being several clades of other pterosaurs. And let's not forget the one group of Mesozoic flying reptiles that survived the Mesozoic. (Hint: feathers.) By the way, if anyone is paying attention. I've tried many times to sign in on Pharyngula, and been told I'm not allowed access. It sure would be nice to fix that.

didymos · 15 October 2009

eric said: And they call us a conspiracy. What self-respecting conspiracy argues in public about whether to allow argument in public?

Novparl loses. Eric wins. At least a couple internets.

harold · 15 October 2009

Stevaroni said -

No, PZ, don’t do that. Disruptive as it is to have to tediously point out over, and over, and over again that the emperor still has no clothes, it’s even worse to let them have the slightest taste of censored martyrdom.

I'm neutral. I will note that this site has become much less combative, for better or for worse. A few years ago it was extremely common for blathering "big word creationists" to put up extremely long posts that must have taken hours to construct, and so on. Most of them actually surrendered. They kept getting answered back, it gave them cognitive dissonance, they didn't have that precious, precious ID court victory that they so desperately hoped for, so they sulked off to the echo chambers. Those who keep coming back tend have no capacity to even recognize feedback, let alone respond to it, except with aggressive repetition of sound bites. And of course, certain very effective but impolite defenders mainstream science have been relegated to the so-called "bathroom wall" as well. Is PT better now? I don't know. The tone is certainly more civil, and actual science can sometimes be discussed without interruption. It's kind of like asking whether Dodge City is better now than it was in 1880.

Matt Young · 15 October 2009

Novparl will not be posting on this thread anymore -- any of his comments will be deleted as I discover them. So please ignore him. I think he just lost.

I think PT's policy is to banish disruptive commenters to the Bathroom Wall, rather than delete their comments. That policy is intended to ameliorate their martyrdom complex.

Henry J · 15 October 2009

eric said: And they call us a conspiracy. What self-respecting conspiracy argues in public about whether to allow argument in public?

Isn't "self-respecting conspiracy" an oxymoron? ;) Henry

Yopienso · 15 October 2009

DS said: Far be it from me to feed the troll (any more). However, I should point out a few things: 1) He didn't "win" on the other thread. All he did was ask irrelevant and ridiculous questions that everyone ignored. For example, anyone who understands how developmental genetics works would not seriously ask how bilateral symmetry could evolve. Anyone who knows anything about limb development would not seriously suggest that limbs could not evolve. Anyone who knew anythng about brain development would not ask how millions of connections in the brain could evolve. Any one who knew anything about evolution would not seriously demand to know exactly how humans are going to evolve. 2) He never answered any of my questions or read the references I provided. As you say, he is only an irritant with no hope of ever learning anything. 3) This guy has never made an on-topic post. Until he posts something relevant to pterosaurs on this thread he should be ignored. Assuming of course that PZ is not going to continuously police the thread and immediately send all off-topic posts to the bathroom wall where they belong.

Hi, DS. I really DON'T know anything about limb development, though I've tried. I know this is off-topic, but I don't know how else to contact you to ask for a simple explanation or a link to one. This one http://dev.biologists.org/cgi/reprint/127/24/5233.pdf is too advanced for me. Thanks! Yp

DS · 16 October 2009

yopienso,

Actually, the question is somewhat on-topic, considering the way in which pterodactyl limbs have evolved. I would recommend the following reference:

Hinchliffe, J. R. 2002 Developmental Basis of Limb Evolution. Int J. Dev. Bio. 46:835-845

This article is available free on the web. I can try to get the link for you, but all I did was google the term "limb evolution". There are thousands of hits for this topic, most of them involving palentological evidence. The link for the reference above comes up in the first page. It shows how changes in developmental pathways can lead to the production of novel morphological features such as limbs. If the article is too technical then there is a lot more information out there.

I would also recommend the talkorigins archive which usually has excellent review articles written for those with no advanced training in biology. I'm sure that PZ also has some excellent posts on this topic over at Pharyngula. Both web sites have search functions.

Good luck and enjoy.

DS · 16 October 2009

yopienso,

I really liked the link you provided to a great review article. Unfortunately, these things do tend to be a little complicated. It might be a good idea to study some genetics and developmental biology before trying to tackle such complex issues.

The basics however are quite clear. We are beginning to understand the molecular mechanisms by which limbs are generated during development. This has lead to an investigation of the origin of the genes and regulatory mechanisms involved. This research has shown how the morphological changes observed in the fossil record could be brought about by relatively simple genetic changes involving gene duplications and the evolution of regulatory pathways. Once in place, these basic pathways hve undergone modifications that have lead to the diversity of vertebrate limbs that we see in living organisms today.

I know it sounds complicated, but this is exciting research that might lead to treatments and possibly even cures for congenital malformations. Sure beats GODDIDIT as an explanation.

fnxtr · 16 October 2009

This research has shown how the morphological changes observed in the fossil record could be brought about by relatively simple genetic changes involving gene duplications [my emphasis - f] and the evolution of regulatory pathways.

Um, you were saying, about photocopies, Mr. Wells...?

Kattarina98 · 16 October 2009

KP said:
stevaroni said: No, PZ, don't do that. Disruptive as it is to have to tediously point out over, and over, and over again that the emperor still has no clothes, it's even worse to let them have the slightest taste of censored martyrdom.
I'm with steve, I think there is some value in having novparl's flailing on display for lurkers who are on the site to learn. One of the things this site has to teach is how vacuous and irrelevant the creationist position is with respect to the science. I didn't see the other thread, but asking about the Big Bang in a thread about Ardipithecus should to any reasonable observer, look as ludicrous as questions about Obama's birth certificate, for example.

Feedback from a longtime lurker: Somehow, my childhood fascination with paleontology has survived, and as a layperson I am happy to get it explained by experts - thanks! However, I am always disappointed when novparl yet again derails the discussion of a topic I am interested in. For once, he did not succeed. Your policy of sending posts to the Bathroom Wall seems a good solution.

Frank J · 16 October 2009

My 2c on whether or not to ban trolls:

1. PT's record of allowing real creationists like FL and Ray Martinez to make their case is impeccable. If anything, the thread-hijackers make it harder for the real creationists.

2. Anti-evolution activists will accuse us of "censorship" no matter what we do, and will ignore the real censorship on their blogs. Everything about them involves a double standard.

3. Banning even the most disruptive trolls may not even be necessary. Instead of taking their bait, ignore their PRATTs and keep asking them for their position on the age of life and common descent (and related, increasingly specific questions if it comes to that). Eventually they give up and go away rather than let any lurkers know what a confused cacophony of opinions there are under the big tent.

Frank J · 16 October 2009

Another 1c:

Does Uncommon Descent have the equivalent of a Bathroom Wall? If not, why not?

ravilyn sanders · 16 October 2009

It is a pity, the lucid and compact summary posted here by PZMeyers is deemed to be "too technical and incomprehensible to general public" by the likes of editors of CNN and other such popular news vendors. I am a lowly computer hack, with no formal education in biology beyond high school, even that biology was not taught in English. I did not even know the English names for most organs of the body when I graduated from high school. Still I am able to follow the logic and the science explained in this post. The whole tone of this CNN article reeks with evolution is controversial, the scientists don't understand it, they are constantly confounded by new discoveries, and they constantly change and tweak their theory with bond wire, balsa wood and band aids. They open with

Scientists say a very rare find of some 20 fossilized pterodactyls has produced the first clear evidence of a controversial theory of evolution.

. Contact them, and they weasel out with, "oh! no! we did not say the WHOLE theory of evolution is controversial, but just this little bit, note the all too crucial indefinite article 'a' we slipped in!". But in the mind of people the axon or whatever that connects 'controversy' with 'theory of evolution' is strengthened. And they are ably aided and abetted by scientists who indulge the news vendors with juicy quote mines. "Darwinopterus came as quite a shock to us". Why does it shock an eminent fossil hunter an intermediary life form was found? The shock was at the finding part, the quality and quantity of the fossils, not the existence of such a life form. That was already predicted. But it was not clear from the article. Probably David Urwin did say it clearly, but it was perhaps edited for brevity and to enhance "juciness".

Henry J · 17 October 2009

Another 1c: Does Uncommon Descent have the equivalent of a Bathroom Wall? If not, why not?

Certainly, only instead of putting it on the wall, they flush it down the commode. Henry

Novparl · 17 October 2009

This comment has been moved to The Bathroom Wall.

jkc · 17 October 2009

The CNN article also has this quote:

"The researchers say more study is needed to substantiate the idea of [sic] that evolution could occur relatively quickly, and that whole parts of a plant or animal's body could change at once."

This is another fallacy of the Cambrian "explosion" variety: making it sound like the evolution in this case happened "quickly" without actually saying that it was still millions of years. In addition, the second part of the quote makes it sound like saltationism, while no one really, of course, is claiming that the skull or other body parts evolved all at once.

Toidel Mahoney · 21 October 2009

Well, I can hear the calliope heralding the arrival of the evolutionists' big tent now! I wonder how they made this one? Is it just a crude paper mache model or something they might have put some more effort into--something like the Cardiff Giant?

ben · 21 October 2009

Toidel Mahoney said: Well, I can hear the calliope heralding the arrival of the evolutionists' big tent now! I wonder how they made this one? Is it just a crude paper mache model or something they might have put some more effort into--something like the Cardiff Giant?

Dumbass, the Cardiff Giant was a hoax perpetrated by an atheist to fool credulous Christians. Scientists denounced it as a hoax immediately.