Tetrapod Zoology

Ok, here we are: welcome to the fifth and final part of that ‘month in dinosaurs (and pterosaurs)’ series (for previous parts: part I, part II, part III, part IV). This time, more pterosaurs…

In the previous article we looked at the new tiny pterosaur Ningchengopterus (a juvenile Eosipterus?), and at new ideas on nyctosaur crest function. However, another very interesting pterosaur paper appeared in January 2009: Dyke et al.’s (2009) look at morphological disparity in pterosaurs across time. For the record, ‘disparity’ refers to the degree of morphological variation seen within a clade. It is not the same as diversity: this term applies to phylogenetic richness and is not necessarily linked to disparity [adjacent picture, by Mark Witton and from here, shows the weird suspension-feeding ctenochasmatoid Pterodaustro].

Dyke et al.’s (2009) paper looks a several things. By analysing pterosaur distribution over time, they argue that the pterosaur fossil record is not strongly biased by a ‘lagerst�tte effect’ as has been argued* and that future pterosaur finds may well come from any Mesozoic rock unit. By looking at discovery rates over time, the authors also report that the pterosaur discovery curve has not yet reached the plateau phase: that at which discovery rates have slowed and are levelling off. This indicates that loads more pterosaurs are yet to be discovered. However, discovery rates that are plotted on the basis of new species descriptions assume that the ‘new species’ really are just that, and the authors also note that the discovery curve data might be affected by the possibility that ‘taxonomists working in this area are dedicated splitters’ (p. 3). Are certain pterosaur workers ‘dedicated splitters’? This depends on who you ask, but do note that there are numerous in-fights among pterosaur workers as to which pterosaur taxa really are valid, and which are not. In fact the discovery curve might differ according to whose taxonomic system is followed [Fig. 1 from Dyke et al. (2009) below. I include it because - in (a) - you can clearly see the impressive cumulative rise of new discoveries made since 1812].

* This is the idea that lagerst�tten – places of exceptional preservation – provide a disproportionately high amount of information on diversity, and that they effectively punctuate large periods where relatively little information is available.

It has often been suggested that pterosaur disparity was correlated with the rise of birds. Perhaps, as birds began moving into habitats previously occupied by pterosaurs, pterosaurs were forced to adapt or die. Unwin (2006) proposed that, during the Cretaceous, the niches left vacant as pterosaur species went extinct were not occupied by new pterosaur species, as they were before, but (opportunistically) by birds instead: ‘Ultimately, the effect of this process was to leave pterosaurs adapted to a relatively narrow range of specialist lifestyles’ (Unwin 2006, p. 264). Over time, this left pterosaurs as a whole vulnerable to extinction [adjacent image shows a selection of ctenochasmatoids. Pterodaustro and Cearadactylus, at the top, are Cretaceous and might be regarded as more specialised than the Jurassic forms Pterodactylus and Gnathosaurus, shown below].

Dyke et al. (2009) set out to test this model of waning pterosaur diversity. They report that Cretaceous pterosaurs are more morphologically disparate than older forms, not less so. This suggests, they conclude, that Cretaceous pterosaur evolution was not constrained by birds, and that new morphologies were appearing among pterosaurs during this time. For a full and excellent discussion of this research do see Al McGowan’s article over at Dave Hone’s Archosaur Musings. I swear, this is the last plug that Dave’s blog is getting for the time being [composite below shows anurognathid and nightjar, apparently similar in ecology].

I confess, however, that part of the logic escapes me here. Dyke et al. (2009) are saying that pterosaurs were not morphologically constrained by birds, and that pterosaur disparity peaked during the Early Cretaceous. This is all clear in their Fig. 5 (p. 7). They then use this data to argue that Unwin was wrong in claiming that pterosaurs were replaced by birds. However, the crux of Unwin’s argument is that the majority of pterosaur lineages were being replaced over time by birds, and that Cretaceous pterosaurs were becoming restricted ‘to a relatively narrow range of specialist lifestyles’ (Unwin 2006, p. 264). The assertion that pterosaurs became restricted to a narrow range of lifestyles is, based on the evidence we have, pretty evident given that the only latest Late Cretaceous pterosaurs were oceanic pteranodontians and the terrestrial stalking azhdarchids. Given that – in birds – those taxa at the edges of morphospace occupy specialised ecological niches (Dyke et al. (2009) cite Ricklefs (2005) in support of this)… haven’t Dyke et al. (2009) actually supported Unwin’s proposal that pterosaurs were becoming more specialised and more ‘peripheralised’ during the Cretaceous? In fact Dyke et al. (2009) state that ‘Early Cretaceous new [pterosaur] taxa appear mainly at the periphery of morphospace’ (p. 5). Your thoughts please!

There is actually a lot more to Dyke et al. (2009) that what I’ve discussed here, and I think it’s a very interesting paper that adds a lot to the debate about pterosaur diversity, disparity and evolution. Like all the other articles and discoveries discussed in this little series, it has provoked a lot of debate and argument.

And on that note I must move on. I have a few new papers coming out over the next few weeks that I’ll be discussing here, and – when and where possible – I’ll be adding a lot of other new stuff. Some very exciting papers have just appeared on such things as extant mammal discovery rates, Late Cretaceous theropod diversity (I’m thinking Hesperonychus), and pliosaur skull mechanics. I turned down a news interview for Channel 4′s More4 News today concerning the last two discoveries as I don’t feel up to travelling to London [UPDATE: Les No� did the interview instead. Well done that Les!]. And, as many of you know, something very neat Mesozoic-wise is due to be announced this Thursday. Circumstances have meant that Tet Zoo has been pretty quiet over the past couple of weeks, but there’s not much I can do about that. I’ve actually made myself ill by working too hard.

Finally, several kind individuals provided various sorts of help that assisted in my production of this series of articles: many thanks to Richard Butler, Andrea Cau, Gareth Dyke, Andy Farke, Dave Hone and David Unwin.

Refs – -

Dyke, G. J. McGowan, A. J., Nudds, R. L. & Smith, D. 2009. The shape of pterosaur evolution: evidence from the fossil record. Journal of Evolutionary Biology doi:10.1111/j.1420-9101.2008.01682.x

Ricklefs, R. E. 2005. Small clades at the periphery of passerine morphological space. American Naturalist 165, 651-659.

Unwin, D. M. 2006. The Pterosaurs From Deep Time. Pi Press, New York.

Comments

#1 Neil
March 17, 2009

Interesting stuff. I can see the anurognathid and nightjar comparision, with the wide gape of their mouths. I assume anurognathids were manoeuvrable but im guessing the ecology does not stretch to nocturnal hunting and sitting dead still in heathland until a birdwatcher nearly treds on you! hehe

As for the pliosaur mechanics paper I was stunned by it – not the crushing a humvee bit, but the fact that the accompanying artwork of a paper on giant Mesozoic reptiles was NOT draw by a certain Dr. M. Witton!
#2 Nathan Myers
March 17, 2009

Triumph of the Damn Theropods. So Dyke is saying that birds didn’t out-compete pterosaurs already occupying a niche and oust them; instead they outcompeted pterosaurs in moving into various vacated niches, one after another. It’s a difference, maybe, but does it matter? It means the differences in fitness were of smaller magnitude, but with the same sign.

But what pervasive (if small) advantage did the birds have? Proper feathers and better feet? Or more plasticity?
#3 Michael P. Taylor
March 17, 2009

I missed this pliosaur-jaw thing completely. Reference?
#4 Metalraptor
March 17, 2009

I think that what most likely happened is that pterosaurs suffered heavily in the end-Turonian extinction, in which only two of the pterosaur lineages survived, the giant azdarchids and the ornithocheiroids (Pteranodon and Nyctosaurus, among others). Pterosaur diversity was huge in the Early Cretaceous, despite the rise of birds, but then it took a hit in the big Early Cretaceous extinction. Pterosaurs were low in diversity during the Late Cretaceous, but more due to unfortunate events, rather than being outcompeted by tiny, chirping theropods. Possibly azdarchids and nyctosaurs could have spread back out into the niches their former kin once occupied (I could easily see a radiation of azdarchid “gruiformes”) but then the K-T event happened, and we all know what went on there.

Anyway, great post Darren, I have always been fascinated by pterosaurs, and these news studies are particularly interesting.
#5 Carlos
March 17, 2009

I think anurognathids were still present at the K-T event; they only occur in laggerstate
#6 Zach Miller
March 17, 2009

I’m with Mike–totally missed the pliosaur thing but would love to know more.

It seems plain to me, too, that pterosaur diversity was shrinking as the Cretaceous went on, and birds increased in diversity a thousand-fold. Does the paper examine avian diversification rates, or just pterosaurs? I do like Unwin’s suggestion that the last pterosaurs may have occupied several ecological niches during their lifetimes, but there’s only so much you can do with the body of a terrestrial stalker. It’s not like these animals went through the kind of niche switch that a dragonfly does.
#7 Dave Hone
March 17, 2009

What’s wrong with pimping the Musings? It’s not my fault I’m supplying you with photos, disussions and advanced pterosaur articles! Thanks for the promos though chief!
#8 Nathan Myers
March 17, 2009

Zach, Mike: There’s a story on the newswires about a pliosaur that could bite through a humvee — check Google News — with four times the bite strength of, e.g., Sue. But how does it compare to the Megalodon shark thing we heard about in August? Seems about the same, maybe less. Funny we didn’t hear, then, that the shark could bite through a humvee, but maybe its mouth wasn’t big enough, or maybe sharks just don’t like humvees.

I think we need to see a pliosaur/megalosaur death match now. Pliosaur has more flippers, but a less maneuverable jaw. Megalosaur’s ancestors faced and survived the pliosaurs, and then got an extra hundred million years of practice.
#9 John Scanlon FCD
March 17, 2009

In the Dyke et al. figure, (a) and (b) illustrate that the first half of C20 was a palaeontological Dark Age. Apart from the World Wars, are there any general explanations for this? I’d be interested in seeing comparative data on dates/rates of description for other vertebrate groups or specific regions. F’rinstance, Paul Willis (1993) plotted the number of publications on Australian fossil reptiles each year over the past centiry and a half, with a similar pattern. I suspect the vast majority of palaeo-tetrapod effort went into mammals over that interval (because they’re easy).

WILLIS, P.M.A., 1993. The Australian Palaeoherpetological Renaissance; a review of Australian Palaeoherpetology, 1990-1993. In Herpetology in Australia a diverse discipline. (D.Lunney and D. Ayers editors). Surrey Beatty and Sons, Sydney. Pp 17-33.
#10 J.S. Lopes
March 17, 2009

It’s a mystery why all pterosaurians and enantiornithes became extinct at K-T event (whatever it was) and neornithes survived.
#11 Jerzy
March 18, 2009

I was once fascinated with the idea of plotting discoveries of species over time to get a curve and predict how much creatures remain undiscovered. But thats failure. It omits (1) geo-political access to fossil beds (2) trends and fashions in science and (3) new methods of discovery.

For example, political opening up of China brought to science lots of new fossils. Who knows, maybe incredible fossils await science in Iraq, Niger, Sudan and Somalia? But who can get there?
#12 Darren Naish
March 18, 2009

John Scanlon wrote…

In the Dyke et al. figure, (a) and (b) illustrate that the first half of C20 was a palaeontological Dark Age. Apart from the World Wars, are there any general explanations for this? I’d be interested in seeing comparative data on dates/rates of description for other vertebrate groups or specific regions.

I note that several other people have commented on this situation, though I can’t offer data in the strict sense. Martill et al. (2001) referred to the lull in research as ‘the quiet period’ and suggested that, in Europe at least, political and financial concerns slowed palaeontological research and essentially put an end to the earlier, more active phase in Mesozoic reptile research. Once things got going again, vertebrate palaeontologists seemed to be of the opinion that there was nothing worth knowing about most fossil reptiles, and interest largely shifted to mammals like horses and rodents.

We were heavily inspired by Ned Colbert’s comments on the situation in North America. Throughout his writings he has referred to the ‘Dark Ages’ of the early 20th century, noting that dinosaurs and other Mesozoic groups became neglected because therapsids were now regarded as far more important. In Colbert (1997) he wrote…

These tetrapods were of significance because, it was said, they were in the mainstream of evolution – from primitive vertebrates to the advanced mammals. The dinosaurs in the eyes of many paleontologists were “gee-whiz” fossils, nice to have in the exhibition halls where they could impress the public, but generally speaking fossils off on an evolutionary sideline, and therefore of lesser consequence to the evolutionist than were the therapsid reptiles’ (p. 31).

There is surely scope for a proper historical study of trends in vertebrate research during the 20th century in which research cycles are related to cultural and socio-political changes. I wasn’t aware of Paul’s paper on this and would be interested if there were others.

Refs – -

Colbert, E. H. 1997. North American dinosaur hunters. In Farlow, J. O. & Brett-Surman, M. K. (eds) The Complete Dinosaur. Indiana University Press (Bloomington, Indiana), pp. 24-33.

Martill, D. M., Naish, D. & Hutt, S. 2001. Introduction. In Martill, D. M. & Naish, D. (eds) Dinosaurs of the Isle of Wight. The Palaeontological Association (London), pp. 11-24.
#13 Jerzy
March 18, 2009

If you accept that pterosaurs and enantiornithes grew over several years, juveniles and subadults OBLIGATELY HAD TO have different ecological niches. Flight aerodynamics changes too radically with size. Eg. hatchling pterosaurs were too small to soar.

BTW – almost all modern flying animals start to fly only when about fully grown. The only exception are galliform birds, which, anyway, use flight only rarely for short escape.

(PS – its mad radical proposals week for me )
#14 Dartian
March 18, 2009

John:

the first half of C20 was a palaeontological Dark Age. Apart from the World Wars, are there any general explanations for this?

I think that the two world wars are the main reasons, actually. Not the only reasons, of course; there is definitely a case to be made on how certain research fields go in and out of ‘fashion’ over time. But global wars tend to interfere badly with any non-military scientific activity, such as paleontology. Scientists join or are drafted to the military, field sites become inaccessible, bombing or looting may destroy museums and specimens, international collaboration becomes difficult or impossible, etc. Not to forget a mundane, but probably very important factor: during WW1 and WW2 (as well as in their immediate respective aftermaths), many countries experienced a severe shortage of paper.
#15 Metalraptor
March 18, 2009

“Once things got going again, vertebrate palaeontologists seemed to be of the opinion that there was nothing worth knowing about most fossil reptiles, and interest largely shifted to mammals like horses and rodents.”

In an ultimate case of irony, its now almost reversed. Mammals and other synapsids are shoved to the side, and now its dinosaurs and especially their avian branch who get all the attention.
#16 Zach Hawkins
March 18, 2009

i always thought the birds where one of the reasens why pterosaurus where so rare at the end of there rain, sorry if this is offtopic but is eny one here going to see “walking with dinosaurus: the live experiance”?
#17 Carlos
March 18, 2009

JS. Lopes:
“It’s a mystery why all pterosaurians and enantiornithes became extinct at K-T event (whatever it was) and neornithes survived”

And Zach:

“i always thought the birds where one of the reasens why pterosaurus where so rare at the end of there rain”

From what I can see, neornithes survived because they were more adaptable than their competitors; in fact, although most clades already had ancestors back then, it is highly likely they were represented as rail/shorebird like forms, while other bird clades and pterosaurs were more specialized.

Also, birds themselves seemed only to be taking the vacant niches; they likely didn’t fought against pterosaurs themselves, and although the later were mostly reduced to pteranodonts and azhdarchids (plus perhaps anurognathids, which were nocturnal insectivores and thus having little competion aside from possible volaticothere relatives), they seemed to still had been decently specious despiste their reductions in the number of clades. Plus, don’t forget that pterosaurs were highly precocial, so a very big pterosaur like Hatzegopteryx would likely occupy many niches throw the lifetime.
#18 Carlos
March 18, 2009

JS. Lopes:
“It’s a mystery why all pterosaurians and enantiornithes became extinct at K-T event (whatever it was) and neornithes survived”

And Zach:

“i always thought the birds where one of the reasens why pterosaurus where so rare at the end of there rain”

From what I can see, neornithes survived because they were more adaptable than their competitors; in fact, although most clades already had ancestors back then, it is highly likely they were represented as rail/shorebird like forms, while other bird clades and pterosaurs were more specialized.

Also, birds themselves seemed only to be taking the vacant niches; they likely didn’t fought against pterosaurs themselves, and although the later were mostly reduced to pteranodonts and azhdarchids (plus perhaps anurognathids, which were nocturnal insectivores and thus having little competion aside from possible volaticothere relatives), they seemed to still had been decently specious despiste their reductions in the number of clades. Plus, don’t forget that pterosaurs were highly precocial, so a very big pterosaur like Hatzegopteryx would likely occupy many niches throw the lifetime.
#19 Dr Vector
March 18, 2009

sorry if this is offtopic but is eny one here going to see “walking with dinosaurus: the live experiance”?

I’ve seen it twice and loved it both times, and so did my brother, who is an English teacher. For more thoughts on the show click on the “Dr Vector” link below and do a search. I’d put in the direct links but the SB spam filter doesn’t like that.

If you accept that pterosaurs and enantiornithes grew over several years, juveniles and subadults OBLIGATELY HAD TO have different ecological niches.

This has been much on my mind lately. I’ve been thinking about the lives of baby sauropods. I’ll be they were omnivores for much of their young lives, and possibly insectivores right out of the egg. You see the same switch from insectivory/omnivory to herbivory in iguanas and tortoises. Baby caimans eat mostly arthropods, and only switch to fish later on. Baby sauropods would have a strong incentive to push down some protein and get big enough to be more than walking hors d’oeuvres.

I also think the flip side of this might explain survivorship curves for extant birds. Most extant birds show a Type II survivorship curve (see the Wikipedia article “survivorship curve” for definitions of the three types), in which the mortality rate is fairly constant regardless of age. It makes a lot of sense when you think about avian life history: most birds grow to adulthood very quickly, and don’t live long enough to become senescent, so in survivorship terms there is little effective difference between the relatively young and the relatively old. To put it in ridiculously anthropomorphic terms, selection usually can’t tell the difference between young and old birds, so it kills them all equally, hence the Type II curve.
#20 David Marjanović
March 18, 2009

Mammals and other synapsids are shoved to the side, and now its dinosaurs and especially their avian branch who get all the attention.

I wouldn’t say so. They now occupy fairly similar-sized portions of SVP meeting abstract volumes and JVP issues.

pterosaur[…]s w[…]ere so rare at the end of the[i]r[…] r[…][e]i[g]n

The question is if we know whether pterosaurs really were rare in the Late Cretaceous — after all, there isn’t much rock known from that time that would preserve their remains in the first place.
#21 Joao S. Lopes
March 18, 2009

What is more mysterious about the K/T extinctions is how some groups died completetly. Why not some weird dinosaur living at some distant land can survive? Could catastrophical events be so selective? Why fragile beings like frogs survived and polar small, robust hypsilophodonts, for example, didn’t?
#22 Andreas Johansson
March 18, 2009

I wouldn’t say so. They now occupy fairly similar-sized portions of SVP meeting abstract volumes and JVP issues

The ornithodires get darn close to all the thunder in the popular press, however.

We should start a non-ornithodiran, non-mammoth exinct tetrapod appreciation society or something.
#23 David Marjanović
March 18, 2009

And, as many of you know, something very neat Mesozoic-wise is due to be announced this Thursday.

Tianyulong has been out for a few hours now. While I haven’t been able to read the Nature paper yet, the abstract is… “very neat Mesozoic-wise” indeed.
#24 Andreas Johansson
March 18, 2009

I wouldn’t say so. They now occupy fairly similar-sized portions of SVP meeting abstract volumes and JVP issues

The ornithodires get darn close to all the thunder in the popular press, however.

We should start a non-ornithodiran, non-mammoth exinct tetrapod appreciation society, or something.
#25 Andreas Johansson
March 18, 2009

Sorry for the double post. I got a “timeout” message on the first post so I foolishly assumed the post hadn’t got through.
#26 Devonian
March 20, 2009

“But what pervasive (if small) advantage did the birds have? Proper feathers and better feet? Or more plasticity?”
I can only assume feathered wings have some advantage over membranous ones, since birds went on to do a pretty good job of restricting the niches available to bats…
#27 Carlos
March 22, 2009

Oh, but pterosaurian wings weren’t like those of bats. Their membranes weren’t as fragile, having a layer of muscle and collagen fibers. Plus, bats walk in a sprawling position, just like basal pterosaurs, while pterodactyloids walked with their limbs under the body, just like dinosaurs and mammals
#28 llewelly
March 22, 2009

I can only assume feathered wings have some advantage over membranous ones, since birds went on to do a pretty good job of restricting the niches available to bats…

Beyond Carlos’s point about pterosaur wings, I thought pterosaurs had more efficient bird-like lungs, versus the less efficient mammal lungs which bats have?
#29 Carlos
March 23, 2009

Indeed. Plus they layed eggs and some developed toothless forms, thus being more adapted to flight than bats
#30 David Marjanović
March 23, 2009

I can only assume feathered wings have some advantage over membranous ones, since birds went on to do a pretty good job of restricting the niches available to bats…

No, the birds were simply already there, so the bats had to take the few niches that were still available.
#31 Carlos
March 23, 2009

After all, bats are quite anatomically different from birds, and I doubt they face direct competion with them
#32 Rhys D.
June 19, 2009

Bats and birds don’t really directly compete, but birds are definitely the better fliers, a huge advantage to the variety of niches that could, theoretically, go either way.

Now, the “large pterosaurs going through different niches in their life times” is very interesting, especially given the time. Seems like Tyrannosaurus rex was also doing something similar, being the only predator at the very end of the Cretaceous in North America. Considering how much more gracile youngsters were compared to adults in T.rex, and now the pterosaur deal, it looks like there was really very, VERY little biodiversity before the K-T, and possibly slightly before the Deccan eruptions. I wouldn’t be surprised if the Tyrannosaurs forced out other predators, or if large pterosaurs were effectively forced INTO a sort of “spread-niche” due to some birds conflicting with the small pterosaur species.

Anyway, that’s just a little pet theory, really more thinking out loud than anything. Still in high school, eager to see where I’m right and wrong with this.