(posted in Evolution)

Radiation causes mutations totally at random -- it never has any bias towards producing beneficial or disadvantageous mutations, because such concepts are very subtle and complex and it would be quite impossible to "tune" radiation to produce one mutation rather than another.  (Exception: in Marvel comics, exposure to gamma rays always causes a mutation resulting in great strength.  Don't try this at home, kids.)

But.  Luckily for evolution, though unluckily for the organisms caught in the collateral damage, evolution doesn't care about the average effect of mutations -- the possessors of disadvantageous mutations simply die and those with beneficial mutations survive and propagate those mutations.  Remember that natural selection is a process that works on the raw material of random change.  In times of more radiation, there will be more random change, and so more material for evolution to work on.  For that reason, evolution will proceed more quickly under these conditions -- so long as the radiation isn't so strong that it kills off the entire population, of course.

I've previously used this analogy to understand how evolution works: random change is like throwing a dice, and natural selection keeps only the sixes.  In times of high radiation, more dice are being thrown, so there are more sixes to keep.  Sadly, there are more ones as well, but evolution doesn't care about them.

(posted in Evolution)

When I read Moby Dick a few years back, I was surprised to read quite a length digression in the middle of the story on the subject of why whales are really fish, and not mammals as a few pesky scientists insist.  (OK, Moby Dick has a lot of digressions, but that was the one that really caught me.)

And another note of caution: this new specimen tells us how the pteroid was oriented in ONE SPECIMEN.  We can legitimately generalise that finding to the species, and somewhat higher, but it's hard to say how far up the tree.  It certainly isn't enough, on its own, for us to draw a firm conclusion about the use of this bone across the whole of Pterosauria, which is a big clade with enormous morphological disparity.

(For the same reason, I am wary of statements like "sauropods were terrestrial rather than aquatic".  Sure, some were; but that's a bit like saying "mammals are terrestrial".  In among all the cows, tigers and rats, there are also hippos, not to mention whales.  Who knows whether some sauropod group -- not yet known, or known but not yet recognised -- was as aquatic as hippos?)

"I would like to point out that the biggest animal we know of is a mammal..."

Well -- the biggest animal that we have a good proportion of the skeleton of.  As you probably know, there is evidence of sauropods on a par with and maybe bigger than blue whales, although nothing wholly convincing yet.  Parts of this story are very well told on Darren Naish's (old) Tetrapod Zoology site:
http://darrennaish.blogspot.com/2007/01 … art-i.html
and
http://darrennaish.blogspot.com/2007/01 … rt-ii.html

Darren wrote: "A 2001 study on body size in giant animals gave _Megistotherium_ a weight of 880 kg and _Andrewsarchus_ a size range of 600 to 900 kg."

Hang on, that's pathetic.  We have bears that weigh more than that, don't we?

Manabu wrote that "biomechanical modeling could predict maximum stresses applied to the skeleton during copulation, thus allowing scientists to say, 'Well dinosaurs couldn't possibly have copulated in this way, because that would excede the maximum stress value, but this other way may have been possible because it's within the safe range.'"

I just want to point out how very approximate such calculations are, though, before anyone gets too excited.  For example, one paper (Economos 1985) calculated the theoretical maximum mass for a terrestrial animal at 15 tonnes, while a followup (Hokannen 1986) yielded estimates between 100 and 1000 tonnes.  Similarly, the masses of the animals involved have been estimated at wildly warying levels: for example, Brachiosaurus brancai has been estimated everywhere between 15 tonnes (Russell et al. 1985) to 78 tonnes (Colbert 1962).  So we need to be very, very careful when working with these numbers.

Taking this to an extreme, I once calculated the safety factor why which sauropod dinosaurs could walk without crushing the articular cartilage in their limbs ... and when I'd evaluated the sources of possible error, concluded that my results were "correct within a factor of 756" :-)  Admittedly that is stretching a little to make a rhetorical point, but it should at least serve as a warning.

For anyone who's interested, by the way, the slides for the talk with that calculation in are at
http://www.miketaylor.org.uk/dino/pubs/ … rength.ppt
I think they should be pretty comprehensible to an intelligent non-specialist.

In summary, Jason's orginal question "Are viruses alive?" might be better expressed as "Is 'alive' defined in such a way as to include viruses?"  Put that way, it's clear that the question is about definitions rather than facts.

Hi, Jason.  Yes, carcinology is a branch of biology -- not necessarily oceanography, since there are also terrestrial crustaceans such as woodlice.  So biology is the subject to concentrate on.  Other sciences, particularly chemistry, are likely to come in useful, and maths is useful in pretty much every field.

You ought to have people at your school who can give you guidance that is more relevant to your own situation.  Talk to your biology teacher for starters: he or she should be able to point you in the right direction.

Still and all, one year sounds like ... a courageous claim.  I would very much like to see the reference for this year-long study.

OK, Jesse, sorry for jumping to the wrong conclusion.  As you'll appreciate, we do get a lot of homework questions, and we have to make a judgement call on some borderline cases.  If it's any consolation, I think you're the first false positive :-)  Let's hope someone can give you a useful answer (I can't).

This has about it the smell of a homework question.  Sorry, but we don't answer those.

(posted in Evolution)

Here is an analogy that someone may find useful.  Variation is random, but the results of the variation are selected non-randomly.  It's like this: each new organism throws a dice to determine its variations, but nature only keeps the sixes :-)

Hi, Wade.  Just wanted to apologise that no-one's taken this question on.  I'd be fascinated to know the answer myself.  Hopefully someone will be stirred into action by this non-response :-)

(posted in Mammals)

As it happens, I saw hyenas being fed at Colchester Zoo (England) twelve days ago.  It was an interesting process.  The hyenas were removed from their usual large enclosure, and zoo staff entered with hunks of meaty bone which they proceeded to drag around on the ground to create a smell-trail.  Then they hid the food in three or four packages around the enclosure.  After they'd left, the hyenas were released back into the enclose, where they ... didn't seem very excited :-)  I suppose this comes of keeping zoo animals well-fed, but they appeared rather bored with the whole process and ambled around in a rather desultory way until they found the packages, then nibbled casually on the contents.

It was a lot more fun watching them feed the mandrills in the next enclosure!

By the way, if anyone gets a chance to visit Colchester Zoo, it's worth it for an experience I've never had before: you can get up close and personal with the elephants, including feeding them little bits of food.  Very exciting.

Yes. but how do the butterflies get in there?

(Sorry.)

... and this example highlight a big problem with the "biological species concept" that, oddly, I've not seen discussed before.  If you use the BSC, then two populations that can interbreed are considered to belong to the same species. but two species that can't interbreed are considered to be two separate species.  Now consider three populations: one each of Chihuahuas, one of Great Danes and one of Basset Hounds.  C and BH can interbreed, so they are the same species; GD and BH can interbreed so they are the same species; but C and GD can not interbreed to they are different species!  That makes no sense; and to me, that means that the biological species concept is fundamentally flawed.

But for some reason, Tyrannosaurus rex, which we'd expect to be very rare, is known from many specimens -- far more than most other dinosaurs.  I'd be interested to know what's going on here -- I think the number of specimens is more than can be accounted for just by collecting bias.

Briefly --

The eminent evolutionary biologist Stephen Jay Gould advocated an understanding that he described as "non-overlapping magisteria", in which science and religion set out to ask and answer fundamentally different kinds of questions.  (Gould also pointed out that these two approaches to the universe by no means encompass all enquiry: poetry, for example, seeks to ask and answer different questions again).

I think that most scientists would say that, while Gould's position is a little over-simplified, it's based on an important insight.  Creationism, and its unacknowledged child Intelligent Design, are what you get when specialists on one magisterium (religion) start to expound their uninformed views on the other (science).  And the same thing happens in the opposite direction: as respected a scientist as Richard Dawkins, when he pronounces on the world of religion (as he increasingly often does, seemingly more often than he does actual science these days) is capable of equally breathtaking mistakes, misapprehensions and misrepresentations.

None of this is to say that you can't be both a scientist and religious: many scientists have proven this by example, including several of the contributors to this forum.  But those who are usefully active in both fields achieve this by carefully avoiding the seductive idea that their expertise in one area necessarily entitles them to comment on the other -- both must be learned.  In the same way, a scientist who is also a poet would be a fool to apply the techniques of scientific writing to his poetry, or to apply his poetic sensibilities in deciding a scientific matter.  Poetry and science certainly have things to say to each other (as Dawkins explains at length in the better sections of his book Unweaving the Rainbow) but the poetic contribution to science must be judged scientifically, and the scientific contribution to poetry must be judged poetically; and the same applies regarding the science/religion interface.

(Did I say "briefly"?  Oh well :-)

The last thing to say is that Ask A Biologist is a science site, and we're here to answer science questions.  Even those among us with religious tendencies are here in our capacities as scientists.  So here you will get scientific answers to your questions.  And judged as science, Creationism and Intelligent Design are badly lacking.

[And finally, finally, I will break the rule I mentioned earlier by speaking with my Christian hat on: as well as being bad science, creationism is in my opinion extremely bad religion.  Even the most committed biblical literalist can surely see that the word "day" in the earliest verses of Genesis cannot mean "period of rotation of the Earth with respect to the Sun" when the sun does not even come into existence until the fourth day! (Gen. 1:16).  Genesis is simply not a science textbook: it was never intended to be read as one.]

Hope some of that helps.

(posted in Evolution)

Hmm.  I am inclined to think that the "humans are not superior" responses are a bit on the Politically Correct side.  Of course it's true that lots of molluscs beat us in terms of species longevity, many bacteria beat us for total biomass and so on, but even with all that said, it seems clear what the questioner means by "superior", that is, with a greater ability to affect and even deliberately manipulate the environment to the detriment of other specieis.  And as the questioner implied, yes, I do think that the emergence of humans is (considered from the point of view of biodiversity) "a flaw in evolution" -- it could hardly be otherwise given the rate at which species are going extinct right now, most of them as a direct result of things we're only able to do because of our superior minds (e.g. building cities).

Then again, you don't have to be humanocentric to think that humans are in some positive senses superior to all the other species we know of: we are certainly way ahead of them all in terms of culture, art, and (most pertinently to those of us who run this site) the ability to do science.

The question goes on, "Does that mean that humans, being superior to everything else, will be caught up or wiped out?"  I don't know of any theory that the very fact of mental superiority on the part of humans will cause us to be at a disadvantage.  Then again, it's no guarantee of success -- any more than T. rex's physical superiority guaranteed it survival after the Big Rock Of Doom hit.

What we [i]can[/] say is that our intelligence has given us whole new ways to wipe ourselves out that the "lower" animals can only dream about.  We're not likely to be outcompeted by any other species in our niche.  But we're quite capable of dying out by making the whole planet uninhabitable, and taking many other species down with us.

Who's superior then?

Well, this has all got very metaphysical.  It may be best to summarise by saying that the particular kind of superiority that we have -- mental -- makes us the only species that can actively manipulate its own evolution.  It's too early to say what the results of that will eventually be, since we've only been doing it for a length of time that's negligible by usual evolutionary standards.  But we're certainly capable of fouling up on a scale that no other species can rival.

Doesn't that make you feel proud?

One mechanism is that tiny fish eggs attach to the legs of birds that are visiting populated ponds, then come off into the new ponds and hatch there.  Of course, from the point of view of a single egg, the chances of this working a tiny -- but fish lay so many eggs that the chance of getting one or two eggs to a new pond in this way is not bad.

(posted in Birds)

Oh, my, you've done it now :-)

The definition of the word "bird" is one of the most argued topics in biology.  Well, actually, the argument is over the formal name Aves, which is the name used by biologists to denote the group of all birds.  Like any word, it can be defined in different ways, and something like a dozen different definitions have been proposed and used by different scientists over the years.  In 2001, Jacques Gauthier and Kevin de Queiroz wrote a 35-page paper just on that subject!

One thing that most biologists do agree on is that names for groups of animals should usually be attached only "natural groups" or clades -- that is, groups consisting of an ancestor and all its descentants.  For this reason, whatever ancestor is chosen as the first bird, all its descendants -- including penguins and ostriches -- would also be included in the group.

Although the notion of a clade, and the tendency name only clades, has only been clearly stated in the last few decades, this kind of evolutionary thinking has been around for much longer, even if it wasn't often spelled out.  It's for this reason that everyone has always agreed that ostriches are birds: it's always been apparent (at least since evolution has been widely accepted) that they are the desdendants of flying birds, and it's always been felt that this is enough to make them birds themselves.

One of the many proposed definitions of Aves corresponds quite closely to your idea that birds are those animals that can "spread their lightweight structured bones, skin and feathers and lift themselves off the ground and into the open air".  That definition is "the first organism that developed powered flight homologous with that in modern birds, plus all its ancestors".  The "homologous with that in modern birds clause" is to specify that the nominated ancestor is not some ancient flying insect!

I usually try to work all my answers around to the subject of sauropods, but on this occasion I admit I am defeated :-)  So I will leave it there.  Birds that cannot fly are still called birds because they are descended from the common ancestor of flying birds.

The Natural History Museum in Kensington is "easily the most beautiful building anywhere for a museum"?  It is certainly very beautiful, but I'm not sure that the Oxford University NHM isn't even better: it's just as grand but less grandiose, and the mostly-glass ceiling means that everything inside is beatifully lit by natural daylight, whereas many parts of the Kensington NHM (especially the dinosaur exhibition unfortunately) are very dimly lit and difficult to see clearly.

Other wonderful museums, if you enjoy being awestruck, include the Humboldt Museum in Berlin, and the Field Musuem in Chicago.  Both of these have life-size Brachiosaurus skeletons, and these are the largest scientifically credible skeletal mounts of any dinosaur anywhere in the world.  (I say scientifically credible because there are other larger skeletons -- notably the "Argentinosaurus" mount in the Fernbank Museum in Georgia, but that is a complete fiction.)

(posted in Birds)

Hi, Mike.  I've not heard of a T. rex-chicken thing.  What exactly are you referring to?

It is true that a great deal of evidence shows that all birds (including chickens) evolved from dinosaurs -- and so, according to the principles of modern classification, we say that they are dinosaurs, just as we say that bats are mammals rather than just that they evolved from mammals.  (Thinking about bats, the flying mammals, is often helpful as an analogy when considering birds, the flying dinosaurs).

But the particular dinosaurs that birds evolved from would have been very different from T. rex -- much more like small fluffy versions of the raptors from Jurassic Park.

So T. rex is not a great grandfather to chickens, but a rather distantly related great uncle.

There are two groups of whales.  One is the mysticetes or balleen whales, which David described, and which filter-feed on tiny creatures.  The other group is the odonticetes or toothed whales, a group which includes dolphins and porpoises as well as killer whales, pilot whales and sperm whales.  The latter do not filter-feed, but catch and eat relativiely large prey -- very large in the case of sperm whales, which like to eat giant squid, and also for killer whales, which sometimes kill baleen whales, though they tend to only eat the tastiest parts.

All dolphins are whales, but not all whales are dolphins.

Well, this is not going to be very helpful, but it depends entirely on what you mean by "the same thing (fundamentally)".  Yes, caviar is fish-eggs, and chicken eggs are eggs, so in that sense they are the same.  But they differ from each other as much as the chicken and the fish do, so you might just as well argue about whether the chicken and the fish as "fundamentally the same thing".  From the persective of an invertebrate, they probably are :-)

If I remember correctly, another giant Argentinian carcharodontosaurid (i.e. a close relative of Giganotosaurus) has been found in a bonebed containing individuals of different ages.  Unfortunately, unless I missed something, this find has not yet been published, so I can't give you any details.  It's possible that I am thinking of either Mupasaurus or Tyrannotitan, but I don't think so.

Hi, Joakim.  You're right that many of the differences between how small and large animals are built, and how they live, are consequences of scaling rules.  The difference between how area and volume scale affects all kind of things, from respiration to breeding rate.  It's one of the most fascinating areas of biology for my money.

The simple answer to your question is just that spiders are an insanely diverse group -- there are something like 40,000 known spider species.  To give you an idea of how diverse that is, there are "only" about 5,800 mammal species, so spiders are about seven times as diverse as we are.  And when you bear in mind that mammals encompass everything from pygmy shrews and naked mole rats, through humans and cheetahs, to elephants and whales, maybe it doesn't seem so surprising that spiders span quite an ecological range, too!

By the way, it's a bit of a fallacy that elephants have fat legs.  Although they're often represented that way in cartoons, their leg bones are surprisingly slender -- much more so than you would expect in comparison to, say, a rhino.  The reason for this seems to be elephants' much more sedate lifestyles: while rhinos gallop around with flexed legs, elephants always keep their legs straight, so they make the best use of their legs' ability to carry their weight.

This is taken to an extreme in (all together now) sauropod dinosaurs, which everyone should know by now are by far the coolest animals there have ever been.  Take a look at the humerus (upper arm bone) of Brachiosaurus brancai at
http://www.miketaylor.org.uk/tmp/brachiosaurus-brancai-humerus.jpeg
(the picture on the left is in rear view, the one on the right in side view).  As you can see, it is ludicrously slender, much more so than for example a horse humerus -- even though the animal weighed something like 35 tonnes in life.

For anyone who wants to look into this subject, I strongly recommend R. McNeill Alexander's wonderful little book Dynamics of Dinosaurs and Other Extinct Giants: there's a brief review at
http://www.miketaylor.org.uk/dino/books/index.html#dod

If you want to go further into this subject, then my very favourite book on the subject -- maybe my favourite book on any subject -- is Dinosaurs, Spitfires and Sea-Dragons, by fellow Ask-a-Biologist member Al McGowan.  Review at http://www.miketaylor.org.uk/dino/books/index.html#dss

Hi, Lottie.  It's impossible to answer this question because the way animals behave doesn't (usually) fossilise.  The best we can do is look at how animals that are alive today sleep, and see if we can make an educated guess about extinct animals.  In general, it seems that pretty much every animals that lives on land can and does lie down, at least sometimes.  It's a "well known fact" that elephants can't lie down because it would stop their blood from circulating, but like many well known facts it turns out not to be true!  Elephants can and do lie down.  Since a T. rex weighed about the same as an elephant, it doesn't seem unreasonable to think that it, too, lay down.

Then again, we know that there are animals that sleep standing up -- horses are the best known example.  But I don't know of any bipeds (two-legged animals) that do this, only quadrupeds (four-legged animals), and this seems reasonable since it's so much harder to balance on two legs, and doing so while sleeping would be asking a lot!

So all in all, the balance of evidence seems to suggest that tyrannosaurs and other large meat-eating dinosaurs lay down to sleep.  What's more interesting to me is how the really big dinosaurs slept -- sauropods weighing ten times as much as T. rex.

(posted in Fossils)

Hi, Yashodha.  The answer may surprise you: here we are in the 21st century, and still by far the most common way to find fossils is to wander around looking carefully at the ground!  Nearly always, no instruments are used at all.  Some people just seem to have a knack for seeing fossils, and the tiniest scrap of exposed bone seems to draw their eye; others (like me) are virtually fossil-blind and can't seem to spot them at all.

There have been a few experiemental uses of other techniques, but they have not really led anywhere yet.  For example, once the first few bones of Seismosaurus had been found -- and they were found in the usual way, by people more or less stumbling onto them -- there was an effort to find more of that very large skeleton using sound-waves.  They set off small explosive charges in the ground near where the first bones were found, and measured how the vibrations differed from what would be expected in uniform rock, using this information to zoom in on where rock of different density -- hopefully bone -- could be found.  The results of this experiment can be reasonably described as "encouraging", but not spectacular enough for the technique to have been widely adopted.  At any rate, there's certainly nothing like the underground scanner machine that they use at the beginning of Jurassic Park!

By the way, the use of this technique is part of the reason that Gillette chose the very cool name Seismosaurus ("Earthquake reptile") -- because they set off little "earthquakes" to find the bones.  You can read more about this, and other techniques that they tried, in Gillette's book Seismosaurus, the Earth Shaker, which is freely available online.  The relevant chapter is at http://www.earthscape.org/r3/gillette/gillette14.html

(I should mention that the name Seismosaurus now seems to be lost to science, though: a paper published last year argues that this specimen is merely a species of Diplodocus.  Shame, really!)

Hi, Brando.  I feel bad that more than week after you asked your question no-one's answered -- sorry.  As you probably guessed, the reason for the silence is that we don't know.  I think you know more about this area than we do, so if you would please start an Ask A Statistician site, we'll be pleased to ask your question right back at you :-)

I very much doubt that any statistic technique could give a "proof" of evolution, though.  More evidence, quite possibly -- but then we have plenty of evidence already!

Sorry we can't be more helpful.

(posted in Evolution)

.. and you can see the process that John describes in ankylosaurian dinosaurs.  The ancestor of these dinosaurs had normal scaly skin, so far as we can tell; primitive anklysaurs such as Scutellosaurus and Scelidosaurus had bony nodules;  and more advanced ankylosaurs such as Ankylosaurus itself were esentially covered with a thick, solid shell.

Hi, Adrian.  First of all, most biologists would say that there is really no such thing as "macroevolution" and "microevolution" -- that they are merely convenient labels for large and small amounts of the same stuff: that is, descent with modification.  So  it's convenient to talk about Macro- and microevolution, just as its convenient for physicists to talk about centrifugal force, but the distinction is arbitrary.

So your question comes down to whether evolution is related to the origins of life.  And the answer to that is no: evolution is the branch of biology that deals with how life changes once it's established, and it's pretty well understood (though not without some gaps that we're filling in as fast as we can!)  The origin of life from non-living matter, called abiogenesis, is a separate and less well understood problem.

(posted in Mammals)

The mammals that we have alive today fall into three groups.  Placental mammals are the ones we're most familiar with - dogs, cows, monkeys, whales, rabbits, and so on. Marsupials are the second group, and the third is the monotremes, represent only by the platypus and the echidna.  (Knuckles, from the Sonic the Hedgehog games and vidoes, is an echidna, which means that unlike Sonic and Tails, he lays eggs.  Well, he would if he were a female echidna.)

It seems that the placentals and marsupials are more closely related to each other than to monotremes.  In the past, there were other mammal groups (triconodonts, multituberculates) but these are all extinct now.

In most continents, placental mammals have been much more successful than marsupials, but the big exception to this rule is Australasia, where the great majority of modern marsupials live.  Until relatively recently, there were some pretty cool marsupial carnivores, including the "marsupial lion" Thylacoleo and giant killer rat kangaroos (yes, really!) but these all seem to be gone now.

(posted in Evolution)

I had to look this up to understand the question ... for anyone else who's interested, http://en.wikipedia.org/wiki/Maturana has an overview.  So far as I can tell from this, Maturana's work would be better classified as philosophy, psychology or even metaphysics rather than biology.  And since we're all biologists here, I'm afraid we're probably not going to be much use to you.  Sorry.

I'm no expert on this, but since no-one else has taken a crack at it, I may as well: I imagine it's just a matter of practicalities.  If you can brew ethanol, which is a liquid, it's easy to capture and store.  But the practical difficulties of capturing cow-generated methane would be challenging to say the least!  (Quite funny, though).

Hi, Brysen.  Although the term "living fossil" sounds good on TV documentaries, it's really so vague that it could mean anything or nothing.  It's true that there are some well-known examples of creatures still alive today that very closely resemble ancestors from long ago - but how old the ancestor has to be, and how closely the living descendant has to resemble it to qualify as a "living fossil" is a matter of sheer interpretation.

If you're up for some slightly technical reading, there's an excellent essay on this subject (among others) on Darren Naish's Tetrapod Zoology blog: http://darrennaish.blogspot.com/2006/09 … iving.html

(posted in Fossils)

Sadly, weathering does not contribute to the formation of fossils at all.  It does have two important effects on fossils that have already been formed, though.  First, weathering of sediments that cover fossils is what exposes them so that they can be uncovered, excavated and studied; and then it destroys the fossils themselves if they're not found in time.  This is one reason why it's unusual to find a complete dinosaur skeleton: usually, by the time the skeleton has weathered out enough to be discovered, much of it has weathered clean away.

(posted in Fossils)

Hi, Dave.  Excellent questions.  If we had really good answers, then palaeontology would be a lot easier.

First of all, each newly discovered fossil is interpreted in the light of what we know about living animals.  When we're dealing with animals pretty similar to what's alive today, things are easier: for example, a bone from a sabre-tooth cat should have enough similarities to the same bone in modern cats that a good mammalogist can identify it pretty easily.  With older groups, comparisons to modern animals are less useful, but still have a role to play: for example, there are similarities between the vertebrae of sauropod dinosaurs and modern birds -- even though some sauropods had individual vertebrae longer than nearly all birds' whole necks!

Second, each newly discovered fossil is interpreted in the light of all the other fossils already known.  This means that the occasional very complete specimen is valuable as a sort of "Rosetta Stone" for interpreting related fossils.  Until something like that is available, all guesses are open to error, and all kinds of mistakes do get made.  To pick a couple of famous examples, the dinosaur Iguanodon was first reconstructed as a heavy rhinoceros-like quadruped with a nose-horn, rather than as the occasionally bipedal dinosaur with thumb-spikes that we are familiar with today; and the long-necked sea reptile Elasmosaurus was originally put together with its head at the wrong end!

Even now, we make plenty of mistakes.  To pick a recent example, the vertebra from which the dinosaur "Ultrasauros" was named was first thought to be from the shoulder region of a brachiosaurid sauropod.  Restudy showed it to be from lower back region of a completely different kind of sauropod.

So the answer to your second question, "how certain are fossil experts that the configurations are correct?" is often "not very".  But thankfully there are exceptions such as the very complete T. rex skeleton "Sue" where we can be very, very confident.

Finally, I should say that these problems are much less acute with smaller animals than with the giant sauropod dinosaurs that I like to work on.  It's not that unusual to find whole skeletons of little lizards, fish and suchlike.  But in general, the bigger the animal, the worse the chancer of finding all of it.

Yes, plenty of theropod dinosaurs (and sauropods!) have clear indications of air-spaces in their bones.  The reason that this was widely reported when it was discovered in Majungatholus is that this animal is a member of rather a primitive theropod group, whereas air-spaces are much more often seen in more advanced theropods, including spinosauroids, allosauroids, and many coelurosaurs.

What's more interesting is that, because similar air-spaces are found in the vertebrae of sauropods, it seems likely that they were produced by the same mechanism in both groups.  This suggests that the air-sacs in the soft-tissue that carry air to the hollow bones in birds (and other theropods) were in place at the time of the sauropod-theropod split.  And the presence of those air-sacs in turn suggests that even the most primitive sauropods and theropods (and their common ancestor) likely had a bird-style respiratory system, with a flow-through lung.  Now isn't that an impressive amount of conjecture to get from a few holes in some bones?  :-)

Hi, Ron.  As a policy matter, we don't want to get into this subject -- whatever we say, we know we will alienate someone, and we can't give such subjects the in-depth treatment they merit in a forum like this.  There are many useful and informative articles on this and related questions on the TalkOrigins web-site:
http://www.talkorigins.org/

I will just say that several members of the Ask A Biologist team are Christians, and many others are not.  In general, it seems fair to say that team members' religious beliefs do not dictate their scientific understanding.

What makes you think that hadrosaurs had more efficient digestion than sauropods?  Just because they lived more recently is not a compelling reason - evolution doesn't usually work in straight lines.  Actually, sauropods were probably the most efficient digesters that ever walked the earth, due simply to their size.  The food ingested by a 40-tonne Brachiosaurus would have spent a very long time making its way through the digestive tract, so that nearly all the nutrients would be absorbed.

It's one of the luxuries of being big that you can manage on lower-grade food - which is why small animals tend to eat nuts, fruit and other high-quality food, whereas elephants eat all kinds of plant material, including bark and even small branches.  It's just as well, though - big animals wouldn't be able to find enough high-quality food if that was all they could eat.

A question as general as this is best answered by general research: Google is your friend, Google Scholar even more so if you need detailed information.