Closed Thread
 
Thread Tools Display Modes
  #1  
Old 08-13-2013, 10:34 AM
breakneck is offline
Guest
 
Join Date: Aug 2013
Posts: 17

why did the dinosaurs not evolve back


why did the dinosaurs not evolve back? why were they so massive? does that mean the plants they fed on were massive too? why didnt the large plants re evolve ? why were they so large ? I have a theory about this and i wanted to see if i was anywhere close. ok the dinosaurs went extict because the metoer that impacted the yucatan penn. . The dinasurs were so large because the plants they ate were too . why? well before the major impact i think the earhs axis of orbit was something much different . Because of this the ice caps would have melted and refroze every half year making ocean levels rise and fall hundreds of feet. so plants adapted to this at least species in lowlieing regions by being very tall with tough bark stems and strong roots like seaweed and soft fiolage on top so they could survive the floods the dinosaurs were huge in high to be able to feed on these and keep heads out of the water . the smaller creatures would run for higher grounds as well as the smaller meat eater dinosuares that fed on them and the floods would reside and the cycle would repeat. then metorite hits kills of dinosuars , changes earths axis to wheere it is now . no floods . no need for plants to adapt . no need for dinosaurs to adapt. i would like to know the real answer to this question thank you for your time
  #2  
Old 08-13-2013, 10:38 AM
Lukeinva is offline
Guest
 
Join Date: Mar 2009
Location: NW Fairfax County
Posts: 2,648
Welcome. Good question.
  #3  
Old 08-13-2013, 10:44 AM
breakneck is offline
Guest
 
Join Date: Aug 2013
Posts: 17
thank you i hope there someone to shed some light on it im very excited to know
  #4  
Old 08-13-2013, 10:48 AM
aNewLeaf is offline
Guest
 
Join Date: Dec 2012
Location: Colorado
Posts: 2,602
They didn't evolve back because they evolved forward and became birds, some of which were pretty big . Not dinosaur big, but bigger than people.
After the extinction event, competition went in a different direction, and intelligence became a major factor in the predator-prey race.

Is my opinion.
  #5  
Old 08-13-2013, 10:50 AM
Chronos's Avatar
Chronos is offline
Charter Member
Moderator
 
Join Date: Jan 2000
Location: The Land of Cleves
Posts: 81,825
The dinosaurs did "evolve back", if by that you mean "evolve to smaller size". The largest dinosaurs alive today max out at about 150 kg, and most of them are less than a kilogram.

You've got to understand how evolution works. It's not a choice between evolving or going extinct: In fact, extinction is one of the major mechanisms by which evolution works. At the end of the Triassic era, you had a wide variety of dinosaurs, some huge, some small. Then the environment changes, in such a way that being huge is suddenly a disadvantage. All the huge dinosaurs die off, but some of the small ones survive. Now all of the dinosaurs that are left are small.
  #6  
Old 08-13-2013, 10:51 AM
Great Antibob is offline
Guest
 
Join Date: Feb 2003
Posts: 5,120
Quote:
Originally Posted by breakneck View Post
why did the dinosaurs not evolve back?
What do you mean by "evolve back"? The descendants of (some of) the dinosaurs are still around and kicking.

You may as well ask why human beings don't go back to being unicellular life forms.

Quote:
Originally Posted by breakneck View Post
why were they so massive?
Most weren't. Most mammals aren't elephant sized, either.

Look at mammal fossils from 10 million years ago. We tend to put the larger specimens in museums and those are the ones we remember. It doesn't mean 10 MYa animals were all large.


Quote:
Originally Posted by breakneck View Post
does that mean the plants they fed on were massive too?
No, and why would they have to be? Do cows eat larger plants than sheep? Do cows eat larger plants than squirrels? Or do they just eat more than their smaller counterparts?

Also, some dinosaurs (just like some modern animals) didn't feed on plants.

Ask the same question for meat-eaters. Do large meat-eating animals always eat larger prey?

The largest single creature to ever exist on planet earth is the blue whale. It's much larger than any other creature that's ever existed. And it exists right now and feeds on some of the smallest animals on earth: krill.



Quote:
Originally Posted by breakneck View Post
I have a theory about this
Study up on physics and biology and get back when you have developed it further. Right now, your theory doesn't hold up to basic facts.

Last edited by Great Antibob; 08-13-2013 at 10:53 AM.
  #7  
Old 08-13-2013, 10:52 AM
Si Amigo is offline
Member
 
Join Date: Nov 2004
Location: North of 8 Mile
Posts: 4,089
Quote:
Originally Posted by breakneck View Post
i would like to know the real answer to this question thank you for your time
There are likely a few "real" answers and and boatload of speculation. One is that some of them evolved into what we now know as birds; so they are really not "gone". Monitor lizards are still around as well. If you are asking why we don't have large land animals roaming the planet anymore it may be because the climate changed enough that it couldn't support the development of the old order and that smaller mammals were able to develop to a point that they successfully kept them from redeveloping.
  #8  
Old 08-13-2013, 10:56 AM
aNewLeaf is offline
Guest
 
Join Date: Dec 2012
Location: Colorado
Posts: 2,602
Quote:
If you are asking why we don't have large land animals roaming the planet anymore it may be because the climate changed enough that it couldn't support the development of the old order and that smaller mammals were able to develop to a point that they successfully kept them from redeveloping.
There were very large mammals in North America until humans arrived. Giant Sloths, Uintatheres, all kinds of huge crazy animals.

Last edited by aNewLeaf; 08-13-2013 at 10:56 AM.
  #9  
Old 08-13-2013, 10:57 AM
Telemark's Avatar
Telemark is offline
Charter Member
 
Join Date: Apr 2000
Location: Yet again, Titletown
Posts: 22,139
Please try using paragraphs and capitalization. It makes reading your questions much easier.

Quote:
Originally Posted by breakneck View Post
why did the dinosaurs not evolve back?
The environment that they evolved in had changed. There were different conditions, different food sources, and different competitors. And as already said they evolved right along with the different conditions and we see them today as birds.

Quote:
well before the major impact i think the earhs axis of orbit was something much different . Because of this the ice caps would have melted and refroze every half year making ocean levels rise and fall hundreds of feet.
The evidence doesn't support this, and ice caps don't melt and refreeze that quickly.
  #10  
Old 08-13-2013, 11:01 AM
Colibri's Avatar
Colibri is online now
SD Curator of Critters
Moderator
 
Join Date: Oct 2000
Location: Panama
Posts: 41,311
Plants during the age of the dinosaurs were no larger than the largest modern ones. During the Mesozoic there was a shift in dominance between gymnosperms (conifers and others) and angiosperms (flowering plants), but this happened in the Cretaceous, in the middle of the dinosaurs' dominance.

It's an open question of why dinosaurs got so much larger in the Mesozoic than mammals did during the Tertiary. Changing levels of carbon dioxide (which could have increased plant productivity) or oxygen (which could have permitted greater activity) could have had something to do with it, but these changed throughout the Mesozoic and this can't be a general explanation.

Sauropods may have been able to get so big because of their highly efficient feeding apparatus and their physiology. Mammals by happenstance have not been able to duplicate this system. The largest carnivores may have evolved to exploit the large sauropods. Dinosaurs in other groups were generally within the range of size reached by the largest mammals, such as Paraceratherium (Indricotherium).

The asteroid strike that killed the dinosaurs did not change Earth's axis. The most recent ice ages did not start until about 2 million years ago, most likely due to changes in oceanic currents triggered by the closure of the isthmus of Panama. There were no ice caps on Earth at any time during the time dinosaurs were dominant, although there had been ice ages before dinosaurs evolved.
  #11  
Old 08-13-2013, 11:02 AM
Maastricht is offline
Charter Member
 
Join Date: Dec 2000
Location: Dutch in the Netherlands
Posts: 9,191
If any floods would have yearly covered the earth, we would have seen that in the fossil records. You can tell if a soil is dry land, marsh-like, a lake, shallow sea, a sea with big fluctuations, or deeper sea. You can tell by the tiny fossils of plants and animals that drift down and make up the soil.

Your scenario of big floods every half year has never been found anywhere.

As for the larger size of insects, that has been linked to the higher levels of oxygen in the air. As Colibri said, the largest tree-like plants back then were the same size they are now.

Last edited by Maastricht; 08-13-2013 at 11:04 AM.
  #12  
Old 08-13-2013, 11:04 AM
Sailboat's Avatar
Sailboat is offline
Guest
 
Join Date: Aug 2005
Posts: 11,842
I don't know anything offhand about biannual flooding, but I'm sure they would have seen something like that in the fossil record if it really happened. (and it seems unnecessarily complex).

As I understand it, the main reasons dinosaurs got so big were 1) they could, and 2) keeping up with the Joneses.

They could grow big because their hollow bones had good strength for their weight, and because (unlike lizards and amphibians) their legs were positioned directly under their mass, which helped support their weight better. Earth's higher oxygen content may have played a part too.

The most likely reason they did grow big was because of competitive advantage with each other. Really big animals are relatively safe from predation -- even huge predators will hesitate to fight something that can hurt them back. Current evidence implies individual dinos grew really fast -- they were probably racing to get big enough to deter predators.

Regarding "evolve back" -- evolution is not directional; there's no forward and backward. Furthermore, it's not purposeful -- creatures don't evolve for a purpose. It's more a process of elimination; some of them die and some don't. A change in conditions that kills some of them before they pass on their genes but permits the survivors to pass on their genes can lead to the evolution of a new species...IF the survivors survived because of some heritable trait, not just random luck.

Some dinos did survive. Birds are dinosaurs -- most likely they survived because they were small. Apparently, when the Chicxulub meteor hit, every land animal on earth heavier than twenty pounds died. All of them.

While it's true that famine (presumably from an "impact winter") hits large animals harder than small ones, the speculation I've heard that makes the most sense to me is that large animals do not tend to burrow (too much work). The meteorite is widely believed have set off worldwide fires, and some experts have said it set earth's atmosphere on fire; burrowing only a few inches underground would have protected an animal from this heat. it may be that ONLY burrowing animals survived (and of course some sea life, although sea life in general also took a big hit).

Some birds burrow today (burrowing owls, for example) and thus might have lived to "radiate" out into all the birds we know today.

Last edited by Sailboat; 08-13-2013 at 11:09 AM. Reason: edit: when I started this, it was going to be the second response. Ninja'ed by lots of people!
  #13  
Old 08-13-2013, 11:20 AM
snowthx's Avatar
snowthx is offline
Guest
 
Join Date: Feb 2012
Location: Sacratomato area
Posts: 3,415
Quote:
Originally Posted by Sailboat View Post
Regarding "evolve back" -- evolution is not directional; there's no forward and backward.
I read the OP as asking why dinos have not evolved "back" to their pre-extinction numbers and variety, based on the few surviving species (not backward evolution). I think the emergence of the mammals exploiting previously occupied niches created competition where there was none before - which prevented re-emergence of large dinosaurs.

My other thought is that the dinosaurs have been gone for only 65 MY, but they existed from about 250 MY ago up until then. Give it some more time and we may see something like them return, depending on how well they compete.
  #14  
Old 08-13-2013, 11:21 AM
Si Amigo is offline
Member
 
Join Date: Nov 2004
Location: North of 8 Mile
Posts: 4,089
Quote:
Originally Posted by aNewLeaf View Post
There were very large mammals in North America until humans arrived. Giant Sloths, Uintatheres, all kinds of huge crazy animals.
Compared to dinosaurs I would label them as medium-large sized animals and the ones we see today as medium-small. Huge yes, dinosaur huge, not really.
  #15  
Old 08-13-2013, 11:22 AM
Stranger On A Train is online now
Member
 
Join Date: May 2003
Location: Manor Farm
Posts: 18,975
First of all, it wasn't just dinosaurs which experienced extinction around the Cretaceous-Tertiary (K-T) boundary; nearly all large animals across the entire kingdom went extinct over an approximately 800,000 year period. There is some evidence that mass extinctions were occurring even before the date of the bolide impact at Chicxulub (and similar large impacts in other locations), and extinctions continued to occur long after the transient climate conditions from the impact should have subsided, indicating that there were more factors than simply dust or soot occluding sunlight. The K-T event is neither the first nor more severe extinction event that has occurred in the history of the Earth; in fact, it was the last such major event of six or seven, and we are statistically overdue for another.

Second, dinosaurs didn't "evolve back" because evolution doesn't work that way. Although there are sometimes a tendency for highly divergent species or even clades to evolve similar mechanisms such as eyes (referred to as convergent or parallel evolution) there is no directed flow of development in evolution, and phenotypical characteristics are selected based upon the ultimate reproductive fitness of the carrier. Evolution sometimes produces regressive phenotypes, such as cave fish losing eyesight, but there is no directed change or modification to return to a particular state, and the record is rife with examples of different species evolving wildly different mechanisms to adapt to similar conditions.

Although we generically lump the dinosaurs into a single clade, Dinosaura, a more careful examination of fossil evidence indicates that there was enormous diversity in basic characteristics such that if we were starting to classify them today we would divide them into separate clades. One particular group of dinosaurs which survived the event and evolved into modern Aves (birds). Others declined precipitously and their ecological niches replaced by presumably more adaptable mammals. Specifically while mammals took over and grew to such enormous sizes (most of the mammalian megafauna have subsequently become extinct or were hunted to extinction) is a point of active research and debate. Very large animals do have some significant advantages in terms of competition and protection from predation, but also come with an evolutionary cost that would seem to be prohibitive, so exactly how they evolved is not well understood.

There is no indication that a bolide impact shifted the Earth's axis significantly, and in fact an impact with sufficient force to do so would likely cause the mantle to erupt over the entire surface of the Earth virtually wiping out all life. The rotational characteristics of the Earth do change over time (by approximately 1° per million years) but there is no indication that it has shifted by such dramatic or rapid change in the geological history since the Cambrian Explosion. It is more likely than an impact or some other event would disrupt ocean currents which would in turn alter the climate of the Earth.

There are a large number of holes in the hypothesis of the o.p., but rather than address them in detail I would recommend obtaining a more thorough layman's knowledge of paleontology and evolutionary processes. The popular science works of the late paleontologist Stephen J. Gould (Ever Since Darwin, The Structure of Evolutionary Theory, The Richness of Life, Ontogeny and Phylogeny) is an excellent place to start learning about the depth and breadth of evolutionary theory and paleontology.

Stranger
  #16  
Old 08-13-2013, 11:40 AM
Telemark's Avatar
Telemark is offline
Charter Member
 
Join Date: Apr 2000
Location: Yet again, Titletown
Posts: 22,139
Quote:
Originally Posted by Si Amigo View Post
Huge yes, dinosaur huge, not really.
Blue whales are more massive than any dinosaur that ever lived. There is a reasonable case for a dino that is as long or longer than a Blue Whale.

http://svpow.com/2008/05/20/sv-pow-s...ods-vs-whales/
  #17  
Old 08-13-2013, 11:46 AM
aNewLeaf is offline
Guest
 
Join Date: Dec 2012
Location: Colorado
Posts: 2,602
Quote:
Originally Posted by Si Amigo View Post
Compared to dinosaurs I would label them as medium-large sized animals and the ones we see today as medium-small. Huge yes, dinosaur huge, not really.
Average size for dinosaurs was really small.
  #18  
Old 08-13-2013, 11:48 AM
Si Amigo is offline
Member
 
Join Date: Nov 2004
Location: North of 8 Mile
Posts: 4,089
Quote:
Originally Posted by Telemark View Post
Blue whales are more massive than any dinosaur that ever lived. There is a reasonable case for a dino that is as long or longer than a Blue Whale.

http://svpow.com/2008/05/20/sv-pow-s...ods-vs-whales/
Well I didn't consider surf vs turf.
  #19  
Old 08-13-2013, 11:56 AM
engineer_comp_geek's Avatar
engineer_comp_geek is offline
Robot Mod in Beta Testing
Moderator
 
Join Date: Mar 2001
Location: Pennsylvania
Posts: 23,368
One reason we know there wasn't periodic flooding caused by the melting ice caps is that at times there wasn't any polar ice. Throughout most of the "dinosaur" era, the earth's average temperature was around 20 to 22 deg C, significantly warmer than it is now (about 14 deg C).

65 million years ago, a hunk of rock slammed down in southern Mexico hard enough that debris from the impact landed as far north as Tennessee and Kentucky. All around the world there's a layer of carbon from the event, indicating that the entire world burned. The only things that lived were things that were small and could burrow under debris and things that could live a long time without food (like crocodiles).

After that, the world went through a cooling phase, eventually dropping all the way down to an average temperature of 12 deg C or so. This is a drastically different climate than what the dinosaurs evolved in, which is why dinosaur-like creatures didn't evolve as the earth recovered from its extinction event as they had after previous extinction events.

While the K-T extinction gets the most focus in schools and elsewhere, it is far from the only extinction event that the earth has faced. Each extinction event has made drastic changes to life on earth. It's just that in all of the previous ones, you had dinosaur-like creatures before the extinction and dinosaur-like creatures evolved to replace them after the extinction. If you take a closer look at what was alive during those times though, you can see that there were pretty drastic changes during those other extinction events as well.
  #20  
Old 08-13-2013, 12:01 PM
Musicat is offline
Charter Member
 
Join Date: Oct 1999
Location: Sturgeon Bay, WI USA
Posts: 20,922
How species evolve is due to millions of factors. If you were to start with two planets, identical in absolutely every respect, plunk a proto-dinosaur pair on each, the chances are they would evolve with major differences between each planet's animals.

It's a little like the "butterfly flapping its wings in Brazil causing a hurricane in Florida." Ecological systems are chaotic and impossible to predict over long periods of time.

So even if the planet was the same when 98% of the dinosaurs were extinct as 100 million years before (it wasn't), the chances of the remaining 2% returning to their roots is highly unlikely.

This principle explains why we have such a diversity of life. Even though all life may have come from one original organism, no two species evolve in the same way, and over time, become more divergent.
  #21  
Old 08-13-2013, 12:03 PM
Little Nemo is offline
Charter Member
 
Join Date: Dec 1999
Location: Western New York
Posts: 79,969
Evolution is not about arriving at some destination. Animals don't evolve until they become humans or tyrannosauri and then declare they've won the game.

Evolution is an ongoing process of change. Animals change in random ways. If the changes are bad, the animals die and the changes disappear. If the changes are good, the animals live and breed and the changes spread through the species.
  #22  
Old 08-13-2013, 12:24 PM
breakneck is offline
Guest
 
Join Date: Aug 2013
Posts: 17
Quote:
Originally Posted by Great Antibob View Post
What do you mean by "evolve back"? The descendants of (some of) the dinosaurs are still around and kicking.

You may as well ask why human beings don't go back to being unicellular life forms.



Most weren't. Most mammals aren't elephant sized, either.

Look at mammal fossils from 10 million years ago. We tend to put the larger specimens in museums and those are the ones we remember. It doesn't mean 10 MYa animals were all large.




No, and why would they have to be? Do cows eat larger plants than sheep? Do cows eat larger plants than squirrels? Or do they just eat more than their smaller counterparts?

Also, some dinosaurs (just like some modern animals) didn't feed on plants.

Ask the same question for meat-eaters. Do large meat-eating animals always eat larger prey?

The largest single creature to ever exist on planet earth is the blue whale. It's much larger than any other creature that's ever existed. And it exists right now and feeds on some of the smallest animals on earth: krill.





Study up on physics and biology and get back when you have developed it further. Right now, your theory doesn't hold up to basic facts.
why would they be hundreds of feet tall to feed on plants that were 10 feet tall?
  #23  
Old 08-13-2013, 12:25 PM
Colibri's Avatar
Colibri is online now
SD Curator of Critters
Moderator
 
Join Date: Oct 2000
Location: Panama
Posts: 41,311
Quote:
Originally Posted by aNewLeaf View Post
Average size for dinosaurs was really small.
No it wasn't. No dinosaur was as small as many modern mammals. Even if you include birds as dinosaurs, the average size of non-avian dinosaurs was far larger than most modern birds.
  #24  
Old 08-13-2013, 12:37 PM
breakneck is offline
Guest
 
Join Date: Aug 2013
Posts: 17

thank you


thank you for the replies and sharing information i wasnt aware of. i will do more reading to become beter educated on the specific eras thank you all
  #25  
Old 08-13-2013, 12:45 PM
bump is online now
Guest
 
Join Date: Jun 2000
Location: Dallas, TX
Posts: 17,061
Quote:
Originally Posted by Stranger On A Train View Post
The K-T event is neither the first nor more severe extinction event that has occurred in the history of the Earth; in fact, it was the last such major event of six or seven, and we are statistically overdue for another.
I thought we were in the early stages or midst of one right now?
  #26  
Old 08-13-2013, 01:13 PM
Colibri's Avatar
Colibri is online now
SD Curator of Critters
Moderator
 
Join Date: Oct 2000
Location: Panama
Posts: 41,311
Quote:
Originally Posted by bump View Post
I thought we were in the early stages or midst of one right now?
Extinction rates now are much higher than they have been in the recent past, mainly due to direct or direct human impacts, but aren't close (yet) to the levels seen in the major mass extinction events in the past. Whether or not we reach them eventually may depend on how severe climate change and other effects become over the next couple of centuries.
  #27  
Old 08-13-2013, 01:20 PM
Stranger On A Train is online now
Member
 
Join Date: May 2003
Location: Manor Farm
Posts: 18,975
Quote:
Originally Posted by bump View Post
I thought we were in the early stages or midst of one right now?
I was distinguishing between the anthropogenic species extinction and those resulting from natural cataclysms. By paelontoligical standards, the current rates of extinction are currently less than what is estimated for historical biotic crises, but the rate of increase of extinctions gives a trend that may be on par with events like the K-T or Triassic-Jurassic extinction event. And it should be noted that we are still actively classifying new species, so actual rates of extinction may be higher than observed.

Stranger

Last edited by Stranger On A Train; 08-13-2013 at 01:21 PM.
  #28  
Old 08-13-2013, 01:21 PM
Colibri's Avatar
Colibri is online now
SD Curator of Critters
Moderator
 
Join Date: Oct 2000
Location: Panama
Posts: 41,311
Quote:
Originally Posted by breakneck View Post
why would they be hundreds of feet tall to feed on plants that were 10 feet tall?
No dinosaur was "hundreds of feet tall." Sauroposeiden, possibly the tallest, may have reached 56 feet. Many trees probably reached hundreds of feet tall in the Mesozoic, and it is speculated that tall sauropods used their long necks to browse on these trees. However, other sauropods probably browsed on low growth. Their necks in general were an adaptation for gathering food over a wide area with minimal effort, rather than for browsing tall trees.
  #29  
Old 08-13-2013, 01:23 PM
Great Antibob is offline
Guest
 
Join Date: Feb 2003
Posts: 5,120
Quote:
Originally Posted by breakneck View Post
why would they be hundreds of feet tall to feed on plants that were 10 feet tall?
Are you being entirely serious?

No dinosaurs were "hundreds" of feet tall. The longest sauropods could probably have reasonably reached up somewhere under 100 feet, which is not a problem for most forests, including the forests that existed back then.

ETA: along the same lines, why aren't more current animals hundreds of feet tall to feet on trees? Why are elephants so large when they don't need to feed on tall trees to survive. The size of the animal is not tightly correlated with the size of its food source. That's true now, and based on everything we've discovered so far, it was true in the age of dinosaurs as well.

Last edited by Great Antibob; 08-13-2013 at 01:27 PM.
  #30  
Old 08-13-2013, 01:24 PM
DrDeth is online now
Charter Member
 
Join Date: Mar 2001
Location: San Jose
Posts: 39,430
Quote:
Originally Posted by Si Amigo View Post
Compared to dinosaurs I would label them as medium-large sized animals and the ones we see today as medium-small. Huge yes, dinosaur huge, not really.
The Baluchitherium aka Paraceratherium, aka Indricotherium weighed in at up to around 16 tons and 16' tall. This comes in there with about the average dinos lumped as Brontosaurus . (I hate the term Apatosaurus, they can suck it )

Trust me, if you saw one of them lumbering towards you, you'd think it was a huge as a dino.
  #31  
Old 08-13-2013, 03:25 PM
Lemur866's Avatar
Lemur866 is offline
Charter Member
 
Join Date: Jul 2000
Location: The Middle of Puget Sound
Posts: 22,358
Quote:
Originally Posted by breakneck View Post
why would they be hundreds of feet tall to feed on plants that were 10 feet tall?
The long-necked sauropods--that is, brontosaur type creatures--were only one type of dinosaur. And we think that only some of them used their long necks to feed on trees. Some of them used their long necks to stand still and eat acres of ground cover while only moving their heads.

And have you ever been to a museum to see dinosaur skeletons? They were really big, but not hundreds of feet.

We used to have the idea that the big sauropods were aquatic or semiaquatic like hippos, but now we know that they lived on dry land and ate land plants.

As for the notion of yearly global floods, think about it. How does a polar ice cap form? When snow falls in the winter, and doesn't melt in the summer. If the snow melts in the summer you don't have an ice cap, you have winter snow. So even if there's a lot of snow in one winter, you're not going to have enough snow to lower global sea levels, and cause floods when that winter's snow melts.

And besides, the current evidence is that the world was generally much warmer in the age of Dinosaurs than it is today. That means that there were no polar ice caps--even at the north and south poles you'd have bare ground (or sea) in the summer. Dinosaurs lived in Antarctica, which was forested at the time. Sea levels were higher because of the absence of polar ice caps, what is now the American great plains was a shallow inland sea during the Cretaceous.
  #32  
Old 08-13-2013, 03:48 PM
Spectre of Pithecanthropus is offline
Charter Member
Charter Member
 
Join Date: Aug 1999
Location: Transplanted!
Posts: 19,210
IANA paleontologist, but couldn't one reasonably argue that evolving into birds, as it were, was the long-term survival tactic of those small dinosaurs that survived the great extinction? Though early mammals were generally small as well, and primitive in comparison to their descendants, they must have been far better equipped to compete and reproduce. Developing feathers and taking to the treetops and the air was a good solution if the small-animal ground-dwelling niches were being taken over by mammals.

Of course, I do realize that birds and mammals didn't just appear at the end of the Mesozoic, so I'm not suggesting there was a simple changeover to mammals and birds at that point.

Last edited by Spectre of Pithecanthropus; 08-13-2013 at 03:49 PM.
  #33  
Old 08-13-2013, 04:08 PM
DrDeth is online now
Charter Member
 
Join Date: Mar 2001
Location: San Jose
Posts: 39,430
Quote:
Originally Posted by Spectre of Pithecanthropus View Post
IANA paleontologist, but couldn't one reasonably argue that evolving into birds, as it were, was the long-term survival tactic of those small dinosaurs that survived the great extinction? Though early mammals were generally small as well, and primitive in comparison to their descendants, they must have been far better equipped to compete and reproduce. Developing feathers and taking to the treetops and the air was a good solution if the small-animal ground-dwelling niches were being taken over by mammals.

Of course, I do realize that birds and mammals didn't just appear at the end of the Mesozoic, so I'm not suggesting there was a simple changeover to mammals and birds at that point.

Umm, no. Altho there is still some dissent on whether or not birds came directly from dinos (some think they just had a common ancestor), they split off around the late Jurassic, 160 some odd millions of years ago or about 100 million years before the KT event. The first “true birds” (and of course this is the subject of some disagreement) occurred about 120 mya. The KT event occurred about 66 mya.
  #34  
Old 08-13-2013, 04:17 PM
Rucksinator is offline
Guest
 
Join Date: Sep 2002
Location: Kentucky
Posts: 1,873
Quote:
Originally Posted by Si Amigo View Post
..... One is that some of them evolved into what we now know as birds; so they are really not "gone". Monitor lizards are still around as well.....
Side question: I hear comments like the one above quite often, but I never hear people say that alligators or crocodiles, et al, are dinosaurs. Are they not considered dinosaurs? Or is it just so obvious that nobody (that I recall reading, anyway) mentions it?

ETA: I did read this part below, but I wanted to ask anyway:
"The only things that lived were things that were small and could burrow under debris and things that could live a long time without food (like crocodiles)."

Last edited by Rucksinator; 08-13-2013 at 04:21 PM.
  #35  
Old 08-13-2013, 04:27 PM
aNewLeaf is offline
Guest
 
Join Date: Dec 2012
Location: Colorado
Posts: 2,602
Dinosaurs were closer to birds than reptiles.
They were an advanced branch that split off from early crocodiles, then developed warm-blooded metabolisms.
  #36  
Old 08-13-2013, 04:39 PM
Lemur866's Avatar
Lemur866 is offline
Charter Member
 
Join Date: Jul 2000
Location: The Middle of Puget Sound
Posts: 22,358
Define "dinosaur". If you mean anything big and scaly and extinct, then crocodiles aren't dinosaurs because they aren't extinct.

If you mean any organism that evolved from the last common ancestor of a sparrow and a triceratops (a common cladistic definition), then crocodiles aren't dinosaurs because their ancestors had split off before that creature existed.

The classic definition to qualify as a dinosaur you have to belong to the order Ornithischia or Saurischia, which are thought to be sister groups. And crocodiles are not part of either group.

Check out this tree that outlines the current consensus view:
http://www.ucmp.berkeley.edu/museum/.../dino_tree.gif
  #37  
Old 08-13-2013, 05:44 PM
Rucksinator is offline
Guest
 
Join Date: Sep 2002
Location: Kentucky
Posts: 1,873
OK. Thanks for the answers.

But there is more ignorance to be fought....

Quote:
Originally Posted by Lemur866 View Post
.....
Check out this tree that outlines the current consensus view:
http://www.ucmp.berkeley.edu/museum/.../dino_tree.gif
The way I'm reading this chart it's saying that birds evolved from theropods, who evolved from "(lizard-hipped dinosaurs)"; and not that they evolved from bird-hipped dinosaurs. Am I not reading this right?
  #38  
Old 08-13-2013, 05:56 PM
AndrewL is offline
Guest
 
Join Date: Sep 2001
Location: USA
Posts: 2,213
Quote:
Originally Posted by Rucksinator View Post
The way I'm reading this chart it's saying that birds evolved from theropods, who evolved from "(lizard-hipped dinosaurs)"; and not that they evolved from bird-hipped dinosaurs. Am I not reading this right?
You are reading that correctly. Birds evolved from "lizard-hipped dinosaurs". I suspect that the two branches of dinosaurs were named before it was known for sure that birds had evolved from dinosaurs, and the resemblance of the hips of "bird-hipped dinosaurs" to the hips of actual birds was coincidental or a case of convergent evolution.
  #39  
Old 08-13-2013, 06:56 PM
Rucksinator is offline
Guest
 
Join Date: Sep 2002
Location: Kentucky
Posts: 1,873
Years ago I was looking through a book my nephew had on dinosaurs and it said that there were 2 types of dinosaurs: Those with a wish-bone, and those without. (I would not have been allowed to read a book that referenced evolution when I was a child.) I assumed that birds evolved from those with a wishbone, and when I saw that chart I assumed that the "-hipped" part referred to the wish-bone. (I really don't know what the wishbone is.)

Could someone clarify the wishbone thing?
  #40  
Old 08-13-2013, 07:02 PM
DrFidelius's Avatar
DrFidelius is offline
Guest
 
Join Date: Mar 1999
Location: Miskatonic University
Posts: 12,392
Wishbones are fused clavicles (collar bones).
  #41  
Old 08-13-2013, 07:11 PM
breakneck is offline
Guest
 
Join Date: Aug 2013
Posts: 17
Quote:
Originally Posted by Lemur866 View Post
The long-necked sauropods--that is, brontosaur type creatures--were only one type of dinosaur. And we think that only some of them used their long necks to feed on trees. Some of them used their long necks to stand still and eat acres of ground cover while only moving their heads.

And have you ever been to a museum to see dinosaur skeletons? They were really big, but not hundreds of feet.

We used to have the idea that the big sauropods were aquatic or semiaquatic like hippos, but now we know that they lived on dry land and ate land plants.

As for the notion of yearly global floods, think about it. How does a polar ice cap form? When snow falls in the winter, and doesn't melt in the summer. If the snow melts in the summer you don't have an ice cap, you have winter snow. So even if there's a lot of snow in one winter, you're not going to have enough snow to lower global sea levels, and cause floods when that winter's snow melts.

And besides, the current evidence is that the world was generally much warmer in the age of Dinosaurs than it is today. That means that there were no polar ice caps--even at the north and south poles you'd have bare ground (or sea) in the summer. Dinosaurs lived in Antarctica, which was forested at the time. Sea levels were higher because of the absence of polar ice caps, what is now the American great plains was a shallow inland sea during the Cretaceous.
So why were there no ice caps at the poles then? maybe because the earths axis was of one of much different as it is now?
  #42  
Old 08-13-2013, 07:26 PM
DrFidelius's Avatar
DrFidelius is offline
Guest
 
Join Date: Mar 1999
Location: Miskatonic University
Posts: 12,392
Quote:
Originally Posted by breakneck View Post
So why were there no ice caps at the poles then? maybe because the earths axis was of one of much different as it is now?
No, the axis was the same. The continents were in a different configuration, though, and the atmosphere had more greenhouse gasses to hold heat.

Really, consider why we have polar ice caps now. At the North, we have a basically circular ocean with limited exchange with the rest of the seas. This allows cold water to build up in the north.

In the south, we have a basically round continent which allows snow to build up into ice sheets.

This configuration was not present during the Mesozoic.

Last edited by DrFidelius; 08-13-2013 at 07:29 PM.
  #43  
Old 08-13-2013, 07:26 PM
Great Antibob is offline
Guest
 
Join Date: Feb 2003
Posts: 5,120
Quote:
Originally Posted by breakneck View Post
So why were there no ice caps at the poles then? maybe because the earths axis was of one of much different as it is now?
No. Temperature alone explains it pretty nicely.

Venus has an almost vertical axial tilt yet no polar ice caps. The massive temperatures have something to do with that.

Likewise, the amount of ice at Earth's poles don't directly depend on its axial tilt.

Look: you're trying to create something that merely sounds plausible. That's nice and all, but you should try looking at actual facts instead of sticking stubbornly to an idea that doesn't fit the data.

For one, the amount of tilt in the Earth's axis has been studied at length. While it has varied a small bit, there's no evidence it has varied nearly as much as you suggest. Worse, a meteor impact that could kill the dinosaurs without killing all life on earth would simply be too small to affect earth's tilt.

And as noted above, theories about absurdly large plants and flooding is pretty easily debunked by the geologic record.
  #44  
Old 08-13-2013, 07:40 PM
DrDeth is online now
Charter Member
 
Join Date: Mar 2001
Location: San Jose
Posts: 39,430
Quote:
Originally Posted by AndrewL View Post
You are reading that correctly. Birds evolved from "lizard-hipped dinosaurs". I suspect that the two branches of dinosaurs were named before it was known for sure that birds had evolved from dinosaurs, and the resemblance of the hips of "bird-hipped dinosaurs" to the hips of actual birds was coincidental or a case of convergent evolution.
Long before. In fact really, we shouldn't be lumping the two together as "dinosaurs"', just a mistake by the guy who coined the name.
  #45  
Old 08-13-2013, 09:00 PM
Wendell Wagner is offline
Charter Member
 
Join Date: Jul 1999
Location: Greenbelt, Maryland
Posts: 14,166
Are there some basic books that we can recommend that breakneck read about evolution, dinosaurs, mass extinctions, the organisms and geology of the K-T period, and the K-T mass extinction? I think that breakneck is going to need to learn a lot more than we can explain in one thread. Also, breakneck, I'd like to suggest that you take a course on writing. I mean ordinary nonfiction expository writing, not fiction writing. You need to do a better job at expressing yourself. If you're out of school, see if you can find some place that will teach adults such classes.
  #46  
Old 08-13-2013, 09:06 PM
snowthx's Avatar
snowthx is offline
Guest
 
Join Date: Feb 2012
Location: Sacratomato area
Posts: 3,415
Quote:
Originally Posted by Wendell Wagner View Post
Are there some basic books that we can recommend that breakneck read about evolution, dinosaurs, mass extinctions, the organisms and geology of the K-T period, and the K-T mass extinction? I think that breakneck is going to need to learn a lot more than we can explain in one thread. Also, breakneck, I'd like to suggest that you take a course on writing. I mean ordinary nonfiction expository writing, not fiction writing. You need to do a better job at expressing yourself. If you're out of school, see if you can find some place that will teach adults such classes.
Never mind reading some books; a scan thru the very high-level Wikipedia article on dinosaurs should at least provide a foundation and launching point to some of the core concepts the smart dopers here have already provided. Even that, tho, will take some effort.
  #47  
Old 08-13-2013, 09:17 PM
Colibri's Avatar
Colibri is online now
SD Curator of Critters
Moderator
 
Join Date: Oct 2000
Location: Panama
Posts: 41,311
Quote:
Originally Posted by DrFidelius View Post
No, the axis was the same. The continents were in a different configuration, though, and the atmosphere had more greenhouse gasses to hold heat.
Yes. The presence of ice at the poles has much more to do with the configuration of continents, how ocean currents transfer heat around the globe, and the level of greenhouse gases in the atmosphere than it does with axial tilt. Since the K-T event the climate has varied from much warmer than today in the Eocene to much colder during the Pleistocene all with essentially the same axial tilt.
  #48  
Old 08-13-2013, 09:36 PM
DSeid's Avatar
DSeid is offline
Guest
 
Join Date: Sep 2001
Location: Chicago, IL
Posts: 21,538
Okay, after the large dinosaurs were taken out of the picture mammals evolved to fill the niche of large animal in both the prey and predator groups. The largest land mammals have been dwarves compared the giants of the dinosaurs.

So question one is what allowed the large land dinosaurs to become so much larger than any land mammal and what factor(s) drove that as an adaptive niche to fill? This article offers the speculation that the air sacs that birds still have were the key in allowing some dinosurs to become so large, allowing for greater cooling and from an engineering standpoint, lightening the load, that and long necks:
Quote:
air sacs like those seen in modern birds. Modified from simpler versions present in the last common ancestor of the sauropod and theropod dinosaur lineages, way back down the evolutionary tree, these air sacs would have been beneficial to the giant dinosaurs in several ways. In addition to allowing the dinosaurs to breathe much more efficiently than a mammal of comparable size, they may have also acted as an internal cooling system that would prevent the large sauropods from overheating. These physiological boons may have allowed the sauropods to have more active metabolisms than previously thought.

From a structural point of view, though, the air sacs significantly lightened the skeletons of these dinosaurs without sacrificing strength. The intrusions of the air sacs reduced the density of sauropod skeletons and gave them enough of an engineering edge to push the limits of body size. Mammals lacked this advantage, as well as the physiological benefits the air sacs brought with them, and this may explain why the biggest mammals of all time were still small compared to the largest dinosaurs.

Air sacs are not the entire picture, though. As argued in a recent, comprehensive review by paleontologist Martin Sander and colleagues, one of the key, early features of the sauropod dinosaurs was a long neck. Rather than extensively chewing food as mammals do – something that requires a big, heavy head and therefore limits possible neck length – sauropods just horked down plant food and processed it inside their digestive systems. By vacuuming up plant food, the heads of sauropods remained small and their necks were able to become longer and longer to reach plants that could not be plucked by other herbivores. In turn, this affected the evolution of air sacs within the body and opened up evolutionary options not available to large mammals. The basic construction of dinosaurs made all the difference.
Fine. It doesn't answer the second part though. What advantage was there to being that large?

That I suspect has a decent WAG out there. The next question though is more interesting, is then informed by the first two, and is close to what I think our op is asking.

Got it that the larger animal niche was mostly filled by some relatively mega-mammals. But birds still have that structural advantage and some birds have indeed gotten a bit big in eras since ... but emu and ostrich big, not much bigger. Birds have also gotten smart. Corvids are freaky smart, especially given the size of their brains. So given that birds have same structural advantages that allowed for mega size that their dinosaur ancestors had, and that they have the capacity to develop significant intelligence out of a brain that does not need to even weigh that much, why in all this time, throughout all the environments and ecological systems that birds have existed in, have no birds evolved to fill the very large animal niche?

Why aren't there any birds that have become even very large mammal sized, let alone megadinosaur sized?

Last edited by DSeid; 08-13-2013 at 09:41 PM.
  #49  
Old 08-13-2013, 09:36 PM
shijinn is offline
Guest
 
Join Date: Feb 2003
Location: waist deep in ink
Posts: 4,197
Quote:
... Baby sauropods did not start out large. They emerged from eggs smaller than a soccer ball and grew at astonishing rates of up to 12 pounds (5.4 kg) per day, according to Michael Novacek, a senior vice president at the museum. ...
how did they arrive at the growth rate? surely fossil records aren't accurate to the day?
  #50  
Old 08-13-2013, 10:41 PM
DSeid's Avatar
DSeid is offline
Guest
 
Join Date: Sep 2001
Location: Chicago, IL
Posts: 21,538
Here's one speculation as to what was special for the dinosaurs that birds have not had:
Quote:
the top carnivorous dinosaurs were still 12 times heavier, and the herbivorous dinosaurs 1.5–3 times heavier, than predicted from the regression lines from Fig. ​Fig.11A for extant ectotherms; therefore, the never-since-surpassed size of the largest dinosaurs remains unexplained. Perhaps the answer has to do with high net primary productivity during the dinosaur era: atmospheric CO2 levels then were up to 10 times the present levels (37) and elevated CO2 stimulates productivity of some plants (38).
Not sure I buy that though.
Closed Thread

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 11:10 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@straightdope.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Copyright © 2018 STM Reader, LLC.

 
Copyright © 2017