Saturday, May 07, 2005

Dinosaurs and their Descendents 2 - Why feathers?

` In my first installment, I've gone over some of the reasons of how and why birds fit in the dinosaur family tree - they are maniraptor theropods close to the dromaeosaurs - and I've mentioned that some dinosaurs had various feathers. In this blog, I will tell you some very good reasons why paleontologists have been putting feathers on their finds for decades before any feathery specimens had even been found!

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` So the first question I'm posing here is; why would dinosaurs have feathers?

` Long ago, I remember the argument was 'were dinosaurs warm-blooded or cold-blooded?' (Actually, the words are endothermic and ectothermic, but I'll let that go.) Basically, reptiles like lizards are 'cold-blooded' (or ectothermic - ecto = 'outside'), meaning they can only stay warm and maintain normal functioning with the help of the sun and surrounding temperature.
` Such animals mainly eat for nutritional purposes, and must actively work to keep themselves from frying or freezing each day. To ensure that they can always let heat in when they need to, their skin is covered with mostly flat, non-overlapping structures that don't block warmth.

` Other reptiles, like birds, are warm-blooded (or endothermic - endo = inside), meaning they can rely on food releasing enough calories of heat energy to keep them warm and allow their muscles and organs to work properly all of the time.
` Because of this, a warm-blooded animal of x weight needs to eat about ten times as much food as a cold-blooded animal of the same weight! This also means that the warm-blooded animal has to use that energy to be a lot more active in order to work for that much more food.
` The payoff is that there is usually more than enough energy to get the next meal as well as the ability to be active in cold weather. This explains why lizards and snakes can only live so far from the equator while polar bears have meanwhile discovered the north pole and many penguins can be found on the iced-over continent of Antarctica.

` Well, ya know what? Dinosaurs lived near the poles in areas that would have been too cold for snakes and frogs during the Mesozoic - which even then included Antarctica. For the small cold-blooded animals, it was too frigid to even hibernate - and crocodiles are even more restricted by temperature, being impractical for going dormant against long periods of snow and ice. As I recall, this is one of many of Bob Bakker's arguments for dinosaur warm-bloodedness in his 1986 book called Dinosaur Heresies. (A neat, if somewhat conjectural, read!)
` As he said more recently (in 1998); "I tramped all over Vancouver, B.C., last time I was there. I did not see a single giant tortoise, not one. It's too cold. But it was that kind of climate in the Cretaceous of Alaska that supported huge herds of duckbills and the tiny Troodons and the tyrannosaurs and horned dinosaurs. That's a rather dramatic climate proof that dinosaurs could thrive in cold air."

` I remember reading Heresies a long time ago: The most significant arguments focused on the fact that, by looking at the bodily structures and other facts about dinosaurs, we can tell that their lives would have had to have been quite similar to that of modern birds and mammals.
` According to their most practical skeletal postures, the bird-like attachment points of muscles, as well as their footprints (direct evidence of movement!), all dinosaurs walked with their feet straight underneath them and their tails raised in the air - and some were capable of great speeds!

` On the other hand, cold-blooded animals can't move fast for long - and I'm talking like, not even thirty seconds, here. Crocodiles can gallop somewhat, as their legs are semi-upright, but having fully-upright legs wouldn't help them out much more as their body chemistry would still cause them to tire quickly.
` Dinosaurs generally were built for walking and/or running for long periods of time, which means that they must have been able to do so. A high metabolism would be needed to supply the energy for all this activity.

` Dinosaurs also moved around and grew faster than cold-blooded animals, as evidenced by their porous (= fast-renewing) bones and generous growth rings. The cute, large-eyed young of duckbills had soft bones, which grew quickly, but were not immediately able to support the animals' weight.
` Mother duckbilled dinosaurs (hadrosaurs) laid nest-fulls of eggs in vast breeding areas, and (like altricial birds such as robins), had to bring food to their young until they were able to forage on their own. (Maiasaura, the 'good mother reptile' was named for this mothering behavior.)

` Some dinosaurs had growth spurts so tremendous that it surprised paleontologists who were very familiar with them!
` Sue, the famous and huge Tyrannosaurus rex specimen, was thought to have died at around eighty years of age because she was so large. But, studying tyrannosaurs of various ages and documenting their growth rings in their non-hollow rib-bones, scientists were able to piece together the following scenario:

` At hatching, Sue was only about ten pounds, beginning life the same size as an adult house cat. By age two, she was only a hundred pounds, a ton as an early teenager, and, inexplicably, widely-spaced growth rings indicate that she had put on about five pounds a day between the ages of fourteen to eighteen, gaining about five tons along the way! After this, the growth rings are very compressed.
` Now, cold-blooded animals do not have growth spurts, though these especially fast periods of growth are completely expected (and incredibly common) in warm-blooded animals!

` Sue was only twenty-eight when she died, riddled with age-related conditions like arthritis, meaning that she had probably only died from old age! (She had not been mauled to death, as was once believed.)
` Elephants, though they have similar teenage growth spurts, can naturally live until 60 or so years of age, partly because of their rather low metabolism (for a mammal, anyway). Evidently, tyrannosaurs grew fast and died young - I wonder if their metabolisms were higher?
` Saltwater crocodiles, on the other hand, barely age at all because they are cold-blooded, and for the same reason, they take about eighty years to reach just half a ton or more.

` So dinosaurs could grow fast with growth spurts, some of them aged fast, and they were all built for continuously moving around, sometimes very quickly! Cold-blooded animals sure aren't like that, but warm-blooded animals are!
` It's just this kind of thing that's been prompting scientists to draw feathers on dinosaurs, especially the maniraptors. This would explain the stunning illustrations Gregory S. Paul did for his 1988 book, Predatory Dinosaurs of the World - a wonderful tome of predatory animals that resembled ground birds. Why?

` The first reason is, like I said, a cold-blooded animal can't have anything that keeps it from absorbing heat - insulation would simply keep it from getting warm. But if an animal generated its own energy, it would need insulation to keep its hard-won heat inside of its body. (This means that insulation would have the opposite effect if it was on a cold-blooded animal!)
` Insulation would allow the animal to be active any time it wanted or needed, not to mention, give it the ability to explore frozen lands, as dinosaurs evidently had.

` The second reason for feathers, specifically, is because birds have always been known to be much closer related to them than mammals, and are indeed classified as dinosaurs. (Mammals are on a distant branch of the 'family tree' from dinosaurs altogether.)

` It turns out that, after decades of feathery drawings, those scientists were right! Very well-preserved soft tissues are hard to come by, but not impossible. (Speaking of soft tissues, see my March 26 entry, which mentions the discovery of stretchy, birdlike Tyranno-tissue!)

` Bird-like internal organs have also been found, such as the heart of Willo the Thescelosaurus. In the stone inside its ribcage, right where you'd expect the heart (made of iron-rich tissue) of such an animal, it has a reddish, iron structure that was first identified by cardiologists as a heart.
` (Go here to find interactive images and scans of this amazing discovery! It's worth seeing!)
` After several more months of studying by other scientists, they agreed that it was indeed a heart and that it had the basic structure found in modern birds with four chambers and one aorta. While cold-blooded reptiles have hearts best suited for a cold-blooded metabolism, Willo's heart is the type found in warm-blooded animals (including us mammals) that is needed for our insanely high metabolisms.
` Birds and mammals evidently did not inherit the 'warm-blooded' type of heart from a common ancestor, but have both evolved it independently because it is a very useful way to modify one's heart to suit the same purpose.

` What you may not know is that dinosaurs like Thescelosaurus - and duckbills! - were not 'birdlike' dinosaurs, as they were on the other main branch of the dinosaur 'family tree'! See, there are two main 'branches', of dinosaurs and both seem to be equally warm-blooded:
` There were the bird-hipped (Ornithischian) dinosaurs, which include Thescelosaurus, Hypsolophodon, Triceratops, Pachycephalosaurus, Ankylosaurus, Stegosaurus, Iguanodon, and duckbills like Maiasaura, Hadrosaurus, and Lambeosaurus.
They are called 'bird-hipped' dinosaurs because for them, the pubis - one of the hip bones - points backwards like that of birds.
* (Personally, I call them the 'chewing dinosaurs' because they ate plants and are especially known for their powerful jaws and grinding teeth.)

` Then, there are the 'reptile-hipped' (Saurischian) dinosaurs, which includes the maniraptors. Most 'reptile-hipped' dinosaurs have a forward-pointing pubis - like a crocodile - though unsurprisingly, most dinosaurs from the maniraptor group actually have more bird-like hips than do 'bird-hipped' dinosaurs!
` Go fig!
` The 'reptile-hipped' dinosaurs include Apatosaurus, Diplodocus, Brachiosaurus, Plateosaurus, Allosaurus, Dilophosaurus, as well as maniraptors like; Oviraptor, Therizinosaurus, Gallimimus, Tyrannosaurus, Velociraptor, and pigeons.
* (The plant-eating members of this branch processed their food through gizzards - usually only evidenced by clusters of smooth stones where you'd expect to find a gizzard - so I like to call them the 'gizzard dinosaurs'.)

` So far, though, only coelurosaurs - mostly maniraptors - are known to have any feathers at all. The smaller 'bird-hipped' dinosaurs, could have had feathers or something similar - large hadrosaurs (duckbills) we know from such things as hadro-mummies had deeply-creased skin and pebbly scales.
` That is likely to be because large warm-blooded animals have trouble dumping heat, and insulation would prevent that from happening. (That's why elephants have such heat-radiating structures as large ears and wrinkled skin - though mammoths had thick fur because they lived on tundra.)

` One bird-hipped dinosaur, Psittacosaurus - the 'parrot reptile' - was a small, primitive relative of Triceratops and other horned dinosaurs (like Chasmosaurus and Torosaurus). It had neither frill nor horns, though it did have about ninety long, hollow, porcupine-style quills on its tail. Why? Nobody knows, but it had them. While they aren't exactly feathers, they might be closely related.

` Nowadays, no respected paleontologist doubts that dinosaurs had a high metabolism like birds and mammals do - though there have been a few odd things. For example, I once heard (WARNING: POSSIBLY POINTLESS!) that since mammals and birds have to breathe ten times more than cold-blooded animals of similar sizes, they have larger, more complex curling structures called turbinates in the sinuses to catch moisture and regulate the temperature in the bloodstream.
` Their smallish sinuses and lack of elaborate nasal turbinates suggest that dinosaurs (including primitive birds!) either didn't breathe like mammals and modern birds or they did not need these turbinates for some reason.
` What that means, nobody knows, but it makes it clear that active animals with high metabolisms (of varying degrees) apparently did not need the turbinates to help them out - perhaps they had a sort of temperature control that went extinct with them. My own personal guess (WARNING: MY OWN GUESSING!) is that the turbinates are for keeping the brain from overheating, and because all dinosaurs had rather small brains compared to birds and mammals, they didn't need a whole lot of brain-cooling assistance.

` It should be clear by now that after a long history of collecting and analyzing data, no respected paleontologist doubts that some nonavian dinosaurs had feathers, either, which means they must have been trapping body heat.
` By now, I bet you're dying to read about the dinosaurs which are known to have feathers and 'protofeathers' - which look like birds' feathers, but lack the miniscule hooklets and barbules.

` In my NEXT 'DatD' blog, I'll describe the first-known dinosaurs to have various forms of feathers. They are:

Beipiaosaurus inexpecticus - therizinosaur

Caudipteryx zoui - oviraptorid

Dilong paradoxus - tyrannosaur

Microraptor zhaoianus - dromaeosaur

Protarchaeopteryx robusta - (?oviraptoroid - does anybody know for sure?)

Shuvuuia deserti - alvarezsaur

Sinornithosaurus milenii - dromaeosaur

Sinosauropteryx prima - compsognathid

(Unnamed) - dromaeosaur

` Undoubtedly, more will be discovered, and when they are, I'll write about them too, so watch for those!

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