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Volume 1, Issue 1 March 7, 2012

Dinosaurios: Herejias y Rumores, elAcalorado Debate

Dinosaurs: Heresies and Hearsaythe Heated Debate

By: Joahna Marie RendonIII-BSBT

The claim is that dinosaurs

were warmblooded, like birdsand mammals, not cold andsluggish like lizards and

snakes. To be specific, theissue is not really whetherthe blood of dinosaurs ran

hot or cold. After all, on ahot day with the sun beatingdown, even a so-called cold-

blooded lizard can bask,heat its body, and, strictlyspeaking, have warm blood

circulating in its arteries and

veins. The issue is not bloodtemperature, hot or cold, but

whether the source of theheat is internal or external.

To clarify this issue, twouseful terms need to bedefined, ectotherm and

endotherm.

Animals that depend largely upon

sunlight or radiation from thesurrounding environment to heat theirbodies are coldblooded, or more

accurately, ectotherms (heat fromoutside). Turtles, lizards, and snakes areexamples. Warm-blooded animals

produce heat inside their bodies bymetabolizing proteins, fats, andcarbohydrates. To be more accurate,

warm-blooded animals are endotherms(heat from within Birds and mammals areobvious examples.

Were dinosaurs ectotherms or

endotherms? The source of their body heat is in

dispute, not their blood temperature. Heatfor ectotherms is cheaply won. They need

only bask in the sun. The trouble withsuch a lifestyle is that the sun is notavailable at night nor is it always available

in cold temperate climates.

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By contrast, heat forendotherms is expensive. A

digested meal, often caughtwith great effort, produces fats,

proteins, and carbohydrates

necessarily spent in part togenerate heat to keep theendotherm body warm. Where

endotherms have an advantageis that their activity need not be

tied to heat vailable from theenvironment. These differentphysiologies are accompanied

by different estyles.Ectotherms bask; on cold

nights they become sluggish;and in freezing winters they

hibernate. Endotherms remain

metabolically active throughouteach day and each season,despite the cold or inclementweather. Certainly there areexceptionsbears and some

small mammals hibernatebutendothermy requires

continuous activity in mostcases.

Thus, the issue of warm-bloodedness in dinosaurs is not

ust about physiology but aboutthe type of accompanyinglifestyle they enjoyed.

Because dinosaurs havetraditionally been classified as

reptiles, they were for many years envisioned to be

ectotherms just like their livingcounterpartslizards, snakes,turtles, and crocodiles. Initially,the case for endothermic

dinosaurs was built aroundfour principal lines of evidence.Lets look at the ar uments.

Insulation. First, some mid- tolate Mesozoic reptiles hadsurface insulation, or at leastthey seemed to. For ectotherms,a surface insulation would onlyblock absorption of the suns

rays through the skin andinterfere with efficient basking.But, for endotherms, a surface

layer holding in their internallymanufactured heat might be anexpected adaptation.

Unfortunately, soft insulation israrely preserved, but in a fewfossils of the Mesozoic,

impressions in the surroundingrock indicate the presence of aninsulating layer of feathers(Archaeopteryx). In fact,

feathers likely first arose asthermal insulation and only

later evolved into aerodynamicsurfaces. Apparently, then,some Mesozoic reptiles hadinsulation like endotherms

rather than bare skin likeectotherms.

Dinosaurs that lived beforehumans

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Large and Temperate. Second, large

Mesozoic reptiles are found in temperateregions. Today, large reptiles such asgreat land tortoises and crocodiles do not

occur in temperate regions. They live inwarm tropical or subtropical climates. Theonly modern reptilian inhabitants oftemperate regions are small or slender

lizards and snakes. The reason is easy tounderstand

When winter arrives in temperate regionsand freezing cold settles in, these smallectothermic reptiles squeeze themselvesinto deep crevices where they safely

hibernate until spring and escape thefreezing temperatures of winter.On the other hand, for a large and bulky

animal, there are no suitably sized cracksor crannies into which they can retreat toavoid the winter cold.

Large animals must be endothermic tosurvive in temperate climates. Eventhough the world of the Mesozoic was

warmer than today, with no polar icecaps, winters in northern temperateregions would have been cool and days

short.Thus, the presence of large reptilesin temperate climates of the Mesozoicsuggests that they were warmblooded.

Like wolves, coyotes, elk, deer, moose,bison, and other large temperatemammals today, the large Mesozoicreptiles depended on heat produced

physiologically to see them through.

Predator-to-Prey Ratios. Thirdthe ratio of predators to pre

argues for endothermic dinosaursEndotherms, in a sense, hav

their metabolic furnaces turneup all the time, day in and daout, to maintain a high bodtemperature. A single endothermi

predator, therefore, requires morfuel, in the form of prey, to keethe metabolic furnaces stoke

than an ectothermic predator osimilar size. Paleontologist RobertBakker thus reasoned that ther

should be few predators but lots oprey (lots of fuel to feed the fewpredators) in ecosystem

dominated by endothermireptiles. But, if ectothermireptiles dominated, theproportionately more predatorshould be present.By selecting strata that steppethrough the rise of dinosaurs

Bakker compiled the ratios. Mesozoic archosaurs werbecoming endothermic, then th

ratio of predator to prey shouldrop. That happens. As this rati

was followed from early reptiles, tpredinosaurs, and to dinosaurs, dropped.

There were proportionately fewpredators and more prey.

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Bone Histology. Fourth, themicroarchitecture of dinosaur bone issimilar to that of endothermicmammals, not to that of ectothermic

reptiles. Bones of ectothermic reptilesshow growth rings, like those of trees,

and for much the same reason a is therings in trees, they grow in seasonal

spurts. Endothermic mammals, with

constant body temperature year round,lack such growth rings in their bones.When various groups of dinosaurs wereexamined, the microarchitecture oftheir bones told a clear storynogrowth rings. Dinosaurs then becameactive animals. They romped and

played, chased prey, and dashed forcover. Endothermically, they wereformidable. They even made it to the

silver screen, snorting warm air fromhot bodies as they made meals ofmammals peoplein the movieJurassic Park.

The important point to keep before us isthat dinosaurs were in their own rightan extraordinary group. These active

animals occupied almost everyconceivable terrestrial habitat.

Their social systems were complex, andthe adults of some species wereenormous. If dinosaurs wereendotherms, their complete demise at

the end of the Mesozoic can only bemore mysterious and the loss of theawesome splendor of this group all themore intriguing. Although dinosaurs

died out, the debate over what kind ofreptile they were continues to evolve.