Why is the definition of dinosaurs not scientifically based?

Question

It seems to me that if the definition of dinosaurs was scientifically based then elephants, hippopotamus, and possibly some other large land mamals would be classified as dinosaurs. Perhaps someone can help me understand what is the basis of the definition of dinosaurs. To say that extinction is one of the defining attributes is not a biological criteria.

Answer 1

Your question is actually a really good one. Basically, you could extend it and ask "What makes any biological group a group?" It's a field of inquiry that biologists call "taxonomy". Its objective is to organize living or extinct species according to their relationship, i.e., their shared evolutionary histories.

As you may know there are different levels of groups, or, as we say in biology, taxa (one taxon, many taxa). The perhaps most easy one to envision (at least seemingly so) is the taxon "species". Our species, Homo sapiens, is one taxon, since all members share certain characteristics, go back to the same origin (ancestral species), and are genetically not isolated (which means, they can produce fertile offspring together). The next level up would be genera. Latin names indicate both the genus and the species. In our case the genus would be 'Homo' and the species 'sapiens'. We have no sister species in our genus, but in other genera there can be numerous species. Take for example fruit flies. The genus is 'Drosophila' and is comprised of a variety of species, with 'melanoptera' being perhaps the best known.

The problem with taxa is that we are reconstructing their relationship as well as themselves based on our understanding of their biology. And that is of course never complete. Using a particular method of comparison of numerous characteristics, ancestral trees can be created to represent how the species evolved and how they are related. In the genus Drosophila there are numerous species, but throughout their evolutionary history they all originated in one ancestral species, from which other populations branched off and eventually became their very own species. All members of the taxon 'Drosophila' are monophyletic, which means that they indeed are closely related by origin (i.e., shared ancestor species).

Now, to answer your specific question of what makes a dinosaur. Someone suggested here that they were reptiles. A good idea, if only 'reptiles' were really one of those 'monophyletic' groups. But in fact, they are not. recent research has shown that those critters we once all subsumed under the term 'reptiles' are actually not at all closely related. Crocodiles and alligators, for example, are closer related to birds than to snakes. They form a monophyletic taxon with birds, but not with their fellow 'reptiles'. Ergo, that criterium 'reptile' is not a very good one. Then, what are dinosaurs and what makes them a monophyletic group?

The Dinosauria are a monophyletic group characterized by the following autapomorphies (that are characters shared by all members of the group, in that case all dinosaur species): elongate vomers, three or more sacral vertebrae, scapulacoracoidal glenoid facing fully backwards, low deltopectoral crest running 1/3 to 1/2 way down humerus, three or fewer phalanges on fourth digit of manus, largely to fully open acetabulum, fully offset proximal head of femur with a distinct neck and ball, greatly reduced fibula, well-developed ascending process of astragalus; upright posture, digitigrade stance. In a cladistic investigation of a recent taxon (i.e., one that is still alive and not exitincet) taxonomists would also include genetic characteristics, but for extinct species that is a bit difficult (to say the least).

At any rate, the list of anatomical characteristics given above gives you the main autapomorphies for the Dinosauria I could find. Previous classifications usually considered dinosaurs a polyphyletic group with independent origins from thecodont ancestors. Dinosauria are now considered by many monophyletic, because of these numerous autapomorphies.

If we found a living species that would also have these characteristics, we would indeed have found a dinosaur. Extinction is therefore no criterion for taxonomy. But so far, no living dinosaur has been found.

I hope this helps a little. Contact me if you have further questions.

Answer 2

The definition of dinosaurs IS scientifically based.

Not all the members of the dinosaur family were gigantic, they ranged from less than a foot tall to taller than a five story building.

Deinos-terrible or formidable
Saurua-Lizard

The lizard bit was a reasonable guess for the day, although not quite accurate.

Elephants and Hippo's are mammals which is a separate family, that of 'mammalia'.

Dinosaurs (dinosauria) are a distinct lineage that diverged from the reptile family approximately 230 million years ago.

Their being extinct has nothing to do with the name. There are plenty of extinct mammals, some of which are larger than modern elephants or hippo's. They aren't and never have been referred to as dinosaurs by professionals just because they are extinct.

Answer 3

How about dinosaurs are reptiles and the creatures you just mentioned are mamals! Not all 'dinosaurs' are extinct either... like crocodiles.

Reptiles are cold blooded, Mamals are warm blooded. Mamals have fur. Reptiles don't. There's a huge difference in the size and the way the brains are structured. Reptiles lay eggs, mamals give live birth.

Answer 4

Whly would you say that there is no science behind studying dinosaurs? First of all, (and I am no scientist) I do know that dinosaurs have a different skelotonal structure than mammals. Scientists can trace the evolution of dinosaurs from reptiles and there is a big school of thought that says birds are descended from dinosaurs.

Answer 5

The name dinosaur means ''thunder-lizard'' and like all organisms were classified based on their anatomy. But often times dinosaurs were given names that did not reflect their anatomy specifically such as Tyrannosaurus Rex or ''tyrant lizard king.''
Archosaurs lived alongside, and were closely related, to dinosaurs. Living archosaurs include crocodiles and birds, but extinct archosaurs are the famous marine reptiles (plesiosaurs, mosasaurs, pliosaurs) and the pterosaurs, also called ''flying reptiles''(pteranodon, dimorphodon, etc...).
Dinosaurs fit in to this group by being distinct from the earlier ''mammal-like reptiles'' thanks to a special hip joint which aloud them to hold their legs directly under their bodies and perpendicular to the ground.
The biggest debate about the classification of dinosaurs arises with their portrayal in modern-culture. Originally they were thought of as sluggish tail-dragging behemoths, but today paleontologists consider them active and animated animals much like mammals are today. A large part of this revolution comes from the general belief in the bird-theropod connection and the general acceptance among many paleontologists that dinosaurs had the ability to control their temperature, and so were not limited to basking in the sun all morning like many reptiles today.

All in all dinosaurs are considered their own branch of life like mammals, or reptiles, birds, or fish, dinosaurs are a unique type of vertebrate that flourished for millenia.

Answer 6

We, like all other living creatures, descend from prehistoric bacteria; and we are still programmed by DNA. Carrying increasingly complex messages, this substance has transmitted life through thousands of millions of years from the first beginnings down to creatures alive today.

There is no direct evidence of living creatures (even big ones) being descended from specific dinosaurs. For more info, look up a book called "The Blind Watchmaker". It is a real simple step-by-step explanation of how evolution works.

Answer 7

Dinosaurs are animals that dominated the terrestrial ecosystem for over 100-million years. Non-avian dinosaurs became extinct 65 million years ago (Mya) at the end of the Cretaceous period. Dinosaurs are known from both fossils and nonfossils including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin and internal organs, and "soft tissues, including blood vessels and cells lining them"

Answer 8

Dinosaurs are very carefully defined by a number of features, including the structure of their hip bones. So it is as scientifically based as grouping all the butterflies together.

Answer 9

I don't think elephants and hippos could be classified as dinosaurs.

Dinosaurs refers to a group of prehistoric species of large reptillian creatures. (that's my definition)

I suppose if you look on an encyclopedia such as wikipedia.org you will find a more substantial biological criteria for what would be considered a "dinosaur"

Answer 10

Because Dinosaurs are just less specialized big birds. Feathers are modified scales.