I'm having trouble with dinosaur theories...

Question

I watched this show on National Geographic saying dinosaurs evolved into birds, reptiles and mammals...then what the heck were they?

Answer 1

Okay, first of all...scientists don't think that dinosaurs evolved into reptiles or mammals.  Most scientists DO think that SOME dinosaurs evolved into birds.  Let's rewind the evolutionary clock a bit to clarify some things.

Before there were dinosaurs or mammals, there were amphibians, which were animals capable of living on land, but which still needed water to reproduce.  From the amphibians came the first amniotes.  Amniotes are animals that bear young surrounded by protective membranes.  For the first time in Earth's history, vertebrates could give birth on land, either by laying eggs (as most modern birds and reptiles still do) or by bearing live young (like most modern mammals).

The amniotes diversified into two major groups: synapsids and sauropsids.  Synapsids were "mammal-like reptiles" and the forerunners of all modern mammals.  Sauropsids were the ancestors of all modern reptiles and birds.  From the point when they split until the present, mammals have followed a completely independent evolutionary track from reptiles.

The sauropsids diversified several times throughout the next several hundred milliion years.  One early split-off gave rise to the ancestors of modern turtles.  A later split gave rise to the ancestors of modern lizards and snakes, and a still-later split gave rise to crocodiles and dinosaurs.

Dinosaurs themselves represent an incredibly diverse group, and one that extended over a period of about 160 million years.  Many dinosaur groups rose and fell during the Mesozoic era, and they carried many different evolutionary adaptations.  Some of those would be passed on to modern birds, but many would not.  There is debate in the paleontology community today about when and how homeothermy (so-called "warm-bloodedness") first evolved, with many scientists arguing that it is quite possible that some dinosaurs were actually warm-blooded.

The birds themselves apparently descended from a class of dinosaurs called Maniraptora, which were small agile dinosaurs, many of which were probably feathered themselves.

To be complete, I should point out that there is some dissention in the scientific community regarding the evolutionary heritage of birds.  While most sources I've seen regard birds as dinosaur descendents, some believe that birds actually descended from archosaurs (reptiles that predated dinosaurs and also gave rise to crocodiles).  If that were true, then birds would be more like the cousins of dinosaurs than their direct descendents.

So to answer your question: what the heck were dinosaurs?  Here's what you have to be in order to be called a dinosaur:

1. You have to be a vertebrate.

2. You have to live on land.  Swimming and flying reptiles (like Ichthyosaurs and Pterodactyls, respectively) were not dinosaurs, even if they lived at the same time.

3. You have to walk with an upright posture.  No sprawling stances allowed in this club.

4. And, unless you're a bird, you have to be dead, and have been that way for more than 65 million years.

I hope that helps.  Good luck!

Answer 2

All dinosaurs were reptiles, and one line gave rise to birds.  However, reptiles predate dinosaurs.

The ancestors of mammals weren't dinosaurs.  They were non-mammalian synapsids, specifically cynodont therapsids.  The last common ancestor of synapsids and reptiles lived around 315 million years ago at the latest, and that's around 85 million years before the oldest known dinosaur.

I hope National Geographic doesn't actually say what you state.  Perhaps the unfamiliar names are the source of the problem.

Answer 3

You must have misunderstood.  Dinosaurs did not evolve in to anything.  Reptiles evolved in to birds, some reptiles, not all reptiles.  Same goes for the other forms of evolution.

Understanding Science and Evolution

http://www.wiu.edu/users/mfb100/evolutio...

Answer 4

They were bireptilemals. Apparently.