Dinosaur Evolution -- May 13, 2019

Three eras:

• Paleozoic
• Mesozoic
• Cenozoic

Reptiles arose about 310–320 million years ago during the Carboniferous period. In the Paleozoic era, seven periods:

• Cambrian
• Orvodician
• Silurian
• Devonian
• Carboniferous
• Permian

From wikipedia, reptiles:

Reptiles, in the traditional sense of the term, are defined as animals that have scales or scutes, lay land-based hard-shelled eggs, and possess ectothermic metabolisms.

So defined, the group is paraphyletic, excluding endothermic animals like birds and mammals that are descended from early reptiles.

A definition in accordance with phylogenetic nomenclature, which rejects paraphyletic groups, includes birds while excluding mammals and their synapsid ancestors. So defined, Reptilia is identical to Sauropsida.

Though lots of reptiles today are apex predators (top of the food chain), many examples of apex reptiles have existed in the past. Reptiles have an extremely diverse evolutionary history that has led to biological successes, such as dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and ichthyosaurs.

Paraphyletic:  descended from a common evolutionary ancestor or ancestral group, but not including all the descendant groups. I guess this means that reptiles were only one group (one phylum) that descended from another ancestral group.

Later: we need to look up the other reptilian paraphyletic groups (reptilian siblings, as it were).

For me, "reptilia" is a grab-bag of animals that encompasses everything that is not a fish (bony/cartilage), mammal, or amphibian. Thus, "reptilia" includes extinct dinosaurs and extant birds. However, note: wiki separates "birds" and "mammals" from "reptilia. And I think that's correct: if one separate mammals from "reptilia," then one must separate birds from "reptilia."

But if one does that, then dinosaurs must also be separated from "reptilia" because it is agreed that birds are direct descendants of dinosaurs.

It's actually pretty cool.

Some could divide the chordates into three groups, instead of the "classical" five groups (which, by the way, has changed since I was in elementary school).

The three groups of chordates:

• fish (bony, cartilage)
• amphibians (fifteen minutes of fame)
• reptilia

And that's it.

Science fiction movies portraying aliens as reptiles is not particularly far-fetched.

Egg-laying and live birth. Is that a big difference? Looking at cross-sections of mammalian wombs and reptilian eggs: very, very similar. A chick pecking its way through an egg shell; a dinosaur pecking its ways through its egg shell; and, a mammalian "infant" coming through the birth canal all very analogous.

Fur, feather, scales: big difference.

Warm-blooded vs cold-blooded: big difference.