Kuu

These might be very silly questions but what the heck, I didn't finish highschool so I can blame it all on that.

As far as evolution go why is there such a distinct division between classes? Shouldn't there be more overlap?

You do have a couple of mammals that lay eggs but why no birds with fur? etc, etc.

pz

Because those classes correspond to lineages, and because lineages don't intersect again (with a very few exceptions). It's a branching tree, not an anastomosing one.

Also, the range of possible solutions to any problem is sufficiently large that it is unlikely that any two lineages will independently arrive at the same answer.

Kuu

But surely animals should exist today that existed at the time the branching off took place? Animals that didn't become extinct?

At one point birds branched off from the reptiles. They must have existed at that time some animals that shared traits with both birds and reptiles. Say reptiles with feathers. Why did such animals become extinct?

pz

Not necessarily. One estimate for the average life span of a species is 10 million years, very roughly. Even lineages that seem fairly stable and long-lived go through extensive changes over time -- the modern coelacanth, for instance, has some huge differences from its ancient crossopterygian ancestors. Because almost every species eventually goes extinct. More than 99% of all the species that ever existed are now gone. It might be more sensible to ask why birds were among the small percentage that survived and thrived. (My answer would be the advantage of flight. Feathers survived as a trait because they happened to be borne by a species that colonized a unique environment and subsequently diversified.)

Kuu

Then would you consider it an anomaly that the egg-laying mammals survive to this day?

pz

I consider it an anomaly that we survive to this day, so yes.

beastmaster

There are some birds with "mammalian" characteristics. For example, the kiwi has bone marrow (unlike most birds which have hollow bones) and has a complex olfactory system. Not aware of any birds with fur. Does a bat count?

Duvenoy

Does a bat count what?

A little off topic perhaps, but the kiwi lays a single egg at a time. It is the largest egg as compared to body size in the world. I saw an xray of a kiwi carrying one of these cannonballs, and all I could think of is: OUCH!! I was not cruel enough to show it to my ex-ol' lady when she had our second daughter.

Also, a kiwi's feathers are so fine that they almost can be compared to hair.

Another bird with an amazing, olfactory system is the turkey vulture.

doov

lpetrich

Bats are not some bird-mammal intermediate but mammals that have done some convergence on birdishness. But overall, bats continue to look much more like mice than birds.

They have the mammalian feature of external ears, they continue to have teeth, which birds have lost, and they do not have beaks, which birds have.

And though the wings of birds and bats are modified front limbs, those limbs were modified in different ways, producing a characteristic bird wing and a characteristic bat wing.

Convergent evolution is very common, but it can often by recognized by looking at details, such as the details of bird and bat wings. Here are some examples:

Camera eyes of vertebrates and squid/octopuses

Flattening of bottom-dwelling fish: rays and flounder

Streamlined shape for traveling underwater (marine reptiles and mammals reinvented it)

Plants have alternation of generations between haploid and diploid phases, but seed plants are mostly diploid with minute haploid phases, thus converging on the all-diploid animal kingdom.

Viti

Off topic a bit

I was just watching a show, then did some reading about T.Rex. Apparently, some scientists now think T.Rex was a scavenger, not a predator, and they used the similarities between the olfactory system in Turkey Vultures and T.Rex brain case molds indicating a huge olfactory system as part of the evidence.