Where are the fossils of the evolutionary ancestors of fish?
Here you go:
Why does the Cambrian explosion resemble an orchard of trees (sudden disparity/almost all the different pyhla appearing in a relatively short period of time) and not the single tree that evolution predicts?
They do have precursors. More fossil evidence has emerged since Darwin, and even since Gould. But to the general point, you’re basically asking why tree branches are thickest closest to the trunk. According to evolution, we should expect the most
generalized animal forms the furthest back, and see things specialize over time. Keep in mind that the different phyla are things like:
Arthropods ← everything from crustaceons to insects to centipedes
Nematoads ← round worms
Molluscs
Flatworms
Sea Urchins
a bunch of other things that are like worms
and
Chordata, which are the first things that have a spine (basically just fish at this point).
You don’t have any mammals or birds or lizards or amphibians. Just
something with a spine that will then diverge over time into all the spine-having animals.
Here’s a pretty good article discussing this stuff:
https://www.nature.com/news/what-sparked-the-cambrian-explosion-1.19379
According to evolution there must exist millions of transitional fossils between dinosaurs and birds. How many have been found?
A lot, but keep in mind that fossilization is a very rare process, especially for land animals. Very, very few animal carcasses will become fossils, and then we have to find them.