We have lots of seashells. We're very short of jellyfish fossils. That's not too surprising.
We have a few T. Rex fossils, but we're short on small, fragile creatures. This is easy to explain. First, it's just easier to find the great big fossils. Second, fragile skeletons are, well, fragile. They are more likely to be scavenged or crushed before they can form a fossil.
But there is another pattern, which is that there just aren't very many really old fossils. Why?
There are at least four reasons. For one, the earth's surface has been rebuilt many times. Regions have been uplifted and then eroded away. Erosion destroys rock, and destroys any fossils in that rock. The new rock that forms contains new fossils. So, much of the earth's surface is recent, compared to the age of the planet itself. Old rocks are rare, so of course old fossils are rare too.
The second reason is that many old rocks have spent time buried. While buried, they experienced great heat and/or pressure, and are now metamorphic rocks. Their fossils have turned to smudges.
Worse again is that the best fossils are found in ocean-bottom sediments. But as the continents move, they ride over the ocean floor. Old floor is sucked down towards the center of the earth at subduction zones, never to be seen again. (Places like the North Atlantic Ridge are creating new ocean floor to replace the old.)
Continents travel at about an inch a year. So, if you look at the size of an ocean, and do some simple arithmetic, you will see that most of the world's ocean floor should be less than 200-300 million years old. But dating methods say that animal life arose 800 million to 1000 million years ago, and it moved onto the land about 400 million years ago. So, this is a frustrating situation. The oceans have been repaved since the really interesting stuff happened. We have to make do with the very few old ocean rockbeds that escaped destruction.
And the fourth reason is that the first creatures didn't have skeletons, and they were tiny, too. We can tell in two ways. First, we've been lucky, and found a few very old deposits that preserved soft things. And secondly, we've found tracks.
Why didn't they have skeletons? Well, because skeletons had to be invented at some point, and that point was about 600 million years ago.