Let's start by meeting some crinoids. They are cool creatures - check this one out. If you're really luck, this is the kind of form you may find in sites with good preservation.
Ordovician crinoid Daedalocrinus. Specimen ~7.5 cm long.
More often than not, though, you'll find crinoid bits that look a bit like this:
Crinoidal limestone where ossicles compise much of the rock. Found in the Lower Carboniferous Keokuk Limestone of Monroe County, Indiana. Specimen ~8.5cm max.
Extinct forms of Echinoderm
The odds of you finding one of these are pretty low, apart from at specific sites. But, they are really nice fossils, so I wanted to include one. Meet the Blastoid Pentremites robustus. This is a group (sometimes called sea buds) had short stalks, and often lose their arms, so you are left with the calyx as you can see here.
Blastoid Pentremites robustus, Lower Carboniferous, found in Indiana. Specimen ~4.5 cm long.
If you would like to learn more about these creatures, there is a good introduction to be found on Palaeontology [online].
Here you can find a fairly typical regular echinoid fossil. Often we'll only find bits, but on occasion we get a nice whole animal like this. Make sure you catch the ambulacra and interambulacra and are comfortable with how these differ in appearance.
Fossil sea urchin Phymosoma texanum: Cretaceous in age, and dicovered in Texas. Specimen ~4.5 cm in diameter.
Now compare that with an irregular echnoid. In particular, note the shift in the position of the mouth and anus (they're labelled for you), and also note the change of the shape of the test. Do you agree that this is better for an infaunal mode of life?
Fossil echinoid Eupatagus antillarum. Well, kind of a fossil, it's Eocene (Ocala Limestone of Levy County, Florida). Specimen length ~7 cm.
Even though these are rare as fossils, let's have a quick look at fossils from this group, which comprise the starfish (Asteroidea) and brittlestars (Ophiuroidea). One of each. Let's start with the former. Here is a fossil starfish:
A fossil starfish, from the Devonian of Tompkins County, New York. max dimension of rock ~6.5 cm.
Contrast this with the less chunky brittlestars - you can tell the difference because there is a clear line/separation between the arms and the central disk in brittlestars.
Fossil brittle stars (Ophiopinna elegans) from the Jurassic (found in Ardeche, France). Rock ~19.5 cm max.