>years ago someone had quoted that "Sea urchins had dwindled to a small number during a particular paleontologic era, .....then they were again resurrected during another era..."
>I cannot remember these Eras and cannot find this reference in the archives.

The sea urchins barely survived the end-Permian extinction but survived and radiated again afterwards in the Triassic.  (These are periods; the Permian is the last period of the Paleozoic Era and the Triassic is the first period of the Mesozoic era.)  This is the largest known extinction.  Crinoids likewise were hit hard; several kinds became extinct, as did several classes of crinoid-like echinoderms.  Among the mollusks, the ammonoids barely survived.

Crinoids appear to be rather distantly related to other extant echinoderms.  Brittle stars (including basket stars) are fairly distinct from true starfish, but closer to them than to crinoids.

All of them can be important components of molluscan diets.  Eulimids and cassids often eat echinoderms.  Other mollusks, such as some galeommatoideans, may be commensally associated with echinoderms.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa