Deuterostomia II: Crown-Group Echinoderms


First, refresh your memories on Echinoderm Systematics:


Deep water crinoid from David Liddell
Department of Geology - Utah State University

Crinoidea:

(Sea lilies and feather stars) (Ord. - Rec.) Finally, a group with living representatives.

The crinoid body plan:

Suspension feeders: The crinoid points its oral surface downstream then arches its arms into the current. Crinoids can quickly orient themselves on their stems using a bizarre trick: The connective tissue running down their stems can be partially emulsified by nervous impulses, thus rapidly altering stiffness. Wow.


from SUNY Cortland
Calyx plate terminology:



Actinocrinites gibsoni
What is your assessment of the morphology of the crinoid at right:

Crinoid phylogeny:

Crinoidea breaks down into three major groups:


Early Carboniferous scene from Illinois State Museum
Ecology:

Eleutherozoa:

(Cam. - Rec.) Eleutherozoa contains the living groups of non-stalked echinoderms. These became the dominant echinoderms of the post-Paleozoic world, and continue to diversify. Do not think, however, that they are of recent origin. The four major Eleutherozoan groups are roughly as old as blastozoans and crinoids. Thus, although relative abundances and diversities have changed, echinoderms diversified quickly into their modern groups during the Ordovician. The illusion of Eleutherozoans being "new" is heightened by the poor preservation potential of all groups except for echinoids.

Eleutherozoan phylogeny:

More recent cladistic analyses support the following tree, however in the last twenty years, enough been suggested that a strict consensus would be a whisk-broom polytomy:

Eleutherozoa characteristics:

Not necessarily synapomorphies:

Eleuthrozoan systematics: The cladogram at right represents a decent consensus of current analyses of eleuthrozoan phylogenies with one huge caveat: Two reasonable hypotheses exist for the placement of Ophiuroidea:

Here we default to the Asterozoa hypothesis, but the issue is unsettled.


from British Chalk Fossils
Ophiuroidea (brittle stars) - (Ord. Rec.)



From Review of the Universe
Asteroidea ("starfish" or "sea stars") - (Ord. Rec.) Taken as "standard" echinoderms for illustrations of tube-feet, the WVS, etc. only because they are familiar and accessible. Actually quite idiosyncratic.

Characteristics:



Hudsonaster matutinus (Ord.)
Fossil Record: Although common taday, aspects of starfish biology limit their preservation potential: And yet we have fossils of Ordovician starfish that are similar to living ones in anatomy. (E.G. Hudsonaster right). The Devonian, Permian, and Triassic extinction events each trimmed their diversity, but after each, they rebounded with a proliferation of new groups.


Somasteroid in oral view
Somasteroidea - (Ord. Dev.) These appeared slightly prior to the appearance of ophiuroids and asteroids, and were extinguished by the Devonian extinctions.

Characteristics:



Galápagos green sea urchin Lytechinus semituberculatus
Echinoidea (sea urchins, heart urchins, sand dollars) (Ord. - Rec.)



Interior of Aristotle's lantern from Microscopy.uk
  • Feeding:


    Evolution and fossil record: Unless otherwise noted, these are probably paraphyletic grade-groups.



    Holothurian shows four of five ambulacra.
    Holothuroidea: (sea cucumbers) (Sil. (maybe Ord.?) - Rec.) Suspension and deposit feeders whose body plan resembles that of a regular echinoid that has been: The result is an animal with a mouth in front, an anus in back, two ambulacra on top and three underneath (on which it walks).

    Characteristics:

    Ophiocistioidea - (Ord. - P.)

    Paleozoic Eleutherozoans close to the ancestry of Echinozoa.