Echinodermata: After molluscs and arthropods, the most commonly fossilized bilatrian taxon. Indeed, disarticulated echinoderm plates are major constituents of Phanerozoic carbonates, especially from the Mississippian. Deterostomes - thus closely related to hemichordates and chordates. Potential synapomorphies of Echinoderms and Chordates:

• Deuterostomous development
• Hollow nerve cord (only a short segement in echinoderms, hollow for whole length in chordates).

Echinoderm characteristics:

• Exclusively marine: Echinoderms lack osmoregulatory mechanisms that might allow them to live in brackish or fresh water.
  

• Skeleton is internal test comprised of individuals plates of porous high-Mg calcite. In life, the pores are occupied by a protein matrix and dermal cells. Such skeletal tissue is known as stereom. Carbonate petrologists typically call the pores meat holes. Calcite crystals are aligned such that optically, each skeletal element which act optically as individual birefringent element.
  

• One portion of the coelom develops into water vascular system. (Dissection)
  • This system is passively involved in gas exchange, maintainance of posture, and locomotion.
  • The latter is effected by outpouchings of the WVS that penetrate the body wall to form podia or tube feet that can be employed in suspension feeding or in locomotion, depending on the critter.
  • Tube feet are arranged into five double-rows termed ambulacra. Typically, these converge on the mouth amd/or anus.
  • Muscles are used to pump water around the WVS, and each tube foot is equipped with small longitudinal muscles to help aim it, yet the the hydrolic force of the WVS is what primarily effects movement in most echinoderms.
  • the WVS obtains water from the outside. It is connnected by a calcite-reinforced stone canal that opens to the exterior in the hydropore. The hydropore is covered by a seive-like plate, the madreporite, that strains incoming water.
  • The lining of the WVS is ciliated, allowing circulation of its fluid. thus, tube-feet function as gas exchange organs.
  • The coelom (including the WVS) contains coelomocytes which attack foreign material and, in some cases, carry oxygen and CO₂.
  • Despite this weirdness, they are proper bilateralians with a mouth, flow-through gut, and anus.
  • NOTE: In addition to the WVS, echinoderms also retain a normal enterocoelic coelom.
• All but most primitive have strong pentameral symmetry.
• Ancestrally suspension-feeders. Living crinoids still suspension feed. Tube feet snare food particles that are then conveyed down ambulacral grooves by cilia to the mouth.

Traditionally all stemmed echinoderms were lumped into "Pelmatozoa", and all non-stemmed to "Eleutherozoa". A more cladistic phylogeny of Echinodermata:

Helicoplacoidea (Early Cam.)

• Body is helically-built (hence name)
• Sessile suspension feeders: ambulacral groove spiralling along body
• If, in your mind, you "unroll" a helicoplacoid, you find three ambulacra converging on the mouth, which is located on the side of the body, about two-thirds of the way up. (The location of the anus is a mystery.)

Edrioasteroid: (Cam. - Penn)

• Sessile, benthic, attached to hard substrates like the surfaces of brachiopods, mollusks, etc.
• The body took the form of a lens-like blister or a bulb sitting on a short broad stalk.
• Have five ambulacra, like more derived forms. Close examination show that two pairs of these actually converge some distance from the mouth. Thus, only three ambulacra actually converge at the mouth. Edrios, therefore, provide a morphological bridge between the triradiate helicoplacoids and the proper, pentamerally symmetrical later echinoderms. NOTE: A line bisecting the edrio mouth and anus shows the primordial plane of bilateral symmetry that can be hard to recognize in other echinoderms.

"Eocrinoids": (Cam - Sil.)

• Paraphyletic group of basal stalked echinoderms
• Ones with brachioles (tiny tentacles surrounded by calcitic rings, only rarely preserved) may actually be basal blastozoans
• Others, with simple arms (which house extensions of the coelomic cavity), may be sister taxa to Crinozoa + Eleutherozoa

Blastozoa

• Ambulacra are lined with brachioles.
• Stalks (although not all are developed for attachment to substrate.)
• Tend to have relatively complex internal breathing structures developed from the WVS.
• Main bodies called thecae may be large but typically shperical or ellispoidal without "arms' containing extensions of normal coelom.

• Three major clades of blastozoans:
  
  • Blastoidea (Ord. - Perm.):
    • Strong pentameral symmetry and beautiful geometric regularity.
    • Ambulacra:
      • Lined by brachioles
      • Ambulacral grooves covered by lancet plates.
      • Ambulacra terminate at the apex of the theca in the center of which is the mouth.
      • Ambulacra lined by inlet pores that led to complex pleated WVS chambers - hydrospires - that functioned in gas exchange.
    • The mouth is surrounded by five spiracles that act as outlets to the hydrospires and exits for gametes from gonads.
    • One spiracle is larger than the others. It is a joint spiracle - anus.
    • Blastoid stalks allowed them to attach to the substrate.
    • Evolutionary trends: Blastoids became common in the Mississippian (a great time for stalked echinoderms generally) declined, then resurged in the Permian before being stomped by the Permian extinction.

    The remaining groups are traditionally termed "cystoids." Non-monophyletic, but similar in having a stem that didn't terminate in a holdfast. Apparantly they crawled around then stuck their thecae up when they found a good place to suspension-feed. Generally, their thecae were somewhat irregular and slightly flattened (i.e with a top and bottom.) We see two clades.

  • Rhombifera: (Ord. - Late Dev).
    

    Respired through pectinirhombs (Rhomboidal arrays of parallel slits in theca that gave access to hydrospire-like infoldings of the WVS)

  • Diploporita:
    

    Early Ordovician to Late Devonian, respired through diplopores (paired holes giving access to WVS.)

    Evolutionary trends: Rhombiferans and dipoporites were never common, but achieved their peak early - in the Ordovician, then declined slowly to be extinguished in the Late Devonian.

Echino-uglies. A problematic group of Early Paleozoic echinoderms. True apples of discord. Mono-, poly-, or paraphyletic, depending on whom you ask.

"Includes":

• Soluta
• Stylophora
• Cincta
• Ctenocystoidea

What we DO know

• Tend to have flattened thecae with a single aulacophore - a feeding or locomotion appendage.
• Some have rough bilateral symmetry, others have no obvious plane or axis of symmetry. NONE are pentamerally symmetrical.
• Paleodrama: Richard Jeffries of the British Museum has interpreted these as ancestral to the vertebrates (making vertebrates a clade within Echinodermata), though most dispute this suggestion. He views the aulacophore as a chordate-like tail, and claims to see pharyngeal slits and a notochord. Objections to his scheme include:
  • Morphological interpretation is very far-fetched.
  • Even if we accept his morphology at face value, his preferred phylogeny of deuterostomes is far from the most parsimonious tree.
• A recent analysis of the aulacophore of a stylophoran suggests that it is a locomotor appendage. Indeed, the most recent work doesn't reject gill slits outright.

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

• The crinoid body plan:
  • A stem consisting of life-saver shaped columnals from which small tentacles - cirri - branch. The stem terminates at a holdfast. Despite this, the crinoid is capable of moving around when it wants.
  • the main body is contained in a cup-shaped calyx.
  • At its "lip," the cup branches into five arms, which often subdivide. Each arm bears an ambulacrum on its upper side and an outpouching of the proper coelom internally. These are lined with calcareous pinnules that assist in filtration and tube-feet that snag food particles and convey them to the ambulacral groove.
  • The top is the oral surface or tegmen. The mouth is in the center with the anus to one side, as in edrioasteroids and blastoids.
  • By convention, the aboral surface is toward the stem.

• 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.

  Calyx plate terminology:

  
  • Minimally, the calyx bears two rows of plates:
    • Radials, which occur directly below the base of the arms
    • Basals, which are below the radials and alternating with them.
  • Sometimes, infrabasals may be below and alternating with the basals. Crinoids with no infrabasals are termed monocyclic, those with infrabsalas are dicyclic.
  • Sometimes, the basal plates of the arms become incorporated inot the calyx as brachial plates. These are directly above the radials.
  • Spaces between brachials may have interbrachial plates.
  • There may be plates on the tegmen covering the mouth and anus. In some cases, the anus is armored with an anal pyramid to guard against parasitism by gastropods who seemed to view crinoid butts as sort of a cornu-copiae.

  Crinoid taxonomy:

  • Inadunates: (Ord. - Tri.) Paraphyletic: Arms start directly above the radials. No brachials or interbrachials.
  • Flexibles: (Ord. - Perm.) The calycal plates are only loosely sutured. Usually have brachials , three infrabasals, a tegmen with an exposed mouth and food grooves. Pinnules absent.
  • Camerates: (Ord. - Perm.) Rigid calycal plates. Covered food groove and mouth. Have fixed brachials, interbrachials.
  • Articulates: (Tri. - Rec.) Flexible articulated arms. Upper part of calyx is reduced. Five infrabasals. This groups includes stalked forms and the stalkless Comatulida - "feather stars."

  Ecology:

  
  • Paleozoic (especially Mississippian) crinoids were common in shallow water, favoring backreef facies and the landward sides of barrier islands - environments with well circulated water but not too much energy.
  • Permian extinction hurt. In post-Paleozoic world, surviving crinoids are rare. Modern crinoids are restricted to deep oceans - depths >100 m, with one exception.
  • Comatulids are the most diverse living group, and are common in reefs. The walk around on their cirri or swim slowly to find a good spot for suspension feeding.
  • The general impression: The post-Paleozoic has been a hostile time. Crinoids have either retreated to realms where predators are rarer (deep oceans) or evolved greater mobility to avoid them.
  • Note, however, Mesozoic Pentacrinitidae. Large crinoids with long stems attached to flotsam. Thus invading pelagic planktonic realm.
  • There are more radical suggestions of an unexpected crinoid radiation (amazed scientists exclaim!!!)

  The final group, Eleuthrozoa will require their own lecture.

Echinoderm origins: Who are the closest relatives or ancestors of echinoderms? Hard to say for fossils that lack echinoderm like calcitic skeletal elements. Some possibilities:

• Echmatocrinus: The so-called "Burgess-shale crinoid." Possibly not even a metazoan.
• Dinomyschus: Another Burgess-shale weirdo. Stalked with a calyx, tentacles of some sort, and a crinoid-like oral surface with mouth and anus.
• Tribrachidium: Ediacaran disk shaped critter displaying a triradiate pattern like we might expect from ambulacra in a very primitive form (indeed, "erase" one of its six "ambulacra" and the result looks a lot like an edrio.
• Arkarua: Another Ediacaran disk, hyped as an echinoderm, but probably an alga.

To Syllabus.

Last modified: 22 August 2008