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, the anispiracle.
- 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.
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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.
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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.
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Crinoidea (sea lilies and feather stars) (Ord. - Rec.): Finally, a group with living representatives.
- The crinoid body plan:
- A stem consisting of poker-chip 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.
- Crinoids don't move much if they have found a good place to feed, but may crawl or swim to good feeding cites on a daily basis.
- 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.
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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.
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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."
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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!!!)
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Homolazoa: (Cam. Ord.)
Echino-uglies. A problematic group of Early Paleozoic echinoderms. True apples of discord. Mono-, poly-, or paraphyletic, depending on whom you ask.
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.
"Includes":
Stylophora (Cambrian - Devonian). With:
- flattened theca
- elongate three-part "tail"
- large openings to the left of the "tail" and at the opposite side from it.
- Numerous small pores on dorsal side.
At most the creatures only approach being bilaterally symmetrical, and often there is nothing like an obvious plane of symmetry. Comprised of two groups:
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Soluta (Cambrian - Devonian). Similar to Stylophora but with:
- an smaller appendage, the aulacophore opposite the "tail," which, to avoid ambiguity, we call the stele.
- elongate "tail"
- large opening near the stele.
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Cincta (Middle Cambrian). Again, similar but with:
- Bilaterally symmetrical ambulacra and mouth on dorsal surface opposite the stele.
- Stele is short and stiff.
- Some indications that the stele may have ended in a holdfast.
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Ctenocystoidea (Ordovician). Strange. No stele or aulacophore. Only an approximation of bilateral symmetry, BUT...
- Anterior and posterior openings.
- Two bilaterally symmetrical ambulacra
- Body opening that could be the hydropore.
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Note that I have only described features of these critters and named a few. No homologies with other organisms have been proposed. This is where the trouble starts. Consider the solute aulacophore. It could be:
- an ambulacrum bearing feeding structure like the arm of a crinoid.
- a tail, used for propulsion, homologous to the chordate tail.
- homologous to the "tail" of stylophorans.
Similar things could be said about any of the openings of the theca, which could be mouths, anuses, pharyngeal slits, hypropores, etc. Into this chasm of ignorance steps the human imagination.
During the 1980s, Richard Jeffries of the British Museum interpreted the various homalozoans as ancestral to the vertebrates (making vertebrates a clade within Echinodermata). This hypothesis (sort of) rests on his convictions about the homologies of the structures. Consider the stylophoran appendage.
Jeffries emphatically views it as a chordate-like tail, with room inside ofr a notochord and myotomes. He also claims to see pharyngeal slits in the openings of the theca. In some ways, these appendages function differently:
For instance, the "tail" is used to pull the theca along over the substrate.
His conclusion: Chordates are derived from these primitive echinoderms. Indeed, in his scheme, specific homalozoans gave rise to specific chordate groups. To emphasize the propinquity of the relationship, he coined the term Calcichordate
. This hypothesis of "calcichordate" phylogeny was developed in the early days of cladistics, and Jeffries does not seem to have used a parsimony analysis.
Objections to his scheme include:
- Morphological interpretation seems far-fetched.
- The transition from them to chordates involves:
- Reorganization of body for 180 deg. change of direction of movement.
- Switch from calcium carbonate to calcium phosphate skeleton.
- Even if we accept his morphology at face value, his preferred phylogeny of deuterostomes is far from the most parsimonious tree.
- Jeffries, himself, seems to approach the issue with the style of a true-believer.
When you add "calcichordates" to the mix, the basic pattern of deuterostome phylogeny seems completely up for grabs.
Today's echinoderm systematists agree that Jeffries' phylogeny is wrong. But are all over the map otherwise. Some maintain that his assessment of homology may, in part, be right. recent analysis of the stylophoran appendage suggests that it is, indeed, a locomotor appendage. Others, such as David et al., maintain that the stylophoran appendage is an ambulacrum on a crinoid-like arm, pure and simple, and that Ctenocystoidea have blastozoan-like brachioles. To them, Homalozoa is polyphyletic and its members belong to better known groups.
Stay tuned.
