BSCI392
11-02-07
Calcichordates or Homalozoans?

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We shift to the last major bilaterian group - Deuterostomia.


Deuterostomia

An odd group of disparate taxa, yet strongly supported by morphological, developmental, and molecular synapomorphies. Includes vertebrates and echinoderms plus minor groups.

The major Deuterostome groups are Echinodermata, Chordata, and Hemichordata. The focus of this presentation is on a particularly enigmatic group of early echinoderms, but first, some preliminaries.

Hemichordata: Including acorn worms and pterobranchs (Remember graptilites? Ancient pterobranchs). Suspension feeders, however food capture is by means of ciliary action.

Ten years ago, the conventional wisdom was that the pharynx and gill slits were synapomorphies of hemichordates and chordates. Since then, several molecular analyses have strongly supported a sister-taxon relationship of hemichordates and echinoderms.

Potential morphological synapomorphies of hemichordates and echinoderms:

But now, Echinoderms....

Major groups

Crinoidea (sea lilies and feather stars) (Ordovician - Recent)
  • Stalked or stalkless suspension feeders of reefs and deep oceans.
  • Formerly very common, especially during Mississippian and Pennsylvanian, but sharply pruned by Permo-Triassic extinction event.


From Norbert Wu's Diving Under Antarctic Ice

Ophiuroidea (brittle stars) - (Ordovician Recent): predators on small things, scavengers, and suspension feeders.

Want to see a live one on campus? Go to the south elevator lobby of Cumberland Hall and look in their aquarium. It's usually hiding in the pile of rocks on the right side.


From Yoichi Kogure's Web Page

Asteroidea (starfish or sea stars) - (Ordovician Recent): Powerful predators on anything they can "outrun."


From Merck's Nature Photos

Echinoidea (sea urchins, heart urchins, sand dollars) (Ordovician Recent): Grazers and deposit feeders.


From Merck's Nature Photos

Holothuroidea (sea cucumbers) (Ordovician Recent): Suspension and deposit feeders. Very common in deep oceans.


From Merck's Nature Photos

Echinoderm characteristics:

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


Deuterostomia

Helicoplacoidea (Early Cam.)

Edrioasteroid: (Cam. - Penn)

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.

"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:
  • Cornuta
  • Mitrata

From Palaeos

Stylophora (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.

From Palaeos

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.

From Problems in Evolution

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.

From Palaeos

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:

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:

When you add "calcichordates" to the mix, the basic pattern of deuterostome phylogeny seems completely up for grabs.

Today's echinoderm systematists now seek to reconcile the "Calcichordate hypothesis" with the "homalozoan hypothesis" that the homalozoans are basal echinoderms with nothing particularly to do with chordates. The emerging consensus:

Adding color to this consensus are recent discover of creatures thought to be near the base of Deuterostomia from the Chenjiang and Burgess Shale faunas:

Sort of resembles what we might expect of a non-calcified stylophoran ancestor in which the thecal openings really are phayrngeal slits and the appendage really is a tail.