We shift to the last major bilaterian group - 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.
Potential morphological synapomorphies of hemichordates and echinoderms:
But now, Echinoderms....
Crinoidea (sea lilies and feather stars) (Ordovician - Recent)
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.
Asteroidea (starfish or sea stars) - (Ordovician Recent): Powerful predators on anything they can "outrun."
Echinoidea (sea urchins, heart urchins, sand dollars) (Ordovician Recent): Grazers and deposit feeders.
Holothuroidea (sea cucumbers) (Ordovician Recent): Suspension and deposit feeders. Very common in deep oceans.
Traditionally all stemmed echinoderms were lumped into "Pelmatozoa", and all non-stemmed to "Eleutherozoa". A more cladistic phylogeny of Echinodermata:
Helicoplacoidea (Early Cam.)
Edrioasteroid: (Cam. - Penn)
Homolazoa: (Cam. Ord.)
Stylophora (Cambrian - Devonian). With:
Stylophora (Cambrian - Devonian). Similar to Stylophora but with:
Ctenocystoidea (Ordovician). Strange. No stele or aulacophore. Only an approximation of bilateral symmetry, BUT...
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 emerginc consensus: