Ambulacraria

From Palaeos

Bilateria
`--Deuterostomia
   |--Vetulicolia
   |--Yunnanozoa
   |--+--Xenoturbella
   |  `--Ambulacraria
   |     |?--Dinomischida
   |     |--Hemichordata
   |     `--+--Vetulocystidae
   |        `--+--Stylophora
   |           `--+--Soluta
   |              `--+--Cincta
   |                 `--+--Ctenocystoidea
   |                    `--+--Helicoplacoidea
   |                       `--+--Edrioasteroidea
   |                          `--Echinodermata
   `--Chordata
      |--Urochordata
      `--+--Cephalochordata
         `--Craniata
            `--Vertebrata

[edit] The Echinoderm Stem: A World of Pure Imagination

Here at Palaeos, we point out right from the start that we are not authorities in the subjects covered, and anyone reading this site should approach its statements with all the caution of a serious burn victim re-potting cacti. Sometimes, though, a particularly abstruse topic is broached that requires us to remind you of this rule. This is one of those times. The echinoderm stem is a repository for what are patently some of the most bizarre organisms ever to clutter the sea-bed. If, indeed, they are stem echinoderms at all. They may be derived crown echinoderms, or stem chordates, or organisms with no real connection to the echinoderms at all. Interpretations of these fossils are so varied and so divergent that one could be forgiven for failing to realize that they referred to the same animals. In the case of the Stylophora, researchers can't even agree as to which end is anterior and which is posterior.

As a result, this section is a pastiche. With so many competing theories available, I have picked and chosen as seems appropriate (or as my whim takes me), and in the process I have quite likely constructed something which no researcher has supported, and to which no-one who has actually examined the fossils in question would give the time of day. My advice is to stop reading. Go straight to the bibliography, and get your information direct from the primary literature. You'll be better off.

For those who have chosen to ignore my advice, a few notes will be given on terminology. In light of the highly debatable nature of the echinoderm stem, I have decided to restrict the name “Echinodermata” to the crown group only, allowing me to be more explicit about the origins of characters leading towards the modern body plan. This differs from the definition (often not explicitly stated) used in many publications.

Probably the majority (including the Treatise on Invertebrate Paleontology) identify Echinodermata by the origin of the stereom, the calcite skeleton with an ultrastructure apparently unique to this clade. However, Jefferies (e.g. 1986) and his supporters, who place the Stylophora as the stem group of chordates, would see the stereom as plesiomorphic for a much larger clade. If defined by the stereom, Echinodermata would then include Chordata.

Another alternative is defining Echinodermata by the origin of pentaradial symmetry. However, this character is often widely altered in both modern and fossil groups of echinoderms. Also, the 2-1-2 system of ambulacra in edrioasteroids demonstrates that pentamery and pentaradial symmetry did not necessarily arise simultaneously.

[edit] The Monophyly of Ambulacraria

As an initial matter, the echinoderms and chordate phyla have been arranged in almost every possible phylogenetic order. Classical taxonomy was of no help, since we lacked enough data on basal deuterostomes to make even educated guesses about interrelationships. It was like trying to do a connect-the-dots picture with most of the dots missing and half the others in the wrong place. This confusion was succeeded by a large number of molecular studies reported by folks who had a whole lot of dots, but were a little unclear on how to connect them. Finally -- really within the last five years -- both new fossils and great improvements in molecular techniques have allowed us to put the big pieces together with confidence. The overall picture looks like this:

Deuterostomia
|--Ambulacraria
|  |--Hemichordata
|  `--Echinodermata
`--Chordata
   |--Urochordata
   `--+--Cephalochordata
      `--Vertebrata

We are particularly encouraged by studies like Furlong & Holland (2002) who re-analyzed prior sequence work with sophisticated Bayesian techniques, and obtained the same result with a great variety of individual sequences. See also Winchell et al. (2002) (same results, concatenating different sequences). Sequence phylogenies are still hit-and-miss; but the new fossils make it much easier to assess which have hit and which have missed.

Credits

The Echinoderm Stem CKT050827; The Monophyly of Ambulacraria ATW051025