Homalozoa

From Palaeos

The Homalozoa, as that term is usually used, are plainly not a clade. Therefore, we will use the term simply to designate a grade of deuterostomes more derived than the Vetulocystidae, but outside the echinoderm crown group. In doing so, we will probably include groups which probably would not be called homalozoans in ordinary parlance -- assuming that one ordinarily cared to discuss homalozoans at all. A more conventional use of the word defines Homalozoa simply as a grade of physical organization, and composed for taxonomic purposes of the Stylophora, Soluta, Cincta and Ctenocystoidea. The four classes have little to unite them other than possession of a flattened theca and the absence of pentaradial symmetry. But, whatever, the definition, Homalozoa is not a terribly useful concept, since the arrangement of this area of phylospace is quite unstable at the moment and it is impossible to say anything of interest about the difference between homalozoans and echinoderms. We simply have too little information about which is which.

One historical point is worth making. Jefferies, e.g. Jefferies (1986), argued long and loudly that some Homalozoa (the Mitrata, in particular) were ancestral to the Chordata. In fact, he continues to do so. See, e.g., Dominguez et al. (2003). The group putatively related to Chordata he called "Calcichordata", the thought being that the calcium carbonate plates which characterize the homalozoans had been exchanged for apatite in chordates. While Jefferies' phylogenetic hypothesis is no longer a serious contender, many of his anatomical observations are very much in play. In any case, it is common to see the term "Calcichordata" used as a synonym for Homalozoa.

Phylogeny

The tree below is derived from a combination of Shu et al. (2004) and Mooi (2001). These two papers differ significantly in their interpretation of homalozoans. Shu et al. (2004) place them on the echinoderm stem, as we have done. Mooi (2001) favours regarding homalozoans as derived echinoderms, within the echinoderm crown group. The seemingly primitive features of homalozoans, such as lack of pentaradial symmetry, would then represent secondary reversals.

<==Homalozoa [Carpoidea]
   |--+--Ctenocystoidea
   |  |    |--Ctenocystidae
   |  |    |    |--Ctenocystis Robison & Sprinkle 1969
   |  |    |    |    |--*C. utahensis Robison & Sprinkle 1969
   |  |    |    |    |--C. colodon Ubaghs & Robison 1988
   |  |    |    |    |--C. jagoi Jell et al. 1985
   |  |    |    |    `--C. smithi Ubaghs 1987
   |  |    |    `--Etoctenocystis Fatka & Kordule 1985
   |  |    |         `--E. bohemica Fatka & Kordule 1985
   |  |    `--Jugoszovia Dzik & Orłowski 1995 [Jugoszoviidae]
   |  |         `--J. archaeocyathoides Dzik & Orłowski 1995
   |  `--+--Cincta [Homostelea]
   |     |    |--Gyrocystis Jaekel 1918 [Gyrocystidae]
   |     |    |    `--*G. platessa Jaekel 1918
   |     |    `--Trochocystidae
   |     |         |--Trochocystoides Jaekel 1918
   |     |         |    `--*T. parvus Jaekel 1918
   |     |         `--Trochocystites
   |     |              |--T. bohemicus Barrande 1887
   |     |              `--T. longifossatus Jaekel 1918
   |     `--+--Echinodermata
   |        `--Soluta [Homoiostelea]
   |             |  i. s.: Rhipidocystis [Rhipidocystidae]
   |             |--Coleicarpus Daley 1996
   |             |    `--C. sprinklei (Ubaghs & Robison 1988) [=Castericystis sprinklei]
   |             `--+--Castericystis vali Ubaghs & Robison 1985
   |                `--+--Minervaecystis
   |                   `--+--+--Girvanicystis
   |                      |  `--+--Maennilia estonica Rozhnov & Jefferies 1996
   |                      |     `--Heckericystis
   |                      `--Dendrocystidae
   |                           |--Dendrocystites
   |                           `--+--Dendrocystoides Jaekel 1918
   |                              |    `--*D. scoticus [=Dendrocystis scoticus]
   |                              `--+--Rutroclypeus
   |                                 `--+--+--Iowacystis
   |                                    |  `--Scalenocystites
   |                                    `--+--Syringocrinus
   |                                       `--+--Belemnocystites
   |                                          `--Myeinocystites
   `--Stylophora [Cornuta]
        |--Ceratocystis Jaekel 1900 [Ceratocystidae]
        `--+--+--Chauvelicystis
           |  `--Prochauvelicystis
           `--+--+--Ankyroida
              |  `--Phyllocystis
              |       |--P. blacyi
              |       `--P. crassmarginata
              `--+--+--Nevadacystis
                 |  `--Galliaecystis
                 `--+--+--Bohemiacystis
                    |  `--Scotiacystis
                    `--+--Procothurnocystis
                       `--Cothurnocystis Bather 1913 [Cothurnocystidae]
                            |--C. bifida Ubaghs & Robison 1988
                            `--C. elizae

* Type species of generic name indicated

References

Domínguez-Alonso, P. 1999. The early evolution of echinoderms: the class Ctenocystoidea and its closest relatives revisited. In Echinoderm Research 1998 (M. D. Candia Carnevali & F. Bonasoro, eds.) pp. 263-268. A. A. Balkema: Rotterdam.

Jaekel, O. 1918. Phylogenie und System der Pelmatozoen. Paläontologische Zeitschrift 3: 1-128.

Jefferies, R. P. S. 1997. How chordates and echinoderms separated from each other and the problem of dorso-ventral inversion. Paleontological Society Papers 3: 249-266.

Mooi, R. 2001. Not all written in stone: Interdisciplinary syntheses in echinoderm paleontology. Canadian Journal of Zoology 79: 1209-1231.

Parsley, R. L. 1997. The echinoderm classes Stylophora and Homoiostelea: Non Calcichordata. Paleontological Society Papers 3: 225-248.

Parsley, R. L. 1999. The Cincta (Homostelea) as blastozoans. In Echinoderm Research 1998 (M. D. Candia Carnevali & F. Bonasoro, eds.) pp. 369-375. A. A. Balkema: Rotterdam.

Shu, D.-G., S. Conway Morris, J. Han, Z.-F. Zhang & J.-N. Liu. 2004. Ancestral echinoderms from the Chengjiang deposits of China. Nature 430: 422-428.

Sprinkle, J., & R. A. Robison. 1978. Addendum to subphylum Homalozoa: Ctenocystoids. In Treatise on Invertebrate Paleontology pt. T. Echinodermata 2. Crinoidea (R. C. Moore & C. Teichert, eds.) vol. 3 pp. T998-T1002. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

Credits

intro CKT050827, ATW050907; phylogeny CKT080203