Scalidophora

From Palaeos

Protostomia
`--Ecdysozoa
   |--Scalidophora
   |?--Protoconodonta†
   |   `--Chaetognatha
   |--+--Nematomorpha
   |  `--Nematoda
   `--o--Lobopoda 
      `--+--Tardigrada
         `--+--Anomalocarida
            `--Arthropoda

Pliciloricus enigmatus Higgins & Kristensen, 1986 (Loricifera,Nanaloricida,Pliciloricidae). Actual size is about one-quarter of a millimeter.

[edit] Introduction

The Scalidophora are a group of marine pseudocoelomates, consisting of the three phyla Kinorhyncha, Priapulida, and Loricifera. The members of the group share a number of characteristics, including introvert larvae and moulting of the cuticle (ecdysis). Kinorhyncha and Loricifera are both minute meiofaunal predators, while Priapulida can reach (for a worm) reasonably large sizes.

[edit] Relationships

Some of the members were originally included in the phylum Aschelminthes, which is now known to be a polyphyletic taxon. They were also formerly considered a single phylum, Cephalorhyncha, with three classes.

The Scalidophora are now considered to be basal Ecdysozoa.

[edit] Fossil Record

The two species in the genus Markuelia, known from fossilized embryos from the middle Cambrian, are thought to be stem Scalidophorans.

Scalidophora have a fairly extensive fossil record for a soft-bodied clade, particularly in the Cambrian. Indeed, scalidophorans seem to have been the major infaunal predators in the Cambrian fauna, only later being replaced by the annelids. While fossil scalidophorans such as Palaeoscolex and Ottoia are often referred to as priapulids, most of them lie outside the priapulid crown group and may not have exactly resembled modern species. The name Priapulida is better retained for the crown group, with Priapozoa used for the wider group including the Cambrian taxa.

Palaeoscolex and its close relatives were covered in an armour of small plates, and disarticulated plates are commonly found as microfossils. The Palaeoscolecida were regarded as probable annelids for many years, until Conway Morris reidentified them as priapulid relatives.

[edit] Phylogeny

<==Scalidophora [Cephalorhyncha, Scalidorhyncha]
   |--Kinorhyncha [Echinoderida]
   |    |  i. s.: Paracentrophyes Higgins 1983
   |    |           |--P. praedictus Higgins 1983
   |    |           `--P. quadridentatus (Zelinka 1928)
   |    |--Homalorhagida
   |    `--Cyclorhagida
   `--Priapozoa
        |  i. s.: Milaculum
        |--Nanaloricus Kristensen 1983 [Loricifera, Nanaloricidae]
        |    `--*N. mysticus Kristensen 1983
        `--+--Priapulida
           `--+--Selkirkia
              |    |--S. columbia
              |    `--S. willoughbyi Conway Morris & Robison 1986
              `--+--+--Maotianshania
                 |  `--+--Cricocosmia
                 |     `--Palaeoscolex
                 `--+--Ottoia prolifica
                    `--+--Louisella pedunculata
                       `--Scolecofurca

* Type species of genus indicated

[edit] References

Briggs, D. E. G., B. S. Lieberman, S. L. Halgedahl & R. D. Jarrard. 2005. A new metazoan from the Middle Cambrian of Utah and the nature of the Vetulicolia. Palaeontology 48 (4): 681-686.

Cavalier-Smith, T. 1998. A revised six-kingdom system of life. Biological Reviews 73: 203-266.

Conway Morris, S. 1998. The Crucible of Creation. Oxford University Press: Oxford.

Dong, X-P., P. C. J. Donoghue, H. Cheng & J.-B. Liu. 2004. Fossil embryos from the Middle and Late Cambrian period of Hunan, south China. Nature 427: 237-240.

Higgins, R. P. 1986. Kinorhyncha. In Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) (L. Botosaneanu, ed.) pp. 110-118. E. J. Brill / Dr. W. Backhuys: Leiden.

Kristensen, R. M. 1986. Loricifera. In Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) (L. Botosaneanu, ed.) pp. 119-121. E. J. Brill / Dr. W. Backhuys: Leiden.

Sutton, M. D., L E. Holmer & L Cherns. 2001. Small problematic phosphatic sclerites from the Ordovician of Iapetus. Journal of Paleontology 75 (1): 1-8.

[edit] Credits