Helcionelliformes

Reconstruction of Yochelcionella cyrano, by Stanton Fink.

Belongs within: Mollusca.
Contains: Trenellidae, Scenellidae, Coreospiridae.

The Helcionelliformes are a group of univalved mollusks known from the early Cambrian. Their affinities have been subject to debate: some authors interpret them as the earliest members of the Gastropoda whereas others regard them as perhaps more basal among the mollusks and/or related to other living molluscan clades. Parkhaev (2002) regarded the turbospiral Pelagielliformes of the early to middle Cambrian as derived from helcionelliform ancestors.

The Helcionelliformes proper can be divided between the Yochelcionelloidea, in which the shell has the parietal train modified into a siphonal groove or tube, and the Helcionelloidea which lack such a feature (Parkhaev 2002).

Characters (from Parkhaev 2002): Shell symmetrical, cap-shaped, planispiral or close to planispiral with few coils. Forms with planispiral shell have elongated aperture, with the length larger, equal or slightly smaller than the shell diameter. Palial caecum probably absent.

<==Helcionelliformes
    |--Yochelcionelloidea [Mellopegmidae] P02
    |    |--Trenellidae P02
    |    |--Stenothecidae P02
    |    |    |--Watsonellinae P02
    |    |    |    |--Eurekapegma MacKinnon 1985 P02
    |    |    |    `--Watsonella Grabau 1900 P02
    |    |    |         `--W. crosbyi SB09
    |    |    `--Stenothecinae P02
    |    |         |--Mellopegma Runnegar & Jell 1976 P02
    |    |         |--Anabarella Vostokova 1962 P02
    |    |         |    `--A. plana KB09
    |    |         `--Stenotheca Salter in Hicks 1872 P02
    |    |              |--*S. cornucopia Hicks 1872 F62
    |    |              `--S. rugosa T10
    |    `--Yochelcionellidae P02
    |         |--Enigmaconus MacKinnon 1985 [Enigmaconidae] P02
    |         |    `--*E. parvus MacKinnon 1985 Y08
    |         |--Runnegarella Parkhaev 2002 P02
    |         |    `--*R. americana (Runnegar & Pojeta 1980) [=Yochelcionella americana] P02
    |         |--Eotebenna Runnegar & Jell 1976 P02
    |         |    |--E. arctica Peel 1989 P04
    |         |    `--+--E. papilio P04
    |         |       `--+--E. pontifex P04
    |         |          `--E. viviannae Peel 1991 P04
    |         `--Yochelcionella Runnegar & Pojeta 1974 P04
    |              |--Y. cyrano Runnegar & Pojeta 1974 PR79
    |              |--Y. daleki Runnegar & Jell 1976 PR79
    |              |--Y. gracile Atkins & Peel 2004 P04
    |              |--Y. greenlandica Atkins & Peel 2004 P04
    |              `--Y. ostentata Runnegar & Jell 1976 P04
    `--Helcionelloidea [Helcionellacea] P02
         |--Scenellidae P02
         |--Igarkiellidae P02
         |    |--Igarkiella Vassiljeva 1998 [incl. Trilobella Vassiljeva 1990 nec Woodward 1924 nec Ivanova 1955] P02
         |    |--Protoconus Yu 1979 [incl. Asperoconus Yu 1979] P02
         |    |--Rozanoviella Missarzhevsky 1981 P02
         |    |--Gonamella Valkov & Karlova 1984 [incl. Patellella Vassiljeva 1990] P02
         |    |--Mastakhella Missarzhevsky 1989 P02
         |    `--Squamiconus Vassiljeva 1990 P02
         `--+--Coreospiridae P02
            `--Pelagielliformes [Orthostrophina, Pelagiellacea, Pelagiellida, Pelagielloidea] P02
                 |--Aldanellidae BR05
                 |    |--Paraaldanella Golubev 1976 P02
                 |    |--Nomgoliella Missarzhevsky 1981 P02
                 |    |    |--N. australiensis P02
                 |    |    |--N. rotunda P02
                 |    |    `--N. sinistrovolubilis Missarzhevsky 1981 P02
                 |    `--Aldanella Vostokova 1962 BR05
                 |         |--A. crassa Missarzhevsky 1969 P02
                 |         `--+--A. utchurica Missarzhevsky 1969 P02
                 |            `--+--A. rozanovi Missarzhevsky 1966 P02
                 |               `--A. operosa Missarzhevsky 1966 P02
                 `--Pelagiellidae BR05
                      |--Costipelagiella Horny 1964 P02
                      |--Tianzhushanospira Yu 1979 P02
                      |--Proeccyliopterus Kobayashi 1939 [Proeccyliopteridae] BR05
                      |--Protoscaevogyra Kobayashi 1939 [Protoscaevogyridae] BR05
                      |--Tannuspira Missarzhevsky 1989 P02
                      |    `--T. magnifica Missarzhevsky 1989 P02
                      `--Pelagiella Matthew 1895 BR05 (see below for synonymy)
                           |--*P. atlantoides (Matthew 1894) [=Cyrtolites atlantoides] KC60
                           |--P. adunca Missarzhevsky 1966 P02
                           |--P. lorenzi Kobayashi 1939 P02
                           `--P. madianensis Zhou & Xiao 1984 P02

Pelagiella Matthew 1895 BR05 [incl. Parapelagiella Kobayashi 1939 KC60, Semicircularia Lochman in Lochman & Duncan 1944 KC60]

*Type species of generic name indicated

REFERENCES

[BR05] Bouchet, P., & J.-P. Rocroi. 2005. Classification and nomenclator of gastropod families. Malacologia 47 (1–2): 1–397.

[F62] Fisher, D. W. 1962. Small conoidal shells of uncertain affinities. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W98–W143. Geological Society of America, and University of Kansas Press.

[KC60] Knight, J. B., L. R. Cox, A. M. Keen, R. L. Batten, E. L. Yochelson & R. Robertson. 1960. Gastropoda: systematic descriptions. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt I. Mollusca 1: Mollusca—General Features, Scaphopoda, Amphineura, Monoplacophora, Gastropoda—General Features, Archaeogastropoda and some (mainly Paleozoic) Caenogastropoda and Opisthobranchia pp. I169–I331. Geological Society of America, and University of Kansas Press.

[KB09] Kouchinsky, A., S. Bengtson, W. Feng, R. Kutygin & A. Val’kov. 2009. The Lower Cambrian fossil anabaritids: affinities, occurrences and systematics. Journal of Systematic Palaeontology 7 (3): 241–298.

[P02] Parkhaev, P. Yu. 2002. Phylogenesis and the system of the Cambrian univalved mollusks. Paleontologicheskii Zhurnal 2002 (1): 27–39 (transl. Paleontological Journal 36 (1): 25–36).

[P04] Peel, J. S. 2004. Pinnocaris and the origin of scaphopods. Acta Palaeontologica Polonica 49 (4): 543–550.

[PR79] Pojeta, J., Jr., & B. Runnegar. 1979. Rhytiodentalium kentuckyensis, a new genus and new species of Ordovician scaphopod, and the early history of scaphopod mollusks. Journal of Paleontology 53 (3): 530–541.

[SB09] Skovsted, C. B., U. Balthasar, G. A. Brock & J. R. Paterson. 2009. The tommotiid Camenella reticulosa from the early Cambrian of South Australia: morphology, scleritome reconstruction, and phylogeny. Acta Palaeontologica Polonica 54 (3): 525–540.

[T10] Taylor, T. G. 1910. The Archaeocyathinae from the Cambrian of South Australia with an account of the morphology and affinities of the whole class. Memoirs of the Royal Society of South Australia 2 (2): 55–188.

[Y08] Yu W. 2008. On the genus Yangtzemerisma and related genera (Mollusca: Merismoconchia). Records of the Western Australian Museum 24 (2): 181–194.