Mimospirida

Taxa on This Page

1. Cliospiridae
2. Mimospirida
3. Onychochilidae
4. Yangtzespiriidae

Representative members of the family Onychochilidae

1. Kobayashiella circe (Walcott 1905) Furongian (= Dolgellian?) China, height about 2 mm;
2. Sinistracirsa altera (Perna 1903), Early Devonian, Czech Republic (Armorica) height about 3.5 cm;
3. Matherella saratogensis (Miller 1889), N.Y. (Laurentia), Furongian (Late Dolgellian) height about 1 cm;
4 Onychochilus physa (Cossman 1915) Middle Silurian (Wenlock) Gotland (Baltica), height about 4 mm;
5 Matherellina walcotti (Kobayashi 1933), Early Ordovician, China, height about 1 cm;
6 Scaevogyra swezeyi (Miller 1889), Furongian (Late Dolgellian) Wisconsin (Laurentia), height about 2.5 cm

Introduction

The Mimospirida (previously Mimospirina) are a group of extinct, superficially snail-like, Paleozoic mollusks with a moderately high-spired hyperstrophic / sinistrally coiled shell. Although conventionally included under the Gastropoda (Knight et al., 1960) it was suggested by Linsley & Kier (1984) that they and a number of related forms are not gastropods, but can be considered as part of a new class, the Paragastropoda. However, the status of the Paragastropoda remains controversial, and at least one recent classification considers the Mimospirida to be Gastropoda incertæ sedis. I have assumed for the present that they are untorted mollusks and represent an early side branch of the post-Helcionelloida and proto-gastropod stem.

Ecology, Relationships, and Evolutionary History

The Mimospirids seem to have evolved from the Pelagiellids. The relationship is indicated not only by the hyperstrophic and other shell features but more specifically by the fact that both onychochilids and pelagiellids are known to share a unique protoconch that is unlike that of any gastropod protoconch. Wagner (2001: 1130). Evolution seems to have occurred away from areas of regular fossilization, as there seems to be a stratigraphic gap of some ten million years or so between the last Pelagiellid (middle Cambrian) and the earliest unambiguous Mimospirid (Trempealeauian = late Dolgellian = Latest Cambrian) . Whereas the pelagiellids were limited to the early and middle Cambrian, the mimospirids continued through to the middle Palaeozoic.

The earliest and least specialized of the Mimospirida were the Onychochilidae, which first appear in the Furongian, and according to Linsley Kier 1984: 248) are inferred from their shell to be mobile browsers.

It seems that these forms began with low spired forms like "Maclurites" thomsoni, Scaevogyra swezeyi, and Kobayashiella circe, which then gave rise to higher spired genera like Matherella and Matherellina (possibly these two genera are synonyms) Wagner, 1999) and their descendents.

The Onychochilids persisted into the Devonian, a late representative being the high spired Sinistracirsa altera. But earlier, during the early Ordovician perhaps, they gave rise to the specialized Cliospiridae, which range from the Ordovician to the Devonian and became sedentary forms, probably filter feeders. In these high spired forms the high center of gravity means that the spire rests of the substrate, holding the animal in place.

According to Linsley & Kier (1984) the Onychochilids also evolved into another sedentary hyperstrophic group, the Macluritidae (both Onychochilidae and Macluritidae are included in the same superfamily - the Macluritoidae - by Knight et al. (1960), but cladistic analysis by Wagner 1999) interprets the Macluritids as true gastropods that developed from Euomphalids. Clearly there was a lot of parallelism of shell morphology among early gastropod and paragastropod groups, which makes determining relationships difficult.

As with a number of Tergomyan groups, the Mimospirida died out without leaving any descendents during the later Palaeozoic. The reason for this is not clear, as both the Onychochilidae and Cliospiridae had survived quite happily for well over a hundred million years.

Development

The Mimospirida had a distinct larval metamorphosis as these mollusks developed a large, smooth, sinistrally coiled (probably non-planktotrophic) larval shell consisting of several whorls and distinct from the teleoconch. Frýda (2001).

Systematics

Under the Order Mimospirida is included the single superfamily Onychochiloidea with the following three families:

• Onychochilidae Koken, 1925
• Cliospiridae Miller, 1887
• Yangtzespiriidae Yu, 1984

The order Mimospirida listed by Jeffery 2001(as "Mimospirina") would seem to be the same as the Superfamily Onychochilacea (= Onychochiloidea) Koken, 1925 of Linsley and Kier 1984, except that it includes the more recently discovered family Yangtzespiriidae.

There is actually some controversy here over whether these actually are paragastropods, and if they are, whether they all belong together. I have followed Linsley and Kier 1984 in assuming they are Paragastropods, Wagner (iNet) in placing them to the Pelagiellida, and Jeffery 2001 in including all three families in the same order, fully aware that future discoveries may overturn this arrangement.

Mimospirida: Untorted hyperstrophic snail-like mollusks

Stratigraphic Range: Furongian (Late Dolgellian) to Early Carboniferous

Characteristics: Paragastropoda with hyperstrophic shells with highly prosocline tangential apertures. Shell form varying from high-spired to moderately low spired.

Taxon: Order Mimospirida

Onychochilidae:

Stratigraphic Range: ?Middle, or Latest Cambrian (Trempealeauian = Late Dolgellian) to Early Carboniferous [Wagner 2001]. According to Curry and Morris 1967, p.426, Matherella and Scaevogyra are known from the Eminence Dolomite, Trempealeauian, of Missouri, and Kobayashiella from the "Upper Cambrian" South Korea

Characteristics: Mimospirida in which the area of the depressed spire is gently rounded into umbilical area with only a gentle angulation to mark exhalant area [Linsley and Kier 1984 p.150]

Taxon: Family Onychochilidae Koken, 1925 Linsley & Kier, 1984).

Genera included: - ?Protoscaevogyra Kobayashi 1939; Matherella Walcott. 1912; Scaevogyra Whitfield 1878; Kobayashiella Endo 1937; Matherellina Kobayashi, 1933; Pervertina Horny, 1964; Invertospira Horny 1964; Helicotis Koken 1925; Laeogyra Perner 1903; Onychochilus Lindström 1884; ?Sinistracirsa Cosamaenn 1908; Hyperstrophema Horny 1864; Versispira Perner 1903; Antispira Pemer 1903

Notes: genus list from Linsley & Kier (1984), who include Scaevogyra in the Macluritidae. Knight et al. (1960) and Wagner 1999 & iNet) both include Scaevogyra under the Onychochilidae, where I have placed this genus here. "Maclurites" thomsoni is also included in this clade by Wagner.

Cliospiridae:

Stratigraphic Range: Ordovician to Devonian

Characteristics: Mimospirida in which the area of the depressed spire with a sharp ridge is on the upper whorl face. [Linsley and Kier 1984 p.150].

Taxon: Family Cliospiridae Miller 1889 Linsley & Kier, 1984; Jeffery, 2001)

Subfamilies included: - Linsley & Kier 1984 divide this family into three subfamilies and I have followed this arrangement here, although it may turn out that these are not natural groups: Cliospirinae, Trochoclisinae, Atracurinae

Mimospira cochleata (Lindstrom, 1884) height about 7mm Late Silurian of Baltica (Sweden) image © xxxx from McLean 1981 from Wängberg-Eriksson, 1979

Cliospirinae:

Stratigraphic Range: Ordovician to Silurian

Characteristics: Low spired clisospirids with sharp ridge surrounding depressed spire at periphery and extended upwards and outwards as a frill Linsley & Kier, 1984: 151)

Taxon: Subfamily Cliospirinae Miller, 1889 (Linsley & Kier, 1984)

Genera included - Cliospira Billings 1865; Ferrogyra Horny 1964.

Trochoclisinae:

Stratigraphic Range: Silurian to Devonian

Characteristics: Medium-spired conical clisospirids. Sharp angulation located at whorl periphery. Angulation may be extended upwards and outwards as a frill. [Linsley and Kier 1984 p.151]

Taxon: Subfamily Trochoclisinae Horny, 1964 (emend. Linsley & Kier, 1984)

Genera included - Conoctisa Horny 1964; Trochoclisa Horny, 1964.

Atracurinae:

Stratigraphic Range: Ordovician to Devonian

Characteristics: Medium-spired conical clisospirids. Sharp angulation located in from the periphery. Linsley & Kier, 1984: 151).

Taxon: Subfamily Atracurinae Horny, 1964 (emend. Linsley & Kier, 1984)

Genera included - Mimospira Koken, 1925; Antigyra Horny 1964; Antizyga Horny, 1964; Atracura Horny, 1964; Bodospira Wängberg-Eriksson, 1979; Angulospira Wängberg-Eriksson, 1979; Tapinogyra Wängberg-Eriksson, 1979; Undospira Wängberg-Eriksson, 1979.

Yangtzespiriidae:

Taxon: Family Yangtzespiriidae Yu, 1984 Jeffery, 2001)

Notes: Apart from the listing in Jeffery 2001) I have no information on the Yangtzespiriidae. One might presume from the name that they include one or several Chinese forms. If anyone has any further information on this group could they please contact me.

note:  In keeping with the convention of the Treatise authors of the volumes on Gastropoda and Cephalopoda, the suffix -ina is used for suborders, and -ida for orders. Since the Mimospirina are here considered of ordinal rank, the suffix -ina has been changed accordingly

"although ICZN does not address ordinal level taxa, the Zoological Sciences Section of the American Association for the Advancement of Science has recommended that zoological orders be given the ending -ida (as has long been done by the Treatise on Invertebrate Paleontology)."

Ken Kinman - Phylocode Discussion