Mollusca		Tergomya
GASTROPODA		Tergomya

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Tergomya

GASTROPODA (stem group)
|--PARAGASTROPODA
`--+--TERGOMYA (paraphyletic)
   |-?Tropidodiscidae  
   |-?Bellerophontida 
   `--Gastropoda (crown group)  
      |--EOGASTROPODA  
      |  |--Macluritoidea
      |  `--PATELLOGASTROPODA
      `--ORTHOGASTROPODA  
         |--NERITOPSINA
         |  |--Neritoida
         |  `--VETIGASTROPODA
         `--+--Apogastropoda
            `--Pulmonata

Introduction

 A word of explanation may be in order.  This unit is distinctly not a clade.  Here we cover a few groups that seem to be very close to the crown group Gastropoda (slugs + limpets).  They may be just outside the crown group, just inside, or some of both.  Like other people interested in this area, we have found that the most detailed information available is the semi-published material on Dr. Peter Wagner's website.  However, Wagner's methodology is obscure, and his cladograms often baffling.  He regularly redefines taxonomic and anatomical terms to suit his needs, and many of the internal links on his site don't work.  Sound familiar?  Absolutely! Were it not for Dr. Wagner's impeccable academic credentials, we would assume that he -- perhaps also like us -- is some sort of amateur fruitcake with a dark  fixation on early mollusks.  But the Field Museum does not hire fruitcakes, amateur or professional; and more's the pity for us.  Therefore we are forced to assume that Dr. Wagner is the James Joyce of molluscan paleontology, and treat his work as a sort of Finnegan's Whelk.   As in Joyce's novels, each critical term on Dr. Wagner's site seems enclouded by a sparkling nimbus of free associations, some of which may be entirely personal.  Thus, the ultimate meaning is variable, depending on which associations one perceives (or imagines), and which of those one chooses to string together as the pertinent subtext.  

But enough.  We begin to sound like one of those sad literary creatures whose hopeless task it is to "teach" sophomores to appreciate Joyce.  It can't be done.  As with Wagner's site, you, personally, wrestle with Joyce; and you usually lose the match, but are vaguely better off for the experience.  We have strung together some traditional taxonomic names in an order which may have something to do with Wagner's phylogeny, or not.  The groups may be monophyletic, or not.  If you need more, try a round or two with the man yourself.  ATW060313.

Furongian to Middle Devonian

Cyrtolites ornatus, late Ordovician, Laurentia (New York). Length about 2 cm
From the shell this animal superficially resembles a gastropod. However, it is untorted and exogastrically coiled, so the spiral faces forward, above the head.

The "Tergomya" are a grade - probably polyphyletic and hence artificial - of primitive, early Paleozoic, cyrtosoma molluscs with limpet-like, conical, or spiral shells. They have previously been (and are often still) included under the Monoplacophora. However, it is now known that "Monoplacophora" refers to a very diverse assemblage of basal molluscs, and is not a natural group. The Tergomya as defined by Peel 1991and others includes the Tryblidiida. I have separated these into two distinct groups, and use the term "Tergomya" to refer to a grade of untorted pre-gastropod molluscs. Cladistic analysis by Dr Peter Wagner indicates they are the sister group of the Paragrastropoda (Onychochilids and Pelagielloids) and related forms. Inverted commas are used when referring to "Tergomya" because they are not a natural group.  

A number of separate "Tergomyan" lineages seem to have evolved in parallel during the Furongian and early Ordovician, representing successive lineages of "gastropodization". This can be compared to the evolution of several independent lines of "batrachosaur" amphibians towards the reptilian condition during the Carboniferous, and of therapsid reptiles towards the mammalian state during the Permo-Triassic. As indicated by Dr Wagner's cladogram, the Tergomya - like the batrachosaurs (anthracosaurs) and therapsids - are a paraphyletic assemblage, rather than a monophyletic clade.

The "Tergomya" reached their greatest diversity in the Middle Ordovician or thereabouts, and continued through to the Middle Devonian.  

"Tergomyans" generally did not exceed several cm in length. The shell was quite variable, ranging from simple cap-shaped limpet-like form of the Kiringellida, to high conical Hypseloconida, to the planispiral shell of the Cyrtonellida. However, in all these forms, as with Tryblidiida, the shell differs from that of true limpets in that it is exogastric and curves forward (toward or over the head); limpets and other gastropods have an endogastric shell that curves backwards (away from the head). This is because, unlike gastropods, "Tergomyans" have not undergone torsion. Some Cyrtonellida, like the Tryblidiida, may have had multiple muscle scars, although this may have been a result of ontogenic development as a single muscle pair attached to different parts of the shell. But most types show only a single ring-like muscle scar.  

The following Technical Diagnosis is from Peel 1991 and refers to the "Tergomya" in general, which is a grade of early, mostly extrogastric molluscs that include both the Tryblidiida and a number of pre-gastropod Tergomyan lineages

Generally bilaterally symmetrical molluscs in which the calcareous shall is usually planispirally coiled through about half a whorl. The shell is often cap-shaped or spoon-shaped usually low, with an anterior apex which may vary from subcentral to overhanging the anterior margin. The aperture is generally planar but may be shallowly arched in lateral view. Muscle scars on the shell interior are grouped into a ring on the dorsal, supra-apical surface, the apex lies outside of the ring. Ornamentation consists of radial and/or comarginal elements; prominent rugae are only infrequently developed. 

Cladogram

<== o "Tergomyans"
    |-- †Kiringella pyramidalis
    |--o Hypseloconoidea
    |  |-- †Hypseloconus elongatus
    |  `-- †Knightoconus antarcticus
    `--+-- †Cyrtolites sp.
       `-+-- †Chippeaella patellitheca
         `--o †Pretorted Bellerophontina
            `-- GASTROPODA

References: Wagner [no date] Outgroups

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