The helcionelloids comprise a little-known but important group of primitive Cambrian conchiferan molluscs with both compressed and spiral shells. Some developed strange "snorkels" through which the animal would breathe, presumably while half-submerged in mud or sand. They include the earliest molluscs that possess a hard shell. Like a number of other early conchiferan stem groups, they have previously been (and are often still) included under the Monoplacophora. However, it is becoming more and more apparent that the "Monoplacophora" are a very diverse assemblage of basal molluscs, and not a natural group. The Helcionelloida were recently named as a distinct class of molluscs by Dr John Peel (1991), who distinguished them from the "Tergomya" (Tergomya + Tryblidiida) by their direction of coiling and other attributes. However, inasmuch as they represent a clearly paraphyletic assemblage (being ancestral to a number of later groups) they cannot be considered a true clade but are a valid taxon a little more advanced than the Tryblidiida. Nevertheless, these creatures constitute a fascinating and very important development in the early evolution of shell-bearing molluscs.
Exogastric or Endogastric
Yochelcionella interpreted exogastrically image © xxxx, from Peel (1991: 163)
Like other basal molluscs, it is certain that the helcionelloids were untorted. Untorted pre-gastropods would have the exogastric shell spiral above the head, rather than towards the tail. Runnegar & Pojeta (1974) in this manner restore the helcionellids as exogastrically coiled Monoplacophora (A above). In those species like Yochelcionella that possess a "snorkel", this would then be used for the inhalent water stream. However Yochelson (1978) suggests that this makes for an unlikely mollusc:
- A general rule for water flow is "in below, across the gills, and out above"...For Yochelcionella the flow would be in above and out below. Additional species of Yochelcionella with an open tube near the apex make even less sense interpreted as functionally inhalent.
Peel argues strongly that the helcionellids were actually endogastric, as shown in the following diagram.
Yochelcionella interpreted endogastrically image © xxxx, from Peel (1991: 163)
In this way the normal molluscan water flow pattern is restored.
In this essay I assume that the position of Yochelson and Peel are correct, and the helcionelloids represent a distinct category of molluscs only very distantly related to conventional Tryblidiida.
Relationships and Evolutionary History
The Helcionelloida are the very first molluscs to develop a mineralised shell. They appear in the very earliest Cambrian, even before most of the other Small Shelly Fauna. As there do not seem to be any intermediate forms betwween the earliest helcionellids and the earliest Tryblidiida, it is likely that these two lineages independently developed a mineralised shell from different soft-bodied ancestors.
The helcionellids are easily distinguished from Tryblidiida such as Neopilina and Tryblidium by the opposite direction of coiling, i.e. they are endogastric rather than exogastric. This also indicates that they have a somewhat different arangement of soft body parts. The Tryblidiida probably began as creeping epifaunal forms (crawling over the surface, perhaps like chitons), whilst the helcionellids were probably semi-infaunal (lying partially buried in sediment, with inhalent and exhalent siphons protuding). While typical helcionelloid shells are usually more strongly coiled and more laterally compressed than tergomyans and tryblidiids; there are also helcionelloids like Scenella which are very tryblidiidan in appearance and life-style. Others resemble planispirally coiled gastropods or tergomyans.
Cladistic analysis by Dr Peter Wagner indicates that the helcionellids are the basal grade from which both bivalves and their kin (subphylum Diasoma) and Tergomya and [[Gastropoda|gastropods] (subphylum Cyrtosoma) developed. It is also now generally thought that they were ancestral to the Cephalopoda and Scaphopoda (Cyrtosoma) as well. Steiner & Dreyer (2002); Runnegar (2002). However, the cladogram by Waller indicates that these two classes developed later (after the gastropods), so someone's cladogram is wrong (ironically, if the Cephalopoda can however be derived from the hypseloconid tergomyans (a position few now hold and is probably quite unlikely) that would resolve this problem nicely).
The helcionelloids had their heyday during the Cambrian, flourishing from the very beginning to the end of the period. By the Ordovician only a single family remained, the Scenellidae, but this continued all the way to the Devonian.
As defined here, the Helcionelloida are a grade of mostly Cambrian molluscs which have an essentially posterior mantle cavity in an endogastric shell. A rather generalised form like Helcionella probably had a mantle cavity extending along the lateral surfaces. Despite its apparently primitive form, this type is probably derived from seemingly more specialised types like Latouchella, which appear earlier in the fossil record. In Latouchella and Yochelcionella, representing the typical helcionelloid form, there is tighter coiling and more lateral compression of the shell. In these animals the mantle cavity was probably concentrated in the posterior portion of the shell, in a manner similar to gastropods and cephalopods, due to restriction on the physiology by the high narrow shell. This reduction of the mantle cavity was probably also associated with a reduction in the number of pairs of gills from the original metameric tryblidiid condition. Peel (1991).
The following technical diagnosis is from Peel (1991):
- Generally bilaterally symmetrical univalves in which the calcareous shell is usually planispirally coiled through about a half to one and a half whorls; the whorls may be in contact or open coiled and are often laterally compressed. The aperture is commonly planar, without re-entrants, but the sub-apical surface may develop a median sinus which may be deep and slit-like or even trematose, with a single perforation at the end of an elongate tube termed the snorkel. In some forms the lateral areas of the aperture may become prosocyrt, extended into weak lateral shields and producing broad emarginations in both the supra-apical and sub-apical surfaces astride the plane of symmetry. Ornamentation may include both comarginal and spiral elements; prominent comarginal rugae are a feature of many forms.
John S. Peel groups the following families in the class Helcionelloida (all from Runnegar & Pojeta 1985 's classification of the Helcionellacea): Scenellidae Wenz 1938, Helcionellidae Wenz 1938, Yochelcionellidae Runnegar & Jell 1976, and Stenothecidae (presumably this last is the same as Procarinariidae Wenz 1938).