Palæos:		Mollusca
METAZOA		Phylogeny

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Molluscan Phylogeny

Lophotrochozoa |--MOLLUSCA | BASAL MOLLUSCA | |--Diasoma | | |--Rostroconchia | | `--BIVALVIA | | |--Protobranchia | | `--HETERODONTA | `--CYRTOSOMA | |--CEPHALOPODA | `--GASTROPODA | |--Eogastropoda | `--APOGASTROPODA | |--Caenogastropoda | `--HETEROBRANCHIA | |--Opisthobranchia | `--PULMONATA `--ANNELIDA

The Ur-Mollusc Paradigm   Cyrtosoma and Diasoma   Yochelson's Archi-mollusk   Stasek's Aculifera - Polyplacophora - Conchifera model   Salvini-Plawen and the Neontological Perspective   The Tergomya and the Helcionelloida   Diasoma-Cyrtosoma vs Helcionellid   Waller's Cladistic Research   Molecular Analysis   The Aplacophora   Bivalves, Rostroconcha, and Scaphopods   A suggestion for a new hypothesis   Comparisons

The Ur-Mollusc Paradigm

The following diagram presents a stylized relationship between the different Molluscan groups, with all evolving from a "hypothetical ancestral mollusk", which people consider to show shared ancestral characters for the group Mollusca

Radiation of the mollusks from the “hypothetical ancestral mollusk

Useful as this diagram may be for illustrating the diversity among extant classes of mollusks, it really does nothing to explain the relationships of the various clades that make up the phylum. For this understanding, we must turn to the various hypotheses that have been proposed by paleontologists and neontologists over the last several decades. We can begin with an idea that has laid the foundations for much of our understanding of molluscan phylogeny, but has itself been later challenged - the Cyrtosoma-Diasoma hypothesis.  

Cyrtosoma and Diasoma

The view of molluscan evolution, which for some time assumed that the various classes developed from a hypothetical ancestral shelled ancestor, was revolutionized by an elegant model proposed by Bruce Runnegar and John Pojeta in 1974, and developed in a series of subsequent papers. This can be best represented by the following diagram:

As indicated here, the Conchiferan (shell-bearing) mollusks are divided into two main sub-phyla, linked by a number of early transitional forms.

The Cyrtosoma (hunchback-body) includes the Monoplacophora, Gastropoda and Cephalopoda. The Monoplacophora gave rise to the Gastropoda, Cephalopoda, and the Rostroconchia (which latter is included in the second group). The Monoplacophora, Gastropoda, and Cephalopoda all are characterized primitively by a conical univalved shell, usually twisted into a spiral. The relatively small single aperture forces the anus to lie close to the mouth, and the gut is bent into a "U"-shape or (with gastropods) twisted on account of torsion. Most are surface-dwelling (epi-faunal) grazers or carnivores.

In the Diasoma (through-body) the gut is essentially straight. This group includes the classes Rostroconchia, Bivalvia and Scaphopoda, with the two latter derived from the Rostroconchia. These three classes form a lineage that diverged from the Monoplacophora in the Early Cambrian. They emphasized a shell form that in all groups is primitively open at both ends, allowing the gut to remain relatively straight, with an anterior mouth and posterior anus. They became burrowing (infaunal) deposit or filter feeders.

But although the Cyrtosomes - including as they do the Monoplacophora - are the ancestors of the Diasomes (and hence not strictly a monophyletic clade), both subphyla began to diversify within a few million years in the Early Cambrian.

Runnegar and Pojeta also suggest that the Polyplacophora can be derived from the Monoplacophora instead of (as usually suggested) a more primitive ancestral stock.

While not universally accepted, the Cyrtosoma-Diasoma paradigm presented an integrated paleontological perspective that complemented neontological discussions.  

Yochelson's Archi-mollusk

Yochelson (1978) rejects the Cyrtosoma-Diasoma model on the grounds that the differences between the shelled taxa remain too great, and that it would be foolish to speculate too much on the soft anatomy of animals known only from hard shells and a few rare muscle scars. Instead of a small number of classes linked by intermediate Cambrian shelled forms, he suggests a large number of different taxa that developed from a non-shelled ancestor or Archi-mollusk. This is represented in the following diagram

Here we are back to the original position at the top of the page, except that the ancestral form lacks a shell, and there are many early short-lived taxa that cannot be pigeonholed into the main classes.

This is reminiscent of the controversy between Gould and Conway-Morris' position regarding the Cambrian explosion and the number of extinct high level taxa. One sees a disorderly bush with a large number of separate early experiments, the other an orderly tree in which the branches can be carefully mapped out.  

Stasek's Aculifera - Polyplacophora - Conchifera model

Stasek theorized that the extant mollusks are the progeny of three separate lineages that separated before the phylum was well established. He stated no known fossil or living genera can bridge the gaps between any two of the three lineages, and treated each as a separate subphylum. These are (i) the Aculifera Hatscheck 1891, containing only the class Aplacophora, derived from the most primitive ancestor; (ii) the subphylum Placophora von Jhering 1876, containing only the class Polyplacophora, and emphasizing the pseudometamerism of its more advanced premolluscan ancestor; and (iii) the subphylum Conchifera Gegenbaur 1878, containing the remaining shelled mollusks - the Monoplacophora and the other classes derived from it.  

Salvini-Plawen and the Neontological Perspective

Salvini-Plawen [1980) provided a reconsideration of the phylogenetic relationships among the various molluscan groups by studying various extant forms, including the little known soft-bodies vermiform types that had previously been called "Aplacophora" and lumped with the chitons under the now rejected class "Amphineura". On the basis of this analysis he proposed a phylogenetic line by which an original archi-mollusc gives rise to the main Conchiferan groups as well as a number of side-branches. All in all there are four stages of progressive differentiation. The first has an early side branch called the Scutopoda dividing from the main line, and eventually becoming the Caudofoveata (a class of Aplacophora). The other line is represented by the Adenopeda, which include the ancestors of all other mollusks. From the Adenapoda the shell-less Solenogastres (the second class of Aplacophora) separate from the shell-bearing Testaria. Within the Testaria the Polyplacophora, or "Placophora", divide from the Conchifera. Finally, the Conchiferan ancestor gives rise to the various extant classes, along with the extinct Galeroconcha (Monoplacophora plus Bellerophontida, the latter here considered to be untorted). The Aplacophora are rejected, as the Placophora and Solenogastres are tied together by a number of shared unique characteristics (synapomorphies), in contrast to the merely symplesiomorphic (shared primitive) characters in Solenogastres and Caudofoveata. This can be represented by the following diagram:

The complete diagram is as follows:

Another arrangement has the mollusks divided into two subphyla, the Aculifera (spiny mollusks) and the Conchifera (shell bearing mollusks).

The subphylum Aculifera comprises the most primitive group - all those mollusks that do not produce a complete shell. Instead they have a hard exterior skin called a cuticula. This cuticula additionally is protected by calcareous spines (spiculi). Two classes belong to the Aculifera:

• Aplacophora
  • Chaetodermomorpha (= Caudofoveata)
  • Neomeniomorpha (= Solenogastres)
• Polyplacophora (Chitons).

The Conchifera includes those mollusks that have a complete shell (although this may be lost in some more derived forms). Included here are the gastropods, bivalves, cephalopods, and other lesser groups, as follows:

• Monoplacophora (Trybliida)
• Visceroconcha
  • Gastropoda
  • Cephalopoda
• Loboconcha
  • Scaphopoda
  • Bivalvia

The Visceroconcha containing the classes Gastropoda and Cephalopoda, and the Loboconcha containing the classes Scaphopoda and Bivalvia, are clearly the neontological equivalents to Runnegar and Pojeta's Cyrtosoma and Diasoma [ Mizzaro-Wimmer and Salvini-Plawen 2001]  

The Tergomya and the Helcionelloida

John S. Peel reinterprets the Cyrtosoma and Diasoma by rejecting the old "wastebasket" taxon Monoplacophora, and replacing it with two distinct and only distantly related early classes. These are the exogastrically-curved Class Tergomya (the superfamily Tryblidiacea), based upon a subclass of the same name described by Horny (1965a, b) and including the order Tryblidiida with the recent Neopilina; and a group of endogastrically coiled univalved mollusks, the Class Helcionelloida (the former superfamily Helcionellacea), including a range of primitive extinct firms, including some with cap shaped shells, others with high shells, even forms with "snorkels". The Helcionelloida is considered to be the direct ancestor of the Class Rostroconchia and members of this latter class are also interpreted here as endogastric mollusks. The model of diasome evolution proposed by Runnegar & Pojeta is replaced by one in which two parallel lineages are recognized within the former Diasoma: Tergomya - Bivalvia, and the Helcionelloida - Rostroconchia.

The scaphopods are tentatively derived from the Rostroconcha (although they may come from either lineage), and the Cephalopoda from the helcionelloids rather than from high-shelled conical Tergomya such as Knightoconus [Yochelson et al 1973). This, together with the derivation of the Gastropoda from the Tergomya or tergomyan-like forms, suggests that the Cyrtosoma is also not monophyletic.  

Diasoma-Cyrtosoma vs Helcionellid

During the 90s the two competing hypotheses of evolutionary relationships among conchiferan molluscs: the Diasoma-Cyrtosoma concept recognizing the clades (Tryblidiida, (Bivalvia, Scaphopoda), (Gastropoda, Cephalopoda)); and the Helcionellid concept with (Tryblidiida, (Bivalvia, (Gastropoda, Scaphopoda, Cephalopoda))). The rostroconches were either stem group diasomes or stem group scaphopods (Runnegar & Pojeta, 1974; Salvini-Plawen, 1980; Steiner, 1992; Engeser & Riedel, 1996; Runnegar, 1996); if the latter the Bivalvia stood alone as a monophyletic clade.

Both of these hypotheses have problems. The Diasoma results in convergent evolution of elongated dorso-ventral body axes in Scaphopoda and Cyrtosoma (Gastropoda, Cephalopoda), of multiple cephalic tentacles and ring-shaped muscle attachment in Scaphopoda and Cephalopoda. The Helcionellid concept has to account for convergent similarities in the development of the mantle-shell, the typical burrowing foot, and the nervous system in Bivalvia and Scaphopoda. [Steiner and Dreyer 2002]  

Waller's Cladistic Research

Recently, Waller (1998) has overturned this traditional view by proposing that the Scaphopoda and Cephalopoda are sister groups within the subphylum Cyrtosoma.

Waller's cladogram is shown below.

o MOLLUSCA
   `--+-- Aplacophora (= Solenogastres + Caudofoveata)
      `--+-- TESTARIA
         `--+-- Polyplacophora
            `--+-- CONCHIFERA
               |-- Tryblidiida
               `--+--o DIASOMA
                  |  `--+--†Stenothecidae
                  |      `--+-- †Rostroconchia
                  |          `-- Bivalvia
                  `--o CYRTOSOMA
                     `--+-- Gastropoda
                        `--+-- Scaphopoda
                           `-- Cephalopoda

This established the new view by which extant conchiferan classes are grouped as follows (Tryblidiida, (Bivalvia, (Gastropoda, (Scaphopoda, Cephalopoda)))).  

Molecular Analysis

Steiner and Dreyer [2002] used molecular analysis and a 18S rDNA data set containing 17 Bivalvia, 6 Gastropoda, 17 Scaphopoda, 4 Cephalopoda, and 3 Polyplacophora. All the resulting trees showed Cephalopoda and Scaphopoda as sister groups with the Gastropoda connecting to their common stem. So far this supports Waller. However Bivalvia here appear paraphyletic at the base of the conchiferan ingroup. This is perhaps more a problem with the unreliability of current molecular phylogeny, due to "long branch attraction", than an actual representation of the early history of the group.  

The Aplacophora

The Aplacophora also are giving up their secrets. Gerhard Haszprunar shows that the two aplacophoran taxa, the Solenogastres and Caudofoveata do not form a single clade. Based on current knowledge and available data, he concluded that the arrangement (Solenogastres (Caudofoveata (Polyplacophora + Conchifera))) is the most parsimonious one, with the Aplacophora as a basic, paraphyletic assemblage. The monophyly of the Testaria (Polyplacophora + Conchifera) is well supported by several characters, and the Aculifera (Polyplacophora + Aplacophora) rejected. The Adenopoda (Solenogastres + Testaria) of Salvini-Plawen is shown to be polyphyletic in this interpretation.

Further insights have been provided by the discovery of the Silurian spiny worm-like soft-bodied plated mollusk Acaenoplax hayae, which combines characteristics of both the Caudofoveata and the Polyplacophora, and can be seen as a sister clade to the Caudofoveata, and a sort of missing link or transitional form between these two groups [Sutton et al 2001], although this interpretation has been challenged by Steiner and Salvini-Plawen 2001, who claim Acaenoplax is a polychaete worm; an interpretation that is unlikely if only because of the similar number of transverse plates and bristles in both Acaenoplax, chitons, and larval Solenogastres, implying a close relationship between all these groups.  

Bivalves, Rostroconcha, and Scaphopods

Steiner and Dreyer 2002 suggest a scenario for the evolution of the higher conchiferan taxa. Acquiring an infaunal life style occurred independently in the early Rostrochonchia-Bivalvia lineage and, later, in Scaphopoda, rendering the relevant characters convergent. After the separation of the gastropod lineage, the stem-group of Scaphopoda and Cephalopoda comprised high-shelled, dorsoventrally elongated helcionellids with a deep mantle cavity, multiple cephalic feeding tentacles, and a circular area of muscle-shell connection. Shell-chambering for buoyancy then led to the first cephalopods, and anterior mantle elongation and transient forms with posterior shell slits or snorkels in response to an increasingly infaunal habit gave rise to the tube-shelled scaphopods.

Thus, all rostroconches and some helcionellacean univalves technically become stem group bivalves. This is because the total group is thought to have originated at the latest common ancestor of the Bivalvia and the Cyrtosoma; Cytosoma here being (Gastropoda (Cephalopoda, Scaphopoda)). [Runnegar 2002].  

A suggestion for a new hypothesis

The problem with the above hypotheses is that the fossil evidence doesn't match the DNA and cladistic results. If Cephalopods, Scaphopods, and Bivalves all evolved from a semi-infaunal Helcionellid ancestor, they should be close together on the molecular tree. In-faunal Helcionellids were neither diverse nor long-lived, and so there is unlikely to be much molecular difference between organisms that diverged from such closely related ancestors. Moreover, the gastropods sprung form a different Helcionellid group, and hence the analysis should give (Gastropoda (Bivalvia + Cephalopoda + Scaphopoda)). This is not the case.

Therefore I would suggest the following tentative hypothesis: that the Bivalves are prior to the Helcionellids, both evolving an semi-infaunal life-style from a common soft-bodied ancestor some time in the latest Ediacaran. In the earliest Cambrian, one lineage developed a bivalved mineralized shell (Fordilla and its relatives), the other an endogastric univalve shell (the Helcionellids). This latter group of primitively compressed shells gave rise to a wider bodied limpet-like line of epifaunal crawlers, which eventually became the paragastropods and Gastropods, while the narrow bodied semi-infaunal primitive forms gave rise to the Cephalopods and the rostroconches, the latter in turn during the Ordovician or the Devonian [Engeser and Riedel 1997] giving rise to the Scaphopoda. Thus (reasserting the Helcionellid hypothesis) the Rostroconchia are not related to Bivalves at all and the Diasoma are diphyletic. The large number of similarities between the rostroconches and the Bivalves (Waller 1998; Wagner 1999) are convergences resulting from adaptations to a similar if not identical life-style. Similar examples of near-identical shells but different soft-parts can be found among opisthobranch gastropods, and in muscle scar evidence from "monoplacophora" and bellerophontids.

In addition, I would suggest that the lineage represented by the Silurian spiny worm-like Acaenoplax hayae can be seen as ancestral to both the extant and degenerate Caudofoveata, and the Testaria with their creeping foot and shell or shells.

The resulting cladogram is as follows:

  o MOLLUSCA
   `--+-- Solenogastres
      `--+--+-- †Acaenoplax hayae
         |  `-- Caudofoveata
         `--o TESTARIA
            `--+-- Polyplacophora
               `--+-- CONCHIFERA
                  |-- Tryblidiida
                  |-- Bivalvia
                  `--+-- †Helcionellidae [Helcionelloida]
                     |--+--+-- †Yochelcionellidae [Helcionelloida]
                     |  |   `-- Cephalopoda
                     |  ` --+--†Stenothecidae [Helcionelloida]
                     |      `--+-- †Rostroconchia
                     |          `-- Scaphopoda
                     `--+--†Coreospiridae [Helcionelloida]
                        |--+-- †Paragastropoda
                        `--+-- †"Tergymya"
                           `--+-- Gastropoda

For a more detailed cladogram, please see the Mollusc cladogram page.  

Comparisons

To sum up, the comparison of the various hypotheses can be shown as follows