VENUS 65 (1-2): 27-49, 2006 Review New Outlook on the System of Chitons (Mollusca: Polyplacophora)* Boris Sirenko Zoological Institute of the Russian Academy of Sciences, Universitetskaya nab.1, St. Petersburg 199034, Russia; [email protected] Abstract: In order to build a natural classification of the chitons, a new approach is proposed that uses not only the shells, as usual, but also other suitable features including aesthetes, girdle, radula, gills, glands, egg hull projections, spermatozoids etc. Several previous classifications are discussed. A brief review of the evolution of the Polyplacophora is given and a new classification of the chitons is proposed. The roles of the articulamentum and the reductions in the tegmentum in chitons are discussed. The evolutionary line of the reduction of slits is shown for the superfamily Cryptoplacoidea. Specifically, the genera Hemiarthrum, Weedingia and Choriplax, which have unslitted valves, have been removed from the order Lepidopleurida and reassigned to the order Chitonida within the Cryptoplacoidea. Affinities of these and other genera within the Cryptoplacoidea are discussed. Keywords: Polyplacophora, taxonomy, evolution, articulamentum, reduction of tegmentum Introduction Creating classifications at its best is all about searching for phylogenetic affinities. The described natural system is a reflection of our ideas about affinities, expressed with the aid of a hierarchy of taxa. In the words of Darwin (1873): “...that Natural System is founded on descent with modification; - that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parents...all true classification being genealogical; - that community of descent is the hidden bond which naturalists have been unconsciously seeking...” For the early classifications of chitons, naturalists usually used a single or a few characters and their classifications were thus artificial. A natural classification can be created by using a number of characters as Darwin (1873) wrote: “... such aggregated characters have special value in classification.” In my work on the revision of chiton classification, I have relied on complex or aggregated characters of different attributes of chitons, such as the shell, girdle, radula, gills, glands, egg hull projections, spermatozoids, etc. History of Recent Systems of Chitons In this paper I will not discuss the previous systems of Gray, Carpenter, Dall, Pilsbry, Thiele and others, because they were all well reviewed in Van Belle (1983). Bergenhayn (1955) created a classification for both fossil and Recent chitons. His emphasis on the significance of variation in the articulamentum layer in chiton valves was a big contribution to the systematics of chitons. The classification of chitons by Bergenhayn, (1955) is as follows: *Invited paper to the special number of Venus for the 2nd International Chiton Symposium, Tsukuba 28 B. Sirenko Class Loricata Subclass Paleoloricata Order Septemchitonina Family Septemchitonidae Order Chelodina Family Chelodidae Family Gotlandochitonidae Family Scanochitonidae Subclass Neoloricata Order Lepidopleurina Family Lepidopleuridae Family Hanleyidae Family Choriplaxidae Order Ischnochitonina Family Subterenochitonidae Family Ischnochitonidae Family Schizoplaxidae Family Callochitonidae Family Callistoplacidae Family Chaetopleuridae Family Mopaliidae Family Schizochitonidae Family Chitonidae Subfamily Chitoninae Subfamily Tonicinae Subfamily Acanthopleurinae Order Acanthochitonina Family Acanthochitidae Subfamily Acanthochitoninae Subfamily Cryptoplacinae Order Afossochitonina Family Afossochitonidae Insertae Sedis: Family Llandeilochitonidae Bergenhayn (1955) emphasized shell characters and practically ignored other features of the girdle, gills, radula etc. The taxonomists who followed him acted in the same way. Allyn G. Smith (1960) wrote: “The radula, though used by Thiele and others for systematic arrangement, has not proved adequate by itself. The chiton classification most useful to paleontologists must of necessity omit girdle characters, important though these may be. For the present at least it would seem logical to use Pilsbry’s system based largely on valve configuration, modified where appropriate by a consideration of the valve structure as developed by Knorre and Bergenhayn”. Smith continued: “Classification of chitons is in a ‘fluid’ state and probably will continue for some time.” He was right. The classifications of Smith (1960), and later those of Van Belle (1975, 1983, 1985) show no considerable differences from Bergenhayn’s classification. The system of Van Belle (1983), as somewhat modified by Kaas & Van Belle (1998), is as follows: Order Paleoloricata Suborder Chelodina New Outlook on the System of Chitons 29 Family Chelodidae Family Scanochitonidae Suborder Septemchitonina Family Septemchitonidae Order Neoloricata Suborder Lepidopleurina Family Leptochitonidae Subfamily Helminthochitoninae Subfamily Leptochitoninae Subfamily Protochitoninae Family Hanleyidae Family Afossochitonidae Suborder Choriplacina Family Choriplacidae Suborder Ischnochitonina Family Ischnochitonidae Subfamily Ischnochitoninae Subfamily Callistoplacinae Subfamily Chaetopleurinae Subfamily Callochitoninae Subfamily Lepidochitoninae Subfamily Schizoplacinae Family Mopaliidae Subfamily Katharininae Subfamily Heterochitoninae Subfamily Mopaliinae Family Schizochitonidae Family Chitonidae Subfamily Chitoninae Subfamily Acanthopleurinae Subfamily Toniciinae Suborder Acanthochitonina Family Cryptoplacidae Family Acanthochitonidae Subfamily Acanthochitoninae Subfamily Cryptochitoninae Ashby (1929), who was widely known for his taxonomic work on chitons, addressed the problem of the taxonomic value of various characters as they related to Polyplacophora. He clearly differed from his colleagues in his assessments of the relative value of particular characters in chitons. Evaluation of Past Works The environment exerts a primary influence upon the shapes of the shell and girdle in chitons. Therefore, it can be expected that the shell and girdle characters might be exposed to convergences and parallelisms more often than other characters. There are several such apparent convergences in Polyplacophora: Amicula (Mopaliidae) and Cryptochiton (Acanthochitonidae); Placiphorella (Mopaliidae) and Craspedochiton (Acantochitonidae), Tonicina, Tonicia 30 B. Sirenko (suborder Chitonina) and Boreochiton, Tonicella (suborder Acanthochitonina), Chaetopleura (Chaetopleuridae) and Lepidozona (Ischnochitonidae)(Fig. 1), and many others. Even more striking are examples of parallel evolution within the same family group. Similar features of shell and girdle have developed among species of the genera Tonicella, Boreochiton, Lepidochitona, Spongioradsia, and Juvenichiton (family Tonicellidae) (Fig. 2). For a long time this prevented an accurate diagnosis of each of these genera. The strong reduction of the tegmentum in Cryptochiton and Cryptoconchus (family Acanthochitonidae), which developed independently from other genera with a reduced tegmentum, also resulted in a similarity between their valves. The apex of Onithochiton and Enoplochiton (family Chitonidae) independently moved to a ter- minal position and, in addition, slits were reduced and a callus was formed in the tail valve. The establishment of a natural system demands the clear determination of all such cases of homoplasy. The taxonomists who followed Bergenhayn likewise placed great emphasis on the development of the articulamentum. As a result, a crisis situation arose by the end of the 1980s continuing until the early 1990s. During that time several new species were described with slitless insertion plates, assigned to the genera Weedingia (Kaas, 1988), Deshayesiella (Xu, 1990) and Xylochiton (Gowlett-Holmes & Jones, 1992). All of these, along with Hemiarthrum, were put in the family Hanleyidae (Kaas & Van Belle, 1990, 1994) despite the fact that they have many more differences than similarities. The family Hanleyidae is thus best recognized as a polyphyletic assemblage comprised of disparate species and genera (Hanleya sinica = Deshayesiella sinica, Xylochiton xylophagus = Ferreiraella xylophaga, Weedingia and Hemiarthrum), relatively unrelated to each other or to species of Hanleya. They were placed together solely on the basis of the absence of a slit in the insertion plates. It would be more logical to assume monophyly, and we should eventually recognize insertion plate convergences or parallelisms in several only distantly related families: Ferreiraellidae (Ferreiraella xylophaga), Protochitonidae (Protochiton granulosus, Deshayesiella sinica), and Hanleyidae (Hanleya spp.). The problem is complex because it was assumed that the articulamentum became steadily more complex through geological time. Therefore, the presence of insertion plates has been recognized as a very important step and this is reflected by its prominence in all post-Bergenhayn classification schemes. However, even though the development of an articulamentum layer is a very important stage of the evolution of at least basal chiton groups, separate characters of the articulamentum, such as insertion plates, appeared not only on the main line of evolution leading to the majority of Recent chiton species but also on lateral branches in specialized genera that survived until the Recent with few changes. In my view, insertion plates appeared independently in different groups and apparently