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Home , Acanthochitona, Acanthochitonidae, Amicula (chiton), Callistochiton, Callochiton, Callochitonidae

VENUS 65 (1-2): 27-49, 2006

Review

New Outlook on the System of (: 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, , , 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 and reassigned to the order within the Cryptoplacoidea. Affinities of these and other genera within the Cryptoplacoidea are discussed.

Keywords: Polyplacophora, , 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 , 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 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 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 Subclass Paleoloricata Order Septemchitonina Family Septemchitonidae Order Family Chelodidae Family Gotlandochitonidae Family Scanochitonidae Subclass Order Family Lepidopleuridae Family Hanleyidae Family Choriplaxidae Order Ischnochitonina Family Subterenochitonidae Family Family Schizoplaxidae Family Family Callistoplacidae Family Family Family Schizochitonidae Family 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 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 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 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: (Mopaliidae) and Cryptochiton (Acanthochitonidae); (Mopaliidae) and (Acantochitonidae), Tonicina, 30 B. Sirenko

(suborder ) and Boreochiton, (suborder Acanthochitonina), (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, , 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 (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 ( 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 in different geological epochs. Adaptive evolution led to a multitude of convergences and parallelisms, within orders or suborders, or within their superfamilies, families, and genera. Similar characters that appeared owing to similar environmental factors are complicated and still complicate placement of chitons into groups based on close relationship. One should avoid using a formal approach devoted to single characters, however important, while ignoring other character complexes. Otherwise taxonomic monsters will appear comprised of genera and families that do not reflect a true system based on intrinsic relationships.

Proposed Stages of Evolution of Polyplacophora

Here I consider the stages of evolution of the Polyplacophora in order to gain a better understanding of the importance of articulamentum development in the taxonomy of chitons. The first chitons probably appeared at the end of the . I will not discuss here the insufficiently substantiated or doubtful placement of Cambrian (Preacanthochiton in Bergenhayn, New Outlook on the System of Chitons 31

Fig. 1. Convergence in chitons. A. Craspedochiton pyramidalis (Is. Taki, 1938). B. Placiphorella stimpsoni (Gould, 1859). C. colimensis (A.G. Smith, 1961). D. Cryptochiton stelleri (Middendorff, 1847). E. Amicula vestita (Broderip et Sowerby, 1829). F. Lepidozona sinudentata (Carpenter in Pilsbry, 1892). G. Tonicina zschaui (Pfeffer, 1886). I. Chaetopleura caboverdensis (Kaas et Strack, 1986). J. Lepidochitona thomasi (Pilsbry, 1898). K. Chaetopleura sowerbyana (Reeve, 1847). L. Tonicella squamigera Thiele, 1909. (A, B: after Saito & Okutani, 1992; C, G, I, J, K: after Kaas & Van Belle, 1985, 1987, 1994; D, E, F, L: original) 32 B. Sirenko

Fig. 2. Parallelism in chitons. A. Tonicella submarmorea (Middendorff, 1847). B. Juvenichiton albocinnamomeus Sirenko, 1975. C. Juvenichiton saccharinus (Dall, 1878). D. Micichiton grandispinus Sirenko, 1975. E. Micichiton kurilensis Sirenko, 1975. F. Boreochiton granulatus (Jakovleva, 1952). G. Nanichiton deplanatus Sirenko, 1975. I. Lepidochitona berryana Eernisse, 1986. J. Spongioradsia aleutica (Dall, 1878). New Outlook on the System of Chitons 33

1960; Luyanhaochiton and Yangtzechiton in Yu, 1984 and in Runnegar et al., 1979) and (Cobcrephora in Bischoff, 1981) remains of in the Polyplacophora. Shell of the first chitons initially consisted of several separate plates. It would be more logical to derive the origin of such shell from plates and spicules, present on the dorsal side of animals similar to from Cambrian Burgess Shale site (Conway Moris, 1985). More reliable records of the first chitons have been known from the Ordovician period. According to Bergenhayn (1955) their shell had three layers: periostracum, tegmentum and hypos- tracum. The first group of the most primitive chitons from the order Chelodida (genera , Euchelodes and Calceochiton) had shell valves that are more similar to spicules of Recent chitons. The valves had a very small area of attachment and did not cover the soft body from above, but rather protected it by their sharp apices. These valves had a simple, more frequently pentagonal shape and a relatively wide apical area, reflecting the high position of the valve apex. The recently described Ordovician chiton Echinochiton dufoei (Pojeta et al., 2003) according to the description has more advanced valve shape, a smaller apical area and antemucronal area, and it is herein with some doubt placed in order Septemchitonida because the valves of chitons of order Chelodida have a large apical area and no antemucronal area. On the other hand, the description of E. dufoei was based mainly on an external mold and in my opinion its reconstruction is not successful, because instead of the upper surface of valves with a projected apex we see the internal part of the valves with incisura. The main evolutionary trend in the first stage from the end of the Cambrian up to the period inclusive was the flattening of the shell and the entire chiton body. This was apparently associated with the growing pressure of and the necessity to hide in narrow refuges, since the shell initially consisted of simple valves looking more like spines that provided an insufficient defense for primitive chitons. In the Lower the fourth layer of the shell (the articulamentum) appeared; it is conspicuous owing to the sutural laminae protruding from beneath the tegmentum. From that moment and until at least the , the main direction of chiton evolution was closely associated with the development and complication of this new layer of the shell. This is the second stage in chiton evolution. All chitons with an articulamentum belong to the subclass Loricata. The appearance of the articulamentum gave ample opportunities for increasing complexity to arise in both the musculature binding shell plates to each other and that binding the plates to the girdle. This in turn greatly increased the mobility of chitons, enhanced their ability to attach firmly to the substratum, and allowed them to better defend themselves from predators. The articulamentum kept developing, and the rudiments of insertion plates appeared in the Upper Carboniferous, first on the intermediate and tail valves as the logical continuation of the sutural laminae. The tegmentum also developed in parallel with the articulamentum. The greatest changes in the tegmentum occurred in the layout of the aesthetes and the associated division of the tegmentum into areas. At a certain moment in chiton evolution, the development of the articulamentum and the arrangement of the aesthetes came in contradiction with each other. This occurred because the widening and thickening of the articulamentum led to the partial isolation of the sense organs (aesthete system) located in the tegmentum from the nerve trunks passing beneath the articulamentum. The contradiction was eliminated by a re-ordering of the arrangement and a considerable increase in the size of the pores in the articulamentum through which the nerve endings passed that connected the aesthetes with the nerve trunks. These pores became arranged like chains on radii from the apices and continued in the form of slits in the articulamentum on the margins of the valves. Therefore, the slit on the insertion plate can be regarded as an incomplete pore in the articulamentum. The first of the fossil chitons having slits was Ochmazochiton comptus from the (Hoare et Smith, 1984). 34 B. Sirenko

Reduction of slits

Cryptoconchus 5/1/5-7 Spongiochiton Choneplax Hemiarthrum 5/1/6 5/1-0/2 3-5/1-0/2-0 3/0/0 Weedingia Craspedochiton Cryptochiton Choriplax 5/1/6 4-7/1-0/2 0/0/0

Fig. 3. Reduction of slits in chitons of different genera of superfamily Cryptoplacoidea.

As the development of the articulamentum continued, chitons appeared in the Jurassic with slits not only on the intermediate valves, but also on the terminal valves. This was the last notable change in the articulamentum, except for the development of small jugal plates connecting the sutural laminae in some species of the genera Lepidozona, Connexochiton, , etc., and reduction of slits on the last valve in some families (Chitonidae, Mopaliidae, and Acanthochitonidae). The improved connection of sensory organs located in the tegmentum with nerve trunks gave a new impetus to the evolution of chitons, which was manifested as increasing complexity in the system of aesthetes and in the structure of the tegmentum. This evolutionary pathway turned out to be progressive, and this led to the appearance of all advanced families and the majority of Recent chiton species. Beginning from the end of the Jurassic, chitons acquired the appearance of Recent chitons and the direction of chiton evolution changed. That was the beginning of the third stage of evolution in chitons characterized by the development of tegmental sensory organs (aesthetes) and the related increasing complexity of the tegmental sculpture and perinotal elements. The first species of the superfamily Cryptoplacoidea originated from one of the species of Lepidochitona as a result of the reduction of the tegmentum in the pleural areas, probably in the middle of the Cenozoic. The reduction of the tegmentum led to a reduction in slits as well.

Evidence and Discussion to Establish the New System

In some cases, an advanced form might superficially resemble its primitive forebears due to a secondary simplification of characters. The reasons for such a simplification are apparent when considering the case of the reduction of slits in representatives of the superfamily Cryptoplacoidea. Such a simplification might be initiated by a reduction of their tegmentum. All species of this superfamily are to a different extent characterized by the degree of reduction of the tegmentum from weakly and moderately pronounced in the majority to strongly pronounced in Choriplax, Cryptoplax and Cryptoconchus. This lineage of reduction is terminated at the Cryptochiton, in which the tegmentum disappears completely. In connection with tegmentum reduction, the number of aesthetes located on the tegmentum decreased abruptly, which implies a decline in the number of nerve endings binding the aesthetes with the nerve trunks beneath the thick articulamentum. For the same reason there is no remaining need for a large number of pores passing from the tegmentum through the articulamentum and forming slits, as they do in other species of the Chitonida. Therefore, the factor that caused multiple slits disappears, their number declines and in some representatives they disappear on the majority of insertion plates. It would be logical to assume that a species that has lost slits on all the insertion plates is at the end of this lineage of slit reduction. The genera Weedingia, Choriplax and Hemiarthrum appear to fit this criterion (Fig. 3). In former systems (Bergenhayn, 1955; Van Belle 1983; Sirenko, 1997), New Outlook on the System of Chitons 35

Fig. 4. Placement of gills. A. Weedingia alborosea Kaas, 1988. B. Hemiarthrum setulosum Carpenter in Dall, 1976. Scale bar = 1 mm.

all these genera or part of them were assigned to the family Hanleyidae because they have no slits. However, species of these genera have several characters that are advanced and draw them together with the species of family Acanthochitonidae. The species of genera Hemiarthrum and Choriplax have abanal types (Gowlett-Holmes, 1987; Sirenko, 1997). Probably Weedingia has the same abanal type of gills because the distribution of its gills is very similar to Hemiarthrum (Fig. 4). Hemiarthrum and Weedingia have innervations of aesthetes only through the pores in the eaves and in a small area under the jugal part of the valve (Fig. 5A, B, C; 6C). The same innervation is characteristic of species of the genera Acanthochitona and Choneplax (Fig. 7C; 8A, B). The several species of the genera and Craspedochiton in the Acanthochitonidae rarely have additional small pores distributed from the apex to a slit. At the same time, all species of the order Lepidopleurida, including the genus Hanleya, have many pores both under the jugal part and under the lateral parts of valves (Fig. 5D-I). The species of the genus Weedingia have a distinct jugal area (Fig. 6A, B; 7A, B) and Weedingia alborosea has tegmentum sculpture in the pleural areas that is rather similar to Bassethullia. Moreover, species of Hemiarthrum and Weedingia have sutural tufts with long spicules, like most of the Acanthochitonidae (Fig. 9). Taking the above-mentioned facts into consideration it is logical to put genera Hemiarthrum, Weedingia and Choriplax in the superfamily Cryptoplacoidea, with the first two genera belonging to the common family Hemiarthridae and the genus Choriplax in the family Choriplacidae. It is interesting to remember that initially Microplax grayi (= Choriplax grayi) was placed in the Lepidopleuridae (Pilsbry, 1892; Hedley, 1918). Then, in the first half of the 20th century, most malacologists (Ashby, 1921; Iredale & Hull, 1925; Thiele, 1929; Cotton, 1964) put Choriplax in the advanced Acanthochitonidae. Later, with authors following Bergenhayn’s work, Choriplax was moved to the Lepidopleurida (Bergenhayn, 1955; Starobogatov & Sirenko, 1975; Van Belle, 1983; Gowlett-Holmes, 1987; Sirenko, 1993). Ashby (1921) wrote: “I propose that the genus Choriplax, Pils., be taken out of its previous setting amongst the less specialized group, the Lepidopleuridae, and be placed under the Family Acanthochitonidae. The apparent absence of slits in the insertion plates is, I suggest, probably due to modifications in a very specialized form brought about by the peculiar habits of the chiton”. The same argument can be applied to Hemiarthrum. The second important question in the classification of chitons was to define the abanal and adanal types of gill placement more precisely. In my opinion only the species having one gill behind the nephropore [or nephridiopore] should be regarded as the abanal type (Sirenko, 1993) 36 B. Sirenko

Fig. 5. Innervation of aesthetes in chitons. A. Hemiarthrum setulosum. B. Weedingia alborosea. C. Weedingia mooreana Kaas, 1988. D. batialis Sirenko, 1979. E. Leptochiton kaasi Sirenko, 1990. F. Leptochiton lukini Sirenko, 1990. G. Leptochiton norfolcensis (Hedley & Hull, 1912). I. Hanleya hanleyi (Bean in Thorpe, 1844). Scale bar = 1 mm. New Outlook on the System of Chitons 37

Fig. 6. Weedingia alborosea. A. Valve IV. B. Latero- pleural area of the same valve. C. Ventral view of valve III. Scale bar = 500 µm (A), 50 µm (B), 300 µm (C). Fig. 7. Weedingia mooreana (A, B) and Acanthochitona fascicularis (Linnaeus, 1767) (C). A. Valve IV. B. Pleural area of the same valve. C. Ventral view of valve V. Scale bar = 500 µm (A, C), 50 µm (B). 38 B. Sirenko

Fig. 8. Ventral view of chiton valves. A. Choneplax indica Odhner, 1919, valve IV. B. Choneplax littlerorum Sirenko, 2003, valve V. Scale bar = 300 µm.

Fig. 9. Hemiarthrid chitons with sutural tufts. A. Weedingia alborosea. B. Hemiarthrum setulosum. Scale bar = 1 mm. New Outlook on the System of Chitons 39

Fig. 10. Type of gill placement in chitons (after Plate, 1901, with changes). I. Adanal, holobranchial (A, C) and merobranchial (B, D) with space (A, B) and without space (C, D). II. Abanal, holobrachial (E) and merobranchial (F). Abbreviations: a, anus; n, nephropore.

(Fig. 10). All other species having 3 or more gills behind the nephropore are to be considered the adanal type. Thus all species of the genera Chaetopleura, Callistochiton, Stenoplax, Onithochiton and others that have a large space between the posterior gill and anus belong to the adanal type. Analysis of the form of the hull projections reveals considerable variability (Eernisse, 1984; Sirenko, 1993) (Fig. 11). Moreover, all the species examined can be divided into two groups both by the form of projections and the size of their bases. The first group of species includes chitons with narrow projection bases (base width from 5-15 up 25-30 µm); these projections are often delicate and elongate, and seldom short. In the second group of species that characterized by a wide projection base (base width from 50 up to 90 µm), the projections are usually bulky. A correlation exists between the shape of the hull projections and the types of gill placement. It is apparent that species of the order Chitonida (suborder Chitonina) with numerous delicate long hull projections and base widths from 5 to 30 µm have the adanal type of gill whereas species of the same order (suborder Acanthochitonina) with few large bulky hull projections and base widths from 50 to 90 µm have the abanal type. A series of works on the morphology of spermatozoids and fertilization of eggs (e.g. Buckland-Nicks et al., 1988; Hodgson et al., 1988; Buckland-Nicks, 1995; Pashchenko & Drozdov, 1998; Buckland-Nicks & Hodgson, 2000) and cladistic analyses of these and other aspects of morphology (Eernisse, 1984; Buckland-Nicks, 1995) also support the proposed classification (Fig. 12).

Some Taxonomical Difficulties

A recent series of molecular analyses of chitons by Okusu et al. (2003) has generally confirmed the main division of the Recent Polyplacophora into two major lineages, the Lepidopleurida and Chitonida, and of the latter into two suborders, the Chitonina and Acanthochitonina. However, I do not find the placement of the genera Callochiton, Cryptoplax, , Acanthochitona and in the trees based on molecules to be supported by morphological characters. First, I agree with previous authors that Acanthochitona, Cryptoplax 40 B. Sirenko

Fig. 11. Hull projections of chiton eggs. A. hakodadensis. B. retifera. 1. Onithochiton hirasei. 2. Callistochiton retusa. 3. Chiton marmoratus. 4. Stenoplax conspicua. 5. Ischnochiton hakodadensis. 6. Chiton cumingsii. 7. Callistochiton periconis. 8. Ischnochiton pectinatus. 9. I. luticolens. 10. . 11. C. grandis. 12. . 13. I. interstinctus. 14. I. petaloides. 15. Stenoplax limaciformis. 16. Chaetopleura angulata. 17. Tonicia (Lucilina) sp., Tonga Is. 18. Stenosemus albus. 19. Stenochiton cymodocialis. 20. Chiton kurodai. 21. brevispinosa. 22. A. granulata. 23. Lepidozona multigranosa. 24. L. albrechti. 25. Callistochiton gabbi. 26. Acanthopleura japonica. 27. Stenosemus sp., Kerguelen Is. 28. S. exaratus. 29. S. golikovi. 30. S. sp., South Georgia. 31. Ischnochiton rissoi. 32. I. smaragdinus. 33. Acanthopleura gemmata. 34. Mopalia seta. 35. Plaxiphora kamehamehae. 36. . 37. Plaxiphora (Mercatora) sp., Pacific . 38. Acanthochitona rubrolineata. 39. Boreochiton granulatus. 40. Amicula vestita. 41. Lepidochitona hartwegii. 42. L. cinerea. (A, B, 1-40: Sirenko, 1993; 41: Eernisse, 1988; 42: original) New Outlook on the System of Chitons 41

Fig. 12. Maximum parsimony analysis of phylogenetic relationships among the Polyplacophora (after Buckland-Nicks, 1995).

and Cryptochiton should be placed together in a common group (Acanthochitonidae or Cryptoplacoidea). I examined a juvenile of Cryptochiton stelleri (body length about 4.5 mm) and found a tegmentum (Fig. 13), sutural tufts with longer spines, and four tufts at the front of the head valve, all just as in Acanthochitona. In a slightly larger juvenile (body length about 6.5 mm), numerous additional tufts with small spines have already appeared, and somewhat later all tufts become of equal size. I think these similarities are evidence for the affinity of Cryptochiton and Acanthochitona. Second, I would also maintain that the genera Nuttallochiton and Plaxiphora are more closely related to Mopalia and than to Cryptoplax and Acanthochitona as they were found to be in the molecular analyses. Third, I think that the two genera Callochiton and Schizochiton are still unclear in their systematic position. Callochiton has several very characteristic morphological features, including its spermatozoid. Schizochiton has a deep caudal sinus in its tail valve, , and a rather large (up to 30 µm) base on each egg hull projection. It is difficult to find relatives among the Recent groups of chitons for these two genera, but taking the adanal type of their gills into account it is logical to retain them in the suborder Chitonina.

Conclusion: New Classification of the Class Polyplacophora

The results of these investigations have allowed me to revise my own classifications of the chitons (Sirenko, 1993, 1997). I had previously proposed substantial revisions to a current alternative system based largely on Van Belle’s 1983 classification, as most recently revised by Kaas & Van Belle (1998). The genera Deshayesiella and Oldroydia were moved from the family Leptochitonidae into the family Protochitonidae. The order Lepidopleurida was divided into two suborders, the Cymatochitonina (with a weakly developed articulamentum and narrow sutural laminae) and the Lepidopleurina (with well developed sutural laminae and slitless insertion plates in some species). However, the main changes were made in the Recent advanced chitons. The order Chitonida was divided into two suborders: the Chitonina (with adanal gills and narrow-based hull projections) and the Acanthochitonina (with abanal gills and wide-based hull projections). The advanced genus Hemiarthrum was moved from the order Lepidopleurida into the order Chitonida (suborder Acanthochitonina, superfamily Cryptoplacoidea). In the present paper genera Weedingia and Choriplax are also moved from the order Lepidopleurida into the 42 B. Sirenko

Fig. 13. Age variation of valves in Cryptochiton stelleri. A. Valves I, V and VIII of juvenile specimen (body length ca. 6.0 mm). Scale bar = 1 mm. B. Valves I, V and VIII of adult specimen (body length 195.0 mm). Scale bar = 10 mm. order Chitonida (superfamily Choriplacidae). The members of order Scanochitonida are placed in “”. The condition of the valves in this group of chitons does not allow one to confidently confirm the absence of sutural laminae (and thus the absence of an articulamentum). Sutural laminae are typically among the first parts of the valves to be destroyed in fossil specimens.

Class Polyplacophora Gray, 1821 Subclass Paleoloricata Bergenhayn, 1955 Order Chelodida Bergenhayn, 1943 Family Chelodidae Bergenhayn, 1943 Chelodes Davidson & King, 1874 Ord.-Dev. Euchelodes Marek, 1962 Ord. Calceochiton Flower, 1968 Ord. Order Septemchitonida Bergenhayn, 1955 Family Gotlandochitonidae Bergenhayn, 1955 Gotlandochiton Bergenhayn, 1955 Ord.-Sil. New Outlook on the System of Chitons 43

Family Helminthochitonidae Van Belle, 1975 Kindbladochiton Van Belle, 1975 Ord.-Dev. Diadelochiton Hoare, 2000 Dev. Helminthochiton Salter in Griffith & M’Coy, 1846 Ord.-Dev. Echinochiton Pojeta, Eernisse, Hoare & Henderson, 2003 Ord. Family Septemchitonidae Bergenhayn, 1955 Septemchiton Bergenhayn, 1955 Ord. Paleochiton A. G. Smith, 1964 Ord.-Sil. Thairoplax Cherns, 1998 Sil. Subclass Loricata Shumacher, 1817 Order Lepidopleurida Thiele, 1910 Suborder Cymatochitonina Sirenko & Starobogatov, 1977 Family Acutichitonidae Hoare, Mapes & Atwater, 1983 Acutichiton Hoare, Sturgeon & Hoare, 1972 Carb.-Perm. Elachychiton Hoare, Sturgeon & Hoare, 1972 Carb. Harpidochiton Hoare & Cook, 2000 Carb. Arcochiton Hoare, Sturgeon & Hoare, 1972 Carb. Kraterochiton Hoare, 2000 Perm. Soleachiton Hoare, Sturgeon & Hoare, 1972 Carb. Asketochiton Hoare & Sabattini, 2000 Perm. Family Cymatochitonidae Sirenko & Starobogatov, 1977 Cymatochiton Dall, 1882 Carb.-Perm. Compsochiton Hoare & Cook, 2000 Carb. Family Gryphochitonidae Pilsbry, 1900 Gryphochiton Gray, 1847 Carb. Family Lekiskochitonidae Smith & Hoare, 1987 Lekiskochiton Hoare & Smith, 1984 Perm. Family Permochitonidae Sirenko & Starobogatov, 1977 Permochiton Iredale & Hull, 1926 Perm. Suborder Lepidopleurina Thiell, 1910 Family Ferreiraellidae Dell’ Angelo & Palazzi, 1991 Glaphurochiton Raymond, 1910 Carb. ?Pyknochiton Hoare, 2000 Perm. ?Hadrochiton Hoare, 2000 Perm. Ferreiraella Sirenko, 1988 Rec. Family Glyptochitonidae Starobogatov & Sirenko, 1975 Glyptochiton Konninck, 1883 Carb. Family Leptochitonidae Dall, 1889 Colapterochiton Hoare & Mapes, 1985 Carb. Coryssochiton DeBrock, Hoare & Mapes, 1984 Carb. Proleptochiton Sirenko & Starobogatov, 1977 Carb. Schematochiton Hoare, 2002 Perm. Pterochiton (Carpenter MS) Dall, 1882 Carb.-Perm. Leptochiton Gray, 1847 Eoc.-Rec. Thiele, 1909 Mioc.-Rec. Jaeckel, 1900 Trias. Pseudoischnochiton Ashby, 1930 Mioc. Risso, 1826 Eoc.-Rec. Hanleyella Sirenko, 1973 Olig.-Rec. 44 B. Sirenko

Family Camptochitonidae Sirenko, 1997 Camptochiton DeBrock, Hoare & Mapes, 1984 Carb. Pedanochiton DeBrock, Hoare & Mapes, 1984 Carb. Euleptochiton Hoare & Mapes, 1985 Carb. Pileochiton DeBrock, Hoare & Mapes, 1984 Carb. Chauliochiton Hoare & Smith, 1984 Perm. Stegochiton Hoare & Smith, 1984 Perm. Family Nierstraszellidae Sirenko, 1992 Nierstraszella Sirenko, 1992 Rec. Family Mesochitonidae Dell’ Angelo & Palazzi, 1989 Mesochiton Van Belle, 1975 Trias. Pterygochiton Rochebrune, 1883 Jura. Family Protochitonidae Ashby, 1925 Protochiton Ashby, 1925 Eoc. Deshayesiella (Carpenter MS) Dall, 1879 Rec. Oldroydia Dall, 1894 Rec. Family Hanleyidae Bergenhayn, 1955 Hanleya Gray, 1857 Rec. Order Chitonida Thiele, 1910 Suborder Chitonina Thiele, 1910 Superfamily Chitonoidea Rafinesque, 1815 Family Ochmazochitonidae Hoare & Smith, 1984 Ochmazochiton Hoare & Smith, 1984 Perm. Family Ischnochitonidae Dall, 1889 Ischnochiton Gray, 1847 Eoc.-Rec. Stenochiton H. Adams & Angas, 1864 Rec. Stenoplax (Carpenter MS) Dall, 1879 Eoc.-Rec. Lepidozona Pilsbry, 1892 Mio.-Rec. Stenosemus Middendorff, 1847 Rec. Subterenochiton Iredale & Hull, 1924. Rec. Thermochiton Saito & Okutani, 1990 Rec. Connexochiton Kaas, 1979 Pleist.-Rec. Tonicina Thiele, 1906 Rec. Family Callistoplacidae Pilsbry, 1893 Ischnoplax Dall, 1879 Rec. Callistochiton (Carpenter MS) Dall, 1879 Mioc.-Rec. Callistoplax Dall, 1882 Rec. Calloplax Thiele, 1909 Rec. Family Chaetopleuridae Plate, 1899 Chaetopleura Shuttleworth, 1853 Olig.-Rec Dinoplax (Carpenter MS) Dall, 1879 Rec. Family Loricidae Iredale & Hull, 1923 H. & A. Adams, 1852 Cret.-Rec. Loricella Pilsbry, 1893 Mioc.-Rec. Oochiton Ashby, 1929 Mioc. Family Callochitonidae Plate, 1901 Callochiton Gray, 1847 Olig.-Rec. Eudoxochiton Shuttleworth, 1853 Rec. Vermichiton Kaas, 1979 Rec. New Outlook on the System of Chitons 45

Family Chitonidae Rafinesque, 1815 Subfamily Chitoninae Rafinesque, 1815 Chiton Linnaeus, 1758 Cret.-Rec. Radsia Gray, 1847 Rec. Thiele, 1893 Olig. -Rec. Tegulaplax Iredale & Hull, 1926 Rec. Mucrosquama Iredale, 1893 Rec. Subfamily Toniciinae Pilsbry, 1893 Tonicia Gray, 1847 Mioc.-Rec. Onithochiton Gray, 1847 Pleist.-Rec. Subfamily Acanthopleurinae Dall, 1889 Acanthopleura Guilding, 1829 Mioc.-Rec. Liolophura Pilsbry, 1893 Mioc.-Rec. Enoplochiton Gray, 1847 Rec. Squamopleura Nierstrasz, 1905 Rec. Superfamily Schizochitonoidea Dall, 1889 Family Schizochitonidae Dall, 1889 Incissiochiton Van Belle, 1985 Paleoc. Schizochiton Gray, 1847 Mioc.-Rec. Suborder Acanthochitonina Bergenhayn, 1930 Superfamily Mopalioidea Dall, 1889 Family Tonicellidae Simroth, 1894 Subfamily Tonicellinae Simroth, 1894 Lepidochitona Gray, 1821 Paleoc.-Rec. Particulazona Kaas, 1993 Rec. Boreochiton Sars, 1878 Rec. Tonicella Carpenter, 1873 Mioc.-Rec. Ceratozona Dall, 1882 Rec. (Carpenter MS) Dall, 1871 Pleist.-Rec. Spongioradsia Pilsbry, 1894 Rec. Oligochiton Berry, 1922 Olig. Subfamily Juvenichitoninae Sirenko, 1975 Juvenichiton Sirenko, 1975 Rec. Micichiton Sirenko, 1975 Rec. Nanichiton Sirenko, 1975 Rec. Family Schizoplacidae Bergenhayn, 1955 Schizoplax Dall, 1878 Rec. Family Mopaliidae Dall, 1889 Subfamily Heterochitoninae Van Belle, 1978 Heterochiton Fucini, 1912 Jura. Allochiton Fucini, 1912 Jura. Subfamily Mopaliinae Dall, 1889 Plaxiphora Gray, 1847 Eoc.-Rec. Placiphorina Kaas & Van Belle, 1994 Rec. Nuttallochiton Plate, 1899 Rec. Mopalia Gray, 1847 Mioc.-Rec. Placiphorella (Carpenter MS) Dall, 1879 Mioc.-Rec. Katharina Gray, 1847 Plio.-Rec. Amicula Gray, 1847 Pleist.-Rec. 46 B. Sirenko

Superfamily Cryptoplacoidea H. & A. Adams, 1858 Family Acanthochitonidae Pilsbry, 1893 Subfamily Acanthochitoninae Pilsbry, 1893 Acanthochitona Gray, 1921 Olig.-Rec. Craspedochiton Shuttleworth, 1853 Mioc.-Rec. Spongiochiton (Carpenter MS) Dall, 1882 Rec. Notoplax H. Adams, 1861 Olig.-Rec. Pseudotonicia Ashby, 1928 Rec. Bassethullia Pilsbry, 1928 Rec. Americhiton Watters, 1990 Rec. Choneplax (Carpenter MS) Dall, 1882 Rec. Cryptoconchus (De Blainville MS) Burrow, 1815 Plio.-Rec. Subfamily Cryptochitoninae Pilsbry, 1893 Cryptochiton Middendorff, 1847 Plio.-Rec. Family Hemiarthridae Sirenko, 1997 Hemiarthrum Carpenter in Dall, 1876 Rec. Weedingia Kaas, 1988 Rec. Family Choriplacidae Ashby, 1928 Choriplax Pilsbry, 1894 Rec. Family Cryptoplacidae H. & A. Adams, 1858 Cryptoplax de Blainville, 1818 Mioc.-Rec.

Incertae sedis Family Scanochitonidae Bergenhayn, 1955 Scanochiton Bergenhayn, 1955 Cret. Family Olingechitonidae Starobogatov & Sirenko, 1977 Olingechiton Bergenhayn, 1943 Cret. Family Haeggochitonidae Sirenko & Starobogatov, 1977 Haeggochiton Bergenhayn, 1955 Cret. Family Ivoechitonidae Sirenko & Starobogatov, 1977 Ivoechiton Bergenhayn, 1955 Cret.

Acknowledgements

I would like to express my sincerely thanks to all the organizers of the Second International Chiton Symposium and especially to my good friends Dr. Hiroshi Saito, Dr. Masami Hamaguchi, Dr. Miho Sasaki, Dr. Shirou Nishihama, Dr. Kenji Okoshi, Dr. Chiya Numako and Dr. Eiji Yoshioka. I wish to express my appreciation to my friend Dr. Douglas J. Eernisse for his very important critical comments and for tidying my English up. My thanks also go to Mr. Taimuraz K. Tsogoev (Zoological Institute, St. Petersburg) for his valuable assistance with the SEM and Mrs. Tatjana Konina (the same Institute) for her skilled help in preparing the text of this article.

References

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(Received April 2, 2004 / Accepted May 10, 2005) New Outlook on the System of Chitons 49

多板類分類体系の新しい視点

B. I. シレンコ

要 約

多板類の分類体系はこれまで主に殻板の形質をもとに構築されていたが（Bergenhayn, 1955; Van Belle, 1983 など），本研究では殻板の形質を再検討し，さらに枝状器官，肉帯，歯舌，鰓，各種の腺，卵殻の 突起，精子の形態など新しい分類形質を加えて多板類の分類体系を構築した。 まず従来の分類で重要視されてきた殻板の形質，特に連接層の形質を再検討した。カンブリア紀から 出現した多板類の進化史上，石炭紀後期における連接層の獲得と連接層のその後の発達は極めて重要な できごとであったと考えられる。したがって多板類の系統を考える上で連接層の重要性は変わらないが， それに付随した形質，たとえば着生板や歯隙などは殻表の形態とともに変化しやすく，平行現象が起き ているため，系統解析で用いるにあたっては注意が必要である。着生板に歯隙がないナンキョクヒザラ ガイ属 Hemiarthrum, ナンヨウヒザラガイ属 Weedingia, マボロシヒザラガイ属 Choriplax が旧分類では原 始的なサメハダヒザラガイ亜目 Lepidopleurina に置かれてきたことはこのような平行現象が誤って解釈さ れた例である。 新しい分類形質としてはまず鰓の形態と配列があげられる。鰓の配列はこれまでも離肛型（abanal type：最後端の鰓と肛門の間が離れる）と近肛型（adanal type：最後端の鰓が肛門に近接する）に分け られてきたが，離肛型を腎口の直後に原則として 1 つの鰓のみをもつもの，近肛型を同様に 3 つ以上 もつものと再定義すると，これまでと異なったグルーピングがなされる。さらに鰓の配列と卵殻突起 の形態には関連があることが判った。すなわち離肛型のものは卵殻突起の基部が小さく細長い突起をも ち，近肛型は基部が大きく，塊状の突起をもつ。このことから現生多板類のうちサメハダヒザラガイ 目 Lepidopleurida を除くすべての種を含むクサズリガイ目 Chitonida は離肛型のケハダヒザラガイ亜目 Acanthochitonina とクサズリガイ亜目 Chitonina とに分類される。この分類は精子の微細構造の違いによ っても支持され，また分子系統学的研究の結果によっても一部支持される。 以上の結果をまとめ，著者自身がこれまで提案した分類体系を再検討して図 12 に示す分類体系を提 案した。要点は以下のようになる。サメハダヒザラガイ目を連接層の発達程度の低い Cymatochitonina 亜 目と発達程度の高いサメハダヒザラガイ亜目に 2 分した。サメハダヒザラガイ亜目の Deshayesiella と Oldroydia をサメハダヒザラガイ科 Leptochitonidae から Protochitonidae 科に移動した。最も重要な点は上 記のようにクサズリガイ目をケハダヒザラガイ亜目とクサズリガイ亜目に 2 分したことである。これに 関しては，すでに移動されていたナンキョクヒザラガイ属に加え，ナンヨウヒザラガイ属と マボロシヒ ザラガイ属も同様にサメハダヒザラガイ目からクサズリガイ目のケハダヒザラガイ亜目に移動した。な お所属が不明なものとして絶滅群では Scanochitonida，現生群ではハチノスヒザラガイ属 Callochiton とサ ケオヒザラガイ属 Schizochiton が残っている。