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Phylogenesis and the System of the Cambrian Univalved Mollusks P

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Phylogenesis and the System of the Cambrian Univalved Mollusks P Paleontological Journal, Vol. 36, No. 1, 2002, pp. 25–36. Translated from Paleontologicheskii Zhurnal, No. 1, 2002, pp. 27–39. Original Russian Text Copyright © 2002 by Parkhaev. English Translation Copyright © 2002 by åÄIä “Nauka /Interperiodica” (Russia). Phylogenesis and the System of the Cambrian Univalved Mollusks P. Yu. Parkhaev Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117997 Russia Received May 30, 2000 Abstract—The history of the study and the development of ideas about the systematic position of the Cambrian univalved mollusks is reviewed. On the basis of the functional morphology of shells (Parkhaev, 2000a, 2001a), the phylogenetic scenario of the evolution of the group is reconstructed and its systematics is determined. Sev- eral new taxa are described, i.e., Archaeobranchia subclas. nov., Khairkhaniiformes ordo nov., Igarkiellidae fam. nov., Trenellidae fam. nov., Watsonellinae subfam. nov., and Runnegarella gen. nov. HISTORY OF THE STUDY OF THE CAMBRIAN This new method of fossil preparation resulted in an UNIVALVED MOLLUSKS increased number of publications on the Cambrian fauna and, therefore, in an increased number of described taxa The first mollusks from the Cambrian deposits were of small shelly fossils (including mollusks). described in the mid-19th century by Hall and d’Orbigny. In the second half of the 19th century, the The number of specialists that studied the Cambrian number of taxa of the Cambrian mollusks significantly mollusks also increased, and different, in some cases increased due to the numerous publications focused on even contradictory, opinions on the systematics of this the fauna of that geological period (Barrande, 1867; group appeared. Billings, 1872; Shaler and Foerste, 1888; Tate, 1892; Missarzhevsky (Missarzhevsky and Rozanov, 1966; Matthew, 1895, 1899). The authors put the described Rozanov et al., 1969; Missarzhevsky and Mambetov, species and genera within already existing families and 1981; Missarzhevsky, 1981, 1989) supported this view, orders of gastropods: the cap-shaped forms were which had become classic since the works of Wenz and grouped together with patelloids, and the spirally Knight. He placed all cap-shaped Helcionella-like coiled ones were grouped together with trochids or forms into the class of monoplacophorans, while those other primitive gastropods. Such an approach to the spirally coiled, like Pelagiella or Aldanella, were systematic position of the Cambrian mollusks domi- assigned to gastropods. In one of his last monographs, nated even later, in the first half of the 20th century, in Missarzhevsky (1989) suggested the establishment of a the works of Cobbold (Cobbold, 1921, 1935; Cobbold conventional group of the ordinal rank for the hel- and Pocock, 1934), Kobayashi (1933, 1935, 1937, cionelloideans and to name it Eomonoplacophora. He 1939), and other malacologists. justified the separation of this group from typical In the mid-20th century, Odhner (Odhner in Wenz, monoplacophoran mollusks by the absence of multiple 1940) introduced the term Monoplacophora; later, muscle scars on the interior surface of the shell. Wenz (Wenz in Knight, 1952) used this name for the In the early 1970s, Runnegar, Jell, and Pojeta order of cap-shaped Paleozoic gastropods. At the begin- described the rich molluskan assemblage from the ning of the 1950s, deep-sea dredging in the Atlantic lower Middle Cambrian of Australia (New South Ocean discovered live monoplacophorans. In 1957, Wales, Coonigan Formation). The excellent preserva- Lemche (1957) used the term Monoplacophora as a tion of the material from this locality (silicified shells) class name. In the molluskan volume of “Treatise on and its diversity gave the opportunity to study the func- Invertebrate Paleontology,” Knight and Yochelson also tional morphology of mollusks and make assumptions raised the Monoplacophora to the rank of class and on their ecology and systematic position. The authors included all the known Cambrian mollusks with cap- proposed to place the cap-shaped helcionelloid mol- shaped shells in this taxon (Knight and Yochelson, 1960). lusks among monoplacophorans. This proposal was Since the mid-1960s, a new period of the study of based on the interpretation of the first finds of an aston- the Cambrian fauna has begun. At the Geological Insti- ishing mollusk, Yochelcionella, which possessed a tute (Moscow), a laboratory for the mass chemical tubular structure (snorkel) on the subapical part of the extraction of organic remains from the Lower Cam- shell. The authors believed that the snorkel took the brian rocks was organized. It was discovered that anterior position and was used for water input inside deposits of this age contain a rich assemblage of small the palial cavity. Having such a position of the snorkel, shelly fossils (SSF), including numerous mollusks. the shell appears to be exogastrically coiled, and this 25 26 PARKHAEV corresponds with the monoplacophoran diagnosis. This as well (Starobogatov, 1970, 1976, 1977; Golikov and reconstruction of the general morphology of helcionel- Starobogatov, 1975; Minichev and Starobogatov, 1979; lids was used as a base for the phylogenetic scheme that Starobogatov and Moskalev, 1987). This study resulted was proposed by the authors (Runnegar and Pojeta, in a new and, to my mind, rather harmonious and tena- 1974, 1985; Runnegar and Jell, 1976, 1980; Pojeta and ble system of gastropods. The class Gastropoda was Runnegar, 1976, 1985; Runnegar, 1981, 1983, 1985). divided into eight subclasses in the framework of this Another opinion on the systematic position of the system (Golikov and Starobogatov, 1988), and the sys- Cambrian mollusks was expressed by Yochelson. Sev- tematic position of some taxa was revised, including eral taxa were completely removed from the Mollusca; several groups of Early Paleozoic mollusks. The major he considered spirally coiled Aldanella to be the tubes part of the Cambrian univalved mollusks was inter- of worms (Yochelson, 1978; Bockelie and Yochelson, preted as gastropods. The helcionelloid mollusks were 1979) and cap-shaped wide shells, such as Scenella or put into the most primitive gastropod subclass, Cyclo- Palaeacmaea, to be imprints of cnidarian medusoid branchia, along with the patelloid gastropods. Based on organisms (Yochelson and Gil Cid, 1984; Webers and the similarity in their shell forms, the pelagiellids were Yochelson, 1999). The morphology of Yochelcionella lumped together with the Early Paleozoic macluritids was also interpreted in a different way. Yochelson into the subclass Divasibranchia, which also contains believed these mollusks to be endogastric and untorted the recent siphonariids. The family Onychochilidae with a posterior exhalant snorkel. In a similar way, i.e., was assigned to the subclass Dextrobranchia, which with the posteriorly directed apex, all the other hel- contains the recent pteropods. cionelloid shells were reconstructed. Thus, Yochelson was the first to suggest a separate taxonomical position PHYLOGENESIS AND SYSTEM for the helcionelloids. In addition, he proposed a rather OF THE CAMBRIAN UNIVALVED MOLLUSKS unconventional treatment of the Pelagiella systematic position. He regarded this form to be an asymmetric, In my recent papers (Parkhaev, 2000a, 2001a), endogastric but untorted mollusk and, therefore, to be I supported the opinion of Starobogatov and his coau- removed from both Gastropoda and Monoplacophora thors (Golikov and Starobogatov, 1975, 1988; Mini- and placed into a special group (Yochelson, 1978). chev and Starobogatov, 1979), considering the hel- A similar conclusion was later reached by Runnegar cionelloid mollusks to be gastropods. (1981), who proposed the consideration of Pelagiella Let us try to determine the system of this group. We as an intermediate form between Monoplacophora and should base this system on the commonly accepted Gastropoda that already evolved the asymmetric shell hypothesis that monoplacophoran mollusks are ances- but failed to complete torsion. tral to gastropods. A simple cap-shaped shell with a Linsley and Kier (1984) studied the morphology of central or subcentral apex could be considered to be the the Paleozoic hyperstrophic shells from the functional most primitive shell type, like in Helcionella, Bemella, aspect and came to the conclusion that those forms or Ilsanella. These genera form the backbone of the experienced no torsion. Thus, they had to remove sev- extensive family Helcionellidae Wenz, 1938. Its mem- eral Paleozoic taxa from the class Gastropoda, includ- bers had the shell of the above-mentioned type and ing the Cambrian Onychochilidae, Pelagiellidae, and were characterized by the symmetrical palial complex Aldanellidae, and establish for them a new class, Para- and postero-anterior water circulation: water entered gastropoda. the shell through the posterior edge of the aperture and was discharged over the mollusk head through the ante- Peel developed on Yochelson’s view in his works on rior edge. The possibility of this type of circulation pat- the functional morphology, evolution, and systematics tern was claimed by Golikov and Starobogatov (1988). of the Early Paleozoic univalved mollusks (Peel and According to their assumption, the postero-anterior cir- Yochelson, 1987; Peel, 1991). He showed that the gross culation type of the primitive gastropods was inherited morphology of helcionelloid mollusks is that of the from monoplacophoran ancestors. Monoplacophorans endogastric but untorted organisms
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