Mollusks Polyplacophorans

The first probable multiplated mollusks appeared during the latest Late Cambrian (Bergenhayan 1960; Stinchcomb and Darrough 1995). Early Cambrian Triplicatella, previously interpreted as the earliest chiton (Yates et al. 1992), is an operculum (Conway Morris and Peel 1995). The morphology of the Late Cambrian multiplated mol-lusks, probable members of the class Polyplacophora, is the subject of some debate. They may be reconstructed as metamerized sluglike animals bearing about eight mid-dorsal plates (Pojeta 1980; Stinchcomb and Darrough 1995). A Late Cambrian multiplated mollusk, Matthevia, has been described in detail, based on co-occurrence of three morphologic types of matthevian shells (valves) (Runnegar et al. 1979). Each shell possesses two large ventral holes; no multiple muscle scars were found. All the valves, when clustered in situ, are of essentially the same shape. The armor might have consisted of more or less than eight shells. Hemithecella and Elongata, which were described by Stinchcomb and Darrough (1995), differ from representatives of the post-Cambrian order Paleoloricata (class Polyplacophora) and Matthevia. The assignment of such forms to the Polyplacophora is questionable because the number and arrangement of scars are similar to those of monoplacophorans.

Conical shells of the multiplated mollusks were robust enough to withstand storm-wave activity. Like Recent chitons, the Late Cambrian multiplated mollusks possibly were scrapers or grazers that fed on algal and bacterial mats (figure 15.2:9, 10) (Taylor and Halley 1974; Runnegar et al. 1979). Shells of multiplated mollusks are associated with stromatolite cores that show little abrasion and rarely breakage. This suggests that they occupied stromatolitic reef areas and may well have lived on firm substrates of stromatolitic buildups (Runnegar et al. 1979; Stinchcomb and Darrough 1995). Like Recent chitons, they possibly lived in intertidal and shallow subtidal environments.

Figure 15.1 Generalized reconstruction of the Early Cambrian community of mollusks, hyo-liths, stenothecoids, and coeloscleritophorans (background = calcimicrobial-archaeocyathan mounds). Helcionelloids: 1, Oelandiella; 2, Anabarella; 4, Yochelcionella; 5, Ilsanella. Paragastro-

pod: 3, Aldanella. Stenothecoid: 6, Stenothe-coides. Rostroconch: 7, Watsonella. Pelecypod: 8, Fordilla. Orthothecimorph hyoliths: 9, La-datheca; 10, Conotheca. Hyolithomorph hyolith: 11, Burithes. Coeloscleritophorans: 12, Chancelloria. 13, Halkieria.

Figure 15.1 Generalized reconstruction of the Early Cambrian community of mollusks, hyo-liths, stenothecoids, and coeloscleritophorans (background = calcimicrobial-archaeocyathan mounds). Helcionelloids: 1, Oelandiella; 2, Anabarella; 4, Yochelcionella; 5, Ilsanella. Paragastro-

pod: 3, Aldanella. Stenothecoid: 6, Stenothe-coides. Rostroconch: 7, Watsonella. Pelecypod: 8, Fordilla. Orthothecimorph hyoliths: 9, La-datheca; 10, Conotheca. Hyolithomorph hyolith: 11, Burithes. Coeloscleritophorans: 12, Chancelloria. 13, Halkieria.

Figure 15.2 Generalized reconstruction of the Late Cambrian community of mollusks and hyoliths (background = stromatolithic mounds). Gastropods: 1, Sinuopea; 2, Strepso-discus; 3, Matherella; 4, Spirodentalium. Tergo-myans: 5, Proplina; 7, Hypseloconus. Helcionel-

loid: 6, Scenella. Cephalopod: 8, Plectronoceras. Polyplacophorans: 9, Matthevia; 10, Hemithe-cella. Rostroconchs: 11, Pleuropegma; 12, Oepi-kila; 13, Ribeiria. Orthothecimorph hyolith: 14, Tcharatheca. Hyolithomorph hyolith: 15, Linevitus.

Figure 15.2 Generalized reconstruction of the Late Cambrian community of mollusks and hyoliths (background = stromatolithic mounds). Gastropods: 1, Sinuopea; 2, Strepso-discus; 3, Matherella; 4, Spirodentalium. Tergo-myans: 5, Proplina; 7, Hypseloconus. Helcionel-

loid: 6, Scenella. Cephalopod: 8, Plectronoceras. Polyplacophorans: 9, Matthevia; 10, Hemithe-cella. Rostroconchs: 11, Pleuropegma; 12, Oepi-kila; 13, Ribeiria. Orthothecimorph hyolith: 14, Tcharatheca. Hyolithomorph hyolith: 15, Linevitus.

Helcionelloids and Paragastropods

The majority of Cambrian univalves (helcionelloids) fall into three main morphologic categories. These reflect adaptive strategies but are also important evolutionarily, giving rise to pelecypods, rostroconchs, and, subsequently, scaphopods.

The earliest helcionelloid, Bemella, is a small caplike shell, with the apex usually lying outside by a slightly elongate apertural ring. Planispirally coiled Latouchella-like and Bemella-like shells, with relatively broad apertures, are abundant and diverse in the lowermost Lower Cambrian and also subsequently. They exhibit a compromise between a flattened shell with broad aperture and a tightly coiled shell with a small aperture (Runnegar and Pojeta 1985). They occur in various facies worldwide and, based on their low-spired and widely expanded shell (Linsley 1978), would have had a broad foot, which characterizes sluggish epifaunal deposit feeders (figure 15.1:1,5) (Kruse et al. 1995; Gubanov and Peel 1999). They were probably an ancestor for other morphologic-adaptive lineages of helcionelloids.

The principal morphologic trend among helcionelloids is lateral compression of the shell and aperture and loss of strong comarginal ornamentation, often followed by the development of emarginations such as sinus, internal ridges, and snorkel. Such shells have an elongate narrow aperture and a high rate of expansion, with rather smooth but often plicate walls (e.g., Anabarella, Stenotheca). Peel (1991) has reconstructed Eote-benna as a transitional range of forms from sinus-bearing to elongated with snorkel. These emarginations are assumed to have had an exhalant function (sometimes both exhalant and inhalant) and were oriented posteriorly (Peel 1991). Some reconstructions place them anteriorly (Runnegar and Jell 1980), but the small cross-sectional area of the snorkel in Yochelcionella, and the development of the snorkel in Eotebenna and Oelandia, suggest its posterior direction and exhalant function (Peel 1991). Lateral compression of the shells may be consistent with a vagrant semi-infaunal living mode and with suspension or detritus feeding (Runnegar and Pojeta 1985). Using the criteria of Linsley (1978) and McNair et al. (1981), laterally compressed and widely umbilical helcionelloids with a long aperture, such as Bemella, Anabarella, and Yochel-cionella, are inferred to have been actively mobile on soft substrate in low-energy conditions and thus to have been semi-infaunal filter feeders (Peel 1991; Kruse et al. 1995; Gubanov and Peel 1999) (figure 15.1:2, 4).

However, the distinction between suspension and deposit feeding, as well as between semi-infaunal and epifaunal habitats, may be meaningless in such small animals, approaching interstitial sizes. Among modern macrofauna, deposit-feeding invertebrates feed principally upon bacteria, whereas suspension feeders ingest phytoplank-ton (Levinton 1974). For diminutive Early-Middle Cambrian mollusks, such a distinctive difference might be inappropriate.

Another main adaptive strategy of helcionelloids is shell elongation and subsequent compaction by means of coiling into a bilaterally symmetric, or dissymmetric, spiral. This mode of development is seen in low-spired bilaterally symmetric Latouchella-like forms when the beak deviates to the left (e.g., Pseudoyangtzespira) or to the right (e.g., Archaeospira), giving rise to dextrally or sinistrally coiled forms, respectively (Qian and Bengtson 1989). Together with increase in the number of revolutions, sculptural relief becomes lower in a succession of dextral forms: Aldanella crassa—A. operosa-Paraaldanella (Golubev 1976). The shell becomes involute or tightly coiled evolute, with more revolutions in groups of sinistral mollusks (Barskovia hemisymmetrica—B. rotunda; Beshtashella-Yuwenia-Kistasella) (Missarzhevsky 1989; Bengtson et al. 1990) and planispiral forms (Khairkhania n.sp.-K. evoluta-K. rotata) (Esakova and Zhegallo 1996). Hook-shaped forms (e.g., Ceratoconus) probably often precede loosely and tightly coiled symmetric or asymmetric conchs with low rates of expansion. Uncoiled, tall, small-apertured shells have a high pressure point and center of gravity (Linsley 1978). To balance such a shell when moving, it is necessary to obtain a lower center of gravity and pressure point and to minimize the frontal cross-sectional area. Curvature and coiling enable a shell held by a snail to be balanced, because movement with a tall or loosely coiled shell is difficult in agitated water. Achievement of a proportionately small cross-sectional area, low pressure point, and low center of gravity favors active locomotion.

Because they were compact, strong, and able to contain a relatively voluminous body, tightly coiled shells could successfully compete with other forms and invade various ecologic niches. Detritus-feeding or grazing is usually assigned to the Cambrian coiled mollusks (Runnegar and Pojeta 1985). Minute shell size, especially in the Early Cambrian, suggests that many Cambrian paragastropods may well have used algae as substrates. Peel (1991) concluded this for Recent and Silurian gastropods of 1-2 mm in size. On the other hand, small paragastropods, with their elongated tangential aperture, have also been inferred to have been mobile epifaunal deposit feeders on soft substrates (Linsley and Kier 1984) (figure 15.1:3).

It is possible that small or large individuals of the same species occurred in different environments in the Early Cambrian, depending on water energy. A large helcionel-loid, Randomia aurorae, was common in microbial mud mounds of the Fosters Point Formation (Landing 1992). Another large helcionelloid, described as Bemella jacutica, was recovered in the vicinity of calcimicrobial-archaeocyath reefs of the Pestrotsvet Formation (Dzik 1991). Peribiohermal facies of the Selinde River calcimicrobial-archaeocyath reefs are surrounded by limestones with abundant Helcionella with diameters up to 1.5 cm. These occur with their apex upright, which is suggestive of their in situ life position (Repina and Zhuravleva 1977). Tannuella elata, a large (23 cm) Atdabanian helcionelloid, occurs in interbiohermal and peribiohermal facies of the Medvezh'ya River archaeocyathan reefs (Sundukov and Fedorov 1986). Shallow subtidal wackestones of the Medvezh'ya Formation abound with Aldanella costata (pers. obs.). This organism probably dominated subtidal muddy soft substrates of the Tommotian Yudoma-Olenek Basin, Siberian Platform (Vasil'eva and Rudavskaya 1989). Very shallow level-bottom environments are indicated by condensed peritidal limestones that form, for example, the tops of shoaling cycles in Member 4 of the Early Cambrian Chapel Island Formation (Myrow and Landing 1992), with numerous firm surfaces containing abundant but small helcionelloids. In general, mollusks that inhabited reefal areas were relatively sizable forms with robust conchs.

Was this article helpful?

0 0

Post a comment