Adamussium jonkersi Quaglio, Whittle, Gazdzicki & Simões, 2010
QUAGLIO, F., R. J. WHITTLE, A. GAZDZICKI & M. G. SIMÕES. 2010. A new fossil Adamussium (Bivalvia: Pectinidae) from Antarctica. Polish Polar Research, 31 (4): 291-302, figs. 1-6 [p. 295, figs 4-6]
2010 Adamussium jonkersi Quaglio, Whittle, Gazdzicki & Simões, 2010
F. Quaglio et al., 2010, figure 5.
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«Etymology: After H. A. Jonkers, for his noteworthy contribution to the paleontological studies of pectinids of the Southern Ocean.
Material. — Holotype, right valve (L3/41). Paratypes, right valves (L3/38, 40, P.2904.10.1) and left valves (L3/39, 42).
Diagnosis. — Radial sculpture of 7-9 broadly undulating, well−developed main plicae, intercalated with faintly developed secondary plicae on the ventral part of the disc of most specimens; commarginal sculpture of distinct but weak lirae, with groups almost equally and closely spaced down the entire shell; RV anterior auricle rounded, with 3-4 radial costae observed internally. Description. — Shell moderate to small in size, suborbicular (chlamydoid to aequipectinoid); dorsal margin short, ventral margin wide and rounded; equant; apparently very compressed, with moderately convex valves, LV more convex than RV; beaks orthogyrate; umbonal angle narrow (ca 110°); radial sculpture of 7-9 broadly undulating, well−developed main plicae, intercalated with faintly developed secondary plicae towards ventral part of disc; commarginal sculpture of distinct but weak lirae, in groups almost equally and closely spaced down entire shell; hinge line straight; RV auricles apparently asymmetrical, anterior auricle rounded, with 3-4 radial costae observed internally. Comparison.— The characters of all known Adamussium species are summarized in Table 2. Assessing all of the characters, each species is unique, however, not many of the characters on their own indicate a clear taxonomic affinity between any two taxa. This can be seen from Table 2, where one species seems to be more related to another considering a given character (e.g. the similar opisthocline shape of the valves in A. c. colbecki and A. c. cockburnensi) and to a different species if another character is considered (e.g. the moderate convexity of the valves and symmetrical auricles in A. c. colbecki and A. auristriatum). Finally, A. auristriatum and A. jonkersi sp. nov. are similar in possessing procline valves and striated auricles. A. alanbeui differs from all other species by lacking radial plicae. Apart from this, Table 2 also shows that only A. c. colbecki and A. jonkersi sp. nov. are morphologically distinct. Therefore, the small morphological differences between the other species possibly indicate either environmentally induced modifications (morphoclines) or gradual, anagenetic evolution.
Despite the poor preservation, Adamussium jonkersi sp. nov. easily can be differentiated from A. auristriatum by its weaker valve convexity, narrower umbonal angle, apparent asymmetry of auricles, and the fewer plicae, which are better−defined and more widely separated that in A. auristriatum, and with intercalated fine plicae observable only at the ventral margin of the valves. Adamussium jonkersi sp. nov. also differs from both subspecies A. colbecki colbecki and A. colbecki cockburnensis in its apparently procline valves, narrower umbonal angle, and the presence of radial costae on the RV anterior auricle. Shell microsculpture.— The commarginal sculpture of Adamussium jonkersi sp. nov. is composed of distinct but weak lirae, in groups almost equally and closely spaced down the entire shell. Unlike other species of the genus, the microsculpture is not observed in SEM images (Fig. 6). Several explanations for this are possible, including the loss of ridgelets through ontogenetic development during life, the loss of this fragile microsculptural pattern during diagenetic recrystallization of the shell, or even because this microsculptural feature is absent (at least at the ventral margin of the available shell material studied under SEM).
Paleoecology.— According to Stanley (1970), pectinids with a swimming habit are characterized by an inequivalve condition, with the lower (right) valve being more convex than the upper valve, the presence of a thin shell with plicae, a broad umbonal angle (around 105 to 130), and gapes adjacent to the auricles. Jonkers (2003), based on Stanley (1970), considered that swimming pectinids have an umbonal angle greater than 108 and a ratio between the anterior and posterior auricle lengths smaller than 1.7. However, Hayami (1991) stressed that the presence of radial folds results in an increase in drag coefficient. Stanley (1970) also affirmed that byssally attached pectinids, on the other hand, possess elongated anterior auricles, a byssal notch with no adjacent gapes, and a narrow umbonal angle. Of the features cited above, A. jonkersi sp. nov. displays an umbonal angle indicating that specimens range from byssate to swimming habits (100 to 110), an apparently slight inequivalve condition, and a rather thin shell with broad undulating plicae, which point to a free−swimming habit. The plicate shell, a character present in all Adamussium species other than A. alanbeui, increases frictional drag in the water, thus reducing swimming capability due to the high−energy demand for swimming (Hayami 1991). The auricles are apparently asymmetrical. However, because of the incomplete preservation of the fossils, the features that would point to a truly byssate habit, such as the presence of a ctenolium and a wide byssal notch adjacent to the anterior auricle, are not observable. Despite the poor preservation, we infer a free−living, non−active swimming habit for A. jonkersi. More specifically,we suppose that it had the ability to clap its valves in order to escape from a potential predator. However, this condition does not exclude the possibility that this scallop lived byssally attached during younger stages, as many pectinids do (Stanley 1970; Gould 1971).
Taphonomy.— The assemblage is composed of thin−shelled specimens, preserved as disarticulated, complete or partially fragmentary valves. The shells are in a convex−up orientation, concordant with bedding (Fig. 4). The sedimentological features of the unit indicates an estuarine environment to lower−energy tidal flat (Birkenmajer 1982). The disarticulation of the shells and their preservation in a convex−up orientation indicate lateral transport of the valves for a short distance due to traction currents. Indeed, as demonstrated by the experimental studies of McKittrick (1987) and Simões et al. (2006), convex shells always settle down in a concave−up (unstable) orientation. Hence, the convex−up attitude (stable orientation) of the shells in relation to bedding is achieved when waves, currents and bioturbation are present in the environment (Simões et al. 2006). Hence, despite their reclinant, free−living mode of life, the studied shells of Adamussium jonkersi are parauthocthonous (sensu Kidwell et al. 1986), and were deposited in a low− to medium−energy environment. Final comments
The modern representative of the genus Adamussium, A. c. colbecki, is an endemic Antarctic species with a circum−Antarctic distribution (Dell 1972, 1990), and is found in almost all sites protected from high−energy water conditions around Antarctica (Schiaparelli and Linse 2006). Some studies have shown that this Antarctic species is highly sensitive to temperature increase, suggesting that it is physiologically adapted to low environmental temperatures (Viarengo et al. 1999; Peck et al. 2004). Because its ancestral lineage can be traced back to the Oligocene, we conclude that this important component of the Recent Antarctic fauna probably had an evolutionary past related to the Cenozoic cooling history of the continent. In this way, the new taxon contributes to the knowledge of the fossil record of the genus and potentially will help future studies of environmental change in Antarctica during the Cenozoic.» FERNANDA QUAGLIO, ROWAN JANE WHITTLE, ANDRZEJ GAZDZICKI & MARCELO GUIMARÃES SIMÕES, 2010
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