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Cycladophora spongothorax (Chen, 1975)

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Cephalis cap-shaped with small circular pores, bearing two three-bladed spines; apical spine about the same length as the cephalis, vertical spine smaller. Thorax campanulate, circumscribed by one or two ridges formed by connecting adjacent horizontal pore bars, commonly sharp changes in contours appearing as steps at these ridges, thoracic wall of medium thickness and covered by spongy meshwork. Thoracic pores subcircular, arranged in transverse rows, increasing in size distally. Abdomen short, expanded, with 1-2 rows of subcricular pores, terminated by numerous short pore bars. Shell either open or closed by a sieve plate in complete specimens. Measurements based on 30 specimens from Samples 265-15-3,10-12cm; and 265-15,CC; length of the shell (excludng apical horn), 101-126; maximum width of the thorax, 81-125.
(Chen) 1975
Description: See Chen (1975).
Comments: Cycladophora spongothorax was described originally as a subspecies of Theocalyptra bicornis by Chen (1975). In addition to transferring it to Cycladophora, we have raised this taxon to species rank. Chen (1975) noted the similarity of C. spongothorax to what he referred to as Theocalyptra bicornis (Popofsky). This latter form is not T. bicornis but C. humerus (Petrushevskaya). Cycladophora spongothorax appears to have evolved from C. humerus in the late Miocene. Early individuals of C. spongothorax have relatively little external sponge, and in general appear more similar to C. humerus. Confirmation of this relationship however will require more detailed sampling and measurement of a much larger number of specimens. Despite this inferred relationship, the two taxa are quite distinct in most samples examined so far, and thus warrant species designation. Cycladophora spongothorax possesses several distinguishing characteristics, the most obvious one being the external spongy layer on the outer shell surface. The outline of the thorax lattice-wall is also distinctive, as it often possesses two or more shoulders or steps in the profile. C. humerus by contrast has only one step in the profile of the thorax, between the upper and lower thorax. These 'steps' are enhanced by the development of external rings in C. spongothorax attached to the outer surface of the shell, a character also seen on occasion, although never so well-developed, in C. robusta. Other characters also serve to distinguish C. spongothorax from C. humerus, particularly the relatively heavy bars of the lower thorax, and larger pores of the thorax lattice-wall (text-fig. 17).
Incompletely developed specimens of C. spongothorax. and specimens with abnormally short lower thoraxes arc fairly common. These specimens are identifiable by the characteristic stepped profile to the upper thorax, and by the development of external rings on some of the specimens.
Occurrence: Late Miocene in Antarctic sediments south of the Polar front, rare in the subantarctic.

Lombari and Lazarus 1988


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