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Eucoronis fridtjofnanseni
Goll and Bjørklund, 1980
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Description - Add description
Eucoronis fridtjofnanseni Goll and Bjorkland, n.sp. Plate 1, figures 4-12; plate 2, figures 1-10; plate 3, figures 14-26
Diagnosis: A species of Eucoronis characterized by a massive lattice shell with short stout lattice spines and well-developed primary lateral bars.
Name: In honor of Fridtjof Nansen, a Norwegian equally famous as oceanographer and humanitarian.
Description: Thick-walled sagittal ring equidimensional and D-shaped (pl. 1, fig. 9; pl. 3, figs 16-17) or tall and subrounded (pl. 3, fig. 18); 78-100 µm high and 63-78 µm thick. Front of sagittal ring bears a broad external longitudinal ridge between the apical and front spines (pl. 2, fig. 10; pl. 3, figs. 14, 21, 23, 25). Back of sagittal ring between apical and vertical spines has slight furrow on some specimens (pl. 1, fig. 6) and is circular in cross section on others (pl. 2, figs. 1, 4). Short, simple apical spine obscure on most specimens (pl. 1, figs. 5, 9-10, 12). Robust vertical spine arises from approximate mid-point of back of sagittal ring height, quite prominent on some specimens (pl. 1, fig. 9; pl. 2, figs. 3-4). Frontal spine 22-45 µm long, simple on some specimens (pl. 1, figs. 9, 12), but most specimens have secondary lateral bars joined to short frontal bar to form 1 or 2 prominent basal pores on the front of the skeleton, and free spine projects downward from distal end of frontal bar (pl. 1, figs. 4, 6, 10-12; pl. 3, figs. 21, 25). Primary lateral bars are stout, well-developed, joined to median bar of sagittal rind and back lateral surfaces of basal ring, frame 2 additional pairs of large basal pores (pl. 1, fig. 4; pl. 2, figs. 6-7). Small tuberculate axobate projects downward from median bar at point of juncture with primary lateral bar. Summit of axobate bears a field of extremely fine microspicules on most specimens (pl. 2, fig. 5), but 1 specimen has been observed with a smooth axobate (pl. 2, fig. 8). No sternal bar or tertiary lateral bars.
Lattice shell discoidal, height and width approximately equidimensional, 115-121 µm in diameter, slightly constricted by top of sagittal ring. Lattice shell consists only of basal ring and a sparse trellis of lattice arches spanning the lateral portions of the basal ring and top of the sagittal ring. Oval basal ring 95-102 µm wide, 70-78 µm thick (pl. 1, fig. 4). Lattice shell does not extend below basal ring on most specimens, but remanents of thoracic lattice occur on rare specimens (pl. 1, fig. 9). Variable number of short simple basal spines project from basal ring (pl. 2, figs. 6-7). Many specimens have a single pair of stout, spinous, out-wardly convex lattice bars projecting from lateral margins of basal ring and branching near contact with top of sagittal ring to form a varibale number of pairs of small apical sagittal lattice pores. Additionally, these lattice arches frame a pair of large, round to D-shaped, sagittal lattice pores 60-82 µm in maximum diameter on each of the front and back of the lattice shell. No sternal bar.
Dimensions based on measurements of 50 specimens from DSDP Site 341, core 25, sections 1 and 2.
Holotype: USNM 258897, DSDP Site 341, core 27, section 3, 14-16 cm; England finder 033/2: plate 3, figures 25-26.
Paratypes: USNM 258898, DSDP Site 341, core 27, section 3, 14-16 cm; England FInder N30/0; plate 3, figures 14-15. USNM 258899m DSDP Site 341, core 27, section 3, 14-16 cm; England FInder 035/0; plate 3, figures 19-20. USNM 258900, DSDP Site 341, core 27, section 3, 14-16 cm; England Finder N31/1; plate 3, figures 23-24. USNM 259101, DSDP Site 341, core 28, section 3, 79-81 cm; England Finder T53/1; plate 1, figure 4.
Remarks: The axobate of Eucoronis fridtjofnanseni is faintly visible when viewed by bright field light microscopy, but the details of this construction can only be observed by SEM. This structure is regarded as having great value for tracing trissocyclid evolution, but it is not ideally suited as a genus level taxonomic criterion because it can not be routinely observed on conventional slide preparations. The absence of a microspicule field on the axobate of one specimen (pl. 2, figs. 7-8) is perplexing. The pitted composition of this specimen appears to represent a slightly weathered surface from which the surface coating has been dissolved, revealing the inner microspherulitic opal texture. In contrast, the smooth outer surface of the specimen shown on plate 2, figure 5 is essentially intact and the microspicule field on the former specimen may be a function of the degree of weathering. The axobates of E. ridtjofnanseni and Eucoronis angulata (Goll, 1972, pl. 63, fig. 3) are quite similar in structure.
Eucoronis fridtjofnanseni is distinguished from the latter species by its smaller overall size and well-developed primary lateral bars. The vertical spine is small and projects from the back of the sagittal ring near the juncture with the basal ring of E. angulata (Goll, 1969, pl. 59, fig. 7) in contrast with the large vertical spine arising high on the sagittal ring of E. fridtjofnanseni (pl. 1, fig. 9).
Specimens of E. fridtjofnanseni, consisting only of the sagittal ring and associated spines (pl. 3, figs. 16-18) are common in Site 341 sediments, particularly near the top of the E. fridtjofnanseni Range-zone. Care must be exercised to prevent misidentifying these specimens as Zygocircus by observing a number of individuals ranging a development from sagittal rings only to complete specimens. Sagittal rings of E. fridtjofnanseni can be distinguished from those of Clathrospyris sandellae (pl 3, figs. 9-10) by the absence of a pair of spines or lattice bars joined to the front midpoint of the sagittal ring between the apical and frontal spines.
Eucoronis fridtjofnanseni is distinguished from E. circumflexa and from the type species of Eucoronis, E. perspicilum, by the absence of large tubercles on the sagittal ring, and from Eucocronis sp. (pl. 1, figs. 2-3) by the absence of a sternal bar or pore.
Occurrence: The stratigraphic range of this species os confined to the Eucoronis fridtjofnanseni Range-zone (table 1 and text-fig. 1), lower Upper Miocene, and the species has been found only in 3 DSDP sites (338, 341 and 348) in the Norwegian-Greenland Sea. | | Goll and Bjørklund 1980 |
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