| Home> Cenozoic> Holocene (Recent)> Gulf of California>
 

List species

Actinomma sp. det. Benson, 2003

Description - Add description

Benson,1966, p.164-166; pl. 5, fig. 5 (not 6):

Actinomma sp.

Remarks. This species or species-group is similar to Actinomma cf. hystrix in number of shells and in the presence of numerous radial beams and spines, but differs in having larger, more irregular, unequal cortical shell pores (7-12 on the half circumference) and a cortical shell with apparently a greater range in diameter. Concerning the latter difference, only a few tests have relatively large cortical shell diameters ranging from 119-153 Ám [Pl. 5, fig. 5]; these specimens can be separated as a group from the remaining tests whose diameters range from 77-107 Ám [Pl. 5, fig. 6]. The cortical shell of the latter group is nearly identical in size and nature of its pores with the third shell of Sphaeropyle 1angii Dreyer but differs from it in the absence of three-bladed spines or beams which arise from its surface but do not continue inward as radial beams. The absence of intermediate forms between the large and small diameter groups of this species-group may be a basis for separation of the two as species, but the former group occurs only rarely in the Gulf. If it had a greater frequency, its complete range of intraspecific variation could be determined. Other differences besides the large vs. small cortical shell diameter are the presence of more radial beams and spines in some specimens of the smaller group (8-25 for the smaller group, 10-16 for the larger group, although in both groups most specimens have 10-15), the smaller diameters of the second (33-46 vs. 48-53 Ám) and first (14-20 vs. 21-25 Ám) shells of the smaller group, and the presence of heavier, three-bladed beams and generally longer radial spines (20-96 vs. 10-49Ám) in the larger group. The differences between these two groups can be emphasized by mentioning that tests of the larger diameter group are similar to the cubosphaerid Hexacontium entacanthum J°rgensen from the Gulf, and differ from it almost solely in the presence of more radial beams and spines, six of which, in a few specimens, are mutually perpendicular. The resemblance of the shells of the smaller group to the first three concentric shells of Sphaeropyle 1angii Dreyer was mentioned above. For this study it was convenient to include both larger and smaller diameter tests in one species-group realizing that future study, including a statistical analysis, of greater numbers of individuals may lead to subdivision.
Because both the larger and smaller diameter groups can, be characterized by the presence of a cortical shell with two medullary shells, this species group was placed within the genus Actinomma Haeckel. It is similar to Cromyomma circumtextum Haeckel (1887, p. 262, P1. 30, fig. 4) but differs from it in the absence of the thin, delicate, outer, spherical veil, although one specimen from the Gulf was observed with its spines branched at a common distance from the surface of the cortical shell; however, this form may be an incomplete specimen of Sphaeropyle langii Dreyer. No other species illustrated in the literature could be identified with either of the two forms of this species-group. Since doubt exists as to whether or not this group represents one or two species, no formal name is proposed.

Distribution. This species-group is rare at all stations in the Gulf except 130, 184, 203, 206, and 214 where it is absent. Its highest frequency is at station 151, whereas Actinomma cf. hystrix is absent at this station. Since station 151 is located within the diatomite facies, this species-group may have its distribution controlled in part by upwelling, because the diatomites in the Gulf are closely associated with areas of upwelling. Its second highest frequency is at station 81 which is in the axial part of the Gulf but is located near a region of upwelling off the coast of Baja California. At the other stations in the Gulf it undergoes no significant changes in its frequency. It appears to be a species-group that is cosmopolitan in the Gulf.

Benson 1966


Description

 

Images

 

Synonyms

 

References

 

Distribution

 

Discussion / Comments

 

Web links