TAXONOMIC
NOTES
- The taxonomic concepts
are those adopted by Rio, Raffi, et al. (1990), Raffi et al. (1993), and
Raffi and Flores (1995). A summary of the most important taxa concepts is
reported below.
Gephyrocapsids
- The taxonomic concepts
used for this group are those adopted by Rio, Raffi, et al. (1990)
and modified by Raffi et al. (1993). Within gephyrocapsids, three
taxa have been considered on the basis of biometric taxonomic entities:
- gephyrocapsids with sizes
4
µm labeled "small" Gephyrocapsa
spp.;
- gephyrocapsids with sizes between 4 and 5.5 µm with an open
central area labeled Gephyrocapsa
oceanica s.l.; and
- gephyrocapsids with sizes
5.5
µm labeled "large" Gephyrocapsa
spp.
- Moreover, two
morphospecies have been distinguished: Gephyrocapsa
sp. 3 following Rio (1982) and Rio, Raffi, et al. (1990) and G.
caribbeanica,
a morphotype with a closed central area.
Reticulofenestrids
- Within the
reticulofenestrids two groups have been considered: the Reticulofenestra
spp., which includes small-sized (<7 µm) reticulofenestrids such as Reticulofenestra
minuta, R. haqii,
R. minutula,
and R.
pseudoumbilicus, which, following the taxonomic concepts
proposed by Raffi and Rio (1979) and Backman and Shackleton (1983),
includes morphotypes >7 µm.
Calcidiscus
- Within the genus Calcidiscus
three species have been distinguished following the taxonomic concepts
expressed in Rio, Fornaciari, et al. (1990) and Fornaciari et al. (1990):
- rounded Calcidiscus
specimens with sizes
11
µm labeled Calcidiscus
macintyrei;
- rounded Calcidiscus
specimens with sizes
11
µm labeled Calcidiscus
leptoporus;
and
- elliptical Calcidiscus
specimens labeled Calcidiscus
premacintyrei.
- Discoasterids show
a variable distribution and preservation pattern at different sites and
stratigraphic intervals.
- Pliocene species of
Discoaster
have been identified following Backman and Shackleton (1983).
- The following late
to middle Miocene five-rayed species of Discoaster
have been recognized:
D. bellus
group, D.quinqueramus/berggrenii,
D. hamatus,
and Discoaster
sp. 1 sensu Rio, Fornaciari, et al. (1990).
- The Discoaster
bellus group is characterized by five-rayed discoasterids
with a small size (6-8 µm), a poorly developed central area, and by
morphotypes with intergraded features between
D. bellus and D.
hamatus and between D.
bellus and D.
berggrenii. Five long rays with a spine extending and bending
sharply near the tip characterize D.
hamatus. In the California margin this species is present
with specimens relatively smaller than average.
- Discoaster
berggrenii and D.
quinqueramus are two distinct species with a distinct
stratigraphic distribution. D.
berggrenii appears and becomes extinct before D.
quinqueramus, but because intergraded forms between the end
members of these two species make their distinction difficult, they have
been lumped together.
- Discoaster
sp. 1 is a small (6-8 µm) five-rayed Discoaster
with a poorly developed central area and a very small knob evidenced by
Rio, Fornaciari, et al. (1990) in the equatorial Indian Ocean that
appears discontinuously within *CN9a.
- The main six-rayed
species of Discoaster observed during Leg 167 in the late-middle Miocene
interval are: Discoaster
variabilis, D. exilis, D.
surculus, D. brouweri, D.
intercalaris, D.
neohamatus, D.
micros, D.
calcaris, and D.
kugleri
sensu Rio, Fornaciari, et al. (1990).
- Discoaster
brouweri, D.
intercalaris, D.
neohamatus, D.
micros, D.
calcaris, and D.
kugleri
have been determined adhering strictly to the original descriptions. D.
variabilis and D.
exilis have been intended in latu sensu. A large taxonomic
concept has been used for D.
exilis that includes transitional forms between D.
variabilis and D.
exilis. Discoasterids possessing "trifurcated" tips
are assigned to D.
surculus.
Ceratolithids
- This
horseshoe-shaped group of calcareous nannofossils is a minor component
of the assemblage during the late Miocene-early Pliocene. Among
nonbirefringent ceratolithids (Amaurolithus),
only A. primus
and A. delicatus
have been recognized.
- A.
primus has two morphotypes: a primitive form with a thick arc
comparable to the holotype (Bukry and Percival, 1971) and a more
delicate crescent-shaped form that occurs together with A.
delicatus. During Leg 167, only the more evolved form of A.
primus has been observed. Also intergraded morphotypes
between A. primus
and A. delicatus
are present, which make it difficult to distinguish between the two
species and consequently to detect the LO of A.
primus. Ceratolithus
acutus, C.
armatus, C.
rugosus, and C.
telesmus have been found in low frequencies and with
discontinuous distribution. Therefore it has not been possible to
recognize the biohorizons marked by these ceratolithids. A rare and
discontinuous transitional morphotype between genus Amaurolithus
and genus Ceratolithus
has been observed and labeled Ceratolithus
sp. 1. This form appears as partially birefringent; namely, one arm is
birefringent like Ceratolithus
and the other arm is not birefringent like Amaurolithus.
Probably this form could be the result of hybridization between species
of the two genera.
![NEXT](next.gif)