Page 6 - Rosso_et_al.2010.Bertorsonidra

Rosso A. et al.
5 in a single one on the type material) and, most
obviously, the presence or absence of avicularia in
large parts of the colony.
Differences in skeletal preservation caused by
biostratinomic mechanical stresses and by partial
dissolution and extensive re-crystallization during
fossilization often hamper the reliable attribution
of fossil specimens to species-level taxa, as recently opposite to the umbo, acute to the frontal plane,
remarked by Berning (2006). Nevertheless, in the
fossil specimens of R. prenanti, the well-preserved
distinctive frontal wall morphology, together with
the partial or complete preservation of orificial
spines and even of condyles (usually absent from
most zooids, seemingly due to their thinness and/
or a different mineralogical composition) on at
least single zooids in a colony, allow for an iden-
tification to species level. Representatives of the
fossil populations appear very close to the living
ones. Only few differences between the fossil and
the Recent specimens have been detected, which
have been considered as intraspecific variability.
They include the slightly distal position of the
lateral-suboral avicularia, which are laterally or
slightly disto-laterally directed; and the relatively
depressed ovicells, being less distinct from the dis-
tal zooid frontal wall (seemingly due to secondary
calcification), tending to form a more V-shaped
proximal border.
Interestingly, fossil specimens from Pianometa
exhibit some ovicells, which have prominences in
their central part, thus being somewhat reminiscent
of the tubercolate ovicells of “Ramphostomella ar-
gentea”, as figured by Zabala & Maluquer (1988).
Nevertheless, and apart from this character, both
diagnosis and drawing of Zabala & Maluquer (1988:
123, fig. 275), are probably a conflation.
REMARKS
The species was formerly erected by Gautier (1955)
as Tremopora prenanti although subsequently syno-
nymised by the same author (Gautier 1962: 261)
with Schizoporella argentea Hincks, 1881 from
the Indo-Pacific, following personal suggestions
by M. Prenant and A. B. Hastings. Nevertheless,
the Mediterranean species is only reminiscent of
Robertsonidra argentea as described and figured by
Ryland & Hayward (1992: 261, fig. 19b) and by
462
Tilbrook (2006: 261, pl. 57E, F). In fact, Bertorso-
nidra prenanti n. comb., exhibiting a wide shallow
sinus laterally marked by squared condyles, clearly
differs from R. argentea, which is characterized by
a primary orifice with a deep concave sinus not
flanked by condyles. Furthermore, R. argentea shows
two different kinds of lateral-suboral avicularia:
1) a smaller and relatively more common one
distally hooked and laterally directed; and 2) a
larger and rarer but more diagnostic one, normal
to the frontal plane and proximolaterally directed.
In contrast, a single avicularium type seems to
be present in the Mediterranean species: lateral
suboral, laterally directed, with a hooked rostrum
on a large and prominent cystid. Further differences
include the colour of living tissue (yellowish-white
in R. argentea), the zooidal outline, and the number,
extent and morphology of the marginal pores. But
most important differences relate to the presence of
an evenly porous zooidal frontal wall in B. prenanti
n. comb. (see discussion below).
Bertorsonidra prenanti n. comb. also differs from
“R. argentea” recorded by Powell (1967: 169, pl. 2,
fig. 10) from the Red Sea, which have rare obliquely
and proximally directed large avicularia, and is
actually a different further species, as suggested
by Tilbrook (2006). Moreover, photo observation
of the specimen labelled as Robertsonidra argentea
from an unknown Mediterranean locality in the
Busk collection (BMNH 1963:4.18.33) but dif-
ferent from this species (see discussion in Tilbrook
2006: 262), proved that it is not even conspecific
with B. prenanti n. comb. More detailed analyses
are needed for its determination.
Bertorsonidra prenanti n. comb. seems also different
from R. oligopus (Robertson, 1908), the type species
of Robertsonidra, also synonymised with R. argentea
by Gautier (1962). Robertsonidra oligopus, seemingly
restricted to the west coast of North America, is
characterized by a convex frontal wall with a sub-
oral, medially-placed prominent umbo, a lateral
oral avicularium usually distal-laterally directed,
and an ovicell with a crenate margin (Robertson
1908: 292, pl. 20, figs 50-52). Unfortunately, the
primary orifice was not described in detail and
drawings, including opercula, only allow to appreci-
ZOOSYSTEMA • 2010 • 32 (3)