LOWER CARBONIFEROUS BRYOZOA 
| Fig. 58 Rhombocladia dichotoma (M‘Coy, 1844) comb. noy. Line 
drawing of external features; a, obverse surface (BMNH PD9591): b, 
reverse surface (BMNH PD9593); scale bar = | mm. 
|(Viséan, Asbian), Carrick Lough, County Fermanagh. TCD.42547- 
42549, Upper part of the Glencar Limestone (Viséan, Asbian), 
Sillees River, County Fermanagh. 
)M‘Coy’s ORIGINAL DIAGNOSIS. ‘Dichotomous; obverse rounded, 
jwith about six equal, parallel, slender, longitudinal ridges, in the 
\concave furrows, between which are five rows of oval, prominent 
\cells, the marginal furrow on each side free of cells: reverse flat, with 
jnumerous, semicircular, scale-like wrinkles, and about six longitu- 
‘dinal striae.’ 
JEMENDED DIAGNOSIS. Rhombocladia with a ramose zoarium of 
dichotomising branches oval to elliptical in cross-section. Seven to 
twelve longitudinal rows of autozooecial apertures open onto the 
obverse surface. A single superior hemiseptum is developed on the 
proximal side of apertures and basal diaphragms are rare. Inter- 
apertural areas smooth, or with small pustules. Autozooecial apertures 
oval to rhombic in shape; a single large acanthostyle occurs proxi- 
ally at each autozooecial aperture. Reverse surface barren, with 
parallel semi-circular ridges along the length of branches. 
DESCRIPTION. Zoaria are ramose with dichotomously dividing 
attened branches. In cross-section branches are oval to elliptical in 
shape, and convex frontally. The largest fragment examined meas- 
ures 30.4mm in length. 
Autozooecial apertures open on the obverse surface only, and are 
wranged into 7 to 12 longitudinal rows. The number of rows along 
branch is usually stable until a bifurcation point is reached, where 
he shape and size of apertures becomes variable and the regularity 
wf their organization is lost. Uniformity returns distally beyond 
ifurcations. Narrow, barren marginal zones on the obverse surface 
ire common in most zoaria. Autozooecial apertures are commonly 
val to rhombic, rarely an acute hexagonal shape. Oval apertures are 
ost common along branch margins, with rhombic shapes predomi- 
ating towards the branch centre. 
Interapertural walls are thin and smoothly rounded. They are 
mooth or bear faint pustules, and a single large acanthostyle, up to 
.18mm in length and 0.03—-0.08mm in width, is situated proximally 
f each aperture. In most zoaria the large acanthostyles have been 
braded down to the zoarial surface and are evident only as small 
eas of coarser skeleton. 
The reverse surface, formed by the colony basal wall, is undula- 
ory, very thin (0.1mm), and bears thin, parallel, semicircular lines 
ong the entire length of branches (apparently marking former 
ositions of the growing tips of branches). Where the basal wall is 
“braded a series of up to 10 longitudinal rows, representing the 
_roximal portions of autozooecial chamber walls, is visible. 
_| Autozooecial chambers originate on the basal wall and distally 
145 
Table 19 Measurements of Rhombocladia dichotoma (in mm), N=18. 
————— ee ee eae 
NM Xx Mn Mx CVw CVb 
ZW 71 1.79 0.80 33,113} 7.95 4.39 
ZT 23 0.47 0.30 0.56 8.16 8.97 
AR 41 10 7 12 6.66 8.60 
ZL2 134 4.89 4 ff] 7.26 8.19 
ZT1 97 3.87 3 6 10.72 8.89 
ADI 160 0.27 0.19 0.42 9.14 6.97 
AD2 160 0.12 0.08 0.18 METS 6.64 
AS1 160 0.09 0.07 0.32 19.81 4.50 
AS2 160 0.07 0.04 0.13 14.13 4.63 
AW 87 0.04 0.03 0.08 15.26 4.13 
AH 21 0.12 0.02 0.18 17.29 0.65 
HL 6 0.15 0.10 0.19 9.82 7533) 
ET 9 0.33 0.30 0.38 6.01 15.79 
TE 8 0.13 0.09 0.17 10.86 3.81 
AWT 7 0.01 0.01 0.02 16.67 9.19 
—_—_—_—_———-_———————————— 
curve upwards at a low angle. At the junction of the endozone and 
exozone the chamber bends abruptly upwards. Here a prominent 
superior hemiseptum (average length 0.15mm) is developed, and is 
a little reflexed distally. Large acanthostyles originate at the base of 
the exozone. Endozonal walls are thin (0.01mm). Diaphragms are 
thin and are found only in the basal areas of the endozone. 
DISCUSSION. M‘Coy (1844) described and figured Vincularia 
dichotoma from the Carboniferous of Ireland. On the basis of 
M‘Coy’s types and conspecific specimens from the Viséan of County 
Fermanagh, this species is here reassigned to the genus Rhombocladia 
Rogers, 1900. Of the eleven specimens examined by M ‘Coy and still 
in existence, none show the obverse surface. M‘Coy must therefore 
have had additional specimens available which are presumed lost. 
The lateral margin of specimen NMING:F7058 is slightly worn and 
shows some detail of internal structure, enabling it to be compared to 
material from Carrick Lough, examined in the present study. All the 
material is conspecific and NMING:F7058 is here selected as the 
lectotype for the species Rhombocladia dichotoma. 
In many cases early workers assigned cylindrical Bryozoa to the 
genus Vincularia which is in fact a cheilostome genus. 16 species of 
Rhombocladia have been previously described. Of these 13 occur in 
the Carboniferous and have been recorded from from the United 
States (McKinney 1972, Rogers 1900), the CIS (former Soviet 
Union) (Dunaeva 1961, Gorjunova 1988, Shulga-Nesterenko 1955), 
the Carnian Alps (Ceretti 1963, 1964), and China (Lu 1989). Three 
species have been recorded from the Permian: R. aktashensis from 
the CIS (former Soviet Union) (Lavrentjeva 1985) and R. minor and 
R. spinulifera from Western Australia (Crockford 1944). 
Table 20 shows morphological measurements for all species of 
Rhombocladia. R. dichotoma differs significantly from all species 
except R. delicatula Rogers which has a similar zoarial thickness, 
and number of autozooecial rows. However, in R. delicatula promi- 
nent superior hemisepta are not present as they are in R. dichotoma. 
Rhombocladia dichotoma displays the greatest zoarial width of 
any of the taxa (maximum observed value = 3.13 mm) and the largest 
number of autozooecial rows. Apertural diameters within the genus 
are relatively constant. This explains the large width of branches 
observed in R. dichotoma. 
Zoarial thickness in all species is similar, with the exception of R. 
aktashensis whose branches are sub-circular in cross-section. The 
ratio of zoarial width to zoarial thickness in Carboniferous species is 
approximately 3:1, compared to 3:2 for the Permian species. 
McKinney (1972: 60) postulated an erect growth habit for 
Rhombocladia on the basis of the occurrence of a species of Hederella 
encrusting the reverse surface. This interpretation is questionable, 
