Riscodopa biincisa ( Waters, 1882 )

Cook, P. L. & Bock, P. E., 2002, Notes on astogeny of some Petraliellidae (Bryozoa) from Australia, Journal of Natural History 36 (13), pp. 1601-1619 : 1604-1608

Riscodopa biincisa ( Waters, 1882)

(gures 1–7)

Smittia biincisa Waters, 1882: 272 , pl. 7, gure 1; MacGillivray, 1895: 95, pl. 12, gure 24.

The LECTOTYPE ( NHM D32997) and PARALECTOTYP E ( NHM D32999) from the Miocene of Mount Gambier, South Australia, have been examined, together with numerous other colonies ( NHM D53581) and ( MOV P27749) from the Miocene of Balcombe Bay.

Other material examined

MOV, Miocene, Victoria, Batesford, Cooriemungle , Paaratte and Princetown ; South Australia, Mount Schanck ; Oligocene , Victoria, Bird Rock (see Appendix) .

Description

Riscodopa with non-tatiform ancestrula. Autozooid frontal shield reticulated, with 15–30 large frontal pores and two to four marginal septular pores, often concealed by calci cation. Primary ori ce with paired proximo-lateral denticles, a central lyrula and six oral spines. Lateral-oral avicularia paired, terminal on columnar processes, and a proximal avicularium terminal on a prominent mucro; rostra small, subtriangular or rounded, with a complete bar. Fully developed ovicells not seen. Basal walls with one or more large septular pores.

Remarks

The lectotype includes only three to four zooids, and the paralectotype (Waters’s gured specimen) does not include an ancestrula. Many ancestrulate colonies occur in samples from Balcombe Bay, Princetown and Mount Schanck, and the early astogeny may be traced from these specimens. The ancestrula is not tatiform, but resembles later-budded zooids. The early budding pattern resembles that of R. parva and R. cotyla , and includes a distal ancestrular triad, followed by a proximal triad, completing a circum-ancestrular ring. The zooidal oral spines, lateral denticles and narrow, median lyrula are constantly present, but the avicularian columns are often worn. No ovicells have been reported, or have been found in the colonies examined, but several show that they were about to be developed. These ovicell traces occur in zooids of the fourth and subsequent astogenetic generations (see gure 4). In R. cotyla , fully developed ovicells occur in the fth generation, but there are only signs of early development in third and fourth generation zooids. This reversal of the usual astogenetic sequence in ovicell ontogeny is typical of the Petraliellidae ( Cook and Chimonides, 1981a) . On the basal side of colonies, the ancestrula frequently shows a depression in the calci cation of the basal wall, which may mark the position of the inferred primary rhizoid element. One, and sometimes two, large, basal septular pores occur in the walls of other zooids, and are inferred to mark the site of rhizoids in life. Some colonies also show a raised, curved, basal lamina surrounding the proximal end of each zooid, which also occurs in R. paucipora (see below).

Riscodopa biincisa has been reported from numerous Tertiary Australian localities. Waters (1882) originally introduced Smittia biincisa for specimens from the Miocene deposits of Mount Gambier, South Australia. His record was followed by those of MacGillivray (1895) from Schnapper Point (probably Balcombe Bay, see Appendix) and Muddy Creek, Victoria. Brown (1952: 307) has shown that Waters’s later references, to ‘ Smittia biincisa var. bicuspis ’ (1887: 58), and ‘ Mucronella biincisa ’ (1889, pl. 3, gure 28) actually refer to a completely diVerent species, Umbonula bicuspis ( Hincks, 1883) . As mentioned above, an even later reference by Waters (1925) to ‘ Petraliella biincisa ’ was, in fact, also to U. bicuspis . Waters (1882) noted that the original colony form of fossil ‘ Smittia biincisa ’ was ‘orbicular’. His gured specimen is regenerated and without an ancestrula. Waters described the large distal septular pores which form a crescentic area near the base of each zooidal wall. He did not describe oral spines, but these were noted by MacGillivray (1895), who described the colony as ‘encrusting’. Brown (1952) noted that the specimens labelled ‘ Petraliella tractifera ’ by Bassler (NHM D35472, Balcombe Bay, Victoria, Miocene) belonged to S. biincisa . These specimens are also regenerated colonies lacking ancestrulae, with a diameter of 4–5 mm. They have all the characteristics of Riscodopa biincisa , including basal septular pores, and large lateral and proximal mucrones surrounding a lyrulate ori ce. It is not possible to decide, from the heavily retouched photograph of P. tractifera given by Canu and Bassler (1935: 19, pl. 5, gure 1), whether or not their type material, from the Miocene of Muddy Creek, Victoria, is also identical with R. biincisa . The colony form was described as ‘unilamellar, perhaps orbicular’, and therefore, in spite of the doubts expressed by Brown (1952: 307), it seems probable that the holotype (USNM 85824) like the material in the NHM collections, belongs to R. biincisa .

Riscodopa biincisa diVers from R. cotyla in its smaller dimensions, and in the presence of oral spines on all zooids. It resembles R. parva closely, both in dimensions, and in the narrow, lyrulate ori ce. Riscodopa biincisa diVers from R. paucipora sp. nov. in its smaller dimensions, shape of ori ce and distribution of basal septular pores, and from R. hyalina in the characters of the ancestrula and autozooid ori ce.