Skeletal Ultrastructure of the Early Astogenetic Stages of Some Cyclostome Bryozoans

Abstract

Weedon, M. J. 1998. Skeletal ultrastructure of the early astogenetic stages of some cyclostome bryozoans.-Acra Zoologica (Stockholm) 79: 163-174.The calcitic protoecia of cyclostome bryozoans have remarkably uniform skeletal ultrastructure in three suborders (Tubuliporina. Cancellata and Rectangulata). The basal wall or floor has a fine outer granular layer succeeded by planar spherulitic fabric, internally lined by irregular semi-nacre. The roof of the disc Comprises an outer granular layer with an inner lining of semi-nacre continuous with that of the floor; planar spherulitic fabric is absent. Growth of the floor of the disc is initiated around the circular outer rim and continues centripetally to the centre; the inner lining has no prevailing growth direction. The gently domed roof also initiates around the outer rim, and grows in strips, which grow toward the centre, then distally toward the distal tube of the ancestrula which has a fully adult ultrastructure. Protoecial ultrastructure is independent of adult ultrastructure. The uniformity of skeletal ultrastructure in cyclostomes corresponds with the close similarity of larvae and post-settlement metamorphosis in the order. The fabric suite of the protoecium resembles the skeletal ultrastructure of Palaeozoic stenolaemates. The primitive fabric condition is retained by some tubuliporines and cancellates. Complex multilayered fabric suites may have evolved in the Mesozoic by addition of new fabric types.

Skeletal ultrastructure and phylogeny of cyclostome bryozoans

“…The results reported in the next two sections represent a synthesis, with additions and some reinterpretations, of findings published by Boardman et al (1 992), Taylor &Jones (1993), , , , 1997, and Weedon (1997Weedon ( , 1998. Details of material used and methods can be found in these papers.…”

“…A granular outer layer, with an absence of planar spherulitic fabric, is found in the roof of all protoecia studied (Weedon, 1998;Fig. lA, E, FJ. High magnification reveals the exterior walled outer surface of the protoecium roof to comprise a relatively coarse granular fabric ( Fig.…”

“…The most recent investigation into the skeletal ultrastructure of cyclostomes focused on the earliest-formed skeleton in the protoecium (Weedon, 1998). A remarkable consistency of protoecial fabrics across all suborders was found but with differences in skeletal ultrastructure between roofs and floors.…”

“…Although skeletal walls are effectively continuous between floor and roof, the ultrastructural fabrics are distinct in the two areas and are consistent in every species studied regardless of postprotoecial fabric suite (Weedon, 1998).…”

In search of phylogenetic congruence between molecular and morphological data in bryozoans with extreme adult skeletal heteromorphy

“…Cyclostome protoecia exhibit interspecific variability in the number and distribution of pseudopores (Weedon, 1998 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 20 characterized by protoecia with numerous, somewhat irregularly arranged pseudopores distributed in a crescentic band around the proximal and lateral edges of the protoecium (Figs 54, 57, 60). Pseudopores are lacking from more distal parts of the protoecium but reappear on the ancestrular tube.…”