Figure 1.
Early development of Punctatus deduced from hatchling features.
(A) Hatchling covered with spines and developing striated oral ruffle (Sn17-18). (B) CT-section at the center of a fossil depicting a primitive gut and blastocoel (Sn40-74). (C) Split half portion of a spherical fossil with a blastopore (Sn45-46) showing proposed original structure of the primitive gut and blastocoel. (D) A well-preserved cleaving stage, possibly 64-cell stage, comparable to a typical coeloblastula with clear blastomeres (Sn25-123). (E) CT-section of specimen D showing blastocoel. Note regional differences in thickness of blastomeres. bc, blastocoel; bm, blastomere; bp, blastopore; fe, egg membrane; or, oral ruffle; pg, primitive gut; rr, radial ridge. Scale bar, 0.2 mm.
Figure 2.
Postembryonic growth of Punctatus.
(A) Hatchling of P. triangulicostalis (Sn62-11) and (B) young P. emeiensis with two annular fringes (kp135). Two distinct body parts are recognized in both species; the embryonic body covered with spines, and the postembryonic body with triangular processes in P. triangulicostalis and with striation in P. emeiensis. (C) Rendering from micro-CT data of a young with four annular fringes (Sn47-96), (D) transverse CT-section at gut level, and (E) median section showing a completely empty body cavity (blastocoel) and small gut. (F) Apical portion of mature P. triangulicostalis (Sn21-38) and (G) a mature P. emeiensis with more than 12 annular fringes (Sn30-03). (H) Clearly demarcated embryonic body at the tip of P. emeiensis (Sn27-06). bc, blastocoel (body cavity); eb, embryonic body; g, gut; mo; mouth; pb, postembryonic body. Scale bars (except C), 0.5 mm.
Figure 3.
Oral development in Punctatus emeiensis.
The mouth developed directly from blastopore. (A) Ten radial ridges as just-emerging striations surrounded with spines marginally (Sn13-109). Deca-radial pattern (white lines) conceals original penta-radial pattern. (B) Oral ruffle displaying ten petal-like folds with striation (Sn108-83) developed from stage A. The original penta-radial pattern is discernible as pairs of larger (square) and smaller (circle) folds. The specimen possibly loosened its tightly closed mouth after death. (C) New striated ruffle appears inside of the primary oral ruffle to replace it (kp08-001). (D) Functional mouth surrounded with a collar ruffle ( = secondary oral ruffle), subsequently surrounded with a fringe of the column ( = primary oral ruffle) (Sn24-89). The original penta-radial pattern is retained in the oral apparatus (squares and circles). col, collar; em; egg membrane; fr, fringe; for, functional oral ruffle; 1st, 2nd, 3rd or, primary, secondary and newest oral ruffles. To highlight the oral development, only the oral region is shown and scale of each image is not the same.
Figure 4.
Punctatus developmental pattern based on molecular developmental data.
Egg is polarized with high concentration of maternal factors at a pole, most likely the animal pole, as maternal factors tend to associate with the germinal vesicle. Depending on maternal factors, a growth zone is established at the pole where gastrulation takes place, and then the oral ruffle forms with a basic penta-radial pattern. The circular growth zone at the oral region retains its morphogenetic ability throughout life and periodically renews the oral ruffle. Old oral ruffles move to the periphery and transform into annular fringes of the column, driving the growth of the animal body. With this growth pattern, the embryonic body is retained at the tip of the body.
Figure 5.
Comparison of developmental patterns among non-bilaterian phyla and Punctatus.
The developmental pattern of Punctatus was similar to that of cnidarian actiniarians (sea anemones), but was generally distinct from phylum Cnidaria by the following features; no partitions of the gut, large body cavity, and benthic hatchlings. Blue broken lines in cnidarian and Punctatus larvae represent respective transverse sections. Porifera pattern after [41], [42] and Ctenophora pattern after [36], [43].