1088
Journal of Paleontology
annulations are apparent in the holotype of Y. magnificissimi possibly due to poorer preservation (Fig. 4.6). The posterior section of YKLP 13071 is twisted and deformed (Fig. 4.7, 4.8). Each segment of the posterior section has five to six annulations; junctions between segments are distinguished according to a slight change in relief, and these boundaries are one annulation in width. Both the holotype and the new specimens have annulations. Annulations are apparent in YKLP 13070 (Fig. 5.3) and YKLP 13072 (Fig. 5.2). In the holotype, annulations are clearly present on the first segment of the posterior section and the ventral area of the sixth (Fig. 4.6).
Possible alimentary tract.—The alimentary tract is preserved in the medial part of the posterior section of YKLP 13071 (Fig. 4.7–4.8). This is irregular and curved, narrow, and apparently filled with fine sediment. This structure is incomplete and only preserved at the second segment of the posterior section.
Remarks.—The last subdivision of the anterior section and first few segments of the posterior section are marked by a wrinkled zone with dark coloration and lower relief than the other part of the anterior section (Fig. 3). This may be a preservation feature of the split through the part and counterpart. The holotype was described by Chen et al. (2003), in which
the anterior end of the anterior section was considered smooth and round without any extensions or margin. However, observations on the holotype indicate that there is a visible fold that starts from the beginning of the arched anterior end, surrounded by narrow wrinkles. This provides evidence that the anterior end was compressed, and this morphology is an artifact of decay and/or preservation. That the walls of the anterior section in members of the family Didazoonidae are generally thinner than in Vetulicolidae lends support to this interpretation of a distorted anterior opening. The anatomical differences of the lateral filamentous pouches possessed by the holotype, and the absence of these structures in the new specimens, is most likely due to differences of preservation and to the level at which the rock has split through these structures in different specimens.
Yuyuanozoon differs from other vetulicolians in the
morphology of its bulky, ovoid anterior section, the shape of its anterior opening, and possession of a posterior section with seven segments that are annulated. These features suggest that Yuyuanozoon is closest in general form to Pomatrum and Didazoon.
Contrary to the comment by Chen et al. (2003, p. 282)
that Yuyuanozoon is distinct from other vetulicolians in the number of anterior and posterior segments, the possession of six anterior section subdivisions and seven posterior segments is
characteristic of both Vetulicolidae and Didazoonidae. The anterior opening of Y. magnificissimi was described as ‘long- ellipsoidal’ by Chen et al. (2003). Our new material shows that the anterior end of Yuyuanozoon is widely open, with a circumventing feature. Reinvestigation of the holotype indicates that the anterior section of that specimen is distorted, as evidenced by a series of wrinkles, and the morphology of the anterior opening is obscured by compression. In the holotype, the lateral pouches and associated wrinkles
have been interpreted as gill sacs and external gill filaments (Chen et al., 2003) or as a possible vascular system (Ou et al., 2012). Filaments have not been recognized in the new specimens of Yuyuanozoon, and it is possible that the filaments described by Chen et al. (2003) and Ou et al. (2012) are a product of decay-induced collapse forming wrinkles or the split of the fine-layered sediment, or an expression of internal anatomy.
Inferences about mode of life
By analogy with other vetulicolids, the segmented posterior section of Yuyuanozoon might be interpreted as an adaptation for propulsion. However, Yuyuanozoon has a relatively small
posterior section countered by a bulky anterior section without fin-like projections that seems ill adapted for protracted forward motion. Vetulicolians were globally widespread in early and middle
Cambrian seas and are known from strata that originated in various environmental settings. All share some common features such as a lateral collapse orientation retaining a degree of three-dimensional preservation, suggesting that the body was made of a material with a degree of robustness, even if it was nonbiomineralized. How Yuyuanozoon fed is a matter of conjecture, as currently no feeding apparatus has been preserved.
Conclusions
In this paper we have described new material of Yuyuanozoon magnificissimi. We have attempted to develop a unified terminology for the description of vetulicolians that avoids inferring animal relationships or functional morphology. Therefore, terms such as ‘carapace’ (suggesting an arthropod affinity) or ‘oral end’ (suggesting functionality for feeding) have been replaced with purely descriptive terms. We believe that much of the existing terminology applied to vetulicolians cannot be sustained. There is no convincing evidence to show the homology of the anterior part of vetulicolians with the carapace or head shield of arthropods. Furthermore, there is no evidence in Y. magnificissimi to sustain the notion that the anterior opening was ‘oral’ or even that it functioned in food collection. In deconstructing this taxonomically ‘loaded’
Figure 4. Detailed views of Yuyuanozoon magnificissimi.(1) Anterior end of Y. magnificissimi YKLP 13071; (2) anterior end of Y. magnificissimi YKLP 13072; (3) anterior end of Y. magnificissimi CFM00059. Arrows show wrinkles, providing a hint that the anterior opening is not observable due to distortion through compression. (4) Anterior section view of CFM00059. Arrows show the outlines of the pouches and possible underlying narrow grooves. (5) Anterior section view of YKLP 13071. Arrows show the outlines of the pouches. (6) Posterior section view of CFM00059. Arrows show the annulations within the first segment. (7) Posterior section view of YKLP 13071 (counterpart); (8) posterior section view of YKLP 13071 (part). Scale bars=10mm.
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