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Journal of Paleontology 92(1):99–113
the Burgess Shale (Whittington and Briggs, 1985). Ten large ventral flaps (‘vf1–vf10’ in Fig. 6.5) are preserved on the side that most clearly shows a dorsal flap (‘df1’ in Fig. 6.5), and six large ventral flaps are preserved on the other side (‘vf1–vf6’ in Fig. 6.5), with one dorsal flap preserved there also (‘df1’ in Fig. 6.5). The front pair of flaps is the largest, and they reduce in size sequentially. The flaps associated with body segments 7–11 are overlapping due to the orientation of preservation. There are no flaps associated with the tail (‘t’ in Fig. 6.5). On the part, two dorsal flaps are also preserved at the front of the animal, in addition to the larger ventral flaps (‘df1’ in Fig. 6.5). A partial mouthpart, KUMIP 314095 (Fig. 6.6, 6.7) is
identified as Peytoia because of the visible plate morphology and lack of internal tooth rows. One large plate with large triangular inner spines is preserved, with five smaller plates on one side and seven on the other side of the large plate. These smaller plates are a regular size and overlap each other, with the plate closer to the large plate overlapping the one next closest. The partially preserved central opening shows no evidence of additional rows of teeth. The large plate has 10 small triangular spines pointing inward, the widest of which, at a central point of the plate, is around 2mm. The others are smaller, at around 1mm wide. Some of the smaller plates have a single projection also pointing inward, around 1mmwide. Unusually for Peytoia, this mouthpart has small (diameter approximately 0.3mm) nodes on the surface of the large plate and some adjacent plates (visible on both part and counterpart; Fig. 6.6, 6.7).
Discussion
Morphological interpretations on Hurdia appendages can be influenced by specimen orientation.—Hurdia appendages are preserved in a variety of orientations (see Daley et al., 2013a). Ventral spines of Hurdia are often preserved curved, both anteriorly (e.g., Fig. 2.3, 2.4) and posteriorly (e.g., Fig. 2.1, 2.2), and straight (e.g., Figs. 2.5, 3), sometimes in the same specimen (e.g., Daley et al., 2009, fig. 2C). The appendages have some element of plasticity, and during preservation they can become deformed. In some specimens, the curvature of ventral spines appears to change along the length of the appendage due to the
appendage being preserved at an angle (e.g., Daley et al., 2013a, figs. 12C, E, 24A, where the distal-most ventral spines appear more curved as the appendage is rotated one way, and Daley et al., 2013a, fig. 12G, where the proximal-most ventral spines appear more curved as the appendage is rotated the other way). Appendages not preserved at such angles tend to have the distal- most podomeres more clearly preserved, not overlapping more proximal podomeres (compare the position of the distal-most podomeres in Fig. 3 and Daley et al., 2013a, fig. 12A, to those described as rotated). The effect that these preservational factors might have on
morphological reconstructions and inferred evolutionary affinities can be observed by considering phylogenetic analyses of Radiodonta. Recent phylogenies (Cong et al., 2014; Van Roy et al., 2015) based on the data matrix and analysis of Vinther et al. (2014) consider four distinct representatives of Hurdia: H. victoria, H. cf. victoria Utah,H. sp. B Spence Shale, and H. sp. B Burgess Shale (the latter two were coded identically except for missing character states). Other than missing
character states, H. victoria and H. cf. victoria Utah only differ in the condition of Character 29: Vinther et al. (2014) coded H. victoria as having distally projecting dorsal spines on the terminal segments; these were coded as absent in Hurdia cf. victoria Utah. Vinther et al. (2014) coded Hurdia victoria (including Hurdia cf. victoria Utah) and H. sp. B as differing in three characters. In Character 34, the ventral spines were coded as broader distally than proximally in Hurdia victoria and subequal or narrower distally in Hurdia sp. B. In character 39, the distal tips of the ventral spines are hooked forward in Hurdia victoria but strongly hooked forward and forming a 90° angle with the spine base in Hurdia sp. B. The phylogenetic significance of Characters 29, 34, and 39 may be called into question by the aforementioned preservational variation. Similarly, Character 46 (curvature of ventral spines) may reflect preservational rather than taxonomic variation. Hurdia sp. B was coded as having proximal ventral spines that curve posteriorly, whereas H. victoria was coded as having ventral spines all straight or anteriorly curved. However, H. victoria specimens with straight proximal ventral spines and anteriorly
curving distal ends are common (e.g., Daley et al., 2013a, fig. 12A, C, E, G), and this reflects taphonomic variation. To visualize how the angle of preservation influences
morphological interpretations of Hurdia appendages, a 3D model was created in Blender based on the morphology of the Hurdia appendage presented by Daley and Budd (2010, text-fig. 1D). This 3D model (Fig. 8) suggests that the apparent broadness of ventral spines on distal podomeres will be influenced by how a specimen is oriented when it is preserved, and so the broadness of ventral spines (Vinther et al., 2014, Character 34) is likely not a good character for distinguishing Hurdia species. A small difference in orientation affecting apparent thickness of ventral spines can be seen by comparing KUMIP 314086 (Fig. 5.1, 5.2, with ventral spines of equal thickness) and KUMIP 314042 (Fig. 2.5, where the distal-most ventral spine appears thicker because of its orientation). This is visualized by the 3D model, where Figure 8.1 (no rotation) shows ventral spines of equal thickness, and Figure 8.2 (small rotation) shows an apparently thicker distal-most ventral spine. A more extreme example of the variation in the orientation of appendage preservation can be seen in the two appendages of KUMIP 312405 (Fig. 3). These appendages are presumably from the same animal but preserved at very different orientations. In summary, Vinther et al.’s (2014) characters 29, 34, 39,
and 46, which comprise the evidence to distinguish four different representatives of Hurdia, may be influenced by preservational factors.Aphylogenetic analysis of the data matrix from Van Roy et al. (2015),which is based on the original datamatrix ofVinther et al. (2014), was run in TNT v. 1.5 using implicit enumeration under equal weighting. The data matrix was modified in the following ways: In Character 29, H. cf. victoria Spence is coded as dorsal spines present, and both H. sp. B taxa are coded as unknown; Character 34 was deleted as it has been shown to reflect preservation and not true morphological difference; Character 39 (now Character 38) was changed to being unordered, and both H. sp. B taxa and Stanleycaris were coded as having hooked forward ventral spines; and in Character 46 (now Character 45), both Hurdia sp. B taxa are
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