Zeng et al.—Radiodontan oral cone from Guanshan Lagerstätte
and other known examples, we suggest some minor revisions to the most recent reconstructions of the oral cones of the Burgess
Shale taxa Peytoia and Hurdia (Daley and Bergström, 2012, fig. 1a, b). Our first point concerns the outline of the Peytoia oral cone, while our second relates to the strict symmetry and orientation of the Peytoia and Hurdia oral cones. The original outline of the Peytoia oral cone has been an
open question because of potential taphonomic deformation (Daley and Bergström, 2012, p. 503; Daley et al., 2013a, p. 779). Primarily drawing from one of the most complete and articulated Peytoia specimens (USNM 274143; Whittington and Briggs, 1985, figs. 17, 20–24), a rectangular rather than a circular profile has been favored for oral cone reconstructions (Whittington and Briggs, 1985; Collins, 1996; Daley and Bergström, 2012, fig. 1a). However, the oral cones in another two articulated Peytoia specimens are circular in shape (USNM 57555, 274144, Whittington and Briggs, 1985, figs. 44, 45, 47, 48, 50, 95–98), while the outlines of other isolated or articulated specimens are distorted. In examples we have measured, the length-width ratios of central openings (range = 1.04–1.25, mean = 1.15, standard deviation = 0.08; logarithmic range = 0.040–0.097, mean = 0.060, standard deviation = 0.030; based on USNM 57538, 57539, 57555, 253216, 274144, and 368584; Whittington and Briggs, 1985, figs. 44, 45, 47, 48, 50, 60–67, 79, 80, 93, 95–98; ROM 51216; Collins, 1996, fig. 5.2, 5.3) are statistically significantly less than those of the specimen used in reconstruction (length-width ratio = 1.40, assuming a normal distribution, P[ratio ≥1.40]<.001; logarithmic length- width ratio = 0.146, assuming a normal distribution, P[ratio ≥ 0.146]<.002; on the basis of USNM 274143; Whittington and Briggs, 1985, figs. 17, 20–24), suggesting this is an abnormal, laterally compressed individual. We argue, on the basis of our measurements, that the oral cone of Peytoia more likely had a circular (Fig. 3.3) rather than rectangular outline. This result also implies that a circular oral cone outline characterizes all known radiodontans. While the terms ‘triradial’ and ‘tetraradial’ are very useful
in descriptions of radiodontan oral cones, they can be misleading when the strict symmetrical patterns of these structures are considered. Even though radially symmetrical oral structures are present across a range of ecdysozoan groups, including some stem-group euarthropods, and appear to be a primitive character (e.g., Smith and Caron, 2015; Vinther et al., 2016), this was not necessarily the case for radiodontan oral cones. In the ‘triradial’ oral cone of Anomalocaris, the large anterior plate is larger than its two posterolateral counterparts (Daley and Bergström, 2012, p. 502, fig. 1C); thus, strictly speaking, this structure should be described as bilaterally symmetrical rather than ‘triradial.’ Similarly, in the ‘tetraradial’ oral cone of Peytoia, size differences are evident between the four large plates, with lateral plates being wider than the anterior
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and posterior plates. This variation in size can be seen in articulated (USNM 57555, 274143; Whittington and Briggs, 1985, p. 582, figs. 17, 20–24, 95–98) and isolated specimens
(USNM 57538, 57539, 253216; Whittington and Briggs, 1985, figs. 60–67, 93; ROM 51216; Collins, 1996, fig. 5.2, 5.3) and has been illustrated in reconstructions (Whittington and Briggs, 1985, figs. 102b, 106c, 106f; Daley and Bergström, 2012, fig. 1a). Recognizing that the four large plates are of different sizes made it possible to confidently orient isolated Peytoia oral cones
(USNM 57538, 253216; Whittington and Briggs, 1985, figs. 60, 67), as implied in the original description. Thus, in a strict sense, as in Anomalocaris, the oral cone of Peytoia should be considered as biradially symmetrical rather than ‘tetraradial’ (Fig. 3.3). The same reasoning applies to Hurdia, which has the same Peytoia-type configurations of the outer ring of 32 plates (Daley et al., 2009; Daley and Bergström, 2012, Daley et al., 2013a). In an almost complete, articulated specimen of Hurdia (Daley et al., 2013a, fig. 21C, D) as well as in an oral cone associated with a pair of frontal appendages possibly preserved in the living orientation (Daley et al., 2013a, fig. 22D), the lateral large plates are wider than the anteroposterior ones. Thus, we suggest that the orientation of the oral cone in Hurdia was most likely the same as in Peytoia, with wider large plates arranged laterally and narrower anteroposterior ones oriented longitudinally (Fig. 3.3, 3.4).
Comparative anatomy.—The comparative anatomy of radio- dontan oral cones is summarized (Table 1). In general, the new radiodontan oral cone from the Guanshan Lagerstätte combines features seen in Anomalocaris (Daley and Bergström, 2012; Daley and Edgecombe, 2014) and Peytoia (Whittington and Briggs, 1985; Daley and Bergström, 2012), while lacking the additional rows of inner teeth that are unique to Hurdia (Daley et al., 2009, 2013a; Daley and Bergström, 2012). The Guanshan oral cone resembles the oral cones of Peytoia and Hurdia because it has an outer ring of 32 plates comprising four per- pendicularly situated large plates intercalated with seven small plates in each quarter. In Peytoia and Hurdia, however, the anterior and posterior large plates are narrower than the lateral ones with no apparent size difference, while in the Guanshan oral cone, the anterior large plate is as wide as the plates on the left and right and significantly wider than the posterior plate. The new Guanshan oral cone also exhibits features thought unique to Anomalocaris, including the presence of furrowed folds and scale-like nodes on plates, both of which are absent from Peytoia and Hurdia. Comparing to Anomalocaris, Peytoia, and Hurdia, which have no more than three spines on the inner margins of their large oral cone plates, the number of spines on the large plates of the Guanshan specimen is always at least five, reaching up to eight in some cases. The central opening of the Guanshan oral cone is also subrectangular,
Table 1. Anatomical comparisons of characters between radiodontan oral cones. Asterisks in the last column indicate that the extra inner teeth are not articulated with the oral cone, but they may still be present elsewhere in the buccal or pharyngeal cavity.
Oral cones
Anomalocaris Guanshan Peytoia Hurdia
Large plates 3, anterior one largest Small plates
4, posterior one smallest 28, sizes undifferentiated 4, lateral pair larger 4, lateral pair larger
>28, sizes differentiated 28, sizes undifferentiated
28, sizes undifferentiated
Furrowed folds Scale-like nodes Central opening Inner spines Extra inner teeth Present
Present Absent Absent
Present Present Absent Absent
Irregular Subrectangular 5–8
Maximum 3 Absent* Absent*
Subrectangular Maximum 3 Absent* Subquadrate Maximum 3 Present
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