Pahvantia hastata

Caron, J. - B. & Moysiuk, J., 2021, A giant nektobenthic radiodont from the Burgess Shale and the significance of hurdiid carapace diversity, Royal Society Open Science 8 (9), pp. 210664-210664 : 10-12

publication ID 10.1098/rsos.210664


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Pahvantia hastata


3.2. Morphological reinterpretation of Pahvantia hastata

Lerosey-Aubril and Pates [ 22] recently described a fossil assemblage of Pahvantia (KUMIP 314089) showing a well-preserved tripartite carapace complex, identifying it as a radiodont, as well as what they interpreted as an unusual type of appendage with extremely long setose endites (their figure 3 a–c View Figure 3 ). This appendage was interpreted as having seven endites divided into two broadly different types and sizes. The two proximal endites they identified were short with about seven strong auxiliary spines each, with the second endite being about three times wider compared with the first endite. The five distal endites were long—up to four times the size of the proximal endites—and had up to 50–60 setae.

There are a number of issues with the interpretation of the putative appendage in this specimen [ 2]. In particular, there is no evidence of podomeres associated with the elongate setose ‘endites’, and these ‘endites’ are irregularly bent, curving to varying degrees along their lengths, suggesting they were highly flexible. Endites are well sclerotized in other hurdiids and are therefore not typically deformed in this way [2,5]. Hurdiid endites generally exhibit a smooth mesial curvature [ 2], contrary to the sharp bends seen in the Pahvantia specimen. In addition, no hurdiid shows endites with clearly differentiated secondary spines and secondary setae in the same appendage. One possible exception is Aegirocassis which might have secondary spines on the peduncular endite, but these are not clearly preserved in the figured material ( figure 2 a, b View Figure 2 in [ 5]). Finally, the presence of two rather than a single peduncular endite as well as their small relative size is unprecedented [ 42].

As is typical with Burgess Shale-type preservation, KUMIP 314089 is preserved as a part and a counterpart, with the split going through various superimposed layers of the fossilized tissues in such a way that different structures may be visible on each. The part is the best preserved; however, the counterpart shows some remnants of structures missing on the part ( figure 6 b, c View Figure 6 ). When images of both part and counterpart are superposed, the stacked image shows clearly the presence of a distinct appendage, ca 10 mm in height, partly overlapping a larger array of ribbon-like structures ( figure 6 b View Figure 6 ). The appendage bears five endites of similar lengths, each probably bearing seven to eight robust, hooked secondary spines and terminating distally in a similar hooked spine. These structures were partially described by Lerosey-Aubril and Pates [ 22] who interpreted them as two proximal endites, but we consider their ‘second endite’ to actually consist of three partially stacked endites with visible posterior margins. The most distal elongate endite (number six) was overlooked and is mostly preserved on the counterpart (only its tip is visible on the part) together with the dorsal sections of the podomeres, which are not preserved on the part. Asemicircular structure proximal to the enditebearing podomeres can be identified as the peduncle. Triangular projections dorsal and ventral to the peduncle might represent a dorsal spine and the remnants of a peduncular or shaft endite [ 42], respectively. Additional possible dorsal spines are present more distally. Overall, the newly recognized appendage is about half the maximum dimension of the structure previously described by Lerosey-Aubril and Pates [ 22]. The appendage, from the distal tips of its endites to the outer margin of the podomeres, also represents less than one-third of the length of the associated H-element on the same slab, which would make it more consistent with the carapace/appendage size ratio observed in other hurdiid assemblages such as Hurdia [ 4] and Cambroraster [ 2].

The putative setose endites described by Lerosey-Aubril and Pates [ 22] are more comparable to structures interpreted as gill blades, associated with flaps or body segments in anomalocaridids and hurdiids [4,12,15] including Hurdia ( figure 6 e–h View Figure 6 ). These bands of lamellate structures are not part of an appendage, nor are they likely to have been used for feeding (contra [ 22]), probably functioning instead for respiration [ 15]. In Hurdia , the gill blades are prominent ventrolaterally and can be very elongate as demonstrated in disarticulated or isolated material [ 4]. Stacks of Hurdia gill blades can be preserved in similar ways to those observed in the Pahvantia material, especially in disarticulated carcasses or moult assemblages ( figure 6 View Figure 6 ). In addition, the number of individual elements in the bands in Pahvantia is roughly similar to what is known in the gills of Hurdia [ 4] suggesting they are equivalent structures. The amorphous strand of material overlapping and extending beyond the margin of the appendage peduncle, originally interpreted as a part of the appendage, is more likely associated with the mass of gills and trunk cuticle.

In conclusion, a re-evaluation of KUMIP 314089 based on the photographic material provided by the authors, leads us to demonstrate the presence of a nearly complete and partially unnoticed appendage. This appendage has the greatest similarity in morphology, particularly in terms of the number and form of the secondary spines, to Hurdia [2,4], suggesting an adaptation for capturing larger prey living along or in the sediment and thus probably a nektobenthic lifestyle, contra [ 22]. However, the subequal length of the five elongate endites as well as the shape of the H- and P-elements sets Pahvantia apart from Hurdia . Together with our phylogenetic results (see below), these differences justify the retention of Pahvantia as a distinct genus.