Chancelloria, Walcott, 1920

Bengtson, S & Collins, D, 2015, Chancelloriids of the Cambrian Burgess Shale, Palaeontologia Electronica 10 (14), pp. 1-67 : 10-20

Genus CHANCELLORIA Walcott, 1920

Type Species. Chancelloria eros Walcott, 1920 .

Species Known from Scleritome Preservation. Chancelloria eros Walcott, 1920 , Chancelloria cruceana Rusconi, 1954 (see Beresi and Rigby, 2013), Chancelloria pentacta Rigby, 1978 .

Diagnosis. Chancelloriids with scleritome dominated by star-shaped N +1 sclerites. 4+0 sclerites and (rarely) 3+0 sclerites may be present.

Chancelloria eros Walcott, 1920

Figures 1 ; 5–11 ; 12 (part); 13; 14.2; 15

Chancelloria eros new species ( Walcott, 1920, partim; pls 86:2, 2a, 2b,?2c, 88:1f, non pl. 88:1, 1a— e).

Chancelloria eros Walcott, 1920 ( Goryanskij, 1973, p. 43).

Chancelloria eros Walcott, 1920 ( Rigby, 1978, pl. 2:3).

Chancelloria ( Bengtson, 2000; figure 12, partim).

? Chancelloria cf. eros Walcott, 1920 ( Randell et al., 2005, figure 6).

Lectotype. USNM 66524. Walcott 1920, pls 86:2, 88: 1f. Figure 1 herein. Designation by Goryanskij (1973).

Remarks. This species, with its characteristic “rosettes” of star-shaped sclerites, has become the epitome of the chancelloriids, and so Goryanskij’s (1973) choice of USNM 66524 as lectotype for Chancelloria eros , among the disparate material figured by Walcott, was appropriate even though he did not have access to the material. The species, in fact, differs considerably from the other Burgess Shale chancelloriids, not only in its generally rosette-like sclerites but also in incorporating a variety of sclerite forms within its scleritome.

Diagnosis. Chancelloria species with main sclerites varying from 5+1 to 8+1, the most common forms being 6–7+1. Auxilliary 4+0 sclerites present, sometimes also 3+0 sclerites. Sclerite size variable, their arrangement in the scleritome irregular. Tuft inconspicuous.

Description. The lectotype, USNM 66524 ( Figure 1 ), has not previously been figured in its completeness. The original illustration ( Walcott, 1920, pls 86:2, 88:1f) showed only a central patch of sclerites, and so the species has become known as an assemblage of sclerites rather than as a body fossil. In fact, the specimen is almost complete and shows a well-defined club-shaped body ( Figure 1.1 ), 29 mm long and 11 mm wide at its broadest part. The body narrows gradually to a width of 2.8 mm near the abapical end, but the end appears to be incompletely preserved. A bunch of 2 mm long spines at the apex may represent the apical tuft ( Figure 1.1 , top).

The sclerites are preserved in flattened relief in the proximal 2/3 of the specimen; in the apical part of the body they appear like “ghosts” without relief. The basal surface is seen either in positive ( Figure 1.3 , white arrow) or in negative ( Figure 1.3 , black arrow) relief. These differences in mode of preservation relate to the orientation of the sclerites: the shale is always parted along the basal surface of the sclerite (rather than along the opposite surface carrying the spiny protrusions). This shows that the central ray protrudes away from the body surface, i.e. in the opposite direction to that of Walcott’s sponge-based interpretation. The outward direction of the spines is also apparent from their preservation in profile at the edges of the specimen ( Figure 1.1 ).

There is no clear regularity in the distribution of the sclerites in the body wall, but the sclerite density is roughly uniform along the body. The common sclerite forms are 6+1 and 7+1, with a basal-disk width of 0.45–0.51 mm. They present a bilateral symmetry owing to the fact that the adapical rays have a larger base than the abapical ones (see, for example, the large sclerite above the black arrow in Figure 1.3 ). The abapical rays also appear shorter, which may be due to the fact that their distal parts protrude from the body surface

BENGTSON & COLLINS: CHANCELLORIIDS and are somewhat recurved in the adapical direction The sclerites preserved in profile at the specimen edges suggest that the adapical rays are straight or somewhat recurved outwards, whereas the central rays protrude perpendicularly from the body surface and, like the abapical rays, tend to be recurved in the adapical direction ( Figures 1.1 ; 6.2 ; 7.3 ; 10 ; and 13).

In addition, smaller, cross-shaped 4+0 sclerites are scattered throughout the scleritome (e.g., Figure 1.3 , grey arrow), though they are smaller and less common than the N +1 sclerites. Their rays are at right angle to each other in the plane of the body wall; it has not been possible to observe the degree of protrusion and recurving from the body wall.

Complete or nearly complete specimens in the ROM collection conform to the club-shaped body outline of the lectotype, but there is considerable variation in length/width ratio. Figure 5.3 shows a large specimen preserved almost in its entirety. The body is roughly cylindrical, up to 20 mm in width. The preserved outline undulates somewhat – a narrow constriction near the apex gives a body width of 13 mm, but otherwise the width is constantly about 20 mm except in the abapical region, which narrows to a stalk about 4 mm wide. The initial 2 cm is bent upwards to form a 45° angle with the main body; this may be a folding

PALAEO- ELECTRONICA.ORG due to collapse of the body. If the body were unfolded, the total length would be about 100 mm. The apical end is flat, although this may be an effect of its being cut off by a crack in the rock. This specimen has a dense scleritome, and the shape of individual sclerites is difficult to make out, because of the preservation.

The specimen in Figure 5.1 and 5.2 is slightly wider (23 mm) than the one in 5.3, but only the apical part is preserved. The apical end is evenly rounded and has a small region of denser sclerite matter around an empty area ( Figure 5.1 , arrow), which may represent an apical tuft and orifice. Although sclerite rays are distinctly visible throughout the scleritome, individual sclerites are often difficult to make out. Discernible ones are of the 5+1 and 6+ 1 types, and the width of the basal disk ranges from 0.38 to 0.64 mm.

The specimen in Figure 6 shows a short, slightly tapering body, 45 mm long and 19 mm wide, rounded at both ends. The structure of the sclerites is poorly discernible, but sclerite rays are visible throughout the body. The direction of the ascending rays in profile ( Figure 6.2 ) and the slight taper of the body unambiguously define the apical– abapical polarity. The apical end has a tuft-like

BENGTSON & COLLINS: CHANCELLORIIDS structure. There is no evidence of a stalk at the attached end.

The specimen in Figure 7 has a preserved length of 66 mm and a greatest measurable width of 27 mm; it still appears to be widening toward the broken-off apical part. The body tapers evenly to a rounded abapical end, without any evidence of a stalk. The sclerites are large, indistinctly preserved, of 7+1 and 8+ 1 type, having basal disks up to 1.2 mm in diameter and ascending spines up to 7 mm in length.

The two specimens from the Trilobite Beds in Figure 8 have the abapical parts missing, but the remaining body outline conforms to the usual club shape. No trace of the soft integument is visible, but the sclerites and their individual rays are distinctly outlined. The discernible ray formulas in both are 4+0 and 5–7+1. In ROM 49580 ( Figure 8.1, 8.2 ) the width of the basal disks ranges from 0.13 to 0.43 mm. This is a substantial range, but there is no evidence of separate size orders of sclerites, and the preserved scleritomes do not have any regular arrangement. In ROM 62590 ( Figure 8.3 ) the sclerites are less well-preserved and cannot be measured exactly, but the size range and (lack of) scleritome organization is similar to that of ROM 49580 . ROM 62590 has a circular opening, 2.5 mm in diameter, in the sclerite mass about 5 mm from the preserved apical edge of the specimen. The regularity of the opening suggests that it might represent the apical orifice, but the interpretation is problematic because there is no other indication that the specimen has been obliquely preserved, and the sclerites surrounding the opening do not show any particular modification. Furthermore, other, less regular gaps in the scleritome indicate that the integument may be ruptured. As none of the other specimens shows the same structure we regard the circular opening in this specimen as an artifact of preservation .

The specimen in Figure 9.1–9.3 is mainly cylindrical, and the directionality of the sclerite rays is ambiguous, making it difficult to determine the axial polarity of the body. The end where it tapers to a narrow point (lower left in the picture) is tentatively interpreted as abapical. Specimens of the lingulate brachiopod Acrothyra gregaria occur among the spines in the apical part of the body ( Figure 9.2, 9.3 ). Although no definitive attachment structure is observed, it is likely that they were attached to the spines of the chancelloriid, as the immediately surrounding matrix is otherwise free of them.

Figure 9.4 shows a specimen, also from the Trilobite Beds, with a club-shaped body that is considerably narrower than most of the other specimens. It is 74 mm long and at its widest 13 mm wide, narrowing slightly towards a well-developed stalk, 3–4 mm wide and more than 15 mm long. The sclerites are tightly spaced, forming a jumbled mass both in the stalk and in the more apical parts of the body.

There is considerable variability in the scleritome composition of Chancelloria eros as delimited in the present study. Although the preservational mode frequently makes it difficult to discern the shape and ray formula of most sclerites, the available material contains examples of scleritomes closely adhering to that of the lecto-

PALAEO- ELECTRONICA.ORG type, as well as those that deviate from this pattern. The specimen in Figure 7 is dominated by 8+1 and 7+1 sclerites. The one in Figure 10 has a more complex scleritome, in that 5+1 and 6+1 sclerites, in addition to the 4+0 sclerites present in the lectotype, are accompanied by 3+0 sclerites of a morphology that is difficult to distinguish from that of Allonnia tintinopsis sclerites. A similar scleritome is seen also in the incomplete specimen of Figure 11 . The sclerites are flat, preserved as shiny films, but otherwise they show the morphology well. In addition to 5+1 and 6+1 sclerites with a basal-disk diameter of 0.47–0.53 mm, there are 3+0 sclerites (arrows in Figure 11 ) similar to those of Al. tintinopsis . The Chancelloria 3+0 sclerites are more flat and star-shaped, however, with rays diverging at 120°, not raising themselves much from the plane of the base. (In Allonnia , all three rays are bent towards the apex.) These specimens also have prominent apical tufts ( Figures 10.1 ; 11.1 ), much like those that are frequently seen in Al. tintinopsis .

The Chancelloria eros specimen in Figure 12.4 (right) and 12.6 has a similar association of sclerites, with common 3+0 and 4+0 amidst star-shaped 6+1 and 5+1 sclerites. In this case, however, the presence of the 3+0 sclerites may be due to the superposition on a specimen of Al. tintinopsis .

The specimens from Mount Stephen shown in Figures 13 and 14 preserve the slender spines particularly well and allow an estimate of how ray length changes along the body. Both specimens in Figure 13 show expansion from a narrow stalk-like portion with densely packed, smaller sclerites, to a wider apical portion with more widely spaced, larger sclerites. In Figure 13.1 , the basal-disk diameter in the lowermost 12 mm of the preserved body is 0.47–0.53 mm and in the remaining portion 0.53–0.78 mm. The rays protruding from the right side of the body in this specimen are predominantly central, ascending rays. Although it is seldom possible to determine whether the whole length of the ray is preserved, the shape and tapering suggest that some of them are at least nearly complete, so at least a minimal length of the ray can be obtained. In the abapical portion, up to 12 mm from the preserved end of the body, the longest ray is at least 3.9 mm, whereas the apical portion of the body 22–30 mm from the abapical end has rays up to at least 6.6 mm long. The same trend is seen in Figure 13.2 and in the incompletely preserved specimens in Figure 14 . The specimen in Figure 14.1 has fairly well-preserved sclerites that allow the recognition of 5–6+1 and (questionably) 4+0 sclerites, whereas the one in Figure 14.2 mostly presents isolated rays, which does not allow assignments to a ray formula.

A life reconstruction of Chancelloria eros is shown in Figure 15 .

The specimen shown in Figure 14.3–14.5 is unique among the material of Chancelloria used in this investigation in that its sclerites are strictly limited to the 6+1 formula, with a narrow size range (basal-disk diameter 0.27–0.31 mm) and furthermore, seem to be more regularly arranged (see Figure 14.5 , lower right) than in the other specimens. For this reason, we refer to this specimen as Chancelloria cf. eros .

Discussion. Rigby (1978, pl. 2:3) figured a sclerite assemblage from the Middle Cambrian Wheeler Shale in the Wheeler Amphitheatre, Utah, as C. eros . Because of the disordered arrangement of the sclerites, he interpreted it as a somewhat disorganized cluster, perhaps even a scatological accumulation. In addition to 6+1 sclerites (he actually wrote “six tangential rays and one or two vertical, proximal-distal, rays”, but the presence of a proximal, inner, ray was not documented and seems to have been based on his concept of Chancelloria as a heteractinid sponge), Rigby also identified 4+0 sclerites.

The presence of 4+0 sclerites, as in the lectotype (also observed in ROM 49580 and ROM 62590, Figure 8 ), thus seems well established as a general character of Chancelloria eros . In addition, 3+0 sclerites in the C. eros scleritome have been observed in ROM 62534 ( Figure 10 ), ROM 49599 ( Figure 11 ), and possibly in ROM 62537 ( Figure 12.4–12.6 ).

Whole-body specimens attributed to Chancelloria eros were described by Janussen et al. (2002) from the Wheeler Shale. These have 6–8+1 sclerites, but no observed 4+0 forms. The sclerites are also more regularly arranged than in C. eros and are of consistent size. The Wheeler specimen figured by Bengtson (2000) as Chancelloria , without species assignment, also belongs to this form. Given the differences from the Burgess Shale C. eros , it is likely that these Wheeler forms (but not the one figured by Rigby, 1978, pl. 2:3) belong to a hitherto unnamed species, which may also include ROM 49576 ( Figure 14.3–14.5 ).

In our material identified as Chancelloria eros there are a number of specimens with poor sclerite preservation ( Figures 5.3 ; 6 ; 9.4 ; 14.2 ). As our recognition of the species technically hinges on the composition and variability of the scleritome, a definitive identification of these specimens is not possible. However, with the exception of ROM 49576 ( Figure 14.3–14.5 ), we have not seen any evidence of additional Chancelloria species present in the Burgess Shale material. We therefore

BENGTSON & COLLINS: CHANCELLORIIDS choose not to use open nomenclature for these specimens.

Figures

FIGURE 1. Chancelloria eros Walcott, 1920, lectotype, USNM 66524. Figured by Walcott (1920) as pls 86:2 and 88:1f. 1. Wet. 2. Dry, coated with ammonium chloride. Detail of 1 (position marked by frame). 3. Dry, coated with ammonium chloride. Detail of 2 (position marked by frame). Top arrow (grey): 4+0 sclerite. Middle arrow (black): 7+1 sclerite, preserved in negative relief (ascending ray towards the viewer). Bottom arrow (white): 7+1 sclerite, preserved in positive relief (ascending ray pointing away from the viewer).

FIGURE 5. Chancelloria eros Walcott, 1920. 1. ROM 62538, ST talus, wet. Arrow points to empty area that may represent an apical orifice. 2. Detail of 1 (position marked by frame), wet. 3. ROM 62535A, BW -300, dry.

FIGURE 6. Chancelloria eros Walcott, 1920. ROM 62539, BW -320. 1. Dry. 2. Detail of 1 (position marked by frame), wet.

FIGURE 7. Chancelloria eros Walcott, 1920. ROM 62588B, BW -320. 1. Dry. 2. Detail of 1 (position marked by frame), dry. 3. Detail of 1 (position marked by frame), wet.

FIGURE 8. Chancelloria eros Walcott, 1920. 1, 2. ROM 49580, ST. 1. Dry. 2. Detail of 1 (position marked by frame), dry. 3. ROM 62590, ST, dry.

FIGURE 9. Chancelloria eros Walcott, 1920. 1–3. ROM 57604, BW -130. Specimens of the lingulate brachiopod Acrothyra gregaria appear to be attached to the spines of the Chancelloria. The brachiopods are concentrated to what is interpreted as the apical part of the chancelloriid body. 1. Wet. 2. Detail of 1 (position marked by frame), dry. 3. Detail of 2 (position marked by frame), dry. 4. ROM 63055, ST, dry.

FIGURE 10. Chancelloria eros Walcott, 1920. ROM 62534, RT. 1. Wet. 2. Detail of 1 (position marked by frame), wet. Note the common presence of 3+0 Allonnia-type sclerites (arrows) in a scleritome dominated by 5–7+1 Chancelloria- type sclerites.

FIGURE 11. Chancelloria eros Walcott, 1920. ROM 49599A, RQ 8.0. Arrows point to examples of 3+0 sclerites. 1. Dry. 2. Detail of 1 (position marked by frame), dry.

FIGURE 12. Allonnia tintinopsis n.sp. intergrown with or superimposed on Chancelloria eros Walcott, 1920. 1–3. ROM 49605(2) (intergrown with a specimen of Vauxia), RQ 9.2. 1. Same as figure 12B in Bengtson, 2000; see that paper for details on settings. 2, 3. Details of 1 (positions marked by frames), dry. 4–6. ROM 62537B, RQ 8.4. 4. Wet. 5, 6. Details of 4 (positions marked by frames), dry. Net-like patches are fragments of Micromitra shells.

FIGURE 13. Chancelloria eros Walcott, 1920. 1. ROM 49578, ST, dry. 2. ROM 43126, ST, dry.

FIGURE 14. Chancelloria eros Walcott, 1920 (1, 2), and C. cf. eros (3–5), ST. 1. ROM 49574, dry. 2. ROM 49572, dry. 3–5. ROM 49576, dry. Items 4 and 5 show details of the upper and lower portions, respectively, of 3.

FIGURE 15. Life reconstruction of Chancelloria eros, Walcott, 1920. Artwork Pollyanna von Knorring.