Archiasterella coriacea, Bengtson & Collins, 2015

Bengtson, S & Collins, D, 2015, Chancelloriids of the Cambrian Burgess Shale, Palaeontologia Electronica 10 (14), pp. 1-67 : 38-50

Archiasterella coriacea n.sp.

Figures 4 ; 31.4 ; 38–45

http://zoobank.org/ 3BF76BF9-FA88-48E2-B80A-9561B6C99543

Chancelloria eros new species ( Walcott, 1920, partim; pl. 88:1c). Chancelloria eros Walc. ( de Laubenfels, 1955; figure 76).

Derivation of the Name. Latin coriaceus, leathery, alluding to the resilient integument of this species.

Holotype. ROM 62531 ( Figures 31.4 ; 38 ; 39).

Figured Paratypes. USNM 66527, ROM 49567, 49583, 49617, 49619, 57573, 62528, 62529, 62530, 62532, 62533.

Diagnosis. Species of Archiasterella having a distinct integument with an imbricating scaly surface pattern. Ray formula 4+0. Sclerite rays up to 5 mm long, of approximately the same length within each sclerite; size and spacing of sclerites within the scleritome may vary considerably. Apical tuft inconspicuous.

Description. The specimen USNM 66527 in Walcott’s syntype series of Chancelloria eros ( Walcott, 1920, pl. 88:1c) was used to illustrate the species C. eros in the first Porifera volume of the Treatise on Invertebrate Paleontology ( de Laubenfels, 1955, figure 76). The specimen can be attributed to Archiasterella coriacea and is reillustrated here as Figure 4 . It is imperfectly preserved, with the soft integument visible as a slightly darker stain of the rock surface. The outline of the soft parts is diffuse, and the specimen is probably incomplete, but it suggests a long and narrow body, at least 49 mm long and up to 8 mm wide. The sclerites are 3- dimensional and show the typical Archiasterella morphology but are generally incompletely preserved. The longest preserved ray (near the top of the specimen) is 4 mm.

Archiasterella coriacea has a conspicuous appearance in the Walcott and Raymond Quarries because of its sausage-shaped body and distinct integument, often folded more or less irregularly, sometimes with a finely scaly surface, and with its characteristic 4+0 sclerites, usually sparsely distributed over the integument.

The holotype ( Figures 31.4 ; 38 ; 39) is 170 mm long and 40 mm broad at its widest. It is complete, except for the lower part, which is missing; where the specimen ends abapically the width is 13 mm. The apical end is evenly rounded. It appears to be somewhat obliquely compressed, an apical orifice occupying a non-terminal position with its centre located 5 mm from the apical edge of the fossil.

The apical orifice of this specimen is indistinctly preserved, as is usual in Archiasterella coriacea . It is primarily noticeable through an elevated area of higher reflectance, like that of the sclerites, and a zone around it with a lumpy surface, contrasting with the more smooth preservation of the rest of the integument ( Figure 38.2 ).

The sclerites are preserved as flattened, reflective structures, with the typical swallow-like outline of compressed Archiasterella sclerites ( Figure 38.3 ). They vary considerably in size, the median ray in adjacent sclerites ranging from about 2.5 to 5 mm in length. The lateral rays are slightly shorter than the median ray, form about a 90° angle with each other, and curve slightly in the api-

BENGTSON & COLLINS: CHANCELLORIIDS cal direction. The ascending ray is usually visible only as a faint swelling, but at the edges of the fossil where the body wall is folded, ascending rays may be seen in side view ( Figure 38.3 , left), showing them to curve adapically and being of about the same length as the other rays. The median rays in these positions seem to form an angle of about 15– 25° with the body wall, but no sclerites are preserved with all their rays visible in their length, and so an identification of the individual rays is often not possible.

The sclerite rays commonly have a longitudinal fold, suggesting collapse of the originally hollow tube.

The sclerites are evenly and regularly distributed over the body surface, with a spacing of 4–6 mm. The spacing appears denser, because sclerites from both sides of the flattened body are visible (cf. Figure 39). As a general rule, the sclerites are regularly oriented with their median ray pointing in the apical direction, and the distribution approximately follows a diagonal rhombic pattern.

The holotype has a clearly developed granulosity to the integument ( Figure 31.4 ). The granulae are spaced about 100 µm apart and packed more or less regularly in a rectangular or rhombic pattern, where the longitudinal direction is more pronounced than the transverse one. The alignment of the pattern largely follows the longitudinal orientation of the body.

The granular surface pattern has been investigated in more detail in the fragmentary specimen ROM 57573 ( Figure 40 ). The SEM stereograms ( Figure 40.2, 40.3 ) show the pronounced longitudinal alignment of the granules as well as their relationship to the sclerites of the upper and lower body surface ( Figure 40.2 , left and right sclerite, respectively) in the compressed specimen. Also visible is a pattern of imbrication, where the free tips of the imbricating objects, expressed as granules, appear to point adapically ( Figure 40.3 ).

This specimen too has a sclerite spacing of 4– 6 mm in the main body regions, with more crowded

PALAEO- ELECTRONICA.ORG sclerites in the preserved portions of the abapical part.

The specimen ROM 62529 ( Figure 41.1, 41.2 ) is 111 mm long and 44 mm wide. Like the previous specimens, it lacks the lowermost part, but the rest of the body is well preserved and shows a sausage-shaped outline, almost cylindrical with a rounded apex. This specimen is less obliquely compressed than the holotype, and the apical tuft is expressed at the edge as an area with short, dense sclerite coverage, markedly differing from the clean edge with the occasional body sclerite visible elsewhere ( Figure 41.2 ) .

The sclerites are similar in shape and size to those of the holotype, and their disposition is similarly regular. Figure 41.2 shows three body sclerites in side view, demonstrating the orientation of the straight median ray (nearest to the body wall) and the ascending ray.

The integument is represented as a smooth film; no granulosity is visible. On one side of a joint in the rock (lower part of Figure 41.1 ), the integument and sclerites are covered with a thin, yellow veneer of limonite.

The specimen ROM 62530 ( Figure 41.3 ) is of medium size, 47 x 28 mm, egg-shaped in outline. The sclerites are preserved in 3D relief; the shape, size, and arrangement of the body sclerites is similar to that of the holotype. The apical end shows a region with dense sclerite rays; the shape of those sclerites is not apparent, but the region clearly represents the apical orifice. There is no obvious base, but at the rounded abapical end (lower left in Figure 41.3 ) there is an area with more irregularly arranged sclerites than in the main part of the body; this may indicate proximity to a constricted stalk-like portion of the body such as is evident in other specimens of Archiasterella coriacea ( Figures 42 ; 43.3 ), as well as Chancelloria eros and Allonnia tintinopsis . The integument is smooth and covered with a thin veneer of limonite.

A similar specimen, ROM 62532 ( Figure 41.4 ), has only the apical part preserved. The sclerites are three-dimensionally preserved and agree in shape, size, and arrangement with those of the holotype . The compression appears to have been non-oblique, and an apical tuft is indicated by a region of dense sclerite rays capping the rounded apical end of the body. The integument has a faint granulosity, which in dimensions and orientation agrees with that of the holotype.

The remarkable specimen ROM 62528 ( Figure 42.1, 42.2 ) is complete, except perhaps for the very attachment end. It is sausage-shaped, 57 mm long, and up to about 15 mm wide, but the shape is distorted by what appears to be irregular coarse folds of the integument, mostly in a transverse direction. A “head” is set off by one such fold about a centimetre from the apex, suggesting a slight sagging collapse of the body.

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This specimen also has a stalk-like abapical part preserved, similar to that found in Allonnia and Chancelloria , 3–5 mm thick. The stalk carries a dense sclerite cover and has an irregular texture; brachiopod shell fragments are juxtaposed to the stalk (creating the swelling in the lower part of the stalk; cf. Figure 42.1, 42.2 ).

The sclerite cover in this specimen is very uneven ( Figure 42.1 ). Large portions in the middle part of the body lack sclerites altogether, and the “head” and lower part of the body have only scattered, irregularly placed sclerites. Their orientation, however, is roughly “correct”, i.e., the median ray points apicalward. The sclerite density increases toward the lower part of the body, and the stalk appears fully sclerite-bearing, though its contorted nature makes it difficult to observe individual sclerites. The sclerites are preserved in a reflective material, with some 3D relief remaining.

The integument in this specimen is smooth in the lower part of the body; in the upper part a faint granulosity similar to that of the holotype is visible.

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FIGURE 39. Archiasterella coriacea n.sp., holotype, ROM 62531A, BW -130, superposition of sclerites from two sides of body. △ – basal side down; ▼– basal side up. Cf. Figures 31.4 and 38 .

The apical end of this specimen is marked by a concentration of reflective matter ( Figure 42.1 , top). There is a broad, indistinctly preserved apical tuft, consisting of a faint striation parallel to the longitudinal axis of the body as well as some coarser arcuate structures of a similar orientation. No morphology of individual elements can be made out, but the structure, dimensions and orientation indicated similarities with the apical tuft seen in numerous specimens of Allonnia tintinopsis .

Another nearly complete specimen, 29 mm long and up to 12 mm wide, is shown in Figure 42.3 (ROM 62533). The sclerites are preserved in semi-3D, some of them with distinct longitudinal folds suggesting collapse of the hollow ray, and evenly distributed over the body. The integument is smooth, but a few patches preserve a granulosity similar to that of the holotype. The apical region is somewhat flattened in outline and marked by a more irregular structure; no discrete tuft can be made out. There is a concentration of limonite in the apical end suggesting the former presence of pyrite, but an even more prominent patch of limonite is seen in the midpart of the body. The abapical end tapers to about 3 mm width and has a smooth termination defined by the lowermost sclerites. No attachment structure is evident.

The incomplete specimen ROM 49583 appears to have the lower end preserved, in proximity to a Vauxia that it may have been attached to ( Figure 42.4 ). The preserved length of the specimen is 50 mm, and in the abapical part it tapers from 15 to 6.4 mm width. The lowermost sclerites are distinctly smaller (up to 1.1 mm ray length) than those of the more apicalward part (up to 2.9 mm ray length).

Specimen ROM 49617 shows two somewhat contorted, sausage-shaped individuals with integument preserved as a light, shiny surface, well set off from the surrounding matrix ( Figure 43 ). The most complete specimen ( Figure 43.1 , top) is 180 mm long and up to 38 mm wide. Sclerites are unevenly distributed in the integument, sometimes with up to 10 mm distance between them ( Figure 43.2 ). No apical tuft or orifice is visible. The second specimen ( Figure 43.1 , bottom) is of a similar appearance, but incompletely preserved in the apical portion. In the abapical part, it tapers abruptly to a thin, bristly, sclerite-studded stalk, about 3 mm in diameter. The stalk makes a U-turn and appears to join the side of the first specimen ( Figure 43.3 ).

Similar specimens with distinctly preserved integuments are shown in Figure 44 . ROM 49619 ( Figure 44.1 ) is incomplete, and probably represents a single specimen that has been folded at the middle. This is indicated by the direction of the sclerites, the rays of which are pointing in different directions in the adjacent parts of the body. The length of the combined parts is 160 mm and the largest width is 34 mm. The specimen ROM 49567 ( Figure 44.2 ) has a contorted shape, with a folded and twisted integument. The specimen appears to have been twisted clock-wise, as indicated by the abapical part, where the regular curvature of the folds is clearly visible ( Figure 44.2 , bottom). This specimen has a broad oval patch at the apical end with a small, sclerite-dense region (arrow) that may represent an apical tuft.

In none of the specimens with preserved integument is there any evidence of pores.

A life reconstruction of Archiasterella coriacea is shown in Figure 45.

BENGTSON & COLLINS: CHANCELLORIIDS

Comparison. The species differs from Archiasterella pentactina Sdzuy, 1969 , and Ar. fletchergryllus Randell et al., 2005 , in having only 4+0 sclerites.

Figures

FIGURE 4. Archiasterella coriacea n.sp. USNM 66527 (specimen figured as Chancelloria eros by Walcott,1920, pl. 88:1c, and de Laubenfels, 1955, figure 76). 1. Dry. 2. Detail of 1 (position marked by frame), dry.

FIGURE 31. Integument structure of Allonnia and Archiasterella. 1. Allonnia tintinopsis n.sp. ROM 62514B, RQ 9.8, dry. 2, 3. Allonnia tintinopsis n.sp., ROM 49600A, UE. 2. Dry. 3. Detail of 2 (position marked by frame), dry. 4. Archiasterella coriacea n.sp., holotype, ROM 62531A, BW -130, dry.

FIGURE 38. Archiasterella coriacea n.sp., holotype, ROM 62531A (same specimen as in Figures 31.4 and 39), BW - 130. 1. Complete specimen showing alignment of sclerites, wet. 2. Detail of 1 (position marked by frame) showing the apical tuft, wet. 3. Detail of 1 (position marked by frame) showing the attitude of sclerites relative to the body wall, wet.

FIGURE 40. Archiasterella coriacea n.sp, ROM 57573, WQ -130. 1. Wet. 2, 3. SEM stereographs (red/blue anaglyphic images) of latex casts taken of counterpart to show structure of integument. Frames show positions of details 2 and 3.

FIGURE 41. Archiasterella coriacea n.sp. 1, 2. ROM 62529B, BW -130. 1. Dry. 2. Detail of 1 (position marked by frame), wet/pol. 3. ROM 62530A, BW -130, wet. 4. ROM 62532, BW -130, apical part, wet.

FIGURE 42. Archiasterella coriacea n.sp. 1, 2. ROM 62528A, RQ 8.4. 1. Showing distribution of sclerites, wet. 2. Showing integument, wet/pol. 3. ROM 62533A, BW -130, wet. 4. ROM 49583B, RQ 9.0, wet. Specimen attached to a Vauxia.

FIGURE 43. Archiasterella coriacea n.sp. ROM 49617B, RQ 13.8. 1. Complete specimen, wet. 2. Detail of upper specimen in 1 (position marked by frame), wet. 3. Detail of 1 (position marked by frame), wet, showing stalk uniting upper and lower specimen.

FIGURE 44. Archiasterella coriacea n.sp. 1. ROM 49619A, RQ, wet. 2. ROM 49567A, UE, wet. Arrow points to tuft.

FIGURE 45. Life reconstruction of Archiasterella coriacea, n.sp. Artwork Pollyanna von Knorring.