Chalazoscolex, Morris & Peel, 2010

Morris, Simon Conway & Peel, John S., 2010, New palaeoscolecidan worms from the Lower Cambrian: Sirius Passet, Latham Shale and Kinzers Shale, Acta Palaeontologica Polonica 55 (1), pp. 141-156 : 142-148

Chalazoscolex gen. nov.

Specimens are generally incomplete, but this is a result of rock breakage rather than decay. The longest known specimens are each about 90 mm in length ( Fig. 1A 1, B 1 ). The smallest specimens are incomplete, but on the assumption that the body had a similar width to length ratio to the adults they would have been approximately 50 mm long. Specimens are usually more or less straight with at most a gentle curvature ( Figs. 1B 1 , 2A 1, B 1, D 1, F 1 , 4B 2), but on occasion they are either strongly recurved or sharply folded ( Fig. 1A 2). This style of preservation suggests that the cuticle was relatively robust, and that the degree of contractibility was relatively restricted.

The body can be divided into three sections, of which the mid−portion of the trunk occupies the greatest proportion and separates an anterior introvert ( Figs. 1A 3 , B 4 , 2B 2, D 2, E 1 , 4B 1 ) (alternatively named the proboscis) from a distinct caudal region ( Fig. 2A 2, C, G). The most obvious feature of the introvert is a dense array of scalids which on the exterior are posteriorly directed ( Fig. 2B 2, E 1 ). In the majority of cases their arrangement is not very regular, but on occasion a transverse pattern is evident ( Fig. 2B 2). The scalids are simple, tapering to a fine point from an expanded base ( Figs. 4B 1 , 6G ), and are about 500 µm in length. Although sometimes appearing asymmetrical this is most likely preservational. In some specimens the posterior part of the introvert bears a series of narrow, anastomosing grooves and ridges ( Figs. 1A 3 , B 4 , 2D 2, 2E 1 ), each with an approximate width of 100 µm ( Fig. 6B ). The fact they tend to be most prominent where the introvert narrows anteriorly ( Fig. 1B 4 ) suggests that these ridges could have doi:10.4202/app.2009.0058

vert served to engage to accommodate in retraction constriction is uncertain

. The

. However ability

,

of a configura−

the intro− The palaeoscolecidan Xystoscolex tion where the scalids appear to be restricted to the ante− gen. nov.

rior−most region (e.g., Fig. 2D 2) suggests that some degree of withdrawal was possible. In some specimens the introvert This palaeoscolecidan is less common than Chalazoscolex ,

contains a massive structure with positive relief ( Fig. 2B 2, D 2). with relatively few well−preserved specimens. They are often

This presumably represents the pharyngeal area, but in only in a coiled configuration ( Fig. 3A, B 1 , D 2, E), although in

one or two specimens is there a faint quincunxial pattern (Fig. some specimens ( Fig. 3C 2) the anterior section is more or less

2E 1) that presumably reflects the teeth that lined the pharynx. straight. Other configurations are more unusual. They in−

The main trunk region occupies much of the remainder of clude an example of a sinuous disposition ( Fig. 4C 2), whilst

the body ( Figs. 1A 2, B 2, 2B 1, D 1, F 1 ). It shows an obvious seg− another specimen is strongly recurved ( Fig. 5A ). As noted mentation that can overlap with the longitudinal ridges extend− below the coiled disposition may reflect a response to anoxia,

ing from the proboscis ( Figs. 1A 3 , 2D 2, E 1 ). The segments have but the other arrangements could reflect the results of trans−

a width of 0.5 mm, so a typical specimen would have possessed port (although in general there is little such evidence) or approximately 140 segments. The ornamentation of the trunk more advanced decay. In any event these various orienta−

was subdivided into at least two longitudinal zones ( Figs. 1A 2, tions make precise estimates of length somewhat difficult,

B 3, 2B 2, F 2, 4B 1). In one of the zones the segments bear individ− but a typical length for this taxon is in the order of 120 mm

ual sclerites, with two to three occupying the width of each seg− (with a width of 10 mm).

ment. Although poorly preserved ( Fig. 6F 1 ) their individual di− The body is divided into three regions: anterior introvert

ameter ranges between ca. 100–200 µm. Apart from having a (or proboscis), trunk and caudal region, of which the trunk

circular shape, no details of any microstructure are preserved. accounts for most of the length. The proboscis bears two

These sclerite−bearing segments evidently formed a single types of structure. Most obvious is a dense array of scalids.

zone, and in some material (e.g., Fig. 2F 2) the orientation of the They do not have a regular arrangement ( Figs. 3C 1 , 4A, D ,

trunk with respect to the bedding plane means that the upper se− E 2, 5A, 6E) and can extend for up to ca. 12 mm along the

ries of sclerites appear in positive relief while the lower are body. The individual scalids are effectively spinose, with a

seen in negative relief. The other main zone on the trunk bears wide base that tapers to a fine point ( Figs. 3C 1 , 5B , 6E ). In

characteristic short longitudinal ridges (ca. 80 µm wide). These addition, there are hints of a faint longitudinal texture on the

are somewhat irregular ( Fig. 6C ) and overall impart a wrinkled cuticle between the scalids ( Fig. 6A ). Located to the anterior

appearance to the segments ( Fig. 2F 2). It appears also that the of the scalids is a series of relatively stout spines, the orienta−

region that connects the sclerite− and ridge−bearing parts of tion of which is quite variable ( Figs. 3C 1 , 4A , 5 ). In some each segment was more or less smooth and at most displayed specimens the proboscis has a smooth oral cone that projects

only a subdued ornamentation ( Fig. 2B 2). forward, and in one specimen ( Fig. 4E ) the entire pharyngeal

The caudal region bears obvious transverse rows of scler− region is everted in the same manner as a number of Burgess ites ( Fig. 2A 2, C, G), but these appear to have encircled the en− Shale−type priapulids (e.g., Conway Morris 1977: pl. 1: 4, pl.

tire body. This region of the body is distinctive both on ac− 2: 3, pl. 5: 5, pl. 6: 3, pl. 10: 5, pl. 11: 2; Huang 2005: fig. 33).

count of the regular arrangement of the sclerite rows, which Across much of its surface there is a rather faint quincuxial total about 15, and the relatively large size of the individual pattern, whilst at the anterior there are irregular striations sclerites (diameter ca. 200–300 µm). As with the more anterior ( Fig. 4E 2). The quincunxial pattern most likely reflects the sclerites, no microstructural details are evident ( Fig. 6D ). In pharyngeal teeth, whilst the striations are perhaps equivalent one specimen ( Fig. 2C) the trunk appears to extend beyond the to structures interpreted as anterior retractor muscles in segmented region as a short, smooth extension (ca. 6 mm Ottoia prolifica ( Conway Morris 1977: pl. 6: 3, text−fig. 50).

long), possibly equivalent to the bursa seen in other Burgess The trunk is more or less smooth in the anterior region,

Shale−type stem−group priapulids, notably Ottoia prolifica but more posteriorly shows a subdued segmentation (Figs.

( Conway Morris 1977: pl. 8: 5–7, pl. 9: 1, pl. 13: 6). 3B 2, D 2, 4C 2). The segments are narrow (ca. 400 µm) and

Apart from the pharynx located at the anterior end, the only bear a faint ornamentation that includes zones with circular other evidence for internal anatomy is parts of the intestine. sclerites and short ridges ( Fig. 3B 2, D 1 ). It is likely, but can−

These are usually delimited by unidentifiable infill ( Figs. 2A 2, not be proved with the material to hand, that the zones had a

C, 4B 2, G). The intestine can be fairly broad (ca. 2 mm, Fig. similar disposition to those of Chalazoscolex . The caudal re−

4B 2, G), and the anus is assumed to have been terminal. gion ( Figs. 3D 1 , 4C 1 , 5A ) forms a distinct region on account +

doi:10.4202/app.2009.0058

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of a prominent series of closely spaced segments (ca. 45), each of which carries relatively large sclerites (ca. 100–200 µm diameter). They evidently have a more or less circular shape, but no microstructure is visible ( Fig. 5B ).

In terms of internal anatomy apart from the specimen with an everted pharyngeal region ( Fig. 4E 2), the only feature is part of the intestinal tract, which varies in width. For the most part the infill is non−descript ( Figs. 3A , 4C 2, F), but in one specimen there are the remains of the arthropod Isoxys volucris , identifiable on the basis of its characteristic spines ( Fig. 4F ; see Williams et al. 1996: fig. 6).

Figures

Fig. 1. Palaeoscolecidan Chalazoscolex pharkus gen. et sp. nov., lower Cambrian (Series 2, stage 3), Buen Formation, Sirius Passet Fossil−Lagerstätte, Peary Land, North Greenland. A. MGUH 29133, entire specimen (A1) and enlargements of trunk (A2) and introvert (A3); see also Fig. 6B. B. MGUH 29134, entire specimen (B1) and enlargements of anterior (B2), trunk (B3), and introvert (B4). Scale bars 10 mm. All specimens coated with sublimate.

Fig. 4. Palaeoscolecidans Xystoscolex boreogyrus gen. et sp. nov. (A, C–F) and Chalazoscolex pharkus gen. et sp. nov. (B, G), lower Cambrian (Series 2, stage 3), Buen Formation, Sirius Fossil−Passet Lagerstätte, Peary Land, North Greenland. A. MGUH 29147, anterior with introvert. B. MGUH 29148, enlargement of introvert and anterior trunk (B1) and entire specimen with gut trace (B2); see also Fig. 6F. C. MGUH 29149, enlargement of posterior end (C1) and entire specimen with gut trace (C2). D. MGUH 29142, enlargement of introvert, see also Fig. 3A. E. MGUH 29150, entire specimen (E1) and enlargement of introvert with everted pharyngeal region (E2); see also Fig. 6A. F. MGUH 29151, gut trace containing arthropod Isoxys volucris Williams, Siveter, and Peel, 1996 with characteristic spinose extension arrowed. G. MGUH 29137, gut trace; see also Fig. 2C. Scale bars 5 mm in D, G; otherwise 10 mm. All specimens coated with sublimate.

Fig. 3. Palaeoscolecidan Xystoscolex boreogyrus gen. et sp. nov., lower Cambrian (Series 2, stage 3), Buen Formation, Sirius Passet Fossil−Lagerstätte, Peary Land, North Greenland. A. MGUH 29142, entire specimen with introvert at top right; note also gut trace. B. MGUH 29143, entire specimen (B1) and enlargement of posterior (upper) and introvert with anterior spines (B2). C. MGUH 29144, detail of introvert (C1) and entire specimen (C2); see also Fig. 6E. D. MGUH 29145, detail of introvert (upper) and posterior termination (lower right) (D1) and entire specimen (D2). E. MGUH 29146, entire specimen. Scale bars 10 mm. All specimens coated with sublimate.

Fig. 6. Scanning electron micrographs of Xystoscolex boreogyrus gen. et sp. nov (A, E) and Chalazoscolex pharkus gen. et sp. nov (B–D, F), lower Cambrian (Series 2, Stage 3), Buen Formation, Sirius Passet Fossil−Lagerstätte, Peary Land, North Greenland. A. MGUH 291560, scalids of the introvert, see also Fig. 4E. B. MGUH 29133, grooves and ridges on the introvert, see also Fig. 1A. C. MGUH 29136, segments of trunk showing longitudinal cuticular ridges, see also Fig. 2B. D. MGUH 29141, posterior trunk with larger sclerites, see also Fig. 2G. E. MGUH 29144, detail of scalid, distal point to upper right, see also Fig. 3C. F. MGUH 29148, segments with rows of sclerites and intervening unarmoured areas of cuticle (F1) and scalids of introvert (F2), see also Fig. 4B. Scale bars 400 µm (A+F1), otherwise 300 µm. All specimens coated with gold film.

Fig. 5. Palaeoscolecidan Xystoscolex boreogyrus gen. et sp. nov., lower Cambrian (Series 2, stage 3), Buen Formation, Sirius Passet Fossil−Lagerstätte, Peary Land, North Greenland. MGUH 29152. A. Anterior and posterior (upper left) ends of folded specimen. B. Detail of introvert with anterior scalids and in lower right sclerites of posterior (lower left). Scale bars 10 mm. Specimen coated with sublimate.