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Biology Articles » Paleobiology » Paleozoology » The earliest annelids: Lower Cambrian polychaetes » Description

Description
- The earliest annelids: Lower Cambrian polychaetes

The majority of specimens are incomplete, either because of rock breakage or because the extremities remain concealed within the sediment and are difficult to prepare. Even in the relatively few specimens with either end preserved these re− gions are usually indistinct. Where size measurements are possible, values for length are found to range from ca. 25–35 mm (Figs. 2A, 3D), whilst widths (which are more readily measurable) range from 1.8–12 mm. The greatest width is also seen in the longest complete specimen, so by extrapola− tion of a length to width ratio of c.1:3 this suggests that the species did not exceed ca. 40 mm in length. The attitude of the specimens is always parallel to the bedding plane, but otherwise varies from being straight (Fig. 3D) to gently or strongly recurved (Figs. 2D, 5F).

The most obvious feature on all specimens is the chaetae (e.g., Figs. 2B, 5C), which form prominent fans. In many specimens further differentiation is difficult, but in appropri− ate material it is clear that on either side of each segment they form two bundles. As with the majority of polychaetes it seems reasonable to interpret the respective bundles as the standard notochaetae and neurochaetae. Differentiation is based on the assumption that the broad naked area that is flanked by obliquely inclined bundes (Fig. 5D) is the ventral view, whilst the other set of bundles that run sub−parallel to the longitudinal axis are dorsal. As noted below this is con− sistent with the neurochaetae and notochaetae adopting re− spectively a locomotory and protective role. The chaetae of each bundle are relatively robust and may also have been somewhat flattened. Under high magnification the chaetae sometimes show a longitudinal structure. The number of chaetae per bundle is difficult to estimate precisely, but ap− pears to have been typically ca. 15.

Notochaetae and neurochaetae are distinguished on the basis of both position and orientation (Figs. 2A, 3D, E, 6C). The former are generally less completely preserved, but are arranged sub−parallel to the body axis and appear to have been relatively short. The notochaetae are directed posteri− orly, except around the anterior where they appear to have extended forwards (Figs. 2A, 3C). By definition the noto− chaetae are on the dorsal surface, and in appropriately pre− served specimens they are seen to overlie the neurochaetae. In the majority of specimens the extent of coverage by the notochaetae appears to be restricted to the dorsal margins. This may, however, be effectively a preservational artefact because in MGUH 28.884 and MGUH 28.888 (Fig. 3A, E) notochaetae are seen to cover almost the entire dorsum. There is some evidence that the anterior−most notochaetae were not only significantly smaller but were also composed of smaller bundles (Figs. 2A, 3C).

The neurochaetae are almost invariably more prominent, and project at an oblique and posterior angle to the body axis (e.g., Fig. 5B). The parapodia are also sometimes visible (Figs. 5E, 6A, B), and were closely spaced and arose along the ventro−lateral margins, being separated by a broad and more−or−less featureless ventral zone (Fig. 5E). As with the notochaetae the anterior−most bundles appear to be smaller (Fig. 6D), while towards the posterior the neurochaetae were more elongate (Figs. 2D, 3D, 6A).

Little else is known of the external aspect. In no specimen is the anterior clearly preserved. So far as can be told there were no palps, antennae, or other appendages. So too evi− dence is lacking for eyes, and also gills. Estimates of segment numbers must rely on counts of chaetal bundles, and given the degree of overlap and incompleteness of specimen num− bers are somewhat tentative. An average size specimen, however, appears to have possessed ca. 20 segments. Several features of the internal anatomy are preserved, al− though some interpretations are not straightforward. A few specimens show a dark trace (but less evident in the material when coated with sublimate; Fig. 6A) which is interpreted as the alimentary canal. In two cases gut contents have been identified. One (unillustrated) consists of a bolus of fine− grained, indeterminate material, while in the posterior region ofMGUH 28.899 (Fig. 6F) there is a semi−continuous gut fill of more particulate matter. No jaw has been identified in this species. In one specimen (Fig. 6D) an elongate strand ex− tends from close to the anterior. This could be identified as an everted proboscis, but this structure lies at a different level and we consider it more likely to be a fortuitous association. A more prominent feature of the internal anatomy are two parallel strands in the mid−region of the body (Fig. 6B, F; see also Fig. 5A, C) which presumably originally flanked the gut. These strands, assuming they represent the same structure, show a somewhat variable appearance. In MGUH 28.885 (Figs. 3B, 5D) the strands show a fine longitudinal fibrosity, consistent with it representingmuscular tissue (Budd 1998). In other specimens these longitudinal strands may display amore complex arrangement, which in MGUH 28.898 (Fig. 6E) con− sists of a series of irregular, tube−like structures. In MGUH 28.882 (Fig. 2C) the strands have prominent relief, but the in− tervening area is also partially mineralized. A more problem− atic arrangement, only clearly seen in MGUH 28.881 (Fig. 2B), consists of segmentally arranged rod−like structures that in being inclined adaxially serve to define a chevron−like ar− rangement. Closer to the midline in this specimen there is also a series of block−like structures which may represent muscles. The somewhat variable appearance of these strands make it difficult to decide whether they represent taphonomic variants of the same structure or are distinct organs. At the least, how− ever, the prominent fibrous strands probably represent mas− sive internal musculature.


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