Skovsted and Topper—Mobergellans from Greenland and Labrador
fine laminae as previously documented for other mobergellans (Bengtson, 1968; Streng and Skovsted, 2006; Demidenko, 2016). The apex of the shell is subcentral but usually slightly shifted toward the anterior side of the shell. Internally, the shells are smooth, with the exception of a series of (usually) 10 paired muscle imprints (although some specimens may instead have nine or 11 scars), roughly arranged in a circle, slightly shifted toward the anterior of the shell. The size of the muscle scars increases toward the posterior, resulting in a distinctive bilateral symmetry. Each muscle scar is a subcircular to triangular depression often ornamented by low, roughly parallel ribs mirroring the shape of the muscle scar and presumably reflect- ing growth lines.
Larval shell morphology.—The initial shell of Discinellamicans is a low, cap-shaped subcircular structure at or close to the center of the shell (Fig. 2). Its placement is usually slightly shifted from the center toward the anterior (Fig. 2.1, 2.3, 2.5, 2.7). In well-preserved specimens, the initial shell is defined and clearly delineated by the onset of commarginal growth as the initial shell itself lacks growth lines. The initial shell is always convex in lateral profile, even in specimens where growth direction later changes, resulting in a concave adult shell. The structure has a maximum diameter of 210μm (average from22 specimens), and its height typically represents 38%of the diameter (average from 8 specimens). The initial shell is similar in shape and general morphology to the larval shells of various invertebrates (such as gastropods, brachiopods, hyolith opercula, etc.; see Demidenko, 2016) and likely represents the first mineralized growth stage, which may have been formed at or slightly before settling of the larvae on the seafloor. The cap-shaped larval shell of Discinella micans consists
of a central ridge oriented along the anterior-posterior line of symmetry (Fig. 2.2, 2.4, 2.6, 2.8). The ridge is widest in the center and sometimes divided in a series of distinct rounded lobes (Fig. 2.6). From the central ridge extends on both sides a set of six to eight fine ribs separating slightly depressed areas forming a radial pattern (Fig. 2.2, 2.4, 2.6). The larval shell is commonly preserved in specimens from
samples with generally good preservation of phosphatic fossils, but it is not observable in all specimens. This is potentially the reason why this feature has not been noted in previous investigations (Lochman, 1956; Landing and Bartowski, 1996; Conway Morris and Chapman, 1997; Skovsted 2003).
Anterior muscle scars.—The characteristic muscle scars on the interior surface of Discinella micans are variable in number (Fig. 3). Although 10 scars are usually reported in the literature, the anteriormost pair of scars are often fused, resulting in nine obvious scars. Conway Morris and Chapman (1997) implied that fusion of the anteriormost scars may be a feature of late growth stages in D. micans. However, in our material, fusion of the anteriormost muscle scars appears to affect specimens of all sizes. Large specimens sometimes express fused scars (Fig. 3.1– 3.4, 3.11–3.12) and sometimes separate scars (Fig. (3.5, 3.6, 3.9, 3.10). The same pattern is obvious in the smallest specimens recovered where both fused (Fig. 3.17, 3.18) and separate scars (Fig. 3.19, 3.20) occur.
73 Conway Morris and Chapman (1997) reported an extra
scar-like feature on the shell interior close to the fused anteriormost scars in a large and unusually thick-shelled specimen from New York State, and Skovsted (2003) noted that similar extra scars are also present in specimens from Greenland. In our material, extra scars between or behind the anteriormost pair of scars are present in a large number of specimens from both Greenland and Labrador (Fig. 3.6, 3.10, 3.12, 3.14, 3.16). The position of the extra scar is typically between (Fig. 3.6) and slightly behind (i.e., closer to the center of the shell; Fig 3.14, 3.16) the normal anteriormost muscle scars. It is variable in size relative to the other scars of the shell interior but is typically much smaller than the smallest of the anteriormost scars (about 10–50 µm in diameter). The extra scar is subcircular to transversely oval and of variable depth but often appears to be deeper than the normal anteriormost scars (Fig. 3.10, 3.14).
Anomalous muscle scars.—A few otherwise unremarkable specimens of Discinella micans exhibit unusual configurations of muscle scars, here exemplified by a shell from Northeast Greenland (Fig. 4.10). This specimen has a normal set of five muscle scars of increasing size on the right-hand side, but on the left-hand side only four scars are apparent, and only the pos- teriormost two are of normal morphology. In the anteriormost position is a single large, heart-shaped scar. Judging from the position of this scar compared to the scars of the right-hand side, it presumably represents the fused first and second muscle scar of the left-hand side. The following scar on the left-hand side is reduced to a narrow slit-like feature between the unusually large combined first and second scars and the normal posteriormost two scars.
Shell malformations.—Shell growth in Discinella micans is typically very regular with slow increase in shell diameter in concert with the addition of new shell layers. The variation in width of growth increments (i.e., the distance between adjacent growth lines) appears to be small, and the growth rate seems to have changed very little through ontogeny in most specimens (Fig. 2.1, 2.5, 2.7). However, some specimens in our collections show evidence of disruption of the normal growth regimen. In some specimens, the growth vector changes and new shell layers are added without increase in diameter, resulting in thick shells with pronounced marginal rims (Fig. 4.1, 4.2). Other specimens exhibit traces of disruption in shell formation with new shell layers partly detached from the older shell (Fig. 4.7–4.9). In such specimens, new shell material was sometimes added at an angle or slightly off-center compared to previously formed shell (Figs 2.3, 4.7). Growth lines in the new shell layers inserted below the previous shell seem to indicate a reduction in the diameter of the shell-secreting tissue before the new shell was formed (Fig. 4.8), partly at an angle to the older shell (visible as a narrow slot between the old and new shell; Fig. 4.9). The newly formed shell in such regrown specimens shows no other signs of disruption of the shell-secreting tissue. The growth increments are of normal width, and although the new growth may be slightly off-center, the shape of the disc only changes marginally.
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