Skovsted and Topper—Mobergellans from Greenland and Labrador A single specimen of Discinella micans from Labrador


exhibits a gross malformation of the shell in the form of a high dome-shaped growth on the internal surface, close to the edge of the shell in the area adjacent to one of the last muscle scars (Fig 4.3–4.6). This specimen is otherwise unremarkable among specimens of D. micans in our collections. The shell is large but not unusually large (diameter 2.04 mm); except for the anomalous growth, it is almost flat and the anteriormost pair of muscle scars is fused. The growth pattern appears to be perfectly normal except for the formation of this distinct dome-like structure. The dome is smoothly rounded, 0.48mm high and


0.3mm wide near its apex. The abnormal growth however has flared the shell margin on either side of the dome by 0.3mmand in total affects about 44% of the diameter of the shell and increases the total height of the sclerite by 150% (shell height— 0.32mm). The surface of the dome is continuous with the shell interior, and a fracture along the shell edge reveals that the anomalous growth affected a series of shell layers (Fig. 4.6).


Discussion


The Mobergellidae are characterized by disc-shaped shells with an internal surface bearing a variable number of muscle- attachment structures. Although these features are fluid across the group, the morphology of the larval shell appears to be relatively consistent, represented by a subcentrally positioned, low, flat or cap-shaped subcircular structure that frequently exhibits features that reflect the configuration of the muscle scars on the interior surface (Fig. 2). Slight variation does exist; for example, in the Siberian species of Mobergella (Skovsted, 2003; Demidenko, 2016), the larval shell is sometimes flat and ornamented by radial rows of pores, reflecting the porous sur- face of muscle scars on the inside of the adult shell. The larval shell structures in D. micans described here reflect the imprints of the internal muscles on the larval shell as depressed areas between radiating ribs. These areas probably represent the attachment sites of the larval muscles to the shell interior and are formed by the deformation of the thin-shelled (and possibly poorly mineralized) cap-shaped larval shell by the forces induced by muscular contraction. The problematic fossil Hip- poklosma mongolica Conway Morris and Chapman, 1997 from the lower Cambrian of Mongolia has been compared to mobergellans (Missarzhevsky, 1989; Conway Morris and Chapman, 1997), but the lack of distinct muscle scars and the peculiar radiating fields of fibrous bundles (fibroplacodes) on the external surface of the shell seem at odds with the typical morphology of mobergellans, and this taxon has sometimes been excluded from the group (Streng and Skovsted, 2006; Demidenko, 2016). However, the cap-shaped larval shell of H. mongolica is composed of a central axis with arcuate furrows extending on both sides. This configuration of structures is strikingly similar to the larval shell morphology of D. micans described here and may lend further support for the hypothesis that H. mongolica is closely related to the Mobergellidae. The morphological similarity of the larval shell, however,


does not unfortunately assist greatly in revealing the functional morphology or biological affinity of these enigmatic sclerites. To resolve the functional morphology of mobergellans, Topper and Skovsted (2017) compared the relative size of the prominent


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muscle scars of two mobergellans (Mobergella holsti (Moberg, 1892) and Discinella micans) to muscle attachment areas of modern and fossil invertebrate shells of differing function (e.g., bivalves, brachiopods, monoplacophorans, polyplacophorans, annelid elytra, and hyolith and gastropod opercula). The ana- lysis showed that mobergellan muscle scars occupy about 20% of the area of the shell interior, which is very high compared to muscle attachment sites in most invertebrate shells, being only compatible with the muscle attachment area of the opercula of neritic gastropods. Hence, an opercular hypothesis as the func- tion of mobergellan shells for a tube-dwelling organism was considered most likely (Topper and Skovsted, 2017), although the nature of the associated tube and its phylogenetic affinity remain elusive. In Northeast Greenland and Labrador, Dis- cinella micans occur together with the phosphatic tubes of Hyolithellus Billings, 1871 as well as hyoliths and other cal- careous tubes (Skovsted, 2006; Skovsted and Peel, 2010; CBS, personal observation, 2016). However, the size and morphology of these tubes do not fit with the size and morphology of co- occurring D. micans. Following Bengtson (1968), Topper and Skovsted (2017)


hypothesized that the corresponding tube was cemented or firmly attached to the substrate, and after the death of the animal,


the opercula were transported away by prevailing currents. The tendency for mobergellan sclerites to be recovered in high numbers from reworked transported sediments was used as evidence to support this hypothesis (Topper and Skovsted, 2017). Discinella micans material from Labrador was originally documented from debris surrounding archaeocyathan bioherms (Spencer, 1980). Spencer (1980) also noted a decrease in the relative abundance of D. micans away from the archaeocyathan bioherms, contrasting to the simultaneously increasing abun- dance of the co-occurring brachiopod Hadrotreta (with shells of similar size, shape, and composition), and it is possible that the mobergellan animal inhabited shallow water archaeocythid buildups and, upon death, the dissociated opercula accumulated at the base of the biohermal structures. Topper and Skovsted (2017) noted a trend of recovering mobergellan sclerites from reworked sediments including Mobergella holsti, which has been documented from shallow-water clastic sediments asso- ciated with paleoislands in Baltica (Bengtson, 1968); it is plausible that mobergellans preferred inhabiting shallow-water, high-energy environments. The material of Discinella micans described herein yields new information that may be important to further constrain


the functional morphology of mobergellans. The unusual con- figuration of fused muscle scars in specimens of D. micans (Fig. 4.10) points to a considerable plasticity in the muscular system of mobergellans where unusual configurations of muscles appear to have been fully functional. The flat to concave lateral profile of the shells illustrated here (Fig. 5) is, as also noted by Topper and Skovsted (2017), not consistent with representing a univalved or a two-valved organism. This is of course assuming, in a two-valved scenario, that both valves are morphologically recognizable as mobergellan sclerites. Perhaps most important is the unusual specimen documented herein with an internal anomalous growth (Fig. 4.3–4.6). The anomalous growth of this unique specimen is reminiscent of the formation of pearls on the interior of shells of bivalves (e.g., Jacob et al.,


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