Journal of Paleontology, 92(1), 2018, p. 71–79 Copyright © 2017, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.41
Mobergellans from the early Cambrian of Greenland and Labrador: new morphological details and implications for the functional morphology of mobergellans
Christian B. Skovsted1 and Timothy P. Topper2
1Department of Palaeobiology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden ⟨
christian.skovsted@
nrm.se⟩ 2Palaeoecosystems Group, Department of Earth Sciences, Durham University, Durham DH1 3LE, UK ⟨
timothy.topper@
durham.ac.uk⟩
Abstract.—New morphological features of the mobergellan Discinella micans (Billings, 1871) from the lower Cambrian (Stage 4) of Northeast Greenland and southern Labrador are described. The new features include: (1) the morphology of the larval shell, which is shown to be cap-shaped, subcircular, and with impressions of the internal muscle attachment scars; (2) a range of unusual shell deformations (changes in growth direction resulting in thickened shells, partial detachment of shell laminae and subsequent regrowth, internal projections of shell material increasing the depth of the shell by up to 150%, disturbances and irregular fusion of muscle scars). In addition, we provide new details about the variability in number and shape of the anteriormost internal muscle scars, which often fuse and may vary in number from one to three (resulting in nine to 11 scars in total). Together the new observations provide additional strength for the hypothesis that mobergellan shells represent opercula of an as yet unknown tubular organism.
Introduction
The fossil record of the earliest Cambrian (Terreneuvian and unnamed Cambrian Series 2) is dominated by a plethora of small shells, plates, and sclerites, often collectively labeled ‘small shelly fossils’ (SSF; Matthews and Missarzhevsky, 1975; Bengtson et al., 1990). These fossils, usually extracted from carbonate rocks using acid treatment, represent some of the earliest undoubted metazoan body fossils and a vital component for our understanding of early animal evolution during the Cambrian Explosion. Many SSF taxa represent parts of larger skeletons (sclerites, spicules, etc.), and their often-unusual morphology lacks clearly identifiable counterparts among living metazoans. This has historically led to the placement of many SSF taxa among the problematica. However, detailed study of morphology, shell structure, and partly articulated specimens has meant that we can now place many previously problematic SSFs in the stem groups of modern phyla. Examples include the net-like plates of Microdictyon that have been shown to belong to stem onychophorans (Chen et al., 1989) and the cap- or cone- shaped tommotiids that have been interpreted as members of the brachiopod stem group (Skovsted et al., 2008, 2009, 2011, 2014; Murdock et al., 2012, 2014). Yet, some SSF groups remain problematic in terms of both functional morphology and phylogenetic position. One such group is the mobergellans. Mobergellans are highly problematic disc-shaped fossils
from the first half of the Cambrian (Stages 3–5) that are wide- spread and may be locally very common (Lochman, 1956; Bengtson, 1968, Skovsted, 2003; Streng and Skovsted, 2006; Demidenko, 2016). The shells of mobergellans are phosphatic, presumably by original composition, and grew by marginal
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accretion (Lochman, 1956; Bengtson, 1968). The most dis- tinctive features of mobergellans are radiating sets of muscle scars and/or platforms on the supposedly internal surface. The exact nature and number of scars is variable (from eight to 14), but the scars are usually arranged in pairs on either side of a central line of bilateral symmetry. Mobergellans were first discovered almost 150 years ago (Billings, 1871), and their taxonomy and phylogenetic position have been discussed ever since. Originally considered to be the opercula of the phosphatic tubular fossil Hyolithellus (Billings, 1871; Fisher, 1962), mobergellans have also been interpreted as brachiopods (Hall, 1872; Moberg, 1892; Dzik, 2010) and univalved molluscs (Hedström, 1923, 1930; Missarzhevsky, 1989; Conway Morris and Chapman, 1997) as well as the opercula of an as yet unknown tubular organism (Bengtson, 1968; Rozanov and Zhuravlev, 1992; Topper and Skovsted, 2017). Here, we illustrate and describe specimens from extensive
collections of the mobergellan Discinella micans (Billings, 1871) from Northeast Greenland and southern Labrador (Cambrian Stage 4). Discinella micans is known to occur along the present eastern paleomargin of Laurentia (Fig. 1), from North Greenland (J.S. Peel, personal communication, 2016), Northeast Greenland (Poulsen, 1932; Skovsted, 2003), through Labrador and western Newfoundland (Schuchert and Dunbar, 1934), Quebec (Landing et al., 2002), the Taconic allochthons of Vermont and New York State (Walcott, 1886; Lochman, 1956), and into a cratonic setting within the Appalachian region of western New York (Palmer, 1971). The basic morphology of D. micans is relatively well known from previous studies (Lochman, 1956; Landing and Bartowski, 1996; Conway Morris and Chapman, 1997; Skovsted, 2003). However,
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