78


Journal of Paleontology 92(1):71–79


shell layers that drape over and encapsulate the foreign body. However, in these examples, the height of the internal shell malformation is small relative to the height of the valves and does not prevent the closure of the valves (Kefi et al., 2012). The anomalous growth exhibited in the specimen illustrated here (Fig. 4.3–4.6) affected a series of shell layers and must have been a constant feature of the shell interior for a substantial portion of the life of the animal. The growth also increases the height of the sclerite by 150% and indicates that there must have existed a substantial cavity beneath the shell of this mobergellan to accommodate the soft parts of the animal during growth. This observation presents further evidence that the mobergellid animal was substantially larger than the preserved shell itself and speaks against an interpretation of mobergellans as two-valved organisms or limpet-like univalved animals.


Acknowledgments


The authors thank the guest editors Z. Zhang (Northwest University, Xian) and G.A. Brock (Macquarie University, Sydney) for the invitation to contribute to the Cambrian Explosion special issue in Journal of Paleontology.Wealsothank S. Bengtson (Swedish Museum of Natural History, Stockholm) for advice on mobergellan taxonomy and access to collections of mobergellids from Sweden and J.S. Peel (Uppsala University, Uppsala) for information on the occurrence of Discinella in North Greenland. CBS also expresses his gratitude to A.R. Palmer (Boulder, Colorado) for hospitality and access to his Cambrian collections. This work was instigated during a postdoctoral visit by TPT to Stockholm. Funding from SMNH and COFUND fellowship (Durham University) are acknowledged.


References


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Figure 5. Discinella micans (Billings, 1871), showing variability in concavity. All specimens from sample ICS1575, Middle Shale member, Forteau Formation, Fox Cove, southern Labrador. (1, 2) NFM F-2484; (1) lateral view of convex specimen, apex up; (2) internal view. (3, 4) NFM F-2485; (3) lateral view of flat specimen, apex up; (4) internal view. (5, 6) NFM F-2486; (5) lateral view of concave specimen, apex up; (6) internal view. Scale bar=1mm for all images.


2008) and to internal growth malformations in bivalves as a response to exposure to parasites (e.g., Kefi et al., 2012). Both are defensive responses to a foreign object or organism that disturbs the secretory tissues and results in the formation of new


Fisher,D.W., 1962, Small conoidal shells of uncertain affinities, inMoore, R.C., ed., Treatise on Invertebrate Paleontology, Pt. W. Miscellanea. Lawrence, Geological Society of America and University of Kansas Press, p.W98–W143.


Hall, J., 1872, Notes on some new or imperfectly known forms among the Brachiopoda. etc.: Annual Report to the Regents of the University of the State of New York, v. 23, Appendix G, p. 244–247.


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