Morphology, Paleobiology and Systematics of the Mobergellids, a Group of Cambrian Problematic Fossils
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hology, Paleobiology and Systematics of the Mobergellids, a Group of Cambrian Problematic Fossils Yu. E. Demidenkoa, * and P. Yu. Parkhaeva aBorissiak
Paleontological Institute, Russian Academy of Sciences, Moscow, 117647 Russia *e-mail: [email protected]
Received November 11, 2019; revised February 25, 2020; accepted February 25, 2020
Abstract—Possible variants of systematic position of Cambrian problematic fossils of the family Mobergellidae are discussed. An affinity between mobergellids and Sipuncula, a phylum of marine benthic worm-like lophotrochozoans possibly related to annelids, is proposed for the first time. The mobergellid discoidal sclerites are interpreted as caudal shields of sipunculans. Keywords: Cambrian problematic fossils, sclerites, morphology, systematic position, Mobergella, sipunculans DOI: 10.1134/S0031030120050044
INTRODUCTION The fossil record of the Early Cambrian, along with widely known fossils (archaeocyaths, trilobites, brachiopods) is represented by many morphologically diverse, mainly small sclerites (individual elements of the skeleton), less commonly scleritomes (a set of sclerites belonging to one organism), as well as small, whole skeletons (shells, tubes). All of them are described in the literature under the general name, “small shelly fossils” or SSF (Matthews and Missarzhevsky, 1975; Bengtson et al., 1990). An assemblage of such taxonomically diverse fossils comes in the result of a widely used technique for the preparation of Cambrian fossils, i.e., extraction of skeletal elements from carbonate rock by dissolving it in acetic acid, washing the undissolved sediment from the clay fraction, and then selecting the material of interest. Problematics form a significant proportion of Cambrian small shelly fossils (Rozanov et al., 1969; Matthews and Missarzhevsky, 1975; Rozanov and Zhuravlev, 1992; Dzik, 1994), i.e., taxa of unclear taxonomic affinity, due to the lack of morphologically close organisms among modern invertebrates. In recent years, there has been noticeable progress in solving the issues of functional morphology and interpretation of the systematic position of many groups of Cambrian problematic fossils, which have long been mysterious. For example, after finding articulated sclerites (scleritomes) of the tommotiids of the genera Paterimitra and Eccentrotheca and studying their morphology and microstructure of the sclerite wall, it became possible to place these taxa at the base of the brachiopod stem. It has also been shown that Microdyction fossils can be considered as basal representatives of the onychophore stem group (Skovsted
et al., 2008, 2009, 2011, 2014; Balthasar et al., 2009; Murdock et al., 2012, 2014). However, the systematic position of many Cambrian taxa represented only by sclerites in the fossil record remains unclear. Such groups include sclerites of representatives of the family Mobergellidae Missarzhevsky, 1989. This family, established by Missarzhevsky (1989) and conditionally classified as Mollusca, initially included three genera (Mobergella
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