The anatomy, taphonomy, taxonomy and systematic affinity of Markuelia: Early Cambrian to Early Ordovician scalidophorans

Abstract

Markuelia is a vermiform, annulated introvertan animal known as embryonic fossils from the Lower Cambrian to Lower Ordovician. Analysis of an expanded and revised dataset for Introverta shows that the precise position of Markuelia within this clade is dependent on the taxa included. As a result, Markuelia is assigned to the scalidophoran total group to reflect uncertainty as to whether it is a stem-scalidophoran or a stem-priapulid. The taxonomy of the genus is revised to provide an improved taxonomic framework for material assigned to Markuelia. Five species are recognized: M. secunda Val'kov, M. hunanensis Dong and Donoghue, M. lauriei Haug et al., M. spinulifera sp. nov. and M. waloszeki sp. nov. Finally, the preservation of Markuelia is evaluated in the light of both the taphonomy of the fossil embryos themselves and the experimental taphonomy of the priapulid Priapulus caudatus, which has been proposed as both a close relative and an anatomical analogue of Markuelia.

Chronology of early Cambrian biomineralization

“…Non-biomineralized ecdysozoans, however, existed before, as evidenced by: (1) fossilized embryos of Markuelia secunda Val'kov from the basal Tommotian Stage in Siberia, having already a sclerotized cuticle and representing the Scalidophora, a sub-group of the Cycloneuralia (Bengtson & Yue, 1997;Dong et al 2004Dong et al , 2005Dong et al , 2010; embryos and possibly related larvae of Pseudooides prima from the lower Meishucunian Stage of South China, considered as possible stem-group arthropods (Steiner et al 2004b) and (2) the arthropod-type trace fossils in pre-trilobitic strata (Crimes, 1987;Weber & Zhu, 2003;Weber, Steiner & Zhu, 2007).…”

“…According to Zhuravlev & Wood (2008), spicules of first representatives of calcarean sponges are probably highmagnesium calcitic in original composition. Stemgroup sponges, which may combine mineralogy and morphology of calcarean and hexactinellid spicules, are also reported from the Cambrian Series 2 (Botting & Butterfield, 2005;Harvey, 2010; see also Eiffelia Walcott, 1920 in online Appendix 1 at http://journals.cambridge.org/geo).…”

“…These fossils are morphologically similar to nematomorphs (Hou & Bergström, 1994) and priapulids (Conway Morris, 1997). They are broadly considered as a stem group of the Priapulida (Harvey, Dong & Donoghue, 2010) or Cycloneuralia (Budd, 2001;Conway Morris & Peel, 2010). Complete preservation of their worm-like bodies is known, but far more often they occur as disarticulated calcium phosphate cuticular sclerites.…”

Middle Cambrian through lowermost Ordovician conodonts from Hunan, South China

“…Alternating layers of dark-gray, medium-bedded, laminated, argillaceous dolostone, and dark gray, medium-bedded micrite and biomicrite. The argillaceous dolostone does not yield fossils, whereas the limestones yield abundant conodonts, fossil embryos (Dong, 2007a(Dong, , 2009aDong et al, 2004bDong et al, , 2005aDong et al, , 2010Peng and Dong, 2008;Cheng et al, 2011), fossils of typical Orsten-type preservation Liu and Dong, 2007Zhang and Dong, 2009;Zhang et al, 2011aZhang et al, , b, 2012Duan et al, 2012), highly modified sponge spicules (Dong and Knoll, 1996;Chen and Dong, 2008), radiolarians , small shelly fossils (Zhu et al, 2003, as well as trilobites and brachiopods. 3.…”

Experimental mineralization of crustacean eggs: new implications for the fossilization of Precambrian–Cambrian embryos

“…Exceptional three-dimensional preservation of fossilized eggs and embryos from the Neoproterozoic, the Cambrian and the Ordovician provide direct insight into embryology during the eve of animal radiation (Bengtson and Zhao, 1997;Li et al, 1998;Xiao et al, 1998Xiao et al, , 2000Chen et al, 2004;Steiner et al, 2004a, b;Donoghue and Dong, 2005;Donoghue et al, 2006;Liu et al, 2007;Dong et al, 2010). In particular, the terminal Neoproterozoic Doushantuo Formation (635 to 551 Ma; Condon et al, 2005) of South China has attracted considerable attention, first because fossilized algae and embryos occur in nearly rock-forming abundance, and second because radiometric ages of the Doushantuo Formation imply that the putative animal embryos predate the Precambrian-Cambrian boundary (542 Ma) by more than 20 million years.…”