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An Enigmatie Cap-Shaped Fossil from the Middle Cambrian of North Greenland

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An Enigmatie Cap-Shaped Fossil from the Middle Cambrian of North Greenland An enigmatie cap-shaped fossil from the Middle Cambrian of North Greenland John S. Peel Nyeboeconus robisoni gen. et sp. nov., is described from the Middle Cambrian Henson Gletscher Formation of western North Greenland. Some authors have interpreted similar shelIs as chondrophorine hydrozoans ar invertebrate fossils of uncertain sy­ stematic position. The coiled, cap-shaped shell and the presence of an internal plate, or pegma, suggest, however, that this new form is the second genus to be described of the Family Enigmaconidae MacKinnon, 1985 (Mollusca, Class Helcionelloida), otherwise known only from rocks of similar age in New Zealand. J. S. P, Dept of Historical Geology & Palaeontology, Institute of Earth Sciences, Uppsala University, Norbyvagen 22, S-752 36 Uppsala, Sweden (formerly Geological Survey af Greenland). Lower and Middle Cambrian strata yield a variety of mya; both helcionelloids and tergomyans were regarded mainly srnall cap-shaped shelIs which are not readily as untorted. assigned to molluscan classes, such as the Gastropoda or The contrasting reconstructions of helcionelloids both Tergomya, living at the present day (Peel, 1991a, b). rely heavily on interpretations of the mantIe cavity, in Notable amongst these small fossils are shelIs of the particular the pattem of presumed inhalant and exhalant Class Helcionelloida which are characterised by a bilater­ respiratory water currents. Taking into account the gen­ ally symmetrical (isostrophic) cap-shaped form, usually eral small size of most helcionelloids (cf. Runnegar & coiled through less than one whorl (Peel, 1991b). Jell, 1976; Runnegar & Pojeta, 1985), it can not be Helcionelloids are widespread and diverse (e.g. Roza­ assumed that these interpretations are valid. Most of the nov et al., 1969 (= Raaben, 1981); Runnegar & Jell, minute helcionelloids were subject to different physical 1976; MacKinnon, 1985; Geyer, 1986; Yu, 1987; Peel, and biomechanical constraints than the significantly 1988, 1991b; Missarzhevsky, 1989; Bengtson et al., larger molluscs which formed the basis for the mantle 1990; Benton & Erwin, 1993) but their record in post­ cavity reconstructions. While refuting the exogastric re­ Middle Cambrian strata is uncertain, as is their relation­ construction of helcionelloids proposed by Runnegar & ship to the molluscan groups which survive them. Pojeta (1974), Peel (1991b, pp. 19-20) admitted that his Reconstruction ofhelcionelloid anatomy is not without preferred endogastric model accepted the same precept dispute; helcionelloids have been considered to be gastro­ that water transport into and out of the helcionelloid shell pods (i.e., torted molluscs) but at the present day there is was possible. general agreement that they were untorted. In a model The cap-shaped shell form is not restricted to the Hel­ strongly inf1uenced by a search for the origin of the cionelloida in the Lower and Middle Cambrian. Artic­ Bivalvia, Runnegar & Pojeta (1974; see also Pojeta & ulated halkieriids (Conway Moms & Peel, 1990; Peel, Runnegar, 1976; Runnegar & Pojeta, 1985) considered 1991c) from the Lower Cambrian of North Greenland helcionelloids to be exogastrically coiled untorted mol­ preserve a calcareous cap- shaped shell at the anterior and luscs (Le., with the apex lying anteriorly) similar to try­ posterior extremities and the latter, in particular, is not blidiid Tergomya such as Pilina, Tryblidium and the ex­ unlike some molluscan shelIs in gross form. A broadly tant Neopilina. Others have interpreted the apex in hel­ similar spectrum of shell forms of problematic origin has cionelloids to lie at the posterior such that coiling of the been documented by Qian & Bengtson (1989) from the shell is endogastric (Yochelson, 1978, 1979; Geyer, earliest Cambrian of China, demonstrating that a variety 1986; Peel & Yochelson, 1987; Peel, 1988, 1991a, b). of organisms produced shelIs of this type. That such a Peel (1991a, b) asserted this view in formally separating morphology is not necessarily indicative of molluscan the endogastric helcionelloids from the exogastric Tergo- affinity is clearly demonstrated by suggestions that some Bull. Grønlands geol. Unders. 169, 149-155 (1994) © GGU. Capenhagen, 1994 150 50 km A Brønlund Fjord & I Tavsens Iskappe Groups > > > Wandal Valley Fm Amundsen Land > o o O O Amundsen Land Group "O "E Gp "E "E O O O O ...J ...J ~ ~ ""1J I- (!lI-- () i= () E Kap Stanton Formation ~ IIII~ ~<: Kap Stanton (!l Ol :> :> _.g Fm i= () ~ Kocfi~ ~ I- - ~ Fm fl0tSSU Holm cg~ .2 .o .o r--t--"' Dal Fm E E (!l -Sic~" .o Ol ~ Fimbuldal Fm :t~ ~ () i= E Henson Gletscher Formation ~ Hensan Ol (!l :!: Ekspedition Bræ Fm () ~ Gletscher llil-- :!: Sydpasset Fm æ:!: ~ ~ .o Fm al_ Aftenstjernesø Formation E ~ æal Henson Gletscher Fm Ol Ol .o () () E Aftenstjernesø Fm Aftenstj. Fm E ...J Ol Ol Buen Formation ...J () () Buen Fm Buen Fm ...J ...J W N Nyeboe Land NW Peary Land E S SW Peary Land NW Peary Ld N c Localities 1 & 2 Locality 3 B LocalJty 3 D Locality 3 ----?--_. Locality 2 Lajopyga laav/gata Locality 1 Zone c: Fig. 1. Derivation of fossiliferous samples c: o c: o c: - o described in this paper and the accompanying 'E.2 si Plychagnostus (il~ punctuosus papers by Babcock (1994a, b) and Robison c.o i~ Zone --------- ~u.. (1994). A, locality and outcrop map; Nyeboe­ conus robisoni was collected from the Hen­ Plychagnostus li; .c: * Holm Dal son Gletscher Formation of the Brønlund ataws oc: Zone li j~ Formation Fjord Group at locality 1. B, C, geological --------- !§ ~!§ ~ o o type section cross-sections through Cambrian - Lower * ~u.. Plychagnostus li; Ordovician strata in North Greenland. BFG, .c: Q) gibbus :I: lO Brønlund Fjord Group; TIG, Tavsens Iskappe Zone ]i" Cl * Group; RGG, Ryder Gletscher Group (from --------- c: :il Ineson et al., 1994). D, biostratigraphy of Glossopleura c: Q) Zone :I: fossiliferous samples; Nyeboeconus robisoni * was collected from the Ptychagnostus gibbus interval-zone at locality 1. 151 of these Cambrian fossils may represent chondrophorine defined radial grooves and varices; a prominent trans­ hydrozoans (Yochelson & Gil Cid, 1984; Stanley, 1986; verse groove on the internal mould represents a plate Babcock & Robison, 1988). Thus, the cap-shaped fossil (pegma) on the sub-apical surface of the shell interior. in these forms is no longer interpreted by these authors as Shell calcareous and seemingly thin, with an external the external, calcareous, dorsal shell of a mollusc, but as ornamentation of radial ridges which correspond in their an impression of the internal, organic, float (pneuma­ position to the grooves on the internal mouid. tophore) of a medusoid cnidarian. A single enigmatic fossil of Middle Cambrian age Discussion. In terms of the prominent pegma on the shell from the Henson Gletscher Formation of northern Nye­ interior, Nyeboeconus closely resembles Enigmaconus boe Land promotes a discussion of these possibie in­ MacKinnon, 1985 (Class Helcionelloida) from the late terpretations, also the suggestion by Evans (1992) that Middle Cambrian of New Zealand (MacKinnon, 1985, similar fossils may represent an invertebrate of presently pp. 72-74, fig. 7). It is readily distinguished from this unknown affinity. The specimen is associated with a few genus by its prominent ornamentation of rugae and radial non-trilobite fossils which were kindly brought to my grooves on the internal mould and less laterally com­ attention by Richard A. Robison; it is described belowas pressed form; the internal mould (and consequently the Nyeboeconus robisoni gen. et sp. nov. in his honour, in shell interior) in Enigmaconus is smooth. The shell in gratitude for his studies and advice concerning Greenland Enigmaconus also appears to be taller, but this (together Cambrian fossils during the past decade. with some of the lateral compression) may in part reflect Nyeboeconus robisoni is from the Henson Gletscher preservation of just the early part of the New Zealand Formation, west of Hand Bugt, northern Nyeboe Land, shell as an internal mouid; this is supported by compari­ western North Greenland (Fig. l, locality 1). Several son of Enigmaconus and Nyeboeconus in lateral aspect. specimens of the helcionelloid Latouchella are also pre­ In the former (cf. MacKinnon, 1985, fig. 7b, n), the most sent in the same sample (GGU 298970), associated with abapical part of the slot representing the pegma lies much elkaniid, acrotretid and orthoid brachiopods (Lars E. Hol­ closer to the latest preserved apertural margin than the mer, personal communication, 1993), conodontomorphs corresponding point in Nyeboeconus (Fig. 2b). and the mollusc Pelagiella. Pelagiellids also form a char­ The general similarity of Nyeboeconus to Enigmaco­ acteristic group of molluscs in Lower and Middle Cam­ nus in terms of shell form and the pegma warrants place­ brian strata. While possibly related to helcionelloids, they ment of the Greenland form within the Farnily Enig­ are readily distinguished by anisostrophic coiling which maconidae MacKinnon, 1985. It should be noted that gives a superficial resemblance to the gastropod shell. MacKinnon (1985, p. 72) considered the pegma to lie The most conspicuous fossils in the Henson Gletscher nearest to the shell anterior while this surface is here Formation at the collection locality are polymeroid and interpreted as posterior (cf. Peel, 1991a, b). agnostoid trilobites described elsewhere in this volume In terms of the unusual ornamentation of the inner by Babcock (1994a, b) and Robison (1994). An accompa­ surface of
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