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Sexual Dimorphism Within the Stem-Group Arthropod Isoxys Volucris from the Sirius Passet Lagerstätte, North Greenland

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Sexual Dimorphism Within the Stem-Group Arthropod Isoxys Volucris from the Sirius Passet Lagerstätte, North Greenland Sexual dimorphism within the stem-group arthropod Isoxys volucris from the Sirius Passet Lagerstätte, North Greenland MORTEN LUNDE NIELSEN, JAN AUDUN RASMUSSEN & DAVID A.T. HARPER Nielsen, M.L., Rasmussen, J.A. & Harper, D.A.T. 2017. Sexual dimorphism within the stem-group arthropod Isoxys volucris from the Sirius Passet Lagerstätte, North Greenland. © 2017 by Bulletin of the Geological Society of Denmark, Vol. 65, pp. 47–58. ISSN 2245-7070. (www.2dgf.dk/publika- tioner/bulletin). https://doi.org/10.37570/bgsd-2017-65-04 Morphometric analyses carried out on 126 specimens of the early Cambrian (Cambrian Series 2, Stage 3) stem-group arthropod Isoxys volucris from North Greenland reveal a bimodal distribution, where Morphogroup A is characterised by a significantly wider doublure than Morphogroup B. The dimor- phism is interpreted as intraspecific variation related to gender (sexual dimorphism), where the relative Received 15 December 2016 numbers of individuals within the two morphogroups are equal (ratio 1:1). The discovery of established Accepted in revised form sexual dimorphism in Isoxys may have implications for the taxonomic discrimination of other stem- 5 May 2017 group arthropods within the morphologically diverse and disparate animals of the early Cambrian. Published online 5 July 2017 Keywords: Sirius Passet Lagerstätte, Isoxys, stem-group arthropods, sexual dimorphism, early Cambrian. Morten Lunde Nielsen [[email protected]], Natural History Museum of Denmark, University of Co- penhagen, Øster Voldgade 5–7, DK-1350 Copenhagen K, Denmark. Jan Audun Rasmussen [jan.rasmussen@ museummors.dk], Fossil and Mo-clay Museum, Museum Mors, Skarrehagevej 8, DK-7900 Nykøbing Mors, Denmark. David A.T. Harper [[email protected]], Department of Earth Sciences, Palaeoecosystems Group, Durham University, Stockton Road, Durham DH1 3LE, United Kingdom; also Department of Geol- ogy, Lund University, Sölvegatan 12, S-223 62 Lund, Sweden. Corresponding author: Jan Audun Rasmussen The arthropod Isoxys volucris Williams, Siveter & Peel 2006; Wang et al. 2010, 2012; Fu et al. 2011, 2014; Schoe- 1996 is the most common fossil in the lower Cambrian nemann & Clarkson 2011; Huang & Wang 2014); France Sirius Passet Lagerstätte of Peary Land, North Green- (Vannier et al. 2005); Greenland (Conway Morris et al. land (Stein et al. 2010; Peel & Ineson 2011b) except for 1987; Williams et al. 1996; Peel 2010; Stein et al. 2010); the trilobite Buenellus higginsi Blaker 1988. The genus Russia (Ivantsov 1990); Spain (Richter & Richter 1927); is a stem-group arthropod (Legg & Vannier 2013) and U.S.A. (Walcott 1890; Campbell & Kauffman 1969; and is enclosed by an elongate, non-mineralised, Briggs et al. 2008). Isoxys volucris is distinguished from bivalved carapace, although it remains unresolved other species of Isoxys by, for example, its long, narrow whether it is dorsally folded or hinged (see e.g. Stein spines and broad, conspicuous, wrinkled doublure. et al. 2010), and with long antero- and postero-dorsal The soft parts of I. volucris are rarely preserved and spines. Isoxys has a widespread distribution which is are known only from a few incomplete specimens latitudinally restricted to the tropical and subtropical (Stein et al. 2010). This makes comparisons with other regions (Williams et al. 1996; Vannier & Chen 2000). species of Isoxys difficult and its affinities uncertain, Representatives of the genus have been reported from and it has even been surmised that I. volucris is not a the lower and middle Cambrian of several countries: true member of the genus (Vannier et al. 2006, p. 211). Australia (Glaessner 1979; Bengtson et al. 1990; García- However, a cladistic analysis by Legg & Vannier (2013) Bellido et al. 2009a); Canada (Walcott 1908; Simonetta & firmly demonstrated that Isoxys volucris belongs to the Delle Cave 1975; Conway Morris 1979; García-Bellido genus Isoxys. Despite having been collected in great et al. 2009b; Vannier et al. 2009; Kimmig & Pratt 2015); numbers, our knowledge of I. volucris is fairly limited, China (e.g. Jiang 1982; Hou 1987; Hou & Bergström partly because only few morphological characters are 1991; Shu et al. 1995; Vannier & Chen 2000; Luo et al. preserved. However, an elongate, medially placed Sexual dimorphism in arthropod Isoxys volucris from Sirius Passet · 47 structure interpreted as a possible body attachment approximately 15°S (Fig. 1). The Buen Formation is (Stein et al. 2010) occurs sporadically in the studied approximately 210–250 m thick at Navarana Fjord material. I. volucris was described in detail first by less than 40 km south-west of Sirius Passet (Higgins Williams et al. (1996) and subsequently by Stein et et al. 1991). The Sirius Passet locality is situated in al. (2010). The genus Isoxys has been interpreted as a westernmost Peary Land at 82°47.6′N, 42°13.7′W (Fig. pelagic hunter or scavenger (Zhao et al. 2014). 1), close to the eastern shores of J.P. Koch Fjord. The Here, we describe morphometric analyses on 126 geological setting of the Sirius Passet fossil locality reasonably well-preserved specimens of I. volucris col- was described in detail by Ineson & Peel (2011) and lected in situ, which allow a test of recently detected only a few points need to be added here. dimorphism in Isoxys based on material from China The section is steeply dipping and exposes ap- (Fu et al. 2014). Furthermore, we examine a number of proximately 12 m of platy, dark grey to black, lami- possible reasons for the observed dimorphism within nated mudstones belonging to the ‘Transitional Buen Isoxys volucris. Formation’ (Fig. 2). The section was excavated during the 2011 expedition to the locality. It contains finely laminated mudstones and interbedded intervals of non-laminated to weakly laminated mudstones with Geological setting weak to moderate degrees of bioturbation; the Lager- stätte is only preserved in the former. Its base and top The fossiliferous beds containing the Sirius Passet is covered by scree and thus not exposed. The locality Lagerstätte are black slates of the Buen Formation, is wedged between the underlying dolostones of the more specifically the so-called ‘Transitional Buen Portfjeld Formation to the east and an intraformational Formation’ of Peary Land, North Greenland (Ineson thrust along the western to northern boundary. The & Peel 2011; Peel & Ineson 2011a, 2011b). It has been majority of the collected fossil material was sampled correlated with Cambrian Series 2, Stage 3 (Peel & in situ at the main section. In addition, fossils of Isoxys Ineson 2011a), and is roughly contemporaneous with volucris have been reported from four other localities the Chinese Chengjiang Biota. In terms of absolute in the ‘Transitional Buen Formation’ in close vicinity dating, the age of the formation is about 520–515 Ma to the main outcrop, although these localities have a (Gradstein et al. 2012). During this time interval North rather poor fossil yield (Peel & Ineson 2011a). The dep- Greenland was situated just south of the Equator at ositional setting of the Sirius Passet Lagerstätte was A 30°N B 60°W 45°W 30°W Ellesmere Arctic Ocean Nansen Island Equator Land J.P. Koch Fjord Navarana Fjord Laurentia Alaska Peary Land 82°N Sirius Passet Lagerstätte Sirius Passet 30°S Lagerstätte 82°N Greenland Siberia Inland Ice Kara Baltica 30°W 60°S 100 km 45°W Fig. 1. A: Palaeogeographic map (at 520 Ma) computed by the BugPlates software (Torsvik & Cocks 2009). Greenland, which is marked in green, was part of the Laurentia continent. B: Present-day locality map showing the Sirius Passet Lagerstätte situated in westernmost Peary Land near J.P. Koch Fjord. 48 · Bulletin of the Geological Society of Denmark previously compared to that of the more renowned stratigraphic interval. More than 250 specimens of Burgess Shale Lagerstätte; both are mudstones depos- Isoxys volucris were evaluated prior to analysis, and ited in relatively deep water adjacent to a carbonate 126 specimens were found suitable for an analysis of platform escarpment (i.e. the Portfjeld Formation) with the doublure. The specimens include both articulated low-energy, poorly oxygenated bottom conditions carapaces and single valves (sensu Stein et al. 2010). All (Ineson & Peel 2011). Periodic interludes of more oxic specimens are housed at the Natural History Museum conditions on the sea bed have been suggested on the of Denmark, University of Copenhagen, Copenhagen, basis of intervals of less laminated and more biotur- Denmark (numbers prefixed by MGUH). bated mudstones. The fauna is generally believed to Because the internal anatomical characters are be parautochthonous but some fossils appear to have very rarely preserved in I. volucris (Stein et al. 2010), been preserved in situ (e.g. Ineson & Peel 2011; Stein et the morphometric analyses have been restricted to al. 2013). The Sirius Passet biota sediments have been characters related to those commonly preserved: total interpreted as being deposited in the upper slope length, total height, doublure width and overall shape coarse-sediment bypass zone (Peel & Ineson 2011b). of the valve (Fig. 3). In a recent study, however, Strang et al. (2016) inter- Specimens were coated with ammonium chloride preted it as a shelf setting, deposited at or just below and photographed using a digital DeltaPix InfinityX the storm wave base, which – based on the occurrence camera connected to a computer, which allowed of cyanobacterial activity – was interpreted as being images to be processed directly by DeltaPix InSight situated in the photic zone. software. Measurements were made with an inbuilt calibrated scale. The height of a disarticulated valve was measured at the highest distance from the ventral margin to the dorsal hinge line. For open articulated Material and methods carapaces, height was measured as half the distance between the ventral edge of the right and left valves. The majority of the material described herein was The width of the doublure was measured along the collected in situ from the main Lagerstätte locality same plane as the valve height.
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