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An Early Silurian ‘Herefordshire’ myodocope ostracod from Greenland and its palaeoecological and palaeobiogeographical significance

Published online by Cambridge University Press:  20 August 2013

VINCENT PERRIER ,
DAVID J. SIVETER ,
MARK WILLIAMS  and
PHILIP D. LANE
VINCENT PERRIER*
Affiliation:
Department of Geology, University of Leicester, University Road, Leicester, LE1 7RH, UK
DAVID J. SIVETER
Affiliation:
Department of Geology, University of Leicester, University Road, Leicester, LE1 7RH, UK
MARK WILLIAMS
Affiliation:
Department of Geology, University of Leicester, University Road, Leicester, LE1 7RH, UK
PHILIP D. LANE
Affiliation:
School of Earth Sciences and Geography, William Smith Building, Keele University, Keele, Staffordshire, ST5 5BG, UK
*
†Author for correspondence: vp110@leicester.ac.uk
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Abstract

Here we record the occurrence of a new species of the Herefordshire Lagerstätte ostracod genus Pauline from the Lower Silurian (upper Telychian) of North Greenland. Pauline nivisis sp. nov. was recovered from a limestone boulder (Pentamerus Bjerge Formation) collected south of Kap Schuchert, Washington Land. It is reasonable to transpose the palaeobiology known from the Herefordshire Pauline avibella – body, limbs including swimming antennae, lateral eyes, gills and alimentary system – into the carapace of the Greenland species, which represents the oldest cylindroleberidid myodocopid and almost the oldest known myodocope, and is the first record of a Herefordshire Lagerstätte genus from outside the Welsh Borderland locality. Morphological, sedimentological and faunal evidence suggest that the Greenland species was nektobenthic. This is compatible with the notion that ostracods (specifically myodocopids) did not invade the water column until later in the Silurian, in the Wenlock and Ludlow epochs. Pauline is an Early Silurian link between ‘Baltic-British’ and North Laurentian ostracod faunas, endorsing the idea that the UK and Greenland were in close geographical proximity, near a remnant Iapetus Ocean, during late Llandovery time.

Keywords

TelychianFranklinian BasinCylindroleberididaepalaeobiologylifestyle
Type
Original Articles
Information
Geological Magazine , Volume 151 , Issue 4 , July 2014 , pp. 591 - 599
Copyright
Copyright © Cambridge University Press 2013 

1. Introduction

The Silurian has relatively few exceptionally preserved biotas compared to most other periods of Earth history. The Early Silurian (Wenlock Epoch) Herefordshire Lagerstätte (Briggs, Siveter & Siveter, Reference Briggs, Siveter and Siveter1996), UK, known from a single locality in the Welsh Borderland, has yielded unrivalled anatomical data in a diversity of three-dimensionally preserved invertebrates, including brachiopods, a polychaete worm, molluscs, an asterozoan and a range of arthropods including four (myodocope) ostracod species (Briggs et al. Reference Briggs, Siveter, Siveter and Sutton2008, Reference Briggs, Siveter, Siveter, Sutton, Garwood and Legg2012; Siveter et al. Reference Siveter, Briggs, Siveter and Sutton2010, Reference Siveter, Briggs, Siveter, Sutton and Joomun2013; Sutton et al. Reference Sutton, Briggs, Siveter and Siveter2011, Reference Sutton, Briggs, Siveter, Siveter and Sigwart2012). This material has provided unparalleled information about the palaeobiology of Early Palaeozoic ostracods. None of the mostly soft-bodied invertebrate taxa of the Herefordshire Lagerstätte were known outside the type locality in the Welsh Borderland until now. The exceptional preservation in this Lagerstätte results from rare geochemical conditions prevailing locally after an ash fall, producing three-dimensional infill fossils preserved in calcite (Orr et al. Reference Orr, Briggs, Siveter and Siveter2000).

Here we record the occurrence of a new, second species of the Herefordshire myodocope ostracod genus Pauline (Siveter et al. Reference Siveter, Briggs, Siveter, Sutton and Joomun2013) from the Lower Silurian (Telychian Stage) of North Greenland and assess its morphology, likely ecology and palaeobiogeographical significance. The new record demonstrates that the evolving corpus of functional anatomical evidence gleaned from the Herefordshire Lagerstätte fossils has potentially wide-ranging application for interpreting the biology of related taxa and the ecology of Silurian fossil assemblages elsewhere.

2. Material and methods

The ostracods described herein were recovered from a single boulder (GGU 216856) collected in August 1976 by J. M. Hurst during an expedition of the Geological Survey of Greenland (GGU), which is now part of the Geological Survey of Denmark and Greenland. The ostracod-bearing rock comes from the Lower Silurian section at Kap Schuchert, Washington Land, western North Greenland (Fig. 1), at approximately 80°78′N, 64°87′W.

Figure 1. Geological setting of the locality for Pauline nivisis. (a) Location and extension of the Greenland segment of the Franklinian Basin (grey filling) and Washington Land. (b) Geological map of Washington Land and location of the Kap Schuchert outcrop (star): maps after Peel & Sønderholm (Reference Peel and Sønderholm1991).

The ostracods consist of 3D-preserved specimens that are housed in the Geological Museum, Copenhagen University, a part of the Natural History Museum of Denmark (MGUH), with the numbers MGUH 30487–30495. The morphological terminology for the myodocope carapace follows that of Siveter, Vannier & Palmer (Reference Siveter, Holland and Bassett1987). Rock matrix was removed from the specimens mechanically using a vibrotool. The photographs were taken using a Leitz Aristophot mounted with a Canon EOS 5D camera, following the methods outlined in Siveter (Reference Siveter, Briggs and Crowther1990).

3. Geological setting

The Lower Palaeozoic succession of the Franklinian Basin extends over almost 2000 km across the Canadian Arctic Islands to eastern North Greenland and is well exposed over 1000 km in North Greenland (Fig. 1a; Peel & Sønderholm, Reference Peel and Sønderholm1991). The ostracod-bearing rock documented here is from an extended reef belt that formed an important component of the Greenland carbonate shelf margin in Early Silurian times and which survived until the late Llandovery Epoch. The reef formed a patchwork barrier across the region and was limited seawards by a steep shelf margin to the north while a flat carbonate platform was maintained between and behind the reef mounds (Fig. 1b; Sønderholm & Harland, Reference Sønderholm, Harland, Geldsetzer, James and Tebbutt1989). The fossil material was collected immediately south of Kap Schuchert (Fig. 1b). In this region, marine shelf sedimentary deposits are represented by the Pentamerus Bjerge Formation of the Washington Land Group; the marine slope sedimentary deposits are referred to the Lafayette Bugt and Cape Schuchert formations, Peary Land Group (Peel & Sønderholm, Reference Peel and Sønderholm1991).

The new myodocope species was recovered from a pale grey biosparite boulder collected from near the top of the section (Hurst, Reference Hurst1980, fig. 24, section A; Fig. 1b). Thin-section petrological analysis shows fragmented brachiopods, articulated trilobites and possible algae but no corals or coralline algae. Such biosparites occur as blocks in the breccia of the Lafayette Bugt Formation and are considered to be derived from the partly coeval Pentamerus Bjerge Formation (Hurst, Reference Hurst1980). Hughes & Thomas (Reference Hughes and Thomas2011) considered these sedimentary deposits to represent high-energy environments within the euphotic zone. Graptolites indicative of the Monograptus spiralis Biozone sensu lato, conodonts of the Pterospathodus amorphognathoides Biozone and trilobites (Meroperix aquilonaris) indicate a late Telychian, Llandovery age (see discussion in Lane & Owens, Reference Lane, Owens and Peel1982).

4. Palaeobiogeographical significance

There is one previous record of a myodocope ostracod from the Silurian of North Greenland, that of the Telychian Entomozoe aff. E. tuberosa, from Centrum Sø, Kronprins Christian Land, eastern North Greenland (Siveter & Lane, Reference Siveter and Lane1999). Entomozoe is also known from several other locations in the Early Silurian, in Scotland, Arctic Russia and South China (Siveter & Vannier, Reference Siveter and Vannier1990; Siveter & Bogolepova, Reference Siveter and Bogolepova2006) and possibly the Middle Silurian of Australia (Siveter, Vannier & Palmer, Reference Siveter, Vannier and Palmer1991). All currently identified Llandovery myodocope ostracods are referred to either Pauline or Entomozoe. The only known older myodocope is Myodoprimigenia fistuca from the Upper Ordovician, Ashgill Series, Soom Shale of South Africa (Gabbott et al. Reference Gabbott, Siveter, Aldridge and Theron2003). There is probably a minimum of 2 million years of time difference between the occurrence of the new Greenland species of Pauline (late Llandovery) and the Herefordshire Pauline avibella, which comes from approximately the Homerian–Sheinwoodian Stage boundary within the Wenlock (unpublished evidence from chitinozoan associates; G. Mullins, pers. comm.; see Zalasiewicz et al. Reference Zalasiewicz, Taylor, Rushton, Loydell, Rickards and Williams2009, fig. 2 for an approximate estimate of the time difference between these horizons).

At present, Pauline is known only from two localities, both at a low palaeolatitude. During Early Silurian times, North Greenland was situated close to the equator on the northern margin of the Laurentia palaeocontinent while England and Wales were in the southerly subtropics (~ 30°S) on the Avalonia microplate (Cocks & Torsvik, Reference Cocks and Torsvik2002, Reference Cocks and Torsvik2011; Fig. 2). During the late Llandovery, these regions were separated by a vestige of the Iapetus Ocean. Pauline, a supposed nektobenthic ostracod, is an Early Silurian link between ‘Baltic-British’ and North Laurentian ostracod faunas (for which see Siveter, Reference Siveter, Holland and Bassett1989; Perrier & Siveter, in press) and, as pelagic larvae are unknown in ostracods, this endorses the idea that southern Britain and Greenland were in close geographical proximity at that time. The shelf margins of the remnant Iapetus Ocean could have acted as a channel for the migration of Pauline species.

Figure 2. Stratigraphic and palaeogeographic distribution of Entomozoe and Pauline. (a) Stratigraphic position, which is approximate within each formation. (b) Palaeogeographic distribution and possible migration pathways (dashed arrows). Stratigraphy for Greenland after Peel & Sønderholm (Reference Siveter, Vannier and Palmer1991); Scotland and England after Siveter & Vannier (Reference Siveter and Vannier1990); Arctic Russia after Siveter & Bogolepova (Reference Siveter and Bogolepova2006); China after Rong, Wang & Zhang (Reference Rong, Wang and Zhang2012); Australia after Strusz (Reference Strusz1984); palaeogeographic map after Torsvik (Reference Torsvik2009). C. R. Fm. – Cock Rig Formation; L. Fm. – Leijiatun Formation; M. Fm. – Majiaochong Formation; M.P.V. – Mount Painter Volcanics; M.W. Fm. – Much Wenlock Formation; W. Fm. – Wangjiawan Formation; X. Fm. – Xiangshuyuan Formation.

The Late Ordovician – Early Silurian (c. 455–425 Ma) ‘Early Palaeozoic Icehouse’ (see Page et al. Reference Page, Zalasiewicz, Williams, Popov, Williams, Haywood, Gregory and Schmidt2007) is an interval characterized by the waxing and waning of a major Gondwanan ice-sheet. The presence of Pauline within tropical latitudes (Fig. 2) in late Telychian time may be consistent with the recovery patterns of ostracod faunas post-dating glacial advances of the Early Palaeozoic Icehouse. Whilst diverse podocope ostracod assemblages are known from the Lower Silurian (Rhuddanian) of low latitude Laurentia (Copeland, Reference Copeland1974) and Baltica (Sarv, Reference Sarv1968; Truuver et al. Reference Truuver, Meidla, Aainsaar, Bergström and Tinn2012), no ostracods are known from the Rhuddanian strata of Avalonia, and only a few from the Aeronian (Siveter, Reference Siveter, Whittaker and Hart2009). In part this reflects the availability of suitable lithofacies to preserve the fauna, with much of the Rhuddanian succession in southern Britain represented by deep-water facies of the Welsh Basin and northern England. The appearance of Pauline farther south (~ 30°S; Fig. 2) in the Wenlock Epoch is consistent with the hypothesis that low latitude faunas spread out to higher latitudes in the Wenlock, both as the high latitude climate ameliorated and as seas advanced into shelf areas as the Gondwanan ice-sheet decayed. Thus, Hairapetian et al. (Reference Hairapetian, Mohibullah, Tilley, Williams, Miller, Afzal, Ghobadi Pour and Hejazi2011) documented the essentially Laurentian aspect of the earliest podocope ostracod faunas in the Llandovery of peri-Gondwanan Iran. The temporal and geographic distribution of Entomozoe tuberosa and related forms also supports this notion (see Siveter & Vannier, Reference Siveter and Vannier1990; Siveter & Lane, Reference Siveter and Lane1999; Siveter & Bogolepova, Reference Siveter and Bogolepova2006). Entomozoe is known only from low latitudes (~ 0–10°N) in the early Telychian (Greenland, Arctic Russia and South China) and then apparently spread farther south in the latest Telychian (Scotland, ~ 20°S; Fig. 2).

Pauline is known from only two localities, but Entomozoe is more geographically widespread (Fig. 2), with E. tuberosa sensu stricto and sensu lato forms present in tropical regions from Laurentia to Gondwana (Australia). To explain at least a part of this wide distributional pattern, Siveter & Lane (Reference Siveter and Lane1999) suggested that Entomozoe may have spread by ‘island hopping’ (e.g. see Cocks & Fortey, Reference Cocks and Fortey1982); for example, using the Siberia and North China plates to reach South China and possibly Australia (Siveter, Vannier & Palmer, Reference Siveter, Vannier and Palmer1991; Fig. 2). To do so it seems that Entomozoe probably had greater dispersal capacity than Pauline. Such a pattern of distribution is not replicated, for example, in podocopid ostracods (their dispersal capacity is generally regarded as more limited than that of some myodocopes; see Perrier & Siveter, in press; but see Hairapetian et al. Reference Hairapetian, Mohibullah, Tilley, Williams, Miller, Afzal, Ghobadi Pour and Hejazi2011). Another possible explanation for the wide biogeographical occurrence of Entomozoe is that they were distributed by ocean currents, as is the case for some Recent myodocopes. For example, the Japan Current is probably responsible for the extensive distribution of the nektobenthic myodocope Vargula higendorfii, which has rapidly (< 10,000 years) dispersed over 3000 km northward along the Japanese coastline (Ogoh & Ohmiya, Reference Ogoh and Ohmiya2005). Nektobenthic myodocopes live at the water–sediment interface and move into the water column typically at night (e.g. Recent cylindroleberidids; see L. Corbari, unpub. Ph.D. thesis, Univ. Bordeaux 1, 2004, Corbari, Carbonel & Massabuau, Reference Corbari, Carbonel and Massabuau2005). Although their active movement in the water column is limited to a few metres above the sea bed, such buoyant organisms may be transported by bottom currents. This may be one mechanism that nektobenthic myodocope ostracods use to colonize coastal environments over long distances (e.g. Vargula in Japan). If E. tuberosa (sensu stricto/sensu lato) was pelagic this would also explain its global distribution. However, such a conclusion requires vigorous testing of its faunal associates, range of lithofacies and functional morphology.

5. Palaeoecology

5.a. Environmental setting

Within the North Greenland Telychian reef environments (Fig. 2) the myodocope faunal associates include abundant stromatoporoids and pelmatozoan debris, together with rugose and tabulate corals, trilobites, gastropods, cephalopods, rostroconchs, graptolites, brachiopods, bivalves and other, undescribed, mostly non-palaeocope ostracods (Hughes & Thomas, Reference Hughes and Thomas2011). The other ostracods include Euprimitia? sp. and Ceratocypris symmetrica (Poulsen, Reference Poulsen1934) from the Kap Schuchert Formation and Monoceratella mazos, which was recovered from a boulder about 1 m stratigraphically from the Pauline material (block GGU 216855; Lane, Reference Lane and Peel1980). This essentially epibenthic fauna, along with the reef lithofacies, indicates a relatively shallow shelf setting (see Hurst, Reference Hurst1980; Peel & Sønderholm, Reference Peel and Sønderholm1991). By comparison, the Herefordshire Lagerstätte biota comprises a diverse ‘shelly’ fauna (see Section 1) and four large nektobenthic myodocope species (Siveter et al. Reference Siveter, Sutton, Briggs and Siveter2003, Reference Siveter, Briggs, Siveter, Sutton and Joomun2007, Reference Siveter, Briggs, Siveter and Sutton2010, Reference Siveter, Briggs, Siveter, Sutton and Joomun2013), in an environmental setting that is estimated to reflect a water depth of 100–200 m (Briggs, Siveter & Siveter, Reference Briggs, Siveter and Siveter1996).

5.b. Functional anatomy

Pauline nivisis sp. nov. has such a similar carapace morphology to that of P. avibella that it is reasonable to transpose the palaeobiology known from the Herefordshire fossil – body, limbs including swimming antennae, lateral eyes, gills and alimentary system – into the carapace of the Greenland species (reconstruction in Fig. 3b). The preadductorial node of P. nivisis most likely housed a well-developed lateral eye and this also suggests that its carapace was thin and translucent, at least at the position of the eye, as in most Recent myodocopes (see Land & Nilsson, Reference Land and Nilsson1990, fig. 1; Vannier, Abe & Ikuta, Reference Vannier, Abe and Ikuta1998, fig. 4). The large basipod and setate exopod of the second antenna of P. avibella suggests that it was an efficient swimmer (Siveter et al. Reference Siveter, Briggs, Siveter, Sutton and Joomun2013) and the same can be assumed for P. nivisis. A swimming lifestyle would have been supported by the presence of gills and paired epipods, as present in P. avibella and a supposed well-developed cardiovascular system including a heart (see Vannier & Abe, Reference Vannier and Abe1992; Williams et al. Reference Williams, Vannier, Corbari and Massabuau2011). The morphology of the mandible and first maxilla of P. avibella, with well-developed endites, and its clawed furca is similar to that in Recent nektobenthic myodocopids (see Vannier, Abe & Ikuta, Reference Vannier, Abe and Ikuta1998) and suggests a similar feeding strategy for Pauline species (Siveter et al. Reference Siveter, Briggs, Siveter, Sutton and Joomun2013), which are presumed to have scavenged, preyed or were detritovores on or near the substrate. Appendages and the furca would cut and tear the food held by the mandible. Both male (see Colymbosathon, Siveter et al. Reference Siveter, Sutton, Briggs and Siveter2003) and female (see Nymphatelina, Siveter et al. Reference Siveter, Siveter, Briggs and Sutton2007) myodocopes were identified from the Herefordshire Lagerstätte, but it was not possible to determine the gender of P. avibella. The presence of carapace dimorphism in P. nivisis is uncertain, but like living myodocopids P. nivisis presumably reproduced sexually and brooded its young.

Figure 3. Pauline nivisis sp. nov. (a) Reconstruction and measurements of the carapace. (b) Idealized reconstruction in life position.

6. The origin of pelagic ostracods

The limited known palaeogeographical distribution, facies occurrence, faunal associates and morphology of P. nivisis sp. nov. suggest that like most Recent myodocopids it had a nektobenthic rather than pelagic lifestyle. This is consistent with the idea that known pre-late Wenlock myodocopes were probably epibenthic (Siveter & Vannier, Reference Siveter and Vannier1990; Siveter & Lane, Reference Siveter and Lane1999; Gabbott et al. Reference Gabbott, Siveter, Aldridge and Theron2003; Siveter & Bogolepova, Reference Siveter and Bogolepova2006), and supports the notion of a Late Silurian ecological shift for the origin of pelagic (myodocope) ostracods (Siveter, Vannier & Palmer, Reference Siveter, Holland and Bassett1987, Reference Siveter and Vannier1991; Siveter & Vannier, Reference Siveter, Briggs and Crowther1990; Perrier, Vannier & Siveter, Reference Perrier, Vannier and Siveter2011). Early Silurian, Llandovery–Wenlock myodocopes were nektobenthic, with most living with dominantly benthic associates on well-oxygenated marine shelves (e.g. reef environments). Myodocope ostracods appear to have undergone an ecological shift by the latest Wenlock or earliest Ludlow (see Siveter, Reference Siveter, Bassett and Lawson1984; Siveter, Vannier & Palmer, Reference Siveter, Holland and Bassett1987, Reference Siveter and Vannier1991; Siveter & Vannier, Reference Siveter and Vannier1990; Vannier & Abe, Reference Vannier and Abe1992; Perrier, Vannier & Siveter, Reference Perrier, Vannier and Siveter2007, Reference Perrier, Vannier and Siveter2011) an event that may be associated with the recovery interval post-dating the middle Homerian biotic extinction (see Porebska, Kozłowska-Dawidziuk & Masiak, Reference Porębska, Kozłowska-Dawidziuk and Masiak2004). However, the precise environmental trigger and anatomical pre-adaptations that facilitated this transition remain to be elucidated in detail.

The only exception in this scenario is the long-lived Entomozoe tuberosa. If the Australian material belongs to the same species (Perrier et al. in prep.) then it would occur from the Telychian to the Homerian with an almost global tropical distribution. This suggests that Entomozoe might not have been strictly nektobenthic as suggested by Siveter & Vannier (Reference Siveter, Whittaker and Hart1990), Siveter & Lane (Reference Siveter and Lane1999) and Siveter & Bogolepova (Reference Siveter and Bogolepova2006), and that its ability to widely disperse may have been fundamental to its long-lived survival.

7. Systematic palaeontology

Class OSTRACODA Latreille, Reference Latreille1802 (nom. correct. Latreille, Reference Latreille1806).
Subclass MYODOCOPA Sars, Reference Sars1866.
Order MYODOCOPIDA Sars, Reference Sars1866.
Family Cylindroleberididae Müller, Reference Müller and Siboga1906.
Genus Pauline Siveter, Briggs, Siveter, Sutton & Joomun, Reference Siveter, Briggs, Siveter, Sutton and Joomun2013

Diagnosis. Large cylindroleberidid. Carapace elongate with an adductorial sulcus, an anterior lobal complex, a prominent wing-like posterolateral lobal structure and a simple anterior gape (after Siveter et al. Reference Siveter, Briggs, Siveter, Sutton and Joomun2013).

Type species. Pauline avibella Siveter et al. Reference Siveter, Briggs, Siveter, Sutton and Joomun2013.

Pauline nivisis sp. nov.
Figures 3a, b, 4b–n

Derivation of name. Latin nivis, snow + avis, bird; alluding to the northerly locality and the ala.

Figure 4. Pauline avibella, Herefordshire Lagerstätte, UK; Wenlock, Silurian (a) and Pauline nivisis sp. nov. Kap Schuchert, Washington Land, North Greenland; upper Telychian, upper Llandovery, Silurian (b–n). (a) Carapace with soft parts (Oxford University Museum of Natural History C.29613), lateral view. (b–f) Right valve (holotype; MGUH 30487), (b) right lateral view, (c) dorsal view, (d) ventral view, (e) posterior view, (f) anterior view. (g–i) Left valve (MGUH 30488), (g) lateral view, (h) ventral view, (i) dorsal view. (j, k) Right valve (MGUH 30489), (j) ventral view, (k) anterior view. (l, m) Left valve (MGUH 30490), (l) lateral view, (m) dorsal view. (n) Right valve (MGUH 30491), lateral view. All pictures are stereo-pairs, (a) digital reconstruction (‘virtual fossil’), (b–n) photographs. Scale bar is 1 mm across.

Holotype. Geological Museum, Copenhagen University, specimen MGUH 30487 (Fig. 4b–f), almost complete right valve.

Diagnosis. Pauline with a well-developed preadductorial node and a posteriorly projecting posterolateral wing-like lobal projection (ala).

Material. Two carapaces and seven valves.

Measurements. Maximum valve length – valve height of largest and smallest well-preserved specimens: 10.3–5.0 mm (MGUH 30490; Fig. 4l, m), 8.2–4.4 mm (MGUH 30488; Figs 3a, 4g–i).

Locality and stratigraphy. Kap Schuchert, Washington Land, North Greenland; upper Telychian, upper Llandovery, Monograptus spiralis graptolite Biozone sensu lato, Silurian.

Description. Valves elongate, tapering posteriorly; maximum length and width at the tip of the ala, maximum height half way between mid-length and anterior end of the valve. Lateral valve outline strongly curved ventrally, almost straight dorsally, pointed posterodorsally and rounded anteriorly. The valve is bordered by a fine marginal ridge that is weaker posteriorly. A wide, thin, striated, lamella-like feature extends adaxially from the ventral to posterior regions of the marginal ridge of the right valve, giving left over right valve overlap. A small indistinct posterior gape is apparent in one specimen (Fig. 5h). A large anterior lobe is gently rounded dorsally. A small (diameter about 1 mm), distinct preadductorial node occurs just above mid-height. An adductorial sulcus occurs at mid-length, is widest dorsally, weakly z-shaped, extends to just below valve mid-height and has a simple muscle spot ventrally. The ventral two-thirds of the valve posterior of the adductorial sulcus is gently inflated; above is a prominent posterolaterally projecting wing-like ala that has two posterodorsal projections and curves gently above the hinge line. The external surface of the valves is finely reticulate, with fossae about 140 μm wide and muri about 80 μm wide.

Figure 5. Pauline cf. nivisis sp. nov. Kap Schuchert, Washington Land, North Greenland; upper Telychian, upper Llandovery, Silurian. (a–c) Incomplete carapace (MGUH 30492), (a) left lateral view, (b) ventral view, (c) anterior view. (d–g) Incomplete carapace (MGUH 30493), (d) right lateral view, (e) posterior view, (f) ventral view, (g) dorsal view. (h) Left valve (MGUH 30494), posterior view. (i) Left valve (MGUH 30495), lateral view. All are stereo-pair photographs. Scale bar is 1 mm across.

Remarks. That P. nivisis is considered to be a cylindroleberidid myodocopid follows the assignment of Pauline to that family based on soft-part evidence (see Siveter et al. Reference Siveter, Briggs, Siveter, Sutton and Joomun2013). P. nivisis thereby represents the earliest known cylindroleberidid. P. nivisis differs from P. avibella by its more subdued anterior lobe, the presence of a distinct preadductorial node, and its more posterolaterally projected ala. All of the P. nivisis specimens are supposed adults and large instars, which may reflect ‘small sample’ bias or size sorting. Four Pauline specimens (MGUH 30492–30495, Fig. 5) from the single block differ from the other material in having a more prominent anterior lobe, a weaker preadductorial node and ornament consisting of very gentle and fine reticulation in which the fossae and muri are about 50 μm wide (Fig. 5). These differences could represent simple intraspecific variation or reflect dimorphism or a separate species, but the limited nature of the material precludes further attribution. Herein, these specimens are referred to P. nivisis with confer.

8. Conclusions

Pauline nivisis sp. nov. from Greenland is the first ‘Herefordshire’ Lagerstätte genus found outside the type locality and is the oldest known cylindroleberidid myodocopid ostracod. It provides an Early Silurian link between ‘Baltic-British’ and North Laurentian ostracod faunas along a remnant Iapetus Ocean. Its stratigraphic occurrence and supposed lifestyle endorse the notion that pre-Late Silurian myodocopes are nektobenthic. The palaeogeographic and stratigraphic distribution of Pauline and Entomozoe support the hypothesis that low latitude faunas spread out to higher latitudes post-dating the Early Palaeozoic Icehouse.

Acknowledgements

This research was funded by a Leverhulme Trust foremost grant (RP14G0168). We thank the Geological Survey of Greenland for allowing access to the fossil material. We thank Derek J. Siveter (University of Oxford, UK) for his instruction in the methods of macrophotography. This is a contribution to the International Geoscience Programme (IGCP) Project 591.

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Figure 0

Figure 1. Geological setting of the locality for Pauline nivisis. (a) Location and extension of the Greenland segment of the Franklinian Basin (grey filling) and Washington Land. (b) Geological map of Washington Land and location of the Kap Schuchert outcrop (star): maps after Peel & Sønderholm (1991).

Figure 1

Figure 2. Stratigraphic and palaeogeographic distribution of Entomozoe and Pauline. (a) Stratigraphic position, which is approximate within each formation. (b) Palaeogeographic distribution and possible migration pathways (dashed arrows). Stratigraphy for Greenland after Peel & Sønderholm (1991); Scotland and England after Siveter & Vannier (1990); Arctic Russia after Siveter & Bogolepova (2006); China after Rong, Wang & Zhang (2012); Australia after Strusz (1984); palaeogeographic map after Torsvik (2009). C. R. Fm. – Cock Rig Formation; L. Fm. – Leijiatun Formation; M. Fm. – Majiaochong Formation; M.P.V. – Mount Painter Volcanics; M.W. Fm. – Much Wenlock Formation; W. Fm. – Wangjiawan Formation; X. Fm. – Xiangshuyuan Formation.

Figure 2

Figure 3. Pauline nivisis sp. nov. (a) Reconstruction and measurements of the carapace. (b) Idealized reconstruction in life position.

Figure 3

Figure 4. Pauline avibella, Herefordshire Lagerstätte, UK; Wenlock, Silurian (a) and Pauline nivisis sp. nov. Kap Schuchert, Washington Land, North Greenland; upper Telychian, upper Llandovery, Silurian (b–n). (a) Carapace with soft parts (Oxford University Museum of Natural History C.29613), lateral view. (b–f) Right valve (holotype; MGUH 30487), (b) right lateral view, (c) dorsal view, (d) ventral view, (e) posterior view, (f) anterior view. (g–i) Left valve (MGUH 30488), (g) lateral view, (h) ventral view, (i) dorsal view. (j, k) Right valve (MGUH 30489), (j) ventral view, (k) anterior view. (l, m) Left valve (MGUH 30490), (l) lateral view, (m) dorsal view. (n) Right valve (MGUH 30491), lateral view. All pictures are stereo-pairs, (a) digital reconstruction (‘virtual fossil’), (b–n) photographs. Scale bar is 1 mm across.

Figure 4

Figure 5. Pauline cf. nivisis sp. nov. Kap Schuchert, Washington Land, North Greenland; upper Telychian, upper Llandovery, Silurian. (a–c) Incomplete carapace (MGUH 30492), (a) left lateral view, (b) ventral view, (c) anterior view. (d–g) Incomplete carapace (MGUH 30493), (d) right lateral view, (e) posterior view, (f) ventral view, (g) dorsal view. (h) Left valve (MGUH 30494), posterior view. (i) Left valve (MGUH 30495), lateral view. All are stereo-pair photographs. Scale bar is 1 mm across.