Gordia nodosa isp. nov. and other trace fossils from the Cass Fjord Formation (Cambrian) of North Greenland

Green!and. D Dresbachian in age new trace fossi! is Midd!e Late stabs Diplich nites Monocraterion Tore1!, tubularis,

The study of Lower Palaeozoic trace fossils from Greenland is still in its infancy (cf. Pickerill & Peel, 1990). Documentation of most ichnotaxa has essentiaIly relied on material collected on a random basis, commonly over a period of several years by different workers, and with no view to a thorough ichnological analysis. Yet despite the restricted sizeof many of the previously described collections (e.g. Bergstrom & Ineson, 1988;Bergstrom & Peel, 1988;Pickerill. & Harland, 1988;Bryant & Pickerill, 1990;Pickerill & Peel, 1990), study of the often well preserved material has proven to be extremely rewarding. Such is also the case herein, where four slabs collected from the Cass Fjord Formation of Daugaard-Jensen Land, western North Greenland (Figs 1, 2), contain a variety of generaIly well preserved ichnotaxa. Among these is a new ichnospecies, Gordia nodosa isp. nov., which is figured and described in detail. Other trace fossils include the ichnotaxa Cruziana problematica (Schindewolf, 1921), Diplichnites Dawson, 1873, cf. Monocraterion TorelI, 1870 and Palaeophycus tubularis Hall, 1847. For the sake of completeness, these are also briefly described and figured. The fourslabs each possess a Geological Survey of Greenland collection number (GGU prefix, Grønlands Geologiske Undersøgelse) and appropriate specirnen numbers prefixed by MGUH; all are housed in the Geological Museum, University of Copenhagen.
Rapp. Grønlands geol. Unders. 150, 15-28 (1991) Location and stratigraphy Poulsen (1927) gave the name Cass Fjord Formation to a unit in the Cass Fjord area of Daugaard-Jensen Land, western North Greenland which yielded Early Ordovician faunas from its highest beds. Formal proposal was accomplished by Koch (1929), while Henriksen & Peel (1976) gave a brief re-description. Palmer & Peel (1981) described the lower part of the formation in more detail, recording trilobites of late Middle Cambrian (Dresbachian) and younger age; the formation thus ranges from the Middle Cambrian to the Early Ordovician. The Cass Fjord Formation outcrops throughout Daugaard-Jensen Land (Fig. 1), from Humboldt Gletscher in the south-west to Petermann Gletscher in the north-east (cf. Palmer & Peel, 1981;Peel & Christie, 1982).
The Cass Fjord Formation (400-470 m) is dominated by greenish grey, nodular and lenticularly bedded micritic limestones, with frequent thin beds (5-50 cm) of intraformational flat-pebble conglomerates. A variety of other lithologies are represented, however, including yellowish finely recrystallised dolomites, anhydritic shales and lime grainstones; white quartzites (1-30 m) near the top of the formation are referred to the Kap Coppinger Member (Bryant & Smith, 1990). The formation forms part of a Cambrian aggradational platform succession assigned to the Ryder Gletscher Group and widely distributed throughout western North  Palmer & Peel (1~81) recognised [OUf informai mcmhers within tile [ower part of the Cass Fjord Formation. A basal member A (2()..4(J m) is characterised by the micritic iimcstoncs typicai ol' the formation as a whole, and silty intcrbeds. Overlying c1iff-forming carbonates assigned to mcmber B (30-45 m) lack the silty interbeds and. in addition to the rnicritic limestones, also include grainstones with trilobites, and infrequenl oolites. Dipliclinites isp., d. Monocraterioll isp. and Palaeophycus {Uhu laris have been collected from this interval, where the associated trilobites indicatc a latc Middle Cambrian age from possihly pre-Dresbachian to the earliest Dreshacllian Cedaria Zone (Palmer & Peel, 1981).
Memher C consists of aboul 45 m ol' recessive micritic limestones. finely recrystallised dolomites and some reddish shales and siltstones. Cmziana prob!emal;c,,'(I was collected from this interval which lies within the Cerlaria Zone, as indicatcd by trilobitcs occurring in the underlying and overlying mcmbcrs. Member D ol' tht Cass Fjord Formation is a promincnt-wcathcring unit (ahout 45 m) ol' similar lithology lO member B, although the thin grainstone beds are Icss prominent. Gordia nodosa and Palaeophycus tfllm!aris were collccted from this member in beds yielding Cedaria Zone, ar possibly youllger. Dresbachian trilobites (Palmer & Pcel. 1981).
Gorclia nodosa isp. nov.  comprise simple, long and slender, horizontal burrows, that wind or loop irregularly and exhibit a marked tendency to level crossing. Burrow diameter varies from 0.3 to 1.1 mm and is relatively constant in individual specimens. The full course of many individual burrows is difficult to ascertain as commonly they merge with, and apparently follow, the course of their own or pre-existing burrows. The selected holotype (Figs 3a, 4a) clearly demonstrates this phenomenon. Additionally , individual burrows are commonly crossed by separate burrow systems, which precludes arealistic assessment of the actual number of individual specimens. Less commonly an individual burrow purposely avoids true level crossing and, instead, crosses below, or more rarelyabove, itself.
In such examples, however, true level crossing of the burrows is also typically developed, particularly where an extensive horizontal burrow course can be ascertained. Burrow-fill is structureless and of the same grain size as the host rock. Burrow linings are absent; cross-sectional shape, where discernible, is circular. Individual burrows are characterised by the development of nodes or annulations thus giving them a beaded appearance. The annulations are irregularly developed; several burrows are annulate throughout their entire course, but more commonly the annulations are interspersed so that annulate sections alternate with non-annulate sections. Typically, the annulations are extremely small with a density of 1 or 2 per mm. Surfaces of the annuli are smooth.
The holotype exhibits the typical irregular winding, looping and level crossing of the ichnotaxon; only portions of this specimen exhibit annulations. The three selected paratypes exhibit winding, looping, and to a lesser degree, self-crossing,but are considerably more annulate.

Remarks.
At the ichnogeneric level these structures resemble a number of ichnotaxa, most notably Helminthoidichnites Fitch, 1850, Helminthopsis Heer, 1877and Mermia Smith, 1909. Helminthoidichnites was considered a junior synonym of Gordia by Hiintzschel (1962Hiintzschel ( , 1975 but was utilised by Hofmann & Patel (1989) and more recently by Narbonne & Aitken (1990). As noted by these authors, it differs from Gordia in that level crossing of an individual burrow system is only rarely , if ever, developed. Instead, Helminthoidichnites is characterised by irregularly sinuous to meandering burrows with common random crossings of different individuals. Helminthopsis, particulary Helminthopsis tenuis Ksiqzkiewicz, 1968, which Hiintzschel (1975 also assigned to Gordia, is an irregularly meandering or sinuous form that consistently avoids level crossing. As such it is clearly different from Gordia and must not be considered its junior synonym. Mermia, an ichnogenus not considered by Hiintzschel (1962Hiintzschel ( , 1975, is also comparable to Gordia, as also noted by Walker (1985). AIthough Mermia is much thinner (typically <0.5 mm wide) and supposedly possesses more intense looping (see also Pollard & Walker, 1984), the figured neotype of these authors (Walker, 1985, fig. 8a, p. 294;Pollard & Walker, 1984, pI. 2, fig. 6) very c10sely resembles G. marina and a case could reasonably be made to regard it as a junior synonym. Thus, the material described here is regarded as Gordia even though the original scanty description has resulted in general confusion and taxonomic inconsistencies. We adopt Gordia for burrows or trails that exhibit a marked tendency to level crossing (cf. Miller, 1889;Narbonne & Hofmann, 1987;Narbonne & Aitken, 1990;Fillion & Pickerill, 1990), as is evident in Emmons' (1847) original illustration and in topotype material subsequently figured by Hall (1847).
We are currently aware of five previously described ichnospecies assigned to Gordia, namely G. marina Emmons, 1844, G. molassica (Heer, 1864), G. arcuata Ksiqzkiewicz, 1977, G. hanyagensis Yang & Hu in Yang et al. (1987, and G. maeandria Jiang in Jiang et al. (1982). As noted by Pickerill (1981) and reiterated by Narbonne & Hofmann (1987) and Fillion & Pickerill (1990), G. molassica should be regarded as a junior synonym of G. marina since their respective sizes are unknown and their course is virtuaIly identicaI. G. hanyagensis also agrees in all respects with G. marina and was also regarded by Fillion & Pickerill (1990) as a junior synonym. G. maeandria is characterised by loose or even guided meanders (e.g. Crimes & Jiang, 1986, p. 646, fig. 4h) with no level crossings, and as noted by Pickerill & Peel (1990), should be assigned to an alternative ichnogenus. G. arcuata is an ichnospecies in which only the apical arcuate bends are developed (see Ksiqzkiewicz, 1977) and it clearly differs from the material described here.
Irrespective of the acceptance or otherwise of these ichnospecies, the one characteristic common to them all is their smoothness, relatively constant diameter and lack of the annulations which characterise the material described herein as G. nodosa.
The nodes or annulations of Gordia nodosa are reminiscent of those present in the ichnotaxa Torrowangea rosei Webby, 1970, and to some extent Planolites annularius Walcott, 1890 and Palaeophycus tortuosus Hall, 1847 (questionably regarded as a junior synonym of Palaeophycus tubularis Hall, 1847 by Pemberton & Frey, 1982). The annulations perhaps reflect peristaltic movement (cf. Pemberton & Frey, 1982) or small scale  Webby, 1970). It is also worth-whilc nuting that in similar fashion to He!mimhoidic!znires and Helmimhopsis. none of these ichnospecies exhibit self level crassing, though thcy may form irregular meshworks af crossing and branching strands produced as a result af intcrsections by adjacenr burrow systems (see Webby, 1970;Pemberton & Frey, 1982). Thus, these ichnotaxa clearly differ from G. nodosa as described herein. The nature of the producing organisms is of course enigmatic. Fillion & Pickerill (1990) proposed that some Gordia marina were produced by aslender bilaterally symmetrical arthropod-like or vermiform organism, and a similar origin for G. nodosa cannot be ruled out. The remarkable size and morphological similarity to the burrows and trails produced under laboratory conditions by the forarninifer Quinqueloculina impressa Reuss by Severin et al. (1982) also suggests the possibility of production by similar organisms, particularly as the fossil record of benthic foraminifera extends well into the Cambrian (Buzas et al., 1987). Interestingly, the examples figured by Severin et al. (1982) also exhibit sections where the forarninifer follows its own or previously existing burrows or trails similar to that observed in G. nodosa. Trail folIowing (at least in the Gastropoda) may be related to predation (Paine, 1963) or homing (Cook, 1979) as a response to mechano-reception or chemoreception.

Additionai trace fossils
Additional ichnotaxa from the available material from the Cass Fjord Formation are figured but only briefly described.
Description. Specimens are variably preserved in convex relief on the sole of a 1 cm thick calcisiltite. They consist of essentiaIly bilobed, horizontal, symmetrical burrows that follow a straight, curved or slightly flexuous course. Several examples are undulatory, disappearing up into the host stratum and reappearing at a different location. When this undulatory behaviour is conspicuously developed the resultant segments commonly reveal short burrow sections that superficially resemble Rusophycus Hall, 1852, particularly R. didymus (Salter, 1856), though assignment to this ichnotaxon would obviously be unsatisfactory. The undulatory behaviour of the producing organisms makes an accurate assessment of the number of specimens present on the slab extremely difficult, as commonly the com-plete burrow course of an individual specimen cannot be ascertained. More rarely, the bilobed burrows extend horizontally into unilobed segments particularly where such segments are more deeply impressed.
Burrows vary in diameter from 0.6 to 3 mm and up to a maximum length of approximately 6 cm. In bilobed sections, the central groove is well developed. Individual burrows commonly cross previously forrned exam-pIes but never intersect themselves. Lobes are either smooth or possess c10sely spaced, transverse or highly obtuse unifid scratch marks that extend from the central furrow to the margins of the lobes. Commonly, and for reasons not completely understood, the narrower burrows preserve obvious scratch markings whereas the wider burrows are either smooth or possess poorly preserved scratch markings and exhibit a more obvious development of unilobed segments.

Remarks.
We follow the reasoning of Bromley & Asgaard (1979) and Romano & Whyte (1987) to include this material within the ichnogenus Cruziana d'Orbigny, 1842 rather than the morphologically similar ichnogenus Isopodichnus Bornemann, 1889. The suggestion of Pollard (1981Pollard ( , 1985 to separate these ichnogenera on the basis of age, facies association and nature of producer is considered inappropriate; trace fossil nomenclature should be based on morphology alone, or behaviour as evidenced in morphology rather than other considerations. Ichnogenus Diplichnites Dawson, 1873 Diplichnites isp. Material. Six, possibly seven specimens from GGU colleetion 242103 from locality 4 (Fig. 2), Member B, of the Cass Fjord Formation (see Palmer & Peel, 1981).
Description. Specimens are preserved in negative epireliet on a 1 cm-thick, parallellaminated, calcisiltite. Following the terminology of Osgood (1970), each specimen comprises a set of paired uhifid imprints; the width of each set ranges from 2 to 4 cm and length from 2 to 5 cm. Imprints of individual specimens number between six and eleven and are oriented approximately normal to the trace axes. Length of opposing individual imprints is the same but length within a series is variable, the more elongate pairs typically being centrally 10cated. Withdrawal markings occur in association with a single set (Fig. 7a, upper left).  Remarks. Fillion & Pickerill (1990) discussed the taxonomic confusion currently existing in ichnologicalliterature with respect to the arthropod-produced trackways of Diplichnites. Until a thorough study of the ichnogenus and its possibie synonyms is undertaken we prefer to identify the Greenland material only to the ichnogeneric level.
Description. Specimens are preserved as circular, con-centricaIly lined structures on the upper surface of the slab containing the previously described Diplichnites. Two structures are 1.2 cm in diameter, the third 1.0 cm. Each possesses a central sparite-filled core. Rims of the structures are tumed slightly upwards. Longitudinal sectioning of the smaller specimen (Fig. 8a) illustrates the linings, the sparite fill and the marked downward tapering of the burrow. Upward defiection of laminae on one side of the burrow is also apparent.
Remarks. The exact nature of these specimens is enigmatic and we are uncertain whether they represent biogenic or physical (water or gas escape) sedimentary structures despite application of the differentiating criteria comprehensively discussed by Ekdale et al. (1984). Tentatively, however, we regard them as biogenic. This conclusion is based on the fact that both upper and lower surfaces of the slab containing the specimens possess abundant ichnofossils and personal experience that water and gas escape structures typically occur in more abundant concentration, are accompanied by considerably more bedding deformation, are larger in scale and, on the whole, are morphologically dissimilar to the structures here (cf. Wnuk & Maberry, 1990). Assuming that the specimens are biogenic, they are tentatively compared to Monocraterion which we regard as the closest morphological analogue. They also resemble the ichnogenus Rosselia Dahmer, 1937 which, however, is a concentric cone-shaped or funnel-shaped burrow produced by active back-filling. The fact that the described structures were originally open, as indicated by their sparite-filled cores, suggests that they are therefore best compared to Monocraterion.
Description. Simple, straight to slightly curved, rarely slightly sinuous, smooth, horizontal, unbranched burrows preserved in positive hyporelief (GGU collection 242103) or positive and negative epirelief (GGU collection 242096). Burrows are typically 2 to 5 mm in diameter, of variable length, lined or apparently unlined, with a structureless fill of similar grain size to the enclosing host rock.
Remarks. Pemberton & Frey (1982), and more recently Fillion (1989) and Fillion & Pickerill (1990), have discussed in detail the distinction between Palaeophycus and the other relatively simple horizontal burrow systems of the ichnogenus Planolites Nicholson, 1873. Following their recommendations, these specimens can be confidently diagnosed as P. tuhularis.