A problematical trilobite from the Lower Cambrian of Freuchen Land, central North Greenland
Philip D. Lane and Adrian W. A. Rushton
A single specimen of a remarkable trilobite is described from the Early Cambrian Buen Formation of north-east Freuchen Land, central North Greenland. It is re ferred with doubt to Alacephalus Repina, 1960, as a new species A? davisi sp. nov. It possibly lacked eyes, which makes it one of, if not the, earliest non-agnostoid trilobite with this adaptation. Its thoracic segments have a unique morphology. In some respects the morphology resembles that of various trilobites adapted to low-energy benthic environments of low oxygenation; such trilobites tend to be widely distrib uted, and in agreement with this Alacephalus appears to be interprovincial.
P. D. L., Department ofGeology, Vniversity ofKeele, Keele, Staffordshire ST5 5BG, V.K. A. W. A. R., British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K.
The trilobite described below was collected by Neil C. olenellid trilobite Buenellus higginsi Blaker, 1988 (Fam- ~ Davis from laminated, locally bioturbated, mudstones ily Nevadiidae) is associated with an abundant soft- in north-east Freuchen Land (Fig. l; Geological Survey bodied fauna (Conway Morris et al.. 1987; Conway of Greenland collection 319544) which are estimated to Morris & Peel, 1990; Peel, 1990), together with large be not more than 5 m from the top of the Buen Forma articulated sponges such as the demosponge Choia hin tion. This formation has been interpreted as a silic dei Rigby, 1986; the same kinds of sponges are associ ic1astic shelf deposit forming part of the sequence on the ated with the new trilobite in GGU collection 319544. southern margin of the Franklinian Basin in Greenland. However, the soft-badied fauna from Sirius Passet ap In its typical development, in southern Peary Land to pears to be derived from near the base of the Buen the south-east of the present locality, the Buen Forma Formation, in contrast with the collection described tion has two units and varies in thickness from about here from the upper beds af the formation. 425-500 m (Jepsen, 1971; Peel, 1982; Higgins et al., 1991). The lower unit is sand-dominated and is consid Preservation ered to have been deposited under high energy, inshore conditions; bioturbation is common, and there is a large The specimen, preserved in laminated mudstones, is ichnofauna (Bergstrom & Peel, 1988; Bryant & Picke flattened but not tectonically deformed. The poar state rill, 1990). This unit is succeeded by an upper mud of preservation of the specimen makes interpretation of dominated unit, deposited in a 10Wer energy, outer the morphology difficult; our interpretation of the vari shelf environment. It is from this latter unit that the ous structures seen on the cranidium is shawn in Fig. 3a. trilobite described below was collected, although the Diagenetic flattening has removed the original convex bipartite division of the formation is not c1early ex itY, although this could not have been great since the pressed in north-eastern Freuchen Land. pleural portions of the specimen, inc1uding genal and pleural spines, are stretched out in a single plane and Age of the specimen not (even partially) folded under, as would be expected in a trilobite with a very convex cross-section. The ante The Early Cambrian age is inferred from the presence rior, longer pleural spines, as preserved, have longitudi elsewhere in Peary Land of olenellid trilobites within nal cracks, which indicates that they may have been oval and immediately above the Buen Formation (Palmer & or circular in cross-section. In addition they have Peel, 1979; Higgins et al., 1991). About 20 km to the creases at their bases which suggest that they were in life north-east, in Sirius Passet across J. P. Koch Fjord, the directed somewhat down and have been rotated up-
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Fig. l, Locality in central North Greenland from which A!([('('ph(Jlus~ davisi was collccted. The occurrcncc af tlle lri!obite near the top af the Buen Formation is jndic wards Oll compactioll af the sediment: SUdl an oricll c1isplaced hut nol invertecl (JS incllcatcd by the matehing [ a ,, ,, / , / -=::==~~ / /' / , " "- , , " , ,, ~~~-'------C, ~ Ji.. _ J_"'------ fig. 2. a. Rcconslruclion of Alacephal/ts rafl/mlll!l. based on Repin.. (1960. pI. Il. figs 11-13) Sys.ematie description J-IoI01ype. Geological Museum of the Univcrsity af Copcnhagen (MGUII) 21.I·U (ccphalon and ?partial Family unknown thorax. from GGlJ colleetion 31954~): monotype. Genus Alacephalus? Repina, 1960 Vescr;ptioll. Cephaloll transversc. Width (tr.) af crani dillm at posterior margin appears \0 bc nearly threc 7}'J)(! species. A/art'phafus colltorlus Repina. 1960. p. times th(,; lcngth (sag.). Glahel1a apparcntly subparallcl· 222. pI. ll. rig~ Il-U, from thc Lowt'r Call1briall of ~idC'd. broadly rouncled in front, app<.trently rC 1/1Fig. 1. Alf/("f'pha!lIs'I dal'isi sp. IlO\'. a. drawing ol' cephalon. X 1.3 to shuw OUT interpret<:ltion. h. rcconstruction 01' parts // ol' tlle .lrd and 9th thora<.:ic segments. and the cross-scctional view ol' tlle ?14th. indicating the low convexity ol' the ,:'./ anilllal in life. x [. X - position uf creasc at base af antcrior pleural spine; Y- position af inner margin ol' doublurc. II is more Ol' tess parallel-sided, with slllall willgS antc more backwardly-directed towards tile posterior af thc riorly and wirll semicircular outline poslcriorly; il is thorax. The inner margin af the thoracic doublure lies one-half the width anli abollt two-lhirds tlle Icngth of just abaxial to the bases af the pleural spines (Fig. 3b). the prc-occipital glabeJla. 4) The two pairs of pleural spines on each thoracic Remarks OD the thoracic structure segment, of which the anterior are longer than the post erior. The structure of the thorax bears similarities to a range of genera. The bilobed axial rings are, however, The glabellar form and SI furrows, and the lack of long unusual. Henningsmoen (1957) referred to the structure eyes, are features that induce us to refer davisi to Ala of an occipital ring of the same general form as that of cephalus? It differs from A. contortus in having a much davisi as 'composite', as in for exampie Ctenopyge (Cte longer LI and a wider interocular area of the fixigena nopyge) affinis graciIis Henningsmoen (1957, p. 201, pI. (i.e. it is wider to the posterior of the eye ridge); in A. 19, figs 20, 21); that such a structure may have been contortus the interocular area of the fixigena is about present in A? davisi is inferred from the structure of the equal in width to the width of the glabella in the same thoracic axial rings. Its function is unknown, but in line, whilst in A? davisi it appears to be not less that one sertions for musculature is an obvious possibility. and a quarter times as wide as the glabella. The wide Oblique apodemes on the axial rings of Ceratocephala fixigenae resemble those of A. latus Romanenko (1978, barrandii were described by Thomas (1981, p. 95). pI. 32, figs 16-19) which is from the Early Cambrian of These occupy a similar position to the furrows bounding Altai, Russia. In this latter species, however, the gla the lateral axial bilobes in davisi, but the homology of bella tapers anteriorly somewhat and the ocular ridges the structures cannot be assumed. are not oblique but nearly transverse. The librigenae of The thoracic pleurae of A? davisi are wide (tr.) and A. latus lack the strong spine of A? davisi. The pygi flat and the pleural ridges may have accommodated dium assigned to A. latus has a few thoracic segments well-developed propleural and opisthopleural veins. attached and these have normal pleurae, resembling, This structure is similar to certain trilobites which, for example, Solenopleura, and not resembling those of though not closely related, are all thought to be adapted A? davisi. to conditions of low oxygenation, e.g. Papyriaspis lan The shape of the glabella and its contact with the ceola (see Opik, 1961, pI. 68, figs 1, 2), Parabolina heres anterior border furrow, and the thin transverse eye (Rushton, 1982, pI. 2, figs 15-18) and Hedinaspis regalis ridges recall Atops and Pseudatops (Howell & Stubble (Palmer, 1968, pI. 15, figs 21, 22). fieid, 1950, pI. 1, fig. 3; pI. 2, figs 1, 2). A? davisi differs Double pleural spines are present in various unre in having the glabellar furrows more oblique and in its lated trilobites including Xystriduridae, ?Solenopleuri thoracic structure. Gelasene acanthinos Palmer (1968, dae, ?Dorypygidae and Odontopleuridae. In some Xys- 11 triduridae (Polydinotes and Galahetes; Dpik, 1975, p. double pleural spines (A? davisi, O. ovata and the 30, figs 7B, C) the pleural tips have two spines of which odontopleurids described by Chatterton & Perry) occur the anterior is larger, as in A? davisi. The differences in in sediments interpreted on other grounds to have been the cranidia indicate that A? davisi is not closely related deposited in outer shelf to slope environments; post to the Xystriduridae. Double marginal spines occur on a mortem transport can be ruled out since articulated Dorypyge pygidium (Fortey & Rushton, 1976, pI. 11, specimens occur in all three examples. The dorypygid fig. 2). The anterior spine is the smaller; this structure and Galahetes both occur in carbonate sequences, the may prove to be repeated on the thoracic segments as is latter deposited in "euphotic and eutrophic waters hab the case in the thoracic and pygidial spines of Galahetes itable down to the sea floor" (Dpik, 1975, p. 5). The fulcrosus Dpik (1975, text-figs 7c, 13; pis 16-20). As only similarity of all the cases is the fine grained nature noted above, the solenopleurid? Gelasene (Palmer, of the matrix in which the specimens occur; this pro 1968, p. 81, fig. 6) has pleural spines and spines on the vides no explanation of the double spine as astructure, fulcra of each thoracic segment. nor lends any clues to an explanation of their function. Double pleural spines occur commonly also in odon topleurid trilobites; Chatterton & Perry (1983) de Significance in provinciality scribed such spines in 21 species belonging to 10 genera and subgenera of the family. In some, anterior and In reconstructions of Early Cambrian faunal prov posterior pleural spines occur on all thoracic segments inces (e.g. Pillola, 1990) North Greenland lies in the (e.g. Odontopleura brevigena Chatterton & Perry faunal province characterised by olenellid trilobites (1983, text-figure 13). In every case the anterior spine is whereas the Altai-Sayan fold belt of Siberia, from which shorter than the posterior; the opposite of the case in Alacephalus cortortus and A. latus come, lies in the davisi, andeach increases in size in more posterior Bigotinid Province (which also contains redlichiids). segments. In some cases the posterior spine is longer Generally speaking, there are few or no genera of trilo than the articulated portion of the same segment. For bites ir common between these provinces; if A? davisi example, the posterior pleural spine of the posterior proves to be correctly assigned to Alacephalus it consti segment of Diacanthaspis (Diacanthaspis) hollandi tutes an exception. However, trilobit~s which occupied Chatterton & Perry (1983, text-fig. 36) is more than outer-shelf and slope habitats are those most likely to twice as long as its articulated portion. Both these char cross provincial boundaries (e.g. Fortey & Owens, acters, i.e. the progressive increase in size of spines 1978; Cocks & Fortey, 1990), and it is postulated that posteriorly and the consequent greater relative length of A? davisi is an example of such a trilobite. the (posterior) pleural spine compared to the articu lated portion of its pleura, are particularly well dis Acknowledgements. Dr A. K. Higgins kindly made the mate played by the widespread Odontopleura ovata Emm rial available for study. Dr J. S. Peel arranged the loan, pro rich, 1839 (see Prantl & Pfibyl, 1949, p. 138, pI. 1, figs vided Fig. land, together with Professor H. B. Whittington, Dr J. A. Zalasiewicz and Dr N. J. Riley, made helpful sug 1-4, pI. 7, fig. 1; Whittington, 1956, p. 197, fig. 4). This gestions on the manuscript. Chris Lane provided bibliographic species, originally described from the Late Wenlock of help. A. W. A. R. publishes with the permission of the Di Czechoslovakia, also occurs in the murchisoni biozone rector, British Geological Survey (N.E.R.C.). (Sheinwoodian, Early Wenlock) of North Wales, and the Wenlock to Early Ludlow of Silesia, Poland. In other forms, twin pleural spines occur on the posterior segment only; Ceratocephala barrandii (see Thomas, 1981, p. 95, pI. 25, fig. 2b; text-fig. 10) is one such. References Bergstr6m, J. & Peel, J. S. 1988: Lower Cambrian trace fossils Functional morphology from northern Greenland. Rapp. Grønlands geol. Unders. 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