A new genus of nevadiid trilobite from the Buen Formation (Early Cambrian) af Peary Land, central North Greenland

a

lower sandstone unit deposited under high energy, inshore conditions succeeded by an upper, mainly mudstone unit, deposited in a lower energy, outer shelf environment (Jepsen, 1971;Peel, 1982;Higgins et al., in press). Occurring with the non-skeletised fauna in the deeper water mudstones is anevadiid trilobite which is described below.
Derivation of name. From the Buen Formation.
Diagnosis. Nevadiid with elongate glabella tapering gently forward, not reaching border. Wide (sag.) occipital ring. Palpebral lobes of moderate length (exsag.), narrow (tr.); widening as approaching ghibella. Wire-like intergenal and metagenal ridges meet at posterior border at position of intergenal spine. Short, stout genal spines, not advanced. The border and border furrow are weakly defined anteriorly and antero-laterally. Thorax and pygidium of 19 segments of which the pygidium usually forms only the posterior-most segment. Thorax tapers rapidly, posteriorly of the eighth segment. Pleurae sigmoidally curved with short deep furrow. Axial rings with medial node. Pygidium formed of single plate.
Diseussion. In the past 15 years several revisions of olenellid c1assification have been made. Bergstr6m (1973) recognized three families and placed Nevadia Walcott, 1910, Nevadella Raw, 1936, Callavia Matthew, 1897and Kjerulfia Kiaer, 1917 in the Daguinaspididae Hupe, 1953, andHolmia Matthew, 1890 in the Holmiidae Hupe, 1953. Subsequently Repina (1979 split the Daguinaspididae (sensu Bergstr6m) into four families. She placed Nevadia and Nevadella in the Nevadiidae Hupe, 1953, and it is to this family that I tentativelyassign the new genus Buenellus. Repina assigned Holmia, Callavia and Kjerulfia to the Holmiidae Hupe, 1953. Ahlberg et al. (1986) largely accepted Repina's dassification, but it was their opinion that Callavia should be assigned to their new family Callaviidae. The material from the Buen Formation resembles Nevadella Raw, 1936. This genus, however, has much longer (exsag.) and wider (tr.) ocular lobes, that extend back to the level of the occipital ring, and has genal spines that extend back beyond this level. In addition Nevadella lacks the intergenal and metagenal ridges, as well as intergenal spines. The thorax of Nevadella is known to have from 18 to 23 segments; it lacks macropleural segments, and the pygidium is small. In these features Buenellus is similar to Nevadella. Nevadia Walcott, 1910 has very wide (tr.) extraocular cheeks, a strongly furrowed glabella and lacks metagenal ridges. The ocular lobes are of similar length but are much wider (tr.) than in Buenellus, with intergenal ridges extending from their posterior tip. The thorax is considerably different from that of Buenellus having 28 segments, with the pleurae extending into long curved spines. The pygidia, however, are similar. The development of an intergenal spine and ridge are characteristic of species assigned to Callavia Matthew, 1897. For Callavia species, however, the position of this spine is different from that in Buenellus, being very dose to the genal angle, and no metagenal ridge is developed. Additionally the ocular lobes are more strongly crescentic, and of greater relative length in Callavia. In the type species C. broggeri (Walcott, 1888) the occipital ring bears a long, backwards directed, slender spine. an all species assigned to the genus the glabella has four or five welldeveloped pairs of furrows. The thorax and pygidium are similar to those of the new North Greenland material with up to 18 segments in the thorax, each with short, stout, posterolaterally-directed spines, a pleural furrow that extends over the adaxial half of the pleura and axial rings with a short median node. The pygidium is forrned of a single plate with a short median spine on the posterior margin.
Buenellus shows some similarities to Holmia Matthew, 1890 in that for both genera the cephalic border is poorly-defined in front of the glabella and metagenal and intergenal ridges are present. In general, like Buenellus, species of Holmia do not have macropleural thoracic segments, and the distance from the axis to the pleural fulcrum is short. The glabella in Holmia, however, is strongly furrowed, and the expanded anterior lobe extends to the anterior border. The palpebral lobes of Holmia species are considerably wider (tr.), and often marked by an epipalpebral furrow. In the type species, H. kjerulfi (Linnarsson, 1871), the thorax is formed of 16 segments which taper regularly backwards, and have welldeveloped axial spines posterior to the tenth segment. The pygidium of Holmia is unlike that of Buenellus, being forrned of at least two segments and a terminal piece. Both intergenal and metagenal ridges are developed in species of Kjerulfia Kiaer, 1917; these converge on the posterior border at the position of an intergenal spine or node. The taxonomic importance of intergenal and metagenal ridges is uncertain for they are present in genera assigned to different families, for example Fallotaspis Hupe, 1953 and Kjerulfia. Kjerulfia differs from Buenellus in having much wider (tr.) ocular lobes, and a strongly-furrowed glabella. In addition, the cephalic border is wider. The thorax of Kjerulfia has up to 17 segments with distinct medial axial nodes that increase in 1ength posteriorly from the twelth segment, with the posterior two axial rings possessing long, slender spines. The pygidium is comparable in form to Buenellus. Other material. GGU 319571.1-10 (damaged complete specimens), GGU 319571.11-13 (partial cepha-Ion and thorax), GGU 319571.14-16 (partial thorax-pygidium).

Diagnosis. As for genus.
Description. Cephalon semicircular in outline. Elongate glabella tapers gently forward with rounded anterior; it is of low convexity transversely, whilst sagittally the posterior threequarters are horizontal on average with the anterior quarter sloping gently downward to the preglabellar fieid. Occipital ring wide (sag.), horizontal and slightly below levelof rest of glabella; sagittallength approximately one-quarter that of glabella. SO moderately concave posteriorly, being of medium depth distally, shaIIowing over sagittalline. Posterior margin of occipital ring has similar curvature to SO. No apparent occipital spine or node. Up to three pairs of glabellar furrows are developed, aIthough On most specimens it is only the posterior pair that are visible. S1 of moderate length, directed moderately backwards, not connected across glabella but joined to axial furrow. S2 shorter than S1, directed backwards at a similar angle and of about equal depth. S3 only observed On one specimen ( fig. 3B), a shallow expanded furrow shorter than S2. Exsagittal length of lateral glabellar lobes deereases forwards. Glabella defined by narrow (tr.) furrow which is weakly to moderately impressed. Glabella anteriorly separated from border by narrow (sag.) preglabellar field with a sagittal length of about one-fifth that of glabella. Transverse width of intraocular areas between three-eighths and two-fifths of basal glabellar width. Intraocular areas slope gently outward. Palpebral lobes of moderate length (exsag.), narrow (tr.), gently arcuate and centered slightly forward of SI. Palpebrallobes are defined by shallow and narrow (tr.) furrow, or by  Measurements. l, sagittallength; 2, sagittal cephalic length; 3, sagittal thoracic length; 4, sagittal pygidiallength; 5, maximum transverse width of cephalon; 6, basal glabellar width; 7. sagittal glabellar length. All measurements are in millimetres . From the same position on the border a low straight mctagcnal ridge is directed forward at an angle of about 60 degrees to an exsagittal line to the glabella, which ir joins slightly anterior!y of SO. The hvo ridges meel at the position af the intergenal spille which is marked by a slight swelling of the border. Extraocular cbeeks genll)' t:onvcx (tr.), sloping moderate!y ollrwards. Anterior margin of cephalon evenly curved and eontinuous along sllort, slout, postero-tatcraily·direeted genal spine. Posterior border directed gcntly postero·laterally from axis until position of intergena! spine, then slightly antero 4 laterally 10 hase af genal spine. \Veakly defined border af moderate width and border furrow are even less well dcfined anteriorly and antero-lateraIJy. Thorax usually formed af 18 segments (but see discussion below) with no macroplcural segments. Posteriorty af the cighth segment the thorax tapers rapidt y; anteriorly over with axis dcfincd by narrow (tr.) funo\\'. Poslerier margins afaxial rings variable from stnlight transvcrsc to gcntly concave postcriorly. Lateral margins afaxial rings moJcrately curvcd oUlwarcls. Rings have a median Ilode, aften c10ngatccl alnng the sagittal line. Tramjverse wiclth uf ring approximatcly one-third width ef corrcsponding segment. Plcurae geni/y sigmoidal in plan vicw; adaxi,tl half dirCClcd genfly pos[cro-Iaterally. abaxial hal[ inilially antero-Iatcrally then extcnded into postcro-Ialerally-direetcd broad. stout spine. Each pleura marked by wide (exsag.) furrow uf medium depth that extends over adi;lxial half. The anterior ridge extends into aspine on some specimens, whilst the posterior ridge ends at the fulcrum. The more posterior segments have a relatively shorter pleural furrow being only one-third the pleurallength. Pleurae become directed more strongly posteriorly from the tenth segment backwards. Pleurae very slightly convex (tr.), sloping outwards from axial furrow.
Pygidium usually forrned of a single plate that is subtriangular in outline, gently convex (tr. and sag.) and sloping moderately backwards. A transverse furrow that is concave posteriorly and of medium depth, crosses the pygidium.
Sculpture is of fine granules covering an parts of the exoskeleton. This is particularly well developed on a deformed specimen of an incomplete cephalon and thorax ( fig. 6), although most specimens do not show any preserved sculpture.
Discussion. Within the small population available for study some variability of morphology has been observed. Specimens are flattened, and as the amount of distortion can not be determined, the relative convexities of the exoskeleton are not necessarily those of undeformed material. As mentioned in the description, above, there is variation in the number of pairs of glabellar furrows between one and three, which is presumably mainlya preservational effect. Additionally the form of these furrows is not consistant, being diffuse and shallow in some specimens and more slot-Iike in others. an the axial rings a convex, transverse marking is often apparent; I have interpreted this marking as the impression of the underlying articulating half-ring, which is visible due to compression of the specimen. As described above, the pygidium is usually composed of a single subtriangular plate bearing a transverse furrow. an at least two specimens, however, (one of which is illustrated in fig.  4B) it appears that the final thoracic segment is still retained in the pygidium, despite the considerable sagittallength of the specimens. For these specimens the pygidium is forrned of the unreleased segment bearing short, unfurrowed spines that are directed strongly backwards, and the terminal plate. The plate is separated from the unreleased segment by an incomplete furrow that is represented distally by moderately deep pits. It is noted that these specimens with only 17 segments in the thorax, have sagittal lengths greater than those of several specimens with 18; the condition therefore is not merelyafeature associated with late meraspids, but rather with variation within the holaspid.
The new form affords no new biostratigraphic information; as discussed by Palmer & Peel (1979, p. 33) the Buen Formation is considered as being of middle to late Early Cambrian age, with the lower part of the overlying Brønlund Fjord Group yielding late Early Cambrian faunas (Palmer & Peel, 1979;Peel, 1982;Blaker, 1986). In North America this would correlate with the upper part of the Nevadella Zone or the lower part of the overlying Bonnia-Olenellus Zone.