The trilobite Mucronaspis in the uppermost Ordovician of the Oslo Region, Norway

ALAN W.OWEN

Owen, A. W.: The trilobite Mucronaspis in the uppermost Ordovician of the Oslo Region, Norway.

Norsk Geologisk Tidsskrift, Vol. 61, pp. 271-279. Oslo 1981. ISSN 0029-196X.

Mucronaspis mucronata (Brongniart) and allied taxa are common uppermost Ordovician trilobites

throughout the world. A lectotype for M. mucronata mucronata is chosen and illustrated, and the

Norwegian subspecies, M. mucronata kiaeri (Troedsson), redescribed. Limited evidence suggests that

dimorphism in glabellar proportions and palpebrallobe length is developed in the larger specimens and

may indicate that sexual maturity is reached only in the later part of the holaspis phase.

A. W. Owen, Department ofGeo/ogy, The University, Dundee DD14HN, Scot/and.

Mucronaspis and the very closely allied Dalma­

nitina are characteristic elements of late Ordovi- ·

cian shelly faunas in many parts of the world. The

palaeoecological and palaeogeographical signifi­cance of this has been discussed by several recent workers including Lesperance (1974), Cocks & Price (1975), Lesperance & Sheehan (1976) and Jaanusson (1979). These authors have also discussed the relationship of Mucronaspis/ Dalmanitina and associated trilobites to the Hir­

nantia brachiopod association. A similar study in the Oslo-Asker district is being completed by P. J. Brenchley and his colleagues. An unnamed new genus closely related to Mucronaspis is now known as part of a dominantly graptolitic biofa­cies in the uppermost Ordovician of Ireland and

Scotland (Siveter et al. 1980). Many of the Ashgill occurrences of Mucronas­

pis/Dalmanitina have been ascribed to M. muc­ronata (Brongniart, 1822), a species based ori­ginally on material from the 'Dalmanitina' Beds in Vastergotland, Sweden. This species was re­

vised in detail by Temple (1952), who reported that the original specimens were missing. He refrained from choosing a neotype, and the syntypes have since been recovered and were illustrated by Reyment (1976, Fig. 9a) in a cata­logue of types in the Palaeontological Museum at Uppsala University. The se specimens are refi­gured here (Fig. l) and the cephalon UM Vg747 selected as lectotype. The pygidium (UM Vg750) is given paralectotype status.

Norwegian material of Mucronaspis has been

known for over a century but has only been illustrated in a descriptive work by Troedsson

(1918), who ascribed most specimens to 'Dalma-

nites' mucronatus but also proposed a new spe­eies 'D.' kiaeri for specimens with an unusually narrow glabella. All existing museum material and new collections have been examined in the present study, which also includes some consid­eration of holaspid morphogenesis. The speci­mens are housed in the Paleontologisk Museum, Oslo (PMO) and Hunterian Museum, Glasgow.

Terminology and techniques. - Most of the morphological terminology used here is that advocated in Moore (1959), except that the

glabella is taken to include the occipital ring (following Whittington l%2, p. 3) and the terms rachis ( = axis) and dorsal furrow ( = axial fur­

row) are preferred (Jaanusson 1956, p. 36-37). In addition, the term 'field' is used for the cheek areas between the borders (Evitt & Tripp 1977)

and the glabellar Iobes and furrows are num­bered LI, Sl etc. forwards from the occipital furrow.

Specimens were measured under a binocular microscope using a scale graduated in O.l mm intervals or, in the case of large specimens, using vernier calipers. The single orientation method for measuring specimens advocated by Temple

(1975) was adopted. A complete list of speci­mens and measurements is included with the

figured material in the Paleontologisk Museum, Oslo. Specimens for photography were coated with dilute opaque and then whitened with ammonium chloride sublimate. A photographic system similar to that described by Whittington (1956) was used.

Family Dalmanitidae Vogdes, 1890. Subfamily Dalmanitininae Destombes, 1972.

272 A. W. Owen NORSK GEOLOGISK TIDSSKRIFf 3-4 (1981)

B

Fig. l. Mucronaspis mucronata mucronata (Brongniart), 'Dalmanitina' Beds, Vastergotland, Sweden. Originals of Wahlenberg (1818) A. lectotype cephalon (UM Vg747), x 1. 5. B. Paralectotype pygidium (UM Vg7 50), x 1. 5. Photographs kindly supplied by

Prof. R. A. Reyment.

Genus Mucronaspis Destombes, 1963. Types species.- By original designation, Dalma­nitina (Mucronaspis) termieri Destombes, 1963 from the Upper Ktaoua Formation (Ashgill) in the Anti-Atlas region, Morocco. Discussion. - Mucronaspis was first described as a subgenus of Dalmanitina Reed, 1905 by Destombes (1963), who later elevated it to full generic status (1972, p. 5(}....51). Campbell (1973) argued that Destombes' original diagnosis of D. (Mucronaspis) was only fully satisfactory for distinguishing its type species from that of D. (Dalmanitina) and that there was only limited justification for maintaining the two taxa. Ing­ham (1977, p. 111) gave an extensive review of the problems involved and concluded that Muc­ronaspis should be considered as a valid genus with individual species being distinguished from those of Dalmanitina by a combination of all or most of the following morphological features: a less convex cephalon, long genal spines com­monly out of alignment with the lateral border, the posterior branch of the facial suture cutting the lateral margin of the cephalon at about the leve! of the posterior border furrow. large eyes, a less swollen glabella, Sl and S2 subparallel (cf. distinctly convergent abaxially) with S2 only well incised proximally, S3 less divergent fron­tall y, the posterior margin of the hypostoma denticulate, thoracic pleurae extended as long points, pleural ribs on the pygidium more curved and more rearwardly directed posteriorly. In

addition, Mucronaspis commonly has a distinct preglabellar area and poorly incised lateral cephalic border furrows. Ingham's approach is followed provisionally here, although a numeric­al taxonomic study similar to that recently used by Temple & Tripp (1979) for the encrinurids may well throw further light on the matter.

Mucronaspis mucronata (Brongniart) kiaeri (Troedsson, 1918) f,ig. 3 A-R.

O 1865 Phacops mucronata Brongn. - Kjerulf: 20. O 1887 Dalmanites mucronata Ang. - Brøgger: 33. O 1897 Dalmanites mucronarus Ang. - Kiær: 33, 74. O 1897 Dalmanites mucro­natus Brong. - Kiær: 76. O 1902 Dalmanites mucronatus Brong. - Kiær: 79. O 1918 Dalmanites mucronatus Brongniart - Troedsson: 71, 97 (pars- Norwegian material only). O 1918 Dalmanites kiaeri- Troedsson: 71-72, 97-98, Pl. 2, Figs. 23a, 23b, O 1934 Dalmanites mucronata - Strømer: 330. O 1981 Mucronaspis kiaeri (Troedsson)- Owen: Table l. O 1981 M. sp. - Owen: Table l.

Lectotype (here selected). - A complete indi­vidual (PM066296, counterpart numbered 66297). The specimen is from Lindøya, Oslo but there is some doubt as to its precise strati­graphical horizon. Troedsson (1918, p. 72) gave it as '5b' (=Langøyene Fm.) as does one of a number of labels with the specimen, all written at !east a decade after the specimen was col­lected (1891). Most of the other labels, including one indicating the type status of the specimen, gi ve the horizon as '5a' ( = Husbergøya Fm.).

NORSK GEOLOGISK TIDSSKRIFT 3-4 (1981)

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The trilobite Mucronaspis 273

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Fig. 2. Scatter plots for various dimensions in M. mucronata kiaeri. All measurements are in mm. 'x' = 'narrow' glabella morph, ' .' = 'broad' glabella morph. All specimens from the uppermost Ordovician of the Oslo-Asker district.

Occurrence. - The subspecies is common around Oslo in the uppermost few metres of the Husbergøya Fm. and the lowest few metres of the overlying Langøyene Fm. (for stratigraphy see Brenchley & Newall 1975). In Bærum and Asker it is less common, but is found at similar stratigraphical levels which pass westwards into the Langåra fm. The subspecies is not known outside the Oslo-Asker district.

Diagnosis. - Cephalon with well developed anterior border, long genal spines and large eyes. Sl and S2 only very slightly convergent, the latter dying out abaxially. Two morphs based on glabellar length to width proportions are de­veloped amongst the larger specimens. Thoracic pleural tips extended as spines. Pygidium with a distinct border, 9-13 rachial rings and 7-8 (rarely 9) pleural ribs.

Description.- Cephalon gently convex (tr.) with a sagittal length which varies from 50-70% of the width at the posterior borders. This variation does not seem to be related to overall size. Analysis of the outline of the glabella suggests that two morphs may be present. A plot of

18 - Norsk Geologisk Tidsskrift �81

preoccipital length against maximum width (Fig. 2A) indicates that individuals with a pro­portionally narrower glabella are developed amongst the larger specimens. This group in­cludes the holotype and its implications are discussed below. There is no evidence of this dimorphism in the degree of glabellar expansion compared with its posterior width (Fig. 2B) or the proportion of the glabellar length occupied by the frontal lobe (Fig. 2C). All these plots indicate virtually isometric growth and there is no clustering into distinct instars. Glabella ex­pands forwards to the mid parts of L3 in front of which it is slightly constricted before increasing to its maximum width at the frontal lobe. The width at Ll is equal to 56-72% (mean 60%, standard deviation 4, n = 12) of the maximum glabellar width. Occipital ring tapering very gently abaxially, bearing a weak median swelling in its posterior half. Occipital furrow shallow and transversely directed mesially, deepening and directly rearwards abaxially. Sl transversely directed, S2 arched very slightly forwards such that L l ta pers a little abaxially, deeply in ei sed mesially, dying out abaxially. L3 very slightly sinuous, diverging forwards at 140-150°. Frontal

274 A. W. Owen

lobe oval in the 'narrow' morph to lenticular in the 'broad' morph, having a sagittal length equal respectively to 76-83% (n = 4) and 45-68% (n = 13) of the maximum width. Of the 6 broad specimens in the same size range as the narrow ones, this last percentage ratio is 55-67%. In both morphs, the frontal lo be occupies 53-65% of the preoccipital sagittal glabellar length (mean 60%, standard deviation 3, n = 16). The smallest speCimens have a (,listinct preglabellar furrow and a narrow (sag., exsag.) portion of the anter­ior border on the cranidium. In larger specimens, irrespective of morph, this furrow is only very weakly incised laterally and dies out adaxially, where the anterior edge of the gla beila is defined by the facial suture. External surface of glabella bears a dense granulation which becomes finer with increasing absolute size and a median notch on the posterior part of the frontal lobe. This notch becomes deeper on internat moulds and on larger specimens. The frontal lobe on internat moulds also bears a broadly triangular pattern of small swellings similar to those described by Eldredge (1971) in other dalmanitids. These swellings are only very weakly developed on the external surface; internat moulds are otherwise smooth. Dorsal furrows weakly incised but dis­tinet. Posterior border furrows deep and trans­versely directed over most of their length, but curving gently rearwards and shallowing distally where they meet the poorly incised lateral bor­der furrows. Posterior borders expand abaxially (exsag.). Palpebral lobes gently inclined from the field of the fixed cheeks; the anterior en ds abutting the dorsal furrows immediately behind the anterior ends of L3, the posterior ends terminating a short distance away from the glabella. The distance between the ends of each palpebral lobe increases only slightly with in­creasing cranidial size (Fig. 2D). This relative shortening is reflected in the distance between the palpebral lobe and the posterior border fur­row (Fig. 2E), the lobes extending rearwards to opposite the mid-parts of LI in small specimens but only to opposite Sl or even the posterior part of L2 in larger individuals. Such c hang es are well documented in Mucronaspis (e.g. Temple 1952, p. 12). Although there are only limited data, there is a suggestion that the palpebral lobes are proportionally longer in the 'narrow' glabella morph (Fig. 2D) than in the broad form. Post­erior branches of facial suture arched strongly forwards but terminating opposite the proximal parts of the posterior border furrows. Anterior

NORSK GEOLOGISK TIDSSKRIFT 3-t (1981)

branches situated along the outer margins of the dorsal furrows and are convergent mesially on the dorsal surface. Almost all the available head­shields are cephala, sugge sting that over the size range present, the facial sutures were largely non-functional. Eyes large, steep sided with 37-?40 files of lenses and a maximum of 8-10 lenses per file. Field of free cheek narrow (tr.). Lateral border broad, defined adaxially by no more than a break in slope in many specimens. Borders unite mesially as a narrow (sag., ex­sag.), flat anterior border. Genal spines long and slender, diverging a little proximally but curving round to become parallel. On the external sur­face, cheek fields bear a dense, shallow pitting over which there is a fine, dense granulation which is also present on the borders and spines. Internat moulds only bear a subdued pitting of the fields.

Hypostoma known from a single, incomplete, specimen. Sagittal length equal to approximately 125% of the width directly behind the anterior wings. Median body extending to anterior mar­gin, moderately swollen, oval in. outline with a maximum width equal to about 70% of the sag­ittal length. Maculae narrow (exsag.) but dis­tinet. Crescentic posterior lobe occupying 25% of the sagittal length of the median body. Ante­rior margin arched gently forwards. Lateral wings tapering strongly abaxially. Lateral bor­ders ridge-like, defined by deep furrows. Posten­or borderflat-lying, defined anteriorly by a broad, shallow furrow. Posterior border on! y partially preserved but there is an indication of a small spine base a short distance out from the sagittal line. Ventral surface of median body bears large, scattered granules. Wings and borders smooth.

Thorax of I l segments, seen only in the holotype. Rachis tapers rearwards at 15°, strong­ly convex (tr.), occupying approximately 30% of the width of each segment. Articulating half­rings equal in length (sag.) to about 50% of each rachial ring. Dorsal furrows weakly impressed but there is a marked break in slope here. Pleurae arched gently upwards, transversely directed proximally, laterally curving rearwards through about 45° in the posterior segments, much less so anteriorly. Distal parts of pleurae extended as spines, those on the first few seg­ments are very short but the remainder are much more elongate. Pleural furrows deep, transverse­ly directed, dying out and curving rearwards a little at the fulcrum. Pleural bands approximate­ly equal in width (exsag.) and bearing a fine,

NORSK GEOLOGISK TIDSSKR!Ff 3-4 (1981)

dense granulation, best seen on the extemal surface. Rachial rings much more sparsely granular.

Pygidium shield-shaped in outline with a sagit­tal length ( excluding mucro) equal to 84-88% (o= 4) of the maximum width. Convex (tr.) rachis has a maximum width equivalent to 30% (o = 2) of that of the pygidium and tapers rear­wards at 20o over most of its length and at 60° over its posterior 15%. 9-11 rachial rings may be developed along with a short terminal piece, or 12-13 rings may be present without a terminal piece. Rachis extended as a stout spine of un­known length which is directed rearwards and very gently upwards. Dorsal furrows shallow and narrow (tr.). Pleural lobes arched: flat-lying proximally but fair! y steeply declined distally. 7-8 pleural ribs present in most (13) specimens but one is known with 9 ribs. Interpleural fur­rows deep and narrow. The anterior ones are directed trånsversely over most of their length, curving rearwards a little beyond the fulcrum. The more posterior ones become almost parallel. Pleural furrows broader than interpleural fur­rows, directed abaxially from just behind the anteromesial corner of each rib to near the posterolateral corner. A distinct but narrow pygidial border is present. The external surface of the pygidium bears a dense, fine granulation (except in the furrows) but internal moulds are smooth.

A pygidium from S. Skjærholmen, Oslo (Fig. 9G) shows an interesting malformation - the coalescence of furrows from a pair of adjacent ribs on one side of the pygidium. This specimen and other incomplete material is !abel­led as being from the unit below the Husbergøya Fm. and was collected on an excursion in 1914. The lithology, however, is that of the uppermost Husbergøya Fm., and this is assumed to be the correct horizon.

Dimorphism. - As is noted above, four of the available cephala of M. mucronata kiaeri have a greater glabellar length to width ratio than others of comparable size. The ·'narrow·

· and 'broad'

forms occur together and are not confined to an y particular part of the short stratigraphical range of the subspecies. The restriction of the 'narrow' morphotype to the larger specimens and the position of these on a single growth curve strong­ly suggest that the forms represent biological entities and do not simply reflect tectonic or diagenetic elongation parallel to the sagittal line.

The trilobite Mucronaspis 275

The limited data on proportional palpebral lobe length differences between the dimorphs (Fig. 2D) also support this view.

The morphogenesis of a closely allied subspe­cies, M. mucronata olini (see discussion below for taxonomic status), is well documented (Troedsson 1918, Temple 1952 a, 1957), and it seems most likely that the entire size range seen in M. mucronata kiaeri Iies within the holaspis phase of development (perhaps extending back into latest metaspis). Thus the dimorphism, de­veloped only in the !argest individuals, is res­tricted to the later part of the holaspis period. Scatter plots based on glabellar measurements of material assigned to 'Dalmanitina' mucronata from the upper Ashgill of Kazakhstan (Apollo­nov in Apollonov et al. 1980, Text-Figs. 20, 21) indicate similar proportions to the 'broad' Norwegian form and no dimorphism.

Polymorphism amongst trilobites is well known (see Campbell 1977: 10-11 for general discussion) and there are several cases ascribed to sexual dimorphism (e.g. Alberti 1971, Clark­son 1%9, Hahn & Hahn 1971, Hu 1971, Jell 1975, Selwood & Burton 1%9, Temple 1975a). Most such studies comprise little or n� informa­tion on morphogenesis and even in those that do, the implication is that dirmorphism is present at !east throughout the holaspis period. The case described here, therefore, differs in suggesting that M. mucronata kiaeri did not reach sexual maturity until well into the holaspis stage. If an analogy with another arthropod group, the ostracods, were to be drawn, the dimorph show­ing the continuation into adulthood of the juve­nile morphology - in this case the 'broad' glabella form - would be the male and the narrow glabella would typify the female. It is not possi­ble to as sess the validity of such an analogy.

Discussion. - Troedsson (1918, p. 71) noted that he bad examined specimens from Norway which previously bad been ascribed to 'Dalmanites' mucronatus, and he argued that white most belonged to this species, those from '5b' (the LangØyene Fm.) of Lindøya and Skjærholmen warranted specific separation. He distinguished his new species, 'D.' kiaeri, from 'D.' mucrona­ta in ha ving a more elongate outline, its narrower glabella and frontal lobe, in the shape of the dorsal furrows, in the size of the eyes, the course of the facial suture and the breadth of the anterior border. In his extensive revision of 'Dalmanitina' mucronata, Temple (1952) tenta-

276 A. W. Owen NORSK GEOLOGISK TIDSSKRIFT 3-4 (1981)

NORSK GEOLOGISK TIDSSKRIFT 3-4 (1981)

tively included the earlier references of Norwe­gian material in his synonymy but excluded without comment Troedsson's species. Kielan, however (1960, p. 119), synomymized 'D.' kiaeri

with 'D.' mucronata.

The lectotype ofTroedsson's species is a 'nar­row' specimen and thus many of his comments relate to its differences from the 'broad' dimorph and possibly to some differences in absolute size. There is very little to distinguish the 'broad' glabellar form (including the small specimens) of the Norwegian taxon from specimens of M.

mucronata mucronata. The only consistent fea­tures distinguishing all the Norwegian material from D. mucronata mucronata are the develop­ment of a broad (sag., exsag.) anterior border and a slightly wider range in the number of pygidial rings and ribs. These differences, together with the development of dimorphism, are not consi­dered sufficient for full specific separation, and thus M. mucronata kiaeri is viewed as a geo­graphical subspecies.

lngham (1977, p. 113) considered that M. olini (Te .... ple, 1952) may prove to be no more than a subspecies of M. mucronata distinguished only by its shorter (exsag.) palpebral lobes and fewer pygidial rachial rings (commonly 9) and pleural ribs (6-7). These differences also differentiate Temple's form from the Norwegian subspecies. M. olini and M. mucronata mucronata are found together at some levels in northem England (lngham 1977, p. 113) and Poland (Kielan 1960,

Fig. 3. Mucronaspis mucronata (Brongniart) kiaeri (Troeds­son). A-D Leetotype. Original of Troedsson (1918), Husber­gøya or Langøyene Fm. , Lindøya, Oslo. A. Dorsal view of east of extemal mould of eephalon (PM066297), 'narrow' morph. x 1. 2. B. Dorsal view of east of external mould of thorax (PM066297), x I.l. C. Dorsal view of intemal mould of pygidium (PM066296), x I. l. D. Oblique la�eral view of inter­nal mould of eomplete individual (PM066296), X 1.2. E. Ventral view of intemal mould of hypostorna (PM080491), HusbergØya Fm., N. W. Hovedøya, Oslo, x2.0. F. Dorsal view of intemal mould of eranidium (PM0100830), 'broad' morph, 1. (}..1.5 above base of LangØyene Fm. , S. W. Hovedøya, Oslo, x 1.2. G. Dorsal view of east of abnormal pygidium (PM011678), probably uppermost part of HusbergØya Fm. , S. Skjærholmen, Oslo, x0.9. H. Dorsal view of intemal mould of eephalon (PM020540) 'broad' morph, Langøyene Fm., Husbergøya, Oslo, x2.8. l, I, Q. Frontal, dorsal and oblique lateral views of intemal mould of incomplete eephalon (PM011059), Langåra Fm., S. W. Langåra, Asker, all x 2.0.

The trilobite Mucronaspis 277

p. 119), but they appear to be mutually exclusive at other horizons and also in the Swedish succes­sions. There has been considerable discussion about the significance of this (Temple 1952, p. 26-28, lngham 1977, p. 113). Temple argued that, under some conditions, variation in M.

mucronata mucronata approached the condition seen in a narrowly defined M. olini. lngham concluded that M. olini was derived originally from an end member morph within M. mucrona­ta. From the published descriptions and illustra­tions, it would seem that although minor, the differences between the two forms are consis­tent, in spite of the wide range of variation seen in M. mucronata mucronata at some horizons. lngham's suggestion of two subspecies with fair­ly subtle environmental controls on their relative distribution seems to be the most acceptable taxonomic approach.

M. mucronata brevispina (Temple, 1952) is a poorly known form based on specimens from basal Silurian strata in northem England (see also Temple 1969, p. 228-229). The only pub­lished illustration of the cranidium is a specimen (Temple 1952, Pl. 5, fig. 6) which is smaller than any known individual of M. mucronata kiaeri

and thus detailed comparison is not possible. Nevertheless, the palpebral lobe appears to be proportionally shorter (exsag.) and more for­wardly placed with its posteromesial part further from the glabella than in the smallest Norwegian specimens. Pygidia appear to be very similar.

K. Dorsal view of partially exfoliated pygidium (PM011904), Husbergøya Fm., Husbergøya, Oslo, x2.0. L. Dorsal view of intemal mould of eephalon (PMOI01542), 'narrow' morph, 1-5 m above base of LangØyene Fm., Gres­sholmen, Oslo, x 1.2. M. Dorsal view of partially exfoliated eephalon (PMOI01541), 'broad' morph, same horizon and locality as L, x 1.0. N. Ventral view of ineomplete pygidium showing muero (PM0103976), lowest part of Langøyene Fm. , S. W. Hove­dØya, Oslo, x 1.5. O. Dorsal view of partially exfoliated pygidium (PM0100885), Langåra Fm., Marsehmanns Brygge, Blakstad, Asker, X 2.5. P. Dorsal view of small eephalon (PM020535), 'broad' morph, 2.6 m above base of Langøyene Fm. , Husbergøya, Oslo, x 3.8. R. Dorsal view of partially exfoliated eephalon (PM0101549), 'broad' morph, 4.3-0.8 m below top of Husbergøya Fm. , RambergØya, Oslo, X2.5.

278 A. W. Owen

As Ingham (1977, p. 112) noted, M. mucronata

mucronata closely resembles the type species, M. termieri (Destombes, 1963). The Norwegian subspecies differs from M.termieri in possessing a broader (sag., exsag.) anterior cephalic border, the glabella expanding forwards more, S2 much shorter (tr.), longer more stender genal spines and commonly more pleural rii:ls (7-9 cf. 6-?7).

Acknowledgements. - This study was largely carried out during tyhe tenure of a NATO postdoctoral fellowship at the Paleontologisk Museum, Oslo and was completed with the assistance of a fieldwork grant from the Carnegie Trust for the Universities of Scotland. I thank Drs. D. L. Bruton and D. A. T. Harper for their helpful comments on a draft of this manuscript. Drs. J. F. Bockelie, P. J. Brenchley and G. Newall kindly placed their specimens of Mucronaspis at my disposal (now PMO).

Revised manuscript received August 1981.

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