New and unusual upper Llandovery graptolites from Arctic Canada Author Lenz And Anna Kozłowska, Alfred C. text Acta Palaeontologica Polonica 2007 2007-12-31 52 3 489 502 journal article 10.5281/zenodo.13741530 1732-2421 13741530 Genus Mirorgraptus nov. lateral Derivation of the name : From Latin miror , “wonder at”, “be astonished apertural at”; relating to the most unusual morphology, unlike any known retiorod litid or other graptolite. apertural Type species : Mirorgraptus arcticus gen. et sp. nov. Species included : Mirorgraptus arcticus new species . Diagnosis .—Straight nema with some lists similar to con− Fig. 7. Giganteograptus giganteus ( Bouček and Münch, 1944 ) , labelling necting rods in retiolitids arising from more or less the same relevant parts of the skeletal morphology. row along one side of nema and, alternately, curving abruptly left and right, diverging about 180 ° to each other, between well developed, proximolaterally projecting paired spines which are loops arising from near base of each connecting ( Fig. 6A ). Nema delicate, attached to thecal framework by rod, oriented 90 ° . Distally, connecting rods bifurcate and connecting rods, possibly free in distal region, extending be− sometimes trifurcate. Connecting rods and vertically oriyond rhabdosome. Transverse rods well developed, close to ented loops apparently fully covered by fusellar banding nema ( Fig. 6A 2 ). Pustulose micro−ornamentation on all lists, originally. well developed facing outwards ( Fig. 6B 2 ). Discussion .—The lack of any vestige of the proximal region Discussion .—The thin lists forming a very coarse and of the rhabdosome, especially of the ancora umbrella, makes it crudely polygonal meshwork of three or four rows of meshes impossible to distinguish proximal and distal ends; furtherare a characteristic of the species. The isolated study material more, their absence makes it impossible to assign this taxon to is somewhat narrower than the flattened specimens illus− any specific graptolite group with confidence. On the other trated in Fig. 2D, F ( 5 mm versus 6–7 mm ); nevertheless, the hand, the presence of what is, almost certainly, a “free−standspecies assignment is considered firm, since some of the ing” nema, and scattered fusellar tissue and fusellar list seams greater width of the supplementary material is likely the re− ( Fig. 8A 2 , B 2 ), suggest that the taxon may be related to a sult of flattening. retiolitid, albeit a most unusual one. The petalolithids are the The possession of a pustulose micro−ornamentation is, only other graptolite group showing fusellar tissue and list however, entirely unexpected, since the typical Pseudopleg− seams in the ancora region, but apart from the narrow focus matograptus , like other members of the subfamily Retio− studies of Bates and Kirk (1997) and Melchin (1999) , that litinae, possesses a smooth or parallel striae micro−ornamenta− group has received no broad−based SEM studies. tion (see Fig. 1B 2 ). The presence of a pustulose micro−orna− A somewhat similar type of list development is known in mentation and well−developed transverse rods are characteris− the rhabdosome of some Ordovician Archaeoretiolitidae; for tic of the Llandovery and Wenlock genus Paraplectograptus example some specimens of Orthoretiolites hami Whitting− Bouček and Münch in Příbyl, 1948a with similarly developed ton 1954 (see Bates and Kirk 1991: particularly fig. 20a) + Fig. 6. A , B . SEM micrographs of Giganteograptus giganteus ( Bouček and Münch, 1944 ) , generalized Cyrtograptus lapworthi – insectus Biozone , uppermost Telychian, Baillie Hamilton Island, Arctic Canada. A . Stereopairs of fragment of rhabdosome, GSC38906; A 1 , ventro−lateral view showing nema attached proximally, possibly free in distal−most region; A 2 , lateral view; A 3 , disto−lateral view showing thecal spines. B . Fragment of rhabdosome with three thecae, showing thecal spines and well−preserved transverse rods, GSC38907; B 1 , stereopair of lateral view; B 2 , lists with well−developed pustulose micro−ornamentation, and seams inside; B 3 , enlargement of fragment of rhabdosome with nema. C . Stereopairs of fragment of rhabdosome of Oktavites spiralis ( Geinitz, 1852 ) with five thecae, GSC38908, upper Telychian O. spiralis Biozone ; C 1 , dorso−lateral view of entire fragment; C 2 , enlargement showing thecal aperture and well−developed paired spines. D . Stereopair of fragment of Cyrtograptus sp. , showing thecal cladium with two thecae, GSC38909, upper Telychian Oktavites spiralis Biozone. which beautifully illustrate a rhabdosome developed only along one side of the nema. Stratigraphic and geographic range .—Cornwallis Island, Arctic Canada ; Spirograptus guerichi Biozone , lower Telychian, upper Llandovery.