Pileograptus, Lenz And Anna Kozłowska, 2007

Lenz And Anna Kozłowska, Alfred C., 2007, New and unusual upper Llandovery graptolites from Arctic Canada, Acta Palaeontologica Polonica 52 (3), pp. 489-502 : 494-496

Genus Pileograptus nov.

Type and only species: Pileograptus pileatus sp. nov.

Derivation of the name: From Latin pileus, “cap“, “capped”; referring to the prominent broad, cap−like thecal hoods.

Stratigraphic and geographic range.—Found in the upper Telychian, although it is unclear whether from the Oktavites spiralis Biozone or the uppermost Telychian Cyrtograptus lapworthi – C. insectus Biozone , Devon Island, Arctic Canada. Diagnosis.— Ancora umbrella deep, bowl−shaped, composed of uniform, polygonal meshes; prosicula fully preserved; zig−zag forming along one lateral wall of ancora sleeve, transverse rods close to nema, lateral aperture lists moderately curved; thecal aperture capped by prominent, broad meshwork genicular hood that is anchored to lateral apertural lists of succeeding theca; ancora sleeve a fairly dense, polygonal meshwork; stomata chimney−like with meshwork walls; list micro−ornament of weak, parallel striae.

Discussion.— Except for its greater depth, the ancora umbrella of Pileograptus gen. nov. is typical of that of Stomatograptus Tullberg, 1883 in its bowl−shape and possession of a distinctly polygonal and uniform−size meshwork (see Bates et al. 2005: fig. 6d); also typical of Stomatograptus , the immediate post−ancora umbrella part of the rhabdosome is noticeably narrower, imparting a “pinched” profile look, particularly on one side (compare Fig. 3A 1, B 2, B 3 with Fig. 4A 1, A 2). The presence of a complete prosicula is also characteristic (as e.g., illustrated in S. canadensis in Fig. 4A 2) of both genera.

The chief difference between Stomatograptus and Pileograptus gen. nov. rests in the nature of the thecal orifice and the thecal hoods. In the more common species of Stomatograptus such as S. grandis grandis ( Suess, 1851) , S. grandis imperfectus Bouček and Münch, 1944 ; S. grandis maior Bouček and Münch, 1944 ; S. canadensis Lenz, 1988 ; and S.? sinensis Wang, 1965, the thecal profiles are more clearly orthograptid, and none possesses thecal hoods. Furthermore, in these same species, the stomata on lateral walls of ancora sleeves are all fairly similar in possessing thickened rims and in being less, or not at all, elevated above the surrounding ancora sleeve.

There are also similarities with Retiolites Barrande, 1850 in the general arrangement of the rhabdosome, but that genus differs in its much shallower ancora umbrella, absence of a “pinched” profile in immediate post−ancora umbrella region, and straight lateral apertural rods. Thecal hoods are sometimes present in the more proximal region of some species of Retiolites , particularly on mature specimens (e.g., Bates and Kirk 1997, figs. 81a, f–j, 82), but these are merely ventral protrusions of the distal rim of the thecal orifice, rather than being distinct hoods that are attached to the proximal side of succeeding thecal lateral rods as in Pileograptus gen. nov.

Comparisons can also be made with Pseudoplegmatograptus Příbyl, 1948b and Dabashanograptus Ge, 1990 , both of which also retain the complete prosicula (and occasionally part of the metasicula). From the former, Pileograptus gen. nov. differs in its much deeper ancora umbrella, laterally projecting thecal orifices and, particularly, prominent thecal hoods. Dabashanograptus Ge, 1990 differs markedly from Pileograptus in possessing an extremely simple and open ancora region, somewhat similar to that of the much younger Plectograptus ; but on the other hand, it appears to possess some type of ventrally projecting and, possibly, meshwork thecal processes (see Bates et al. 2005: fig. 7b). Unfortunately, because of limited preservation, further comparison is impossible.

Figures

Fig. 4. SEM micrographs of Stomatograptus canadensis Lenz, 1988. A. Immature specimen with five pairs of thecae, with well−preserved, shallow ancora umbrella, GSC38916, Cape Phillips, Cornwallis Island, Nunavut, Canada, probably Cyrtograptus centrifugus–C. murchisoni Biozone; A1, stereopair enlargement showing sicula and ancora umbrella; A2, stereopair of obverse view of rhabdosome; A3, proximal view of the ancora umbrella, showing opening of prosicula. B. Rhabdosome with ten pairs of thecae and nematularium, GSC38917, Cape Phillips, Cornwallis Island, Nunavut, Canada, probably Cyrtograptus centrifugus–C. murchisoni Biozone; B1, enlargement of distal part of rhabdosome with nematularium, B2, latero−ventral view of rhabdosome.