Graptolithoidea Lapworth, 1875

Mierzejewski, Piotr & Kulicki, Cyprian, 2002, Upward curvature of the upper critical field in the Boson- - Fermion model, Acta Palaeontologica Polonica 47 (1), pp. 169-175 : 171-173

Class Graptolithoidea Lapworth, 1875

(in Hopkinson and Lapworth 1875)

sensu Beklemishev, 1951

Order Rhabdopleuroidea Beklemishev, 1951 Family unknown

Genus Diplohydra Kozłowski, 1959

Type species: Diplohydra longithecata Kozłowski, 1959 .

Emended diagnosis.—Stolon system devoid of diaphragms, peduncular stolons and true capsules of the dormant buds. Major stolon with irregularly arranged lateral offshoots. Lateral offshoots form diads composed of the two daughter stolons. As a rule, one of the daughter stolons is strongly inflated and, sometimes, forms an imperfect composite cyst.

Species assigned.— Diplohydra longithecata Kozłowski, 1959 , D. micropedunculata Kozłowski, 1959 , D. solida Kozłowski, 1959 , D. gonothecata Kozłowski, 1959 , D. szaniawskii sp. nov.

Remarks.—The genus Diplohydra was erected by Kozłowski (1959) on the basis of chemically isolated material from Ordovician erratic boulders of Baltic origin, occurring in the glacial (Pleistocene) deposits of central and northern Poland. Originally, it was interpreted as a representative of the hydrozoan coelenterate order Hydroida . Later, Kozłowski’s species of Diplohydra , with the exception for D. gonothecata , were recorded from the Ontikan Limestones (lower Ordovician) of Öland ( Sweden) by Skevington (1965). Mierzejewski (1986) questioned both the hydroid nature of Diplohydra and congeneric character of the four species. All material described here agrees with the original description of Diplohydra , as stated by Kozłowski (1959: 240): “Les espèces groupées dans ce genre sont caractérisées par la présence, le long des rameaux, des thèques de deux catégories, groupées par paires et communiquant entre elles de différente manière.” At present, it is clear that the alleged thecae of two categories should be reinterpreted as stolons, some of which are inflated and similar to capsules of the dormant buds. It is of interest to note that Kozłowski (1959: fig. 20B 2) discovered a “glomérule chitineux” in the “gonotheca” of D. gonothecata which strikingly resembles clusters of minute spherules observed in cysts of Rhabdopleurida and Camaroidea ( Mierzejewski 1986, 2000), as well as in graptoblasts ( Crowther et al. 1987). According to Mierzejewski (1986, 2000), this material appears to correspond to yolk material known from the dormant bud capsules in living Rhabdopleura .

The large stratigraphic gap between the Ordovician species of Diplohydra and Permian D. szaniawskii sp. nov. might argue against congenerity of these forms. However, the Rhabdopleuroidea are undoubtedly a very conservative group. For example, the extant genus Rhabdopleura is known from the Jurassic ( Kulicki 1969, 1971) and some fossils disputably attributed to this taxon were described from the Middle Cambrian ( Durman and Sennikov 1993), Silurian ( Mierzejewski 1977; Rickards et al. 1984) and Carboniferous ( Mortelmans 1955).

The fragmentary condition of our material does not permit a closer determination of the systematic position of this form. The family membership is unknown since it differs widely from other rhabdopleurids. Moreover, it is hardly possible to determine the nature of its resemblance to Rhabopleurites and Stolonodendrum . In our opinion, it seems reasonable that Diplohydra represents an unknown evolutionary line of the order Rhabdopleuroidea . A fact of great interest is that stolonal diads do show a certain degree of dimorphism, a feature unknown in other fossil and living rhabdopleurids. Unfortunately, the interpretation of this phenomenon remains obscure.

Distribution.— Poland and Sweden, Ordovician; Norway (Barents shelf), Upper Permian.