Under this name Ettingshausen described the type-specimen of the genus from Lower Lias strata at Steierdorf in Hungary. He assigned the plant to the Coniferae on the ground of a resemblance of the pinnules to the phylloclades of Phyllocladus. Thinnfeldia rhomboidalis bears a close resemblance to T. odontopteroides, but the pinnules are usually longer and narrower, as shown in the English specimen from the Lower Lias of Dorsetshire represented in [fig. 359]. The darker margin of the pinnules shown in [fig. 360], C, gives the impression of a revolute lamina, but a microscopical examination points to a thicker cuticle at the edge of the segments.
Fig. 359. Thinnfeldia rhomboidalis, Ettings. Slightly reduced. From an English Liassic specimen in the British Museum. [M.S.]
The species is recorded from Jurassic rocks of France, Germany, Italy, India, Australia, and elsewhere[1442].
Palaeobotanical literature contains numerous records of Jurassic, Cretaceous and some Tertiary species referred to Thinnfeldia, but many of these are probably not generically identical with T. odontopteroides or T. rhomboidalis. Mr Berry[1443] in a paper on The American species referred to Thinnfeldia concludes that the genus is “a rather indefinite one ... and badly in need of revision.” He regards the Middle and Upper Cretaceous American species as Conifers related to Phyllocladus and probably forming a link between the Podocarpeae and Taxeae: for these forms he proposes the generic name Protophyllocladus. The opinion has been expressed elsewhere[1444] that this “problematical[1445]” genus rests on an unsatisfactory basis; the available data do not justify the use of a name which implies the existence in North American Cretaceous floras of a type related to the New Zealand and Tasmanian Conifer Phyllocladus. We are not in a position to assign a single species of Thinnfeldia to the Filicales or the Gymnosperms.
A leaflet from Jurassic rocks of Poland figured by Raciborski[1446] shows what this author regards as the impression of a circular sorus: no sporangia have been found. A specimen in the British Museum[1447], which is said to come from Rhaetic beds in Queensland, shows a row of contiguous polygonal prominences on each side of the midrib which resemble the sori of a fern; but until sporangia are discovered we cannot determine the precise nature of this apparently fertile frond.
A species described by Fontaine[1448] from the Potomac beds (Wealden-Jurassic) of North America as Thinnfeldia variabilis affords a good example of a plant which cannot be identified with any degree of confidence either as a fern or a seed-bearing type. Mr Berry draws attention to the former application of this name by Velenovský to a distinct Lower Cretaceous Bohemian species and proposes for Fontaine’s plant the name T. Fontainei; he maintains that no one has doubted the fern-nature of the Potomac plant. T. variabilis may indeed be a fern, but the evidence is not such as to preclude legitimate doubts as to the correctness of this suggestion. Solms-Laubach[1449], in referring to Schenk’s view that Thinnfeldia and its allies may represent a group intermediate between Ferns and Gymnosperms, admits that it is a possible supposition; he is, however, inclined to consider Lomatopteris and Cycadopteris, “genera especially comparable with Thinnfeldia” as more probably ferns.
At this point we may conveniently consider a series of genera which occupy an equally uncertain position and bear a very close resemblance to Thinnfeldia.
Fig. 360.