Thinnfeldia.

The genus Thinnfeldia, founded by Ettingshausen in 1852[1434] on some Hungarian Liassic specimens, though frequently included in the Filicales, cannot be said to occupy that position by virtue of any well-authenticated filicinean features. It is by no means improbable that many of the species referred to this genus are closely allied to Palaeozoic Pteridosperms.

Thinnfeldia may be briefly defined as follows:

Fronds simple and pinnatifid, pinnate or bipinnate: rachis broad and occasionally dichotomously branched. Pinnules often fleshy or coriaceous; broadly linear, entire or lobed, provided with a midrib from which simple or forked secondary veins are given off at an acute angle: or the laminae may be short and broad without a midrib and traversed by several slightly divergent and forked veins.

No satisfactory evidence of reproductive organs has so far been adduced.

The genus is chiefly characteristic of Upper Triassic, Rhaetic, and Jurassic floras, though it was in all probability represented in Permian floras. Several species, many of which are valueless, are recorded also from Cretaceous and Tertiary formations. Search should be made for fertile specimens or for evidence as to the association of seeds with Thinnfeldia fronds.

Some Permian fossils from Kansas which Sellards[1435] has made the type of a new genus, Glenopteris, appear to be indistinguishable generically from leaves of Lower Mesozoic age universally recognised as typical examples of Thinnfeldia.

Thinnfeldia odontopteroides (Morris)[1436]. [Figs. 356–358].

This is a very variable species as regards the shape and size of the ultimate segments and their venation. It is a type of extended geographical range characteristic of Rhaetic or Upper Triassic rocks in Australia, South Africa, India, South America, and various European localities.

Frond bipinnate; the broad rachis may be dichotomously branched. Pinnules with a thick lamina which may be almost semicircular in form, deltoid, broadly oval or broadly linear, and often confluent at the base. Short and broad pinnules occur on some fronds directly attached to the main rachis between the pinnae. The longer and narrower pinnules ([fig. 356], C), resembling those of the Palaeozoic genus Alethopteris, have a well-defined midrib, while the smaller segments are characterised by several slightly divergent veins which spring directly from the rachis ([fig. 356], A). Epidermal cells polygonal or, above the veins, rectangular in shape; stomata, which are slightly sunk, occur on both the upper and lower epidermis. Fertile specimens unknown.

The portion of a lobed pinnule shown in [fig. 356], B, illustrates a form of segment intermediate between the linear type with a midrib and a row of shorter pinnules without a median vein. Fig. 356, D, represents another instance of variation in the arrangement of the veins in segments of different sizes. Various specific and generic names have been assigned to Thinnfeldia fronds of Rhaetic age on the ground of the occurrence of pinnules longer and narrower than those usually associated with T. odontopteroides; but in view of the range of variation met with in a single leaf it is advisable to extend rather than to restrict the boundary of what we are pleased to regard as a specific type.

Fig. 356.

The name Thinnfeldia lancifolia has been applied by Morris to fossils from Australia which may be identified with T. odontopteroides, and the same designation is employed by Szajnocha and by Solms-Laubach[1437] for Rhaetic specimens from South America. Similar fronds are described by Geinitz[1438] as Thinnfeldia tenuinervis from Argentine Rhaetic strata. Odontopteris macrophylla Curran, T. falcata Ten.-Woods, Gleichenia lineata Ten.-Woods, and Cardiopteris Zuberi Szaj. afford other examples of what are probably closely allied forms[1439].

Fig. 357. Thinnfeldia odontopteroides (Morris). ⅘ nat. size.

Some exceptionally large examples of T. odontopteroides are figured by Feistmantel[1440] from the Hawkesbury series of New South Wales in which the bipinnate frond has a breadth of 25–30 cm. A specimen from the Molteno beds of South Africa, probably of Rhaetic age, represented in [fig. 357], illustrates a smaller leaf with pinnules of the linear type, some of which are partially divided into shorter pinnules with forked veins. The example represented in [fig. 358], from Cyphergat (S. Africa), shows two equal branches of a rachis with small contiguous segments.

Fig. 358. Thinnfeldia odontopteroides. From a specimen in the British Museum (v. 2490). 1½ nat. size.

Some specimens figured by Zeiller[1441] from the Rhaetic strata of Tonkin as Pecopteris (Bernouillia?) sp. may be portions of Thinnfeldia fronds, and the large leaves which he refers to Ctenopteris Sarreni differ but slightly from the Australian specimens described by Feistmantel as T. odontopteroides.

Thinnfeldia rhomboidalis, Ettingshausen. Figs. [359], [360], C.

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.

  1. Lomatopteris jurensis. (⅞ nat. size. After Kurr.)
  2. L. Schimperi. (⅞ nat. size. After Salfeld.)
  3. Thinnfeldia rhomboidalis, Ett. (Slightly enlarged. British Museum. No. 52672.)