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.
