The Protolepidodendridae comprising of the single order Protolepidodendrales consist of Palaeozoic fossils extending from the Silurian to the Devonian. They are usually placed within a single family Protolepidodendraceae which may be rather artificial as the details of most of the genera are not known.
The sporophytes were herbaceous with the branches densely covered by microphyllous, eligulate leaves. Sporophylls were dispersed and not localised on definite strobili. The sporangia, where known, were homosporous.
Three genera (Baragwanathia, Drepanophycus, and Protolepidodendron) are better known:
Baragwanathia longifolia Lang and Cookson (Fig. 536A) from the Silurian of Australia is better known. Even this is older than the Psilophytes. The plant was probably larger than the present-day Lycopodium. Microphyllous, narrow, leaves with single median veins up to 4 cm long and 1 mm wide are laxly borne on the stout, dichotomous, stems 1 to 5 cm in diameter.
Reniform sporangia, about 2 mm in diameter, are present in certain areas of the stem in axillary position of leaves intermixed with vegetative leaves. Although it is not definite whether the sporangia were adaxial on the leaves, the general pattern seems to be Lycopodiaceous. The stele, though not well preserved, seems to be actinostelic with annular tracheides in the 12 or more xylem rays.
Drepanophycus (=Arthrostigma of Dawson) has been reported from the Lower and Middle Devonian of Canada, Germany, Norway, Great Britain, Belgium and China. D. spinaeformis (Fig. 536B) shows a creeping stem giving rise to erect, often dichotomously branched shoots up to 45 cm high.
The stems with leaves are 2 to 5 cm in diameter and are clothed with stout, falcate, spiny appendages which are leaf-like in having median veins. While the basal leaves are simple, the upper ones are often forked. Some of these appendages bore single sporangia on the upper surface at about the middle of the leaf.
Protolepidodendron is best known from the Middle Devonian of Germany although it has also been found in the Lower and Middle Devonian rocks of Bohemia, Scotland and Eastern United States. P. scheryanum (Fig. 536C) from the Middle Devonian of Bohemia shows a branched prostrate system from which aerial shoots (simple or with a single dichotomy) arise upwards to a height of 20 to 30 cm.
The tips of the aerial shoots and the prostrate system are closely covered with small, spinous, eligulate leaves which bifurcate at the tips. On the middle part of the stem sporophylls are laxly borne intermixed with vegetative leaves.
The sporophylls (Fig. 536D) resemble the vegetative leaves with bifurcate tips and bear single sporangia on the adaxial face is little above the bases. The stems are protostelic with triradiate exarch xylems. The meta- xylem contains scalariform tracheides. Leaf traces depart from the angles of the xylem and median veins are present in the leaves but it is not clear if the two were continuous.
Leaf scars were not formed on the stem when the leaves were shed.
It is a point of interest, to be discussed again, that none of the Protolepidodendrids show any vascular connection leading to the sporangia.
Mention should be made of a few other fossil genera. Archaeosigillaria (first described by Kidston from Upper Devonian of New York) is an imperfectly preserved type often indistinguishable from Protolepidodendron. Its upper leaves show hexagonal bases resembling those of Sigillaria.
Colpodexylon, also from New York State Middle and Upper Devonian, is also included within this family. This is a herbaceous plant with crowded 3-forked leaves, adaxial sporangia and a solid, lobed xylem.
The discovery of Aldanophyton antiquissimum by Kryshtofovich from the Middle Cambrian of Eastern Siberia in 1953 caused some speculation on the evolution of vascular plants. Although Pteridophytic spores and tracheides with bordered pits had already been reported from the Cambrian rocks of India and elsewhere, Aldanophyton was at first accepted as a vascular plant from that age.
The specimen is, however, poor and reported by only shoots about 8’5 cm long and 13 mm wide covered by microphyllous leaves attaining a length of 9 mm. Internal anatomy and reproductive structures are unknown. The specimen was placed within the Lycopsids on not very complete evidence.
Later considerations however have brought forth the conclusion that Aldanophyton is quite possibly a marine alga and evidences are too inadequate for considering it as a vascular plant.
This reminds the case of another such fossil — Nematothallus from Upper Silurian discovered by Lang. Its 7 feet long and 3 feet diameter ‘trunk’ was discovered earlier and its anatomy formed of tubes led Dawson to name it Prototaxites, a definite vascular plant. Further researches have shown that this is actually some marine alga and no vascular plant at all.