It remains to consider some of the causes to which we may attribute the scarcity of fossil algae, and the possible sources of error which beset any attempt to describe or assign names to impressions and casts simulating algal forms.
In the first place, the delicate nature of algal cells is a serious obstacle to fossilisation. Even in plants in which the woody stems have been preserved by a siliceous or calcareous solution, we frequently find the more delicate cells represented by a mass of crystalline matter without any trace of the cell-walls being preserved. In such plants as algae, where the cell-walls are not lignified, but consist of cellulose or some special form of cellulose, which readily breaks down into a mucilaginous product, the tissues have but a small chance of withstanding the wear and tear of fossilisation.
The danger of relying on external form as a means of recognition is especially patent in the case of those numerous markings or impressions frequently met with on rocks, and which resemble in outline the thallus of recent algae. Among animals, such as certain Polyzoa, the flat branching body of various algae is closely simulated, and in other plants, such as the frondose liverworts, the same thalloid and branched form of body is again met with. Some of the much dissected Aphlebia leaves of ferns (e.g. Rhacophyllum species) bear a striking resemblance to fossil algae; and numerous other examples might be quoted. In palaeobotanical literature we find a host of names, such as Chondrites, Fucoides[224], Caulerpites and others applied to indefinite and indistinct surface markings which happen to resemble in shape certain of the better known genera of recent seaweeds.
The close parallelism in outward form displayed by different genera and families of algae is in itself sufficient argument against the use of recent generic names for fossils of which the algal nature is often more than doubtful. Were external form to be accepted as a trustworthy guide, in the absence of internal structure and reproductive organs, such a genus as Caulerpa[225] would afford material for numerous generic designations. A comparison of the different species of this Siphoneous green alga brings out very clearly the exceedingly protean nature of this interesting genus, and serves as one instance among many of the small taxonomic value which can be attached to external configuration. Caulerpa pusilla Mart. and Her., C. taxifolia (Vahl.), C. plumaris Forsk., C. abies-marina J. Ag., C. ericifolia (Turn.), C. hypnoides (R. Br.), C. cactoides (Turn.), C. scalpelli formis (R. Br.), and others clearly illustrate the almost endless variety of form exhibited by the species of a single genus of algae. We constantly find in the several classes of plants a repetition of the same form either in the whole or in the separate members of the vegetative body, and but a slight acquaintance with plant types should lead us to use the test of external resemblance with the greatest possible caution. To emphasize this danger may seem merely the needless reiteration of a self-evident fact, but there is, perhaps, no source of error which has been more responsible for the creation of numerous worthless species among fossil plants.
Fig. 30. 1. Rill-mark (after Williamson). 2. Trail made by a seaweed dragged along a soft plaster of Paris surface (after Nathorst). 3. Tracks made by Goniada maculata, a Polychaet (after Nathorst). 4. Burrow of an insect. 4a. Section of the gallery (after Zeiller).
There is, however, another category of impressions and casts of common occurrence in sedimentary rocks which requires a brief notice. Very many of the fossil algae described in text-books and palaeobotanical memoirs have been shown to be of animal origin, and to be merely the casts of tracks and burrows. A few examples will best serve to illustrate the identity of many of the fossils referred to algae with animal trails and with impressions produced by inorganic agency.
Dr Nathorst of Stockholm has done more than any other worker to demonstrate the true nature of many of the species of Chondrites, Cruziana, Spirophyton, Eophyton, and numerous other genera. In 1867 there were discovered in certain Cambrian beds of Vestrogothia, long convex and furrowed structures in sandstone rocks which were described as the remains of some comparatively highly organised plant, and described under the generic name Eophyton[226]. By many authors these fossils have been referred to algae, but Nathorst has shown that the frond of an alga trailed along the surface of soft plaster of Paris produces a finely furrowed groove (fig. 30, 2) which would afford a cast similar to that of Eophyton. The same author has also adduced good reasons for believing that the Eophytons of Cambrian rocks may represent the trails made by the tentacles of a Medusa having a habit similar to that of Polydonia frondosa Ag. Impressions of Medusae have been described by Nathorst from the beds in which Eophyton occurs; and the specimens in the Stockholm Museum afford a remarkable instance of the rare preservation of a soft-bodied organism[227]. By allowing various animals to crawl over a soft-prepared surface it is possible to obtain moulds and casts which suggest in a striking manner the branched thallus of an alga. The tracks of the Polychaet, Goniada maculata Örstd.[228], one of the Glyceridae, are always branched and very algal-like in form (fig. 30, 3). Many of the so-called fossil algae are undoubtedly mere tracks or trails of this type. In the fossil-plant gallery of the British Museum there are several specimens of small branched casts, clearly marked as whitish fossils on a dark grey rock of Lower Eocene age from Bognor; these were described by Mantell and Brongniart[229] as an alga, but there is little doubt of their being of the same category as the track shown in fig. 30, 3.
FOSSILS SIMULATING ALGAE.