Fossil plants, Vol. 1: [A text-book] for students of botany and geology

by A. C. Seward

α. Caulerpaceae.

Thallus unseptate, showing an extraordinary variation in the external differentiation of the plant-body. Reproduction is effected by means of detached portions of the parent plant.

The genus Caulerpa, represented by a few species in the Mediterranean and by many tropical forms, has already been alluded to as a striking example of a plant which appears under a great many different forms[271]. As a recent writer has said, “Nature seems to have shown in this genus the utmost possibilities of the siphoneous thallus[272],” Fragments of coniferous twigs, the tracks and burrows of various animals and other objects have been described by several authors as fossil species of Caulerpa. As an illustration of the identification of a very doubtful fossil as a species of Caulerpites, reference may be made to such a form as C. cactoides Göpp.[273] from Silurian and Cambrian rocks. There are several examples of this fossil in the Brussels Museum which probably owe their origin to some burrowing animal, and may be compared with Zeiller’s figures of the tunnels made by the mole-cricket (fig. 30, 4)[274].

Mr Murray, of the British Museum, has recently described what he regards as a trustworthy example of a fossil Caulerpa from the Kimeridge Clay near Weymouth[275]. Specimens of the fossil were first figured in a book on the geology of the Dorset coast as casts of an equisetaceous plant[276].

To this fossil Murray has assigned the name Caulerpa Carruthersi, and given to it a scientific diagnosis. The best specimens have the form of a slender central axis, giving off at fairly regular intervals whorls of short and somewhat clavate branches; they bear a superficial resemblance to such a recent species as Caulerpa cactoides Ag. An examination of several examples of this fossil leads me to express the opinion that there is not sufficient reason for assigning to them the name of a recent genus of algae[277]. To use the generic name of a recent plant without following the common custom of adding on the termination “ites” (i.e. Caulerpites) is as a general rule to be avoided in dealing with fossil forms; and there are, I believe, no satisfactory grounds for referring to these fossils as trustworthy examples of a Mesozoic alga.

In the present case I am disposed to regard the Caulerpa-like casts as of animal rather than plant origin. The clavate branches have the form of very deep moulds in the hard brown rock which have been filled in with blue mud. It is hardly conceivable that the branches of a soft watery plant such as Caulerpa could leave more than a faint impression on an old sea-floor. The specimens occur in different positions in the matrix of the rock and they are not confined to the lines of bedding; in none of the examples is there any trace of carbonaceous matter in association with the deep moulds. On the whole, then, this Kimeridge fossil cannot, I believe, be accepted as an authentic example of a Mesozoic Caulerpa.

It is not improbable that some of the supposed fossil algae may be casts of egg-cases or spawn-clusters of animals. In Ellis’ Natural History of the Corallines[278] there is a drawing representing a number of disc-like ovaries attached to a tough ligament, and referred to the mollusc Buccinum, which bears a certain resemblance to the Weymouth fossil. A similar body is figured by Fuchs[279] in an important memoir on supposed fossil algae.

It is not suggested that the Caulerpa Carruthersi of Murray should be regarded as the cast of some molluscan egg-case attached to a slender axis, but it is important to bear in mind the possibility of matching such extremely doubtful fossils with other organic bodies than the thallus of a Caulerpa. In an example of an egg-case in the Cambridge Zoological Museum, referred to a species of Pyrula, there is a hard, long and slender axis, bearing a series of semicircular chambers divided into radial compartments. The whole is hard and horny and might well be preserved as a fossil.

β. Codiaceae.

The members of this Order present a considerable diversity of form as regards the shape of the plant-body; the thallus of some species is encrusted with carbonate of lime. The order is widely distributed in tropical and temperate seas.