On the Coprolites of the Upper Greensand Formation, and on Flints
Authors: Sollas W. Johnson 
Published: October 2009

Abstract

Part I.

The first part of this paper was principally occupied in an endeavour to explain the perfect fossilization of sponges and other soft-bodied animals. It was shown that the hypothesis which considered that sponges had become silicified by an attraction of their spicules for silica was altogether untenable. Mr. Hawkins Johnson's supposititious reaction, according to which the carbon of animal matter is directly replaced by silicon, was shown to be inconsistent with the known facts of chemistry. The author's explanation was not intended to be final. The first fact pointed out was the very remarkable way in which the silica or calcic phosphate of the fossils under consideration followed the former extension of organic matter. This was explained for silica by the fact that, when silicic acid is added to such animal matters as albumen or gelatin, it forms with them a definite chemical compound; and it was assumed that in process of time this highly complex organic substance would decompose, its organic constituents would be evolved, and its silica would remain behind. In such a way flints might be produced, and dialysis would lend its aid. The same explanation was applied to account for the connexion between calcic phosphate and animal matter in the case of the “Coprolites.”
The Blackdown silicified shells were next explained; and it was reasoned that the state of their silica offered arguments tending to prove a passage of silica from the colloidal to the crystalline state.

Part II.

Coprolites, as has already been stated, are the result of the phosphatization of organic matter. They may be classed according as their origin is known or obscure. The progress of discovery transfers the obscure forms into the class of known forms; but there will always remain a certain number which cannot be thus transferred—those, for instance, which have been produced from softbodied animals in the last stage of decomposition, all traces of their structure having been obliterated. The fossil remains of Reptiles, Fish, and Crustacea, the casts of Mollusks, and the perfectly preserved Ventriculites are instances of coprolites about whose origin there is no uncertainty; but besides these easily recognizable kinds there remain a vast majority of forms which, from their want of any striking characters, have always been obscure. In this paper the author proposes to transfer these in great part among the fossil sponges.

General Character of the Coprolites.

In the Greensand, most of the coprolites are of a black or deep brown colour, while in the Gault they are greyish-white on the surface, but brownish-black internally. By etching the greensand coprolites with acid they change, however, to the same greyishwhite colour as the specimens from the gault; and masses of agglomerated coprolites are met with in the Greensand which, when broken open, reveal nodules of their original light colour.
The surfaces of most of the coprolites are variously marked with (i.) fold-like depressions, (ii.) osculiform pits, (fii.) puncta, and (iv.) contraction cracks.
(i.) The depressions occur as fold-like markings, which sometimes run longitudinally with remarkable constancy in size and direction for nearly the whole length of the fossil; besides this they may take any other direction. These grooves are marked by very minute wrinkles, which give the whole depression the appearance of one of the creases stretching from button to button in the leathern back of an easy chair. The better-marked of these grooves scarcely appear to be due to contraction consequent on fossilization.
(ii.) The circular or oval osculiform pits vary in size, frequently being one-twentieth of an inch in diameter. Their margins are often depressed into a concave border, which is striated by regularly radiating “groovelets.” These commence at the sides of the osculiform pit, pass across the concave border, and either stop there or pass to a greater or less distance on the surrounding surface. In some specimens these little striations are more restricted and betterdefined than in others, and in some they are altogether absent. The osculiform pits are grouped sometimes at the end of one of the wrinkled depressions above described; sometimes they are collected in sieve-like patches, or two or three may be noticed at the bottom of a wider-mouthed pit, while occasionally they are terminal, and frequently dispersed. It may be noticed that in the same specimen these pits occur of very various sizes, and that the striated grooves of their margins are never arranged in any multiple of four—an argument, if arguments are needed, against the alcyonioid origin of these forms.
(iii.) The puncta are minute pore-like marking, which appear in the greensand specimens as mere specks of a different nature from the rest of the fossil; but in the gault coprohtes they are the distinctly open terminations of fine canals.
(iv.) Contraction-cracks are evident on the surfaces of many specimens, generally filled in with lighter-coloured material. In the coprolites from the Gault the oscula, puncta, canals, cracks, and other cavities are either empty or filled with loose clay; in those from the Greensand all these cavities are infiltrated with phosphatized chalk-marl, containing green grains and sometimes diffused glauconite. Since these infiltrated coprolites of the Greensand are derived from those of the Gault, which are not infiltrated, this filling-in must have taken place after the greensand fossils were washed out of their matrix.
Smooth surfaces of attachment are to be seen on some specimens; and in rare cases the shell on which they grew remains adherent to them.

General Appearances under the Microscope
.—Thin sections examined under the microscope vary from colourless to yellowish-brown when transparent, but sometimes they are almost opaque from included earthy matter. Granular patches of a deep red colour are sometimes scattered throughout the lighter-coloured portions. Spicules occur in many sections, presenting some of the most characteristic forms of sponge-spicules, as for example, hexaradiate, triradiate, hamate, sinuate, and connecting forms. These spicules are frequently grouped together in a manner which would seem to indicate that they cannot have been washed in from the sea-bed during fossilization. Globular bodies one-four hundreth of an inch in diameter are numerous; they seem to be gemmules.
Polycystina
and
Xanthidia
occur in some sections. With polarized light the sections appear distinctly cryptocrystalline, presenting an appearance very nearly resembling that of chalk flints when examined in the same way. A very curious phenomenon may be alluded to here. A number of small circles may be seen in some sections, each of which is marked by a black cross, the arms of which radiate from the centre to the circumference. On turning the analyzer the cross revolves and, when he analyzer has been turned round 90° is replaced by a complementarily illuminated cross. The explanation of these appearances seems to be as follows: small
Globigerina
-shells and other similar spaces occur in the coprolite, into which the crystalline apatite which was diffused throughout the fossil has penetrated and crystallized inwards from their walls to their centres, thus forming a radiating mass of crystals; it is well known that crystals arranged in this manner will produce the phenomena described.

Behaviour with Hydrochloric Acid
.—Hydrochloric acid dissolves the coprolites, some undergoing solution more readily than others, owing to their containing a larger quantity of calcic carbonate. While dissolving they emit a smell almost precisely resembling that of petroleum. The hydrocarbons which produce this odour must exist previously as gases, or combined with the calcic phosphate of the coprolite. I believe that they are present in the latter condition. The insoluble residue left on solution very frequently contains spongespicules and siliceous organisms; these may be separated in the same way as Foraminifera are obtained from a piece of chalk.

Enumeration of Genera

Genus 1.
Rhabdospongia
.
Sponge
more or less rod-like, 1" to 2" long, one-third to one-half diameter. Frequently attached at one end.
Cloaca
none, solid throughout.
Spicules
filiform, acerate, sinuous.
Species
: R. communis.

Genus 2.
Bonneyia
.
Sponge
cylindrical to clavate, size variable.
Cloaca
: longitudinal axis always occupied by a cloacal cavity from one-third to two-thirds of the whole diameter of the fossil.
Spicules
few.
Species
: B. bacillfformis, B. cylindrica, B. Jessoni, B. scrobiculata, B. verrongiformis.

Genus 3.
Acanthophora
.
Sponge
massive, lobose.
Spicules
acerate, fusiform, spiculated porrecto-ternate and recurvo-ternate, triradiate, hexaradiate.
Species
: A. Hartogii.

Genus 4.
Polycantha
.
Sponge
ovate.
Cloaca
present.
Spicules
acerate, defensive, triradiate, quadriradiate, hexaradiate.
Species
: P. Etheridgii.

Genus 5.
Retia
.
Sponge
cylindrical or hemicylindrical, marked on surface with a symmetrical fibre-like reticulation.
Spicules
few or absent.
Species
: R. simplex, R. costata.

Genus 6.
Hylospongia
.
Sponge
large and massive.
Cloaca
always present.
Surface
covered by a bark-like exterior, beneath which it is smoothly and longitudinally striated.
Species
: H. patera, H. calyx, H. Brunii.