Part 4

Some persistent archaic genera as Conocoryphe and Elyx inform us how this ridge has originated and how widely different it is from that in the Olenellidæ. On the surface of their head shield no ridge is visible, but on the interior side of it we behold, as the figures show (Pl. VI, figs. 43 & 44) on each side of the glabella a ramifying system as of the most minute vessels, which spread and cover the whole surface of the cheeks. This reticulation issues from a main trunk that goes nearly straight in an oblique direction from the foremost segment of the glabella and emits narrow branches forwards and backwards on its both sides. And these branches go on dividing till they occupy the entire interior surface of the fixed cheeks. The figures representing this are taken from Elyx laticeps fig. 43, and an almost similar image has been obtained from a species of Conocoryphe fig. 44.

Now the question may be asked, what does this network of branching canals mean? I think we cannot gain a more plausible answer than that given us through the inspection of the soft parts that lie hidden behind the glabella of Limulus and its fixed cheeks, the only living crustacean which offers the greatest homologies with the head shield of the trilobites. Next below the shell of the glabella and the cheeks of Limulus there is a complicated stratum of muscles and behind this the heart and the great central circulatory system spreading from that centrum towards the sides of the head shield, the vessels being the more fine and minute, the more they are elongated from the centre. I now suppose that likewise in these trilobites the narrow and prominent glabella has been the receptacle not only of a strong mass of muscles, but also for the central part of the circulation. This centre has there sent out two relatively strong ducts or canals, one on each side of the glabella feeding all soft tissues inside and near the cheeks and probably also other important parts of the body. It is also clearly seen in Elyx how the orifice of the main trunc opens in the hollow of the glabellar apex.

In all crustaceans, as far as is now known, their more or less hard calcareous or chitinous skeleton is moulded by the subjacent tissues and glands. The sculpture of the surface consequently is an outcome of the fashioning procedures showing what has been going on below and what is still going on. If we then on the exterior surface of the head shield of the trilobites see the radiating lines in relief we must conclude that they are due to a subjacent system of almost capillary vessels causing hollow impressions on the inside and elevated ridges exteriorly. In Elyx the vessels have made no strong impression as to be visible on the outside. In a couple of American Conocoryphæ again (C. reticulata and C. trilineata) the main trunc of the vessels has formed a short faintly elevated ridge. In Solenopleura (Pl. VI, f. 45) the facial ridge has been fully developed, and by the casts of the inside it is clearly seen that the main trunc of the vessels makes the inside of the ridge and has been much incrassated. On the other hand the smaller branches issuing from it have been reduced in size and number, but are still visible. Thus, if I am right, the two, the facial ridge and the main trunc, are in causal connection and the former has been moulded by the later, when it had gained in size sufficiently, and when a richer affluxion of nutritive fluid was directed backwards towards the point where the eye at last originated. The further changes in this ridge thus are related to the development of the eye. As far as I am aware, there are as yet no data to tell us whether the appendages or pleura of the glabella in the larval (or primordial) Olenellidæ have the same origin as the ridge of the Olenidæ or are homologous with it. Perhaps it may be so in respect to the foremost one, which also is persisting. It is, however, not known how the pleura in the olenellid glabella have been developed, whereas it is well ascertained by the growth of the glabella in Sao, Liostracus and others that such pleura never have been developed in them.

What the phylogenetic evolution has taught us concerning the formation of the facial ridge is confirmed by the ontogenetic development of individuals of some species as for instance Sao hirsuta BARR., Liostracus sp. BRÖG. and others.

This development can be followed in detail through the excellent figures of the larval stages of Sao hirsuta which BARRANDE has given in his magnificent work, vol. I, pl. 7. In the first stage figured (figs. 1 d-e) the whole animal consists only of the head shield which is completely smooth, the glabella scarcely segmented, no facial ridge, no facial suture. In the second stage, according to me (figs. 2-4 a), the glabella has become distinctly segmented, and the pygidium and partly the thorax have been added. In the third stage, 4 c, d-9, one small narrow string exits from each side of the front of the glabella, making the first faint beginning of the facial ridge. They form a right angle with the glabella. In continued growth they become by degrees a little more curved (figs. 5 c, 6 b) and the lengthened ridge bends parallel to the outer lateral margin of the head (fig. 9 b). In fig. 9 d it has become so far complete that it reaches nearly back to the posterior cephalic margin, but is still of the same narrowness all along. In the fourth stage, in a specimen (fig. 10 a) of 3 mm. in length, the facial suture makes its first appearance, setting in from the posterior margin of the head and meeting in its forward progress the posterior extremity of the facial ridge which now begins to swell out. It seems that both have a mutual influence on each other, the suture being deviated from a straight course to take a bend outside the ridge, and the extremity of the ridge again at this contact to increase in size so as to form the elongated tubercle, often called palpebral lobe (fig. 14 b). This now augments in the same rate as the whole body. The characters of the four stages of the development of the larva consequently are: 1:o The archaic stage, only head shield with ridgelike glabella. 2:o The coming and growth of the pygidium and the complete segmentation of the glabella. 3:o The coming and growth of the facial ridge. 4:o The coming and development of the facial suture.

BARRANDE regards the whole ridge as a prolongation of the eye and the tubercle at its posterior extremity as the eye itself. But, again, in page 399 he says »Sa surface (of the eye) est toujours mal conservée, pour nous permettre de voir si elle était réticulée.» And he also confounds the ridge, »filet» as he names it, with the eye itself, and the first faint beginnings of this ridge in his fourth stage he considers as the eye. By a partly schematical figure of the free cheek (fig. 25) he places the eye on this cheek, and in the same manner in fig. 29, »restaurée d'après divers fragments» he figures a reticulated surface of the eye on the free cheek, outside the tubercle. I have sought for a reticulated surface on sufficiently good specimens, but never found any, and I must consider Sao as one of the blind genera. BARRANDE himself also in the table on the eyes of the trilobites places Sao in the group with »Surface visuelle inconnue», p. 131.

BRÖGGER has also succeeded in finding a series of small larva, which he considers as belonging to a species of Liostracus. As the figures drawn by BRÖGGER twenty five years ago may now be very little known, I here reproduce them with the kind permission of the Editors of »Geologiska Föreningens Förhandlingar», where they were published in 1875. This development proceeds nearly upon the same plan as in Sao. The first stage, however, (I) seems to be much earlier than any of Sao, the rhachis or glabella consisting of an unsegmented ridge of more primitive appearance. Before this first stage of BRÖGGER'S larva a still older phase of development can be imagined, (0) a simple rounded, smooth head shield without any indication of a glabella at all. This stage might correspond to the head of certain species of Agnostus, as A. glandiformis, A. nudus, which have no glabella. This stage 0 is also valid for the larva of Sao. Several stages are evidently wanting between I and II, in the later of which the thickened glabella is divided in four segments. In III we have six segments, all these three stages consisting only of the ovate head with narrow fixed cheeks. In IV the pygidium has been added to the primitive head, but the segments of the glabella have been reduced to four and in V slightly altered in shape In VI, again, we see the head with five glabellar segments and scarcely the first sign of the facial ridge. Between VI and VII there must be links missing, as the change can not be so abrupt, and likewise between VIII and IX as in VIII there are still no free cheeks nor any facial ridge. This interesting discovery of BRÖGGER confirms, together with those of several other authors, the supposition that the development of the later Cambrian and older Silurian forms is a quite different one from that of the Olenellidæ and the Paradoxidæ. They have a rhachis, but no pleura proper, as the single facial ridge has a quite different signification and appears at a comparatively much later stage than the facial ridge of the Olenellidæ, which is present from the earliest stages known.

This form of facial ridge, although prevalent in the middle and later Cambrian times, dates back so far that genera coeval with Holmia, viz. Ellipsocephalus and Arionellus show it along the facial suture. This early occurrence of the ridge coeval with the less developed Olenellidæ leads to the assumption of a different origin of these trilobites as a branch, which already far back in the oldest Cambrian or precambrian times had deviated from a common ancestor and I have tried to give a view in tables of these two coordinated lines of evolution further on, at pages 24 and 25. The most remarkable genera which, as far as I have found, belong to this fourth group are Ellipsocephalus, Arionellus, Liostracus, Olenus, Leptoplastus, Parabolina, Corynexochus, Parabolinella, Sao, Ptychoparia, Doropyge, Oryctocephalus etc.

I have not been able with an absolute degree of certainty to recognize whether some of these genera now enumerated, have been oculate or blind like Sao, like the Paradoxidæ and similar. The precarious state of preservation prevents all definite conclusions in that respect. It seems, however, that the evidence gathered through the examination of numerous specimens rather points in a negative direction. As the free cheeks in these old Cambrian trilobites have been in a very loose connection with the fixed cheeks and generally deciduous, contrary to the condition in the Silurian ones, it is in many instances very difficult to tell whether species with facial ridge, especially those from the earlier Olenus schists have been blind or provided with eyes.

The order of succession of the genera in the Swedish uppermost Cambrian, in the Olenid slates is, according to S. A. TULLBERG'S researches at Andrarum in Scania as follows:

1. Parabolina (oldest division of the Olenus slates). Olenus. Liostracus. 2. Eurycare. Leptoplastus. 3. Peltura. Sphærophthalmus. Ctenopyge. 4. Cyclognathus (uppermost). Acerocare.

It is already in the second division that we find the earliest oculate genus in Eurycare and in the higher strata. Sphærophthalmus and Ctenopyge with enormous hemispherical eye balls, while Olenus and Parabolina, their earliest predecessors, probably were blind. We have not amongst hundreds of specimens of these genera found a single specimen showing an ocular globe covered with facets. Olenus and the nearly related genus Parabolina are found in innumerable specimens in the thinly laminated alumschists of Scania and other provinces of Sweden. But rarely a perfect head shield, or nearly so, is to be found with the free cheeks in place. If so, the semicircular scallop in the free cheek is entirely filled up by the posterior lobe of the facial ridge and there is no place left for any eye.--If we, again, find a non compressed glabella and fixed cheeks likewise, the facial lobe (-»eye-lobe») is in some elevated so much as to leave a little space between it and the scallop of the free cheek, which space must have been an empty lacuna if not filled up by an eye ball. But it may also be that the free cheek has been somewhat put out of its order and that consequently some space has been left between it and the free cheek. It is quite as much with older genera, as Solenopleura especially, in which the posterior lobe of the facial ridge (vulgo »eye lobe») has attained a great development, and in which one just could expect to find a sphærical eye resting between the elevated lobe and the scallop. The elevated rim of this scallop does not in fact constitute a proof for its having clasped an eye as the elevated scallop in Sphærophthalmus did. In Paradoxides, again, in Ellipsocephalus, where there is absolutely no trace of an eye ever having been present, whenever you succeed to find the free cheek in juxtaposition with the fixed cheek it is evident that the elevation of the scallop rim is due to the impact of the posterior lobe of the facial ridge. In many specimens of Dolichometopus and Corynexochus etc. no free cheeks have ever been found and to judge by the shape of the facial ridge it may be concluded that these also were deprived of eyes.

On seeing this great number of trilobites, that on account of their organization must be considered as blind, the first suggestion that strikes the mind, is that they must have lived in abyssal depths of the Cambrian sea, where the most intensive darkness prevailed. Nor does the nature of the strata contradict such an assumption as evidently this fine sediment must have been deposited far beyond the reach of the influence of the wave motion, a depth amounting to more than a thousand metres as now calculated, for else it could not have preserved unbroken such delicate parts of organisms as that free cheek of Ctenopyge pecten with its extraordinary long and delicate horn, figured on plate III fig. 27 and many others. But it seems incredible that such a state of things should have prevailed during the deposition of all the Cambrian strata, although they in Sweden amount to only 160 feet in thickness, according to the evaluation of S. A. TULLBERG, not considering what has been lost through denudation. The length and duration in time can in this instance not be measured by the thickness of the beds, but by the great changes in the faunas which there have succeeded one another. The physical conditions, to judge by the composition of the rocks, seem in the main to have lasted during immeasurable periods and still the fauna has changed in no little degree. During that enormous length of time, embracing in Sweden eight well separated periods, there must, however, have been minor changes in the conditions of depth and consequently in the nature of the depositions. To a certain extent the physical agents must have influenced the organization of the animals, but not essentially. There are sure evidences of another factor being the chief agent and that is the evolution.

We have from the lowest Olenellus beds to the lowest Lower Silurian strata followed the clear traces of the changes from the eyeless Olenellidæ past the Paradoxidæ with the facial suture to the great tribe of the Olenidæ in which the facial ridge with its lobe is so prominent, and to the oculate genera at the top of the formation.

As stated above there seems to be evidence enough for accepting two different lines of evolution in these the oldest trilobites. For the oldest, Olenellus is the type and for the second Sao may be taken as the representative.

These two evolutional lines could be represented with their phases in chronological succession as follows:

A. =The Olenellus line.=

1. The trilobite consisting only of the head with rhachis and pleuræ, no sutures, no ridge, no eyes. Adult forms precambrian. Corresponding larval forms (Olenellus asaphoides) in the Lowest Cambrian. Still older and simpler forms may be presupposed as preceding these.

2. Semilunate facial ridge on the tripartite head-shield, no sutures, no eyes, but a hypostoma fused to the rostrum of the head and provided with maculæ. The adult animal with thorax and pygidium in the oldest Cambrium, the Olenellus zone. Genera: Olenellus, Holmia (with beginning suture), Mesonacis, Schmidtia.

3. The head quinquepartite with facial suture, short semilunate ridge in the adult, a long ridge in the young, no eyes. The genera occur in the Paradoxides-zones of the Cambrian formation. These genera are Paradoxides, Centropleura, Metadoxides, Hydrocephalus, which are direct descendants of the Olenellidæ.

This line of evolution, in which the species never acquired eyes, was extinct in the sixth zone of the Swedish Cambrian, Centropleura Lovéni being the last.

B. =The Sao line.=

I call this so because we see in the development of Sao, as represented by BARRANDE, most of the different' phases in evolution, which the ridge bearing trilobites of this second group have experienced during the Cambrian times.

The body consists only of the head, neither pygidium, nor thorax yet developed.

1. Psilocephali (ψιλος, bald), primeval, precambrian, adult stage not found, supposed to be like the head shield of Agnostus glandiformis and A. biplicatus quite round and bare without any glabella nor sutures. The corresponding stage in development also wanted.

2. Glabellate. The entire animal only head with a mesial ridge, which at first is entire or unsegmented and later is metamorphosed into the segmented glabella.

a. unsegmented., Precambrian, adult form unknown, supposed to be like the first larval stage of Liostracus according to the figure I of BRÖGGER and also as the head of Agnostus parvifrons LINNARSSON. Corresponding larval stages Liostracus (I).

b. segmented. Probably precambrian, not represented as adult in the Cambrian strata. Supposed to be like the larval forms II and III of BRÖGGER'S Liostracus, consisting of the round or oval head shield and the segmented glabella. Also BARRANDE'S first stage of Sao (BARR., fig. 1 pl. 7).