The following is my translation of an article called: Verschwundene Lebewesen (Trilobiten) von Dr Hch. Kirschner. It appeared in a German popular science magazine, Kosmos Handweiser für Naturfreunde 1921, Heft 9, Seiten 244-248. As is my habit, this translation doesn't include the beautiful illustrations although, at various points, they do get mentioned so as to make you jealous. I should mention that I haven't gone to the degree of learning the ins and outs of trilobite anatomy in English, and don't necessarily know the correct technical terms for particular features. That said, the article was intended for the interested lay reader and not for trilobite fiends.
I'm not aware of any previous translations.
Trevor Dykes.

Vanished organisms (trilobites) by Dr Hch. Kirschner
The rambler, attracted by the beauty of the strangely formed rock structures of the Jurassic, only holds the memory of the appealing images of the landscape. Should they open their eyes wider to observe the nature, then they would also find the many shapes of the ammonites, flattened plate-like spiralled shells of long vanished invertebrates, and would perhaps be incited to consider the idea of the Jurassic time. Indeed, they might even have observed that particular ammonites dominate in similarly formed rock layers and, if so, then they would have recognized a fact which serves to allow researchers to subdivide these strata into successions or zones. The individual zones are actually named after the ammonites that predominate in them, while these forms strongly diminish, or are even absent from other zones.

A similar role to the ammonites of the Jurassic is played by trilobites, crustaceans, in the more ancient part of the history of life, the Paleozoic. Traces of them have been reportedly found from the Algonkuium (Additional note: an alternative German word for the Proterozoic.), a still older group of rock formations; however, wholly secure reports are absent. In any case, one finds the first record of the strong radiation of trilobites with the first undoubted appearance in the Lower Cambrian, and that allows it to be concluded that their forerunners existed during the Algonkium. The numbers of species and individuals quickly increased during the Cambrian, and from their number arose trilobites that ruled the seas. The high point of their variety was reached during the Lower Silurian but soon, already from the Upper Silurian, they were increasingly pushed into the background by the radiations of cephalopods and brachiopods. Only a few genera occur in large numbers during the Devonian and Carboniferous, but it is during the Permian that the final representative of their once proud group had to clear the field; they disappeared and do not seem to have lived on in descendant animal forms.

Nevertheless, there have been plenty of attempts to bring trilobites into connection as the ancestral form of animal groups from later times. When many details speak in favour of the probability, many lines of evidence can also be brought against it, and they also require consideration. A layperson could easily be led astray by the external similarity, and internal, and attempt to explain it as a consequence of the history of common descent; they rarely have the time and necessary equipment required to solve such questions. Therefore, it would be appropriate not to present controversial scientific issues to a lay audience until they have been resolved, especially when they concern matters of descent. On the other hand, the public at large not only have a great interest in such issues, but they even have a certain right to be kept informed of the current results of research. However, whoever supports that must also become acquainted with the fact that, especially in the natural sciences, things which have once been "proven" may later be overturned again.

And so, as to the question of whether other animal classes have been descended from trilobites, there is an agreement, in as far as that is actually possible, that the answer is no. The isopods (woodlice), which are part of the cephalopod decapods, the cirripedian (barnacles), a unit of the spiders, insects and shellfish are, according to a now very weakly supported theory, supposed to have been the ancestral lineage of trilobites. Currently, the majority of researchers are distanced from this assumption and, indeed, reject it.

In contrast to this, we have a very detailed understanding of the development of individual trilobites. One has managed to identify an unbroken sequence of development for an impressive number of families extending from the egg to the adult animal. Nevertheless, trilobite eggs are very remarkable things, as the small, round what-nots could just as well have been something else. The first pronounced developmental stage is a 0.5mm small, flat, almost circular body, which has two longitudinal channels dividing the body into 3 segments, as is universally agreed (Ill. 1). This body is equivalent to the head shield of the fully developed animal. To the lower end of this structure, described as a protapsis, traces of a tail shield (pygidium) can also be recognized. Over the course of further shell replacements associated with growth, the tail shield becomes more clearly developed, and body segments grown between it and the head shield (cephalon), and they appear to originate from the pygidium. The more body segments an adult animal has, the smaller the tail shield becomes (Ill. 2 and 3). The number of these segments varies between 2 and 20, but it always remains the same within the same species. As soon as the appropriate number of segments for the species has been attained, the animal then only grows in terms of general length and width. The average size of trilobites found so far ranges from 3 to 10cm, but giant forms are also known, such as Uralichas ribeiroi, that reached about 70cm.

The adult trilobites have a variety of forms, but all conform to a certain archetype; it is rewarding to observe this more carefully (Ill. 2). Especially characteristic is the doubled triple division of the originally chitin back armour, a horn-like substance. This triple division earlier provided the trilobite with its name; naturally, one also made a number of errors at that time, as one often found head and tail shields of muscle shells and described them all as conchae trilobae.

I have already touched upon the triple division according to the head, body and tail shields while outlinging the developmental history of the individual organism. A second triple division will now be emphasized, as longitudinal furrows run through the head, body and tail shields (dorsal furrows). The middle sector is called the rhachis, and the side parts are named pleuren. The various sectors of the tail shield cannot be independently moved, but are rather melted together as a unified shield. It is a different situation for the body shield segments. They grip into each other like roof tiles and leave most trilobites with the possibility of rolling themselves together like hedgehogs or wood lice (Ill. 5), and a few species could close up like boxes. Whether all species used this method to protect themselves against enemies is not known; some species, though, did often have this habit, as shown by many finds of rolled up trilobites. Along the middle of the back, the segments have bumps and spikes, and the pleuren often also end in spikes.

The richly segmented head shield (cephalon) merits particular attention. The middle sector, between the two longitudinal furrows, is subdivided by the traces of 4-5 cross furrows. The cheeks lie to the sides of the glabella, and these are either secure or loose (but nevertheless movable) cheeks subdivided by a suture called a Furche (Additional note: I'd usually translate that as 'furrow', but there's bound to be a better formal term.) The outer rand of the cheek can run into a short or long spike at its end, and this leant the animal a war-like appearance (Ill. 4). As unattractive as this facial line looks, it is of systematic importance; it is mainly on this basis that science has established the orders. However, a system based on the pronouncement of this feature cannot be termed "natural", and objections are sometimes raised; but one has not yet found something more suitable.

The eyes of trilobites are mostly set together from facets, they usually lie on the cheeks, are shaped like half-moons, and sometimes also placed on bumps or stems. The number of facets for a single eye varies greatly; some have only a few facets, others have many thousands (one has estimated a few species as having up to 15,000). Also single-faceted eyes are found (stemmata). Also astonishing is that the eyes are not always positioned in the same place for all species; they can be more or less near to the edge of the head shield, on or under it, and can even be entirely absent. One must not even assume that such 'blind' trilobites lived at such depths in the ocean, that light could no longer reach them, nor that a life spent constantly in the mud had made the need for eyes redundant. Much evidence suggests that some of these 'blind' trilobites possessed no eyes on the upper side of the head shield, but much observation on the development of trilobites has shown that, with time, the eyes wander from the middle towards the edge of the shield, and can also end up placed below it. The underside of the head shield closes at the front edge at an oval, domed shield, the hypostom, and this carries two low bumps (maculae) on the outer side. Microscopic examination has revealed structures there which are similar in build to trilobite eyes, so a pair of eyes would have been placed on the underside of the head as well. I do not want it left unsaid that this interpretation has also found opponents, who view these features as being anchoring points for muscles.

Other than for the hypostom and the chitin skin over the stomach, for a long time one knew nothing of the underside of trilobites. One assumed that the feet must have been soft parts as they had never been found in a fossilized state. It is less than 50 years ago that one discovered the first trilobite feet. Finally, a particularly favourable find from the Lower Silurian Utica Shale of Rome (New York State) allowed a complete picture of the underside of a trilobite to be gained, and it showed all appendages (Ill. 6).

Next to the hypostom emerged a pair of sectioned antennae, and there were also 4 pairs of legs on the jaws below the head shield. In addition, a pair of thick, divided feet sat on the underside of each segment of the body and tails shields, with the longest division being endopodit and the somewhat shorter, equipped with bristles, the exopodit, and that served mainly as a swimming organ. A pair of leaf-shaped structures, on an organism that does not with certainty belong to the trilobites, has been interpreted as gills. Furthermore, pipe-like impressions on the underside of the head shield have been observed, but what these are is not yet clear; some researchers regard them as liver sacks whereas others see them as the impressions of vessels. Above, or on the rear edge of the hypostom, is the mouth opening. The intestinal canal extended from it to the stomach below the glabella, and this then ran between the back armour and skin of the stomach, gradually narrowing along its course, until reaching the anus on the underside of the tail shield.

One can draw conclusions with a high degree of certainty from the external appearance of the trilobites. Undoubtedly, they were capable of swimming, but the small dimensions of the feet suggest they were more dependent upon crawling. This is consistent with the fact that individual trilobite species were more or less restricted to particular areas. They did not wander from one sea to another, but were sometimes carried off by ocean currents. They lived together in great swarms, and seem to have favoured coastal areas or shallow seas. Their food could only have consisted of small and soft animals and plants, as trilobites have no means of breaking up harder objects. It is also to be mentioned that one has found crawl trails and coils (excrement?) in rock layers covered with trilobite remains, and these have been referred to them; in particular, structures described as cruziana and bilobites have been brought into connection with trilobites.

As I already stated in the introduction, the trilobites were restricted to the ancient stage of the history of life, the Paleozoic. Their sudden appearance in a relatively well developed form, and their complete disappearance before the start of the Triassic appear puzzling to us today. The assumption that presently living animals are, as said, descended from trilobites cannot not be easily accepted. Currently, for a decisive conclusion, it is better to await further discoveries and research.

An index of more of my translations of old Kosmos articles can be found at:

Kosmos Translations Archive

A number of Mesozoic (and post-Mesozoic) location summaries can be found at Localities.