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Pathological carapaces

Deformities in the Palaeozoic

Every now and then collectors may come across a trilobite which seems to be „different“, not so much in size or rare occurance but rather in its morphological shape which seems to diverge from the regular pattern within a defined group. Something is “wrong” about the specimen and upon closer inspection it may turn out to be missing a pleural segment, perhaps show an atrophied tail shield or an unusually shortened genal spine. Not always is the anomaly as obvious as in cases where the carapace is missing entire sections due to the attack by a predator. Its the more or less subtle distinctions we care about on this subpage.

The following images were supplied by Dieter Holland, who also gave the idea to address this subject in more detail. All images copyright by Dieter Holland

PlatyscutellumObvious misdevelopment in the morphology of the trilobite Platyscutellum from the Devonian of Morocco. This genus regularly shows ten thoracic segments which should run into long pleural spines as is evident on he unaffected right side of this carapace. On the left side, however, the tenth pleural segment not only appears unusually short but is also fused with the tail shield or pygidium. In addition, the ninth pleural segment is distinctly shortened as are segments three to eight, if somewhat less. It has to be said, though, that the latter observation may be attributed to other reasons as they appear sort of “broken off”. This is more evident in the picture’s original size.

Scutellum The tail shield or pygidium of the Styginidae (Scutelluidae) seems to be frequently affected by pathological developments, while at the same time often give evidence of attacks by predators. Specimens belonging to this family with such notable anomalies are frequently found. Our example of Scutellum shows a distinct loss of shell substance to the right pygidial border which appears unusually smooth. The effacement of the pygidial ribs in the direction of the pygidial border, which can be seen more easily in the large version of this image, seem to indicate a pathological development rather than being the result of a healing process after an attack by a predator. Also, there are no bite marks evident as can sometimes be found in such trilobites.

Thysanopeltis This Thysanopeltis from the Devonian of Morocco has been hit hard by a „failure of nature“. The right side of its pygidial shield is clearly pathological, with a surplus of free pleural segments fused together at their ends. The whole tail shield appears to be completely out of its designated shape. It would seem unlikely that the anomalies in this carapace can be attributed to the impacts of a predator attack as they appear to have been due to a morphological misdevelopment. Nevertheless, the trilobite seems to have been able to live with his handicap as extremities, like in most other animals, are usually not vital to the animal’s survival.

Coltraneia A deformed axis or rhachis in an otherwise very nice specimen of Coltraneia from the Devonian of Morocco (very prominent and nicely preserved compound eyes). The peculiarities in this specimen are not as obvious as in the other carapaces shown on this page. Upon closer inspection you will find that some of the axial rings are distinctly more narrow than usual, in particular rings 5, 6, 9 and 10. This observation can be extended to the tail shield. Whether this is actually due to pathological developments or perhaps just to a reduction of substance in the course of diagenesis is not safe to say, but the overall state of preservation in this trilobite clearly speaks in favour of the first option. .

Diademaproetus Unevenly developed genal spines do not appear to be as rare as might be suggested by the lack of such specimens on the market. This Diademaproetus from the Devonian of Morocco is just one example of many more that might have been sorted out during preparation and never made it to the desk of your local fossil dealer. The large version of this image clearly shows that the left genal spine appears to have failed to develop the regular length rather than being broken off at some time. We may speculate, though, that it may have been broken off at some earlier stage of the trilobite’s development and that succeeding moulting cycles played a part in recreating the original shape, even if the normal length could not be reached.

Leymeriella Naturally, morphological anomalies are not restricted to the class trilobita. Ammonites is another group in which such irregularities can be observed quite frequently. As an example, we would like to present a specimen of the ammonite Leymeriella from the Lower Cretaceous of Vöhrum, Northern Germany. Obviously, this cephalopod suffered a severe injury to its planispiral shell which the animal tried to compensate by continuously building new segments to its housing. A pathological development can be ruled out as there is a clear tearing edge visible that can be attributed to a predator..
Should you own a fossil or fossils with similar anomalies and would like to present it on our website, please feel free to email us with an image of your specimen

Last Update : 01/30/2010 5:07 PM

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