Trilobite Abnormalities
last revised 12 DEC 2010 by S. M. Gon III
Abnormality in Acanthopyge
An abnormality in the pygidium of the
Moroccan lichid trilobite Acanthopyge
Because trilobites are relatively common in the fossil record, occasionally we find various kinds of abnormalities, such as asymmetrical features, healed injuries, or signs of disease. Teratology is the study of malformations or serious deviations from normal form or development, and such studies of trilobite fossils have revealed some very interesting abnormal forms.

Healed injuries
The most common types of trilobite abnormalities are partially healed injuries. Trilobites were victims of many predators in Paleozoic seas. Because an exoskeleton can not heal until molting, abnormalities such as the ones shown here document that the trilobite survived the attack and began to heal the damaged area during its next molt. This kind of repair would often require several molts, with more and more recovery of the injured area restored, so the molts provide a sequential picture of how trilobites repaired their wounds.
Healing pleurae in Phacops
Partial healing of two thoracic pleurae in
Phacops.The rounded edges of the small pleurae indicate the zone of healing. a fresh injury would likely be sharp-edged.

Image courtesy: Marc Behrendt
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Disease-related abnormalities
Disease is another possible cause of damage to trilobite exoskeletons. Infections of small injuries might cause slow-healing ulcers that induce the exoskeletal equivalent of scar tissue or even neoplasms (tumors) such as shown in the exoskeletons of Bohemoharpes and Hydrocephalus at right (from Owen 1983).

In the Bohemoharpes (left), the abnormality is also accompanied by radiating circulatory canals, similar to the way that tumors often attract blood vessel development. The neoplasm in the exoskeleton of Hydrocephalus (right) may have begun as an irritation or small injury in the sensitive thinner exoskeleton between the segments.

In both cases, over sequential molts, scar tissue or thickened exoskeleton was deposited around the infection site, perhaps in a similar way that tree bark surrounds and encloses injury. Typically there is only a single neoplasm on a trilobite exoskeleton. This suggests that trilobites were resistant to such infections, or that major infections proved fatal, with no opportunity for exoskeletal evidence.
malformed Bohemoharpes
Neoplasm in the fringe of Bohemoharpes.
Neoplasm on Hydrocephalus
Neoplasm in the interpleural axis of Hydrocephalus.

Asymmetrical development
Occasionally there are specimens that show asymmetries that are relatively subtle, such as different numbers of lenses between right and left eye, where there is no evidence of injury. Such abnormalities might have arisen as a result of developmental problems. For example, in a specimen of Phacops rana (right), the size and number of lenses in the left eye is much reduced compared to the right, which bears a normal size eye and a full complement of lenses. The pattern of reduction, with smaller eye surface and reduced numbers of lenses of normal size, mimicks evolutionary reduction of eyes in other phacopids, but exhibits it in a single abnormal individual. The fact that this is a holaspid suggests that even with a smaller eye and reduced lenses, the growing trilobite was able to engage in a normal enough lifestyle to survive and grow to adult stage.
abnormal left eye in Phacops rana

To end this page, here are a few more examples of typical injury-related abnormalities in a variety of trilobites. sometimes, fossils bearing injuries are so common that statistical studies can be conducted. One study (Babcock 1993) demonstrated a bias toward injury on one side, suggesting that either the predator attacked in a typical way, or that trilobite evasive maneuvers were consistently to one side, resulting in higher likelihood of injury to a particular side. This is an interesting insight on the behavior and ecology of long-extinct creatures in an ancient ecosystem.
Abnormal Scutellum pygidium Abnormal elrathia Abnormal genal spine
abnormal right pygidium
Scabriscutellum sp.
image courtesy Dieter Holland
abnormal left thoracic pleurae
Elrathia kingi
image courtesy Ross Wetzel
abnormally short genal spine
Diademaproetus sp.
image courtesy Dieter Holland

For more information:

Babcock, L.E. 1993. Trilobite malformations and the fossil record of behavioral asymmetry. J. Paleontol. 67(2):217-229.

Conway Morris, S. & R.J.F. Jenkins 1985. Healed injuries in Early Cambrian trilobites from South Australia, Alcheringa 9:167-177

Jago, J.B. & Haines, P.W. 2002. Repairs to an injured early Middle Cambrian trilobite, Elkedra area, Northern Territory. Alcheringa, 26:19-21

Ludvigsen, R. 1977. Rapid repair of traumatic injury by an Ordovician trilobite. Lethaia 10(3):205-207

Owen, A.W. 1980. An abnormal cranidium of the trilobite Calyptaulax norvegicus. Norsk Geologisk Tidsskrift 60:87-88

Owen, A.W., 1983. Abnormal Cephalic Fringes in the Trinucleidae and Harpetidae (Trilobita). Spec. Pap. Palaeontology 30:241-247

Owen, A.W. 1985. Trilobite Abnormalities. Trans. Royal Society Edinburgh, Earth Sci. 76:255-272

Owen, A.W.; Tilsley, J W. 1996. An abnormal pygidium of the trilobite Brachymetopus ornatus Woodward from the Lower Carboniferous of Derbyshire. Geological Journal 31(4):389-392

Rudkin, David M. 1985. Exoskeletal abnormalities in four trilobites. Can. J. Earth Sci. 22:479-483

Snajdr, Milan 1979. Two trinucleid trilobites with repair of traumatic injury. Vest.Ustred.ustavu geol. 54(1):49-50

Snajdr, Milan 1979. Note on the regenerative ability of injured trilobites. Vest.Ustred.ustavu geol. 54(3):171-173

Whittington, H.B. 1997. Abnormalities of the exoskeleton. In: Whittington, H.B. et al. Treatise on Invertebrate Paleontology, Part O, Arthropoda 1 Trilobita (Revised). The Geological Society of America and the University of Kansas.

Kipping, Michael. Pathologische Trilobiten: from the website
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