Pathology: Bites, Injuries & Healing
Judging by the variety of lethal-looking bite marks and healed injuries that frequently adorn their fossilized exoskeletons, it seems safe to say that life as a trilobite presented some daunting challenges. Such stark evidence offers emphatic proof that the primal seas in which these Paleozoic arthropods flourished for 270 million years rarely provided a hospitable environment. Indeed, those ancient oceans were filled with an ever changing, and ever more dangerous array of predators, all seemingly intent on turning the local trilobite population into little more than an afternoon snack.
From the moment they emerged on the world stage some 521 million years ago, trilobites were in constant peril from a lethal cast of predators, ranging from the legendary Cambrian-age Anomalocaris, to giant Silurian eurypterids, to armored Devonian fish. Current theory even speculates that some trilobites, such as the large Cambrian genus Olenoides (which often exceeded 12 cm in length), may have taken an occasional cannibalistic turn upon some of their smaller trilobite brethren. Fossilized trilobite carapaces sporting either healed or potentially fatal bite marks are pervasive in certain locales, such as the famed Middle Cambrian Elrathia kingi beds of Utah.
By the dawning of the Ordovician, 485 million years ago, a majority of trilobite species had developed the ability to enroll, which provided them with at least a degree of protection from the hostile world which surrounded them. Other trilobites had evolved to grow rows of menacing spines that covered their carapaces and afforded additional defense from predators. Still others apparently were able to generate increasingly thick calcite shells with each successive molt, providing these trilobites with a greatly enhanced ability to survive and pass along their genetic advantages.
Despite their best efforts to protect themselves, however, the pathologic evidence indicates that trilobites were still often the subjects of predatory attack. Even in the Devonian and subsequent Carboniferous, a time when the entire trilobite line was in steep decline, the fossil record reveals evidence of their injuries, many of which the trilobite apparently survived. On those blatantly bitten -- or otherwise injured -- shells, we can often see the appearance of asymmetric spines, indicators of previously healed injuries consistent with bite mark predation. While it's virtually impossible to attribute these later wounds to any explicit attacker, in some earlier cases, especially during the Cambrian, those marks appear to be caused by specific predators such as the previously mentioned Anomalocaris - whose menacing appendages have been found in close association with trilobites everywhere from British Columbia to central Utah to southern China.
As a side-note, since trilobites frequently shed their outer shell - much like many modern arthropods -- the wounded exoskeleton of any surviving creature could not begin to heal until their subsequent molt occurred. Occasionally, the fossilized edges of these next-molt bite marks appear surprisingly smooth, a credible sign that the healing process had already begun. Often however, the jagged but symmetric pattern left upon the trilobite shell showed that the inherent injury may have indeed been fatal.
But if such was the case, the trilobites' loss served as our eventual gain, for the recovery of these flagrantly injured carapaces has provided science with an additional window upon life in the Paleozoic oceans. After all, it's one thing to examine 500 million year old fossils featuring the detailed impressions of ancient life forms… it's quite another to be able to interpret those impressions in a manner that furnishes greater understanding of what the battle for daily existence may have been like during those long-forgotten yesterdays when trilobites ruled the seas.
Here is a look at some trilobites exhibiting evidence of injury.
Bathyuriscus fimbriatus Robison,1964
Late Middle Cambrian
Marjum Formation, House Range
Millard County, Utah, U.S.A.
Note: Defect left pygidium consistent with bite injury. The smooth edges of the defect suggest repair on a subsequent molt.
Coal County, Oklahoma, U.S.A.
2 cm (without spines)
Note: Asymmetric occipital spines consistent with post injury regeneration
Rosella Formation, Atan Group
British Columbia, Canada
Note: Large defect on left side consistent with possible bite mark
Eoredlichia intermedia (Lu, 1940)
Chengjiang Maotianshan Shales
Yunnan Province, China
Note: Defects on left anterior and mid thoracic segments consistent with a bite injury. The smooth rounded edges of the shortened pleurae suggest repair on a subsequent molt.
Glossopleura gigantea Resser, 1939
Note: Defects on right posterior pleural tips and pygidium consistent with bite injury. The rounded edges of the shortened pleural tips suggest subsequent healing.
Hemirhodon amplipyge Robison 1964
Millard County, Utah, U.S.A.
Note: Defect right pygidium consistent with bite injury (? Anomalocaris). The smooth edges of the defect suggest repair on a subsequent molt.
Isotelus maximus Locke, 1838
Bowmanville, Ontario, Canada
3 cm across generals
Note: Regenerative repair to left genal
Athabaskia bithus (Walcott, 1916)
Langston Formation, Spence Shale
Antimony Canyon and Miner's Hollow, Wellsville Mountains, Utah, U.S.A.
Note: bite mark right thorax