Literature Review on the Ultrasructural Variation in Trilobite Exoskeletons

Haley Kempf, “Literature Review on the Ultrasructural Variation in Trilobite Exoskeletons”
Mentor: Victoria McCoy, Geosciences

Trilobites were extremely diverse and abundant arthropods, now extinct, with over 20,000 recognized species ranging in size from one mm to 72 cm. Unlike most living arthropods, trilobites had a heavily calcified exoskeleton, sometimes bearing elaborate ornamentation. The exoskeletal structure and composition in modern arthropods is well-studied and well understood. In trilobites, however, diagenesis (the process by which biological material is altered into a fossil over geologic time) often alters or obscures the original structure and composition of the exoskeleton, resulting in widely varying published descriptions. The aim of the present research is to determine as best as possible the true exoskeletal structure of living trilobites through collating, interpreting and synthesizing data from the literature. Fossil trilobites are reported to have between one and four exoskeletal layers; some of this variation may represent diagenetic variation and some may represent actual biological variation. Based on comparisons to modern arthropods and interpretations of the fossilization process, the majority of trilobites likely had a three-layer exoskeleton, composed of the outer epicuticle, the middle exocuticle, and the inner endocuticle. In these cases, the two commonly preserved layers are interpreted as the exocuticle and endocuticle: the exocuticle is preserved as an outer layer of prismatic calcite and the endocuticle (or “principal layer”) is preserved as calcite with various non-prismatic textures. The epicuticular layer is typically not preserved. There are some trilobites whose fossil exoskeleton structure suggests the living exoskeleton did not conform to this typical three layer structure. Tapinocalymene nodulosa, for example, has four exoskeleton layers in the fossil suggesting more than three layers in life. In living arthropods the structure of the exoskeleton can provide information about functional morphology, so a better understanding of the exoskeletal structure of trilobites may help understand the function of various body parts, including the enigmatic ornamentation.

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