58-4 A Review of Occurrences of the “Dorsal Organ” in Trilobites with Evidence from BSE-SEM Analysis of New Material of the Upper Ordovician Trilobite Cryptolithus tessellatus Green, 1832

Session: Future Leaders in Paleontology

Presenting Author:

Authors:

Abstract:

Trilobite specimens secondarily replaced by silica provide an opportunity for detailed study as they can be completely liberated from the matrix by chemical means. The limestones of Virginia’s Shenandoah Valley have been the source of abundant silicified trilobite remains for almost a century, beginning with seminal works in the 1940-50s by G. A. Cooper, H. B. Whittington, and W. R. Evitt. Here, I discuss some results of Backscattered Electrons-Scanning Electron Microscopy of nearly 1000 cranidia, collected in the lower Martinsburg Formation near Harrisonburg, Virginia, and assigned to the trinucleoid species Cryptolithus tessellatus. Among these is the presence of an array of five small pores in the median glabellar tubercle that are only visible in early ontogeny and under certain taphonomic circumstances. Such a structure is present among extant crustaceans and has generally been referred to as the “dorsal organ.”

Speculation about the function of the glabellar tubercle of trilobites began with Beyrich (1846) who thought that it might be the start of the gut tract. Ruedemann (1916) made thin sections through it in a few different species and concluded that it likely served as an ocellus, or simple eye. Størmer (1930) published a rigorous study of the trinucleoid Tretaspis and generally agreed with Ruedemann, though he later decided that it might be more akin to the enigmatic dorsal organ of some modern crustaceans following the work of Hanström. In the decades since, partly fueled by the inference that trilobites ought to have had median ocelli, many other workers have debated about the nature of the glabellar tubercle. Detailed analysis of the glabellar tubercle of C. tessellatus indicates that the dorsal organ hypothesis is more likely but that preservation, imaging technique, and ontogenetic stage are all factors that determine whether diagnostic microstructures are visible. Recognition of the true identity of such features is important, not only for inferring the sensory capabilities of extinct organisms, but also for determining homology and resolving evolutionary patterns.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6957

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