A Spectacular Devonian Trilobite Specimen, Dalmanites pratteni, From Illinois Helps Shed Light on How Trilobite Eyes Worked

In 1912, the Field Museum purchased Henry Pratten's collection of fossils. One of the specimens was a dalmanitid trilobite with large eyes (Fig. 1) that Pratten collected from the Devil’s Backbone. In 1933, this specimen was described as a new species of trilobite, Dalmanites pratteni (P 16704) (Roy 1933). Delo (1935) described a new genus of trilobite, Dalmanitoides and assigned it to a new family, Synphorinae. He noted Dalmanites pratteni has similar ornamentation to this genus. Delo (1940) later places D. pratteni in the genus Odontochile . Other authors refer to it as Dalmanites pratteni (Clarkson and Levi-Setti 1975;Levi-Setti 1975; Frankie et al. 2008). Holloway and De Carvalho (2009) refer to it as ‘Dalmanites’ pratteni and state that this species “probably belongs to the Synphoriinae”. The stratigraphic position of this specimen was not recorded. Seid et al. (2009) published a detailed geologic map of the Devil’s Backbone. Most of the ridge is Middle Devonian Grand Tower and St. Laurent formations. However, the southern end of the ridge is faulted, and the Lower Devonian Backbone Limestone and Crystal Creek Chert (Emsian) is exposed. Frankie et al. (2008) and Devera et al. (2020) report Dalmanites pratteni occurring from the Crystal Creek Chert, indicating that the original specimen may have been collected here. One of the most remarkable features of this fossil is the record number of lenses for a Phacopida. Roy (1933) reports 42 rows of eyes, however: “The exact number of lenses cannot be determined, as not all of them are preserved, nor are the areas which they occupied distinctly outlined. It is, however, certain that there are not less than 770 lenses on each eye.” (Roy 1933). I used images of the eyes and illustration software to estimate the number of lenses by filling in the geometric pattern they exhibit. I counted 730 lenses on the right eye and 712 lenses on left eye, which is missing a sizable piece (Fig. 2). The structure of some individual lenses in this specimen can be seen due to differential weathering. Clarkson and Levi-Setti (1975) reported the large round lenses were composed of a doublet structure that followed the principles established by Descartes (1637) and Huygens (1690) to eliminate spherical distortion. This doublet structure allowed the lens to gather more light and focus it on a single point, and strongly indicates that these trilobites had better vision then previously established (Clarkson and Levi-Setti 1975).