Abstract
Recent discoveries of fossil nervous tissue in Cambrian fossils have allowed researchers to trace the origin and evolution of the complex arthropod head and brain based on stem groups close to the origin of the clade, rather than on extant, highly derived members. Here we show that Kerygmachela from Sirius Passet, North Greenland, a primitive stem-group euarthropod, exhibits a diminutive (protocerebral) brain that innervates both the eyes and frontal appendages. It has been surmised, based on developmental evidence, that the ancestor of vertebrates and arthropods had a tripartite brain, which is refuted by the fossil evidence presented here. Furthermore, based on the discovery of eyes in Kerygmachela, we suggest that the complex compound eyes in arthropods evolved from simple ocelli, present in onychophorans and tardigrades, rather than through the incorporation of a set of modified limbs.
Highlights
Recent discoveries of fossil nervous tissue in Cambrian fossils have allowed researchers to trace the origin and evolution of the complex arthropod head and brain based on stem groups close to the origin of the clade, rather than on extant, highly derived members
The presence of relatively marked relief in the eyes (Fig. 1d; Supplementary Figs 2b, 8b,d) contrasts with Cambrian lobopodians that show a single or a few clustered visual units lacking such elevation[13,14]. This indicates that there was a visual surface formed by the contiguous arrangement of visual units, suggesting that the visual structures seen in K. kierkegaardi represent primitive compound eyes, further corroborated by their large size
The segmental origin of frontal appendages in stem-group euarthropods is one of the key issues for solving the “arthropod head problem”[4], and identifying their innervation has been regarded as a possible solution to this problem[5]
Summary
Recent discoveries of fossil nervous tissue in Cambrian fossils have allowed researchers to trace the origin and evolution of the complex arthropod head and brain based on stem groups close to the origin of the clade, rather than on extant, highly derived members. We show that Kerygmachela from Sirius Passet, North Greenland, a primitive stem-group euarthropod, exhibits a diminutive (protocerebral) brain that innervates both the eyes and frontal appendages. It has been surmised, based on developmental evidence, that the ancestor of vertebrates and arthropods had a tripartite brain, which is refuted by the fossil evidence presented here. For this study, unweathered fossils of Kerygmachela were collected from the rock exposure of the Buen Formation in Sirius Passet, and these specimens include structures preserved as carbonaceous reflective films in the head These structures conform to eyes and nervous tissue in their anatomy and preservation mode. The new material, provides novel information on the overall anatomy of Kerygmachela
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