Athena Review Image Archive ™
Asaphus tyrannus, A. buchii, and Trinucleus concentricus (trilobites)
Asaphus tyrannus, A. buchii, and Trinucleus concentricus (photo: after Salter et al. fig. 47)
Asaphus tyrannus and Asaphus buchii are trilobites from the Lower Silurian period, both from the Upper Llandeilo Group. The Llandeilo formation comprises the middle subdivision of the British Ordovician rocks. It was first described and named by Sir Roderick Murchison from the neighborhood of Llandeilo in Carmarthenshire. The Upper Llandeilo Group transitions into the Lower Silurian period.
Asaphus tyrnannus is commonly found and was one of the index fossils used by Murchison to define the Llandeilo formation. Asaphus bucchi (at right) is now called Ogygia bucchi. Both are closely similar in overall form and attributes such as eye form.
At bottom is Trinucleus concentricus from the Caradoc phase of the middle Ordovician. It is one of a group called trinucleids which developed a distinct overall form, including protruding head shield segments with a broad pitted margin, along with a much-shortened lower body. Many such Ordovician trilobites developed external spines and nodules to defend against predators such as primitive eurypterids and nautiloids.
References:
Salter; McCoy; Rupert Jones; and Dana
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