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Effaced styginid trilobites from the Silurian of New South Wales


, : Effaced styginid trilobites from the Silurian of New South Wales. Palaeontology (Leeds) 41(5): 853-896, Sept

Eight species of illaenimorph trilobites belonging to five genera of the Styginidae are described from limestones of the mid-late Wenlock to Ludlow Mirrabooka Formation and its stratigraphical equivalents in the Orange district, New South Wales. The morphology of illaenimorph (=effaced) styginids is discussed; the term 'omphalus' is introduced for the socketed, tubercle-like projection present in some genera on the interior of the cranidium at or in front of the anterior end of the axial furrow. Amongst other characters, the gross convexity of the exoskeleton, the form of the rostral plate, the presence of the omphalus, the form of the thorax, and possibly the form of the hypostome are deemed most useful for generic diagnosis; characters used for discrimination at a lower taxonomic level include the proportions of the exoskeleton, the degree of effacement, the pattern of cranidial muscle scars, the size and position of the eye, and the character and disposition of sculpture. New taxa are Excetra iotops gen. et sp. nov., Lalax olibros gen. et sp. nov., L. lens gen. et sp. nov., Rhaxeros synaimon sp. nov. and R. trogodes sp. nov. Bumastus (Bumastella) Kobayashi and Hamada is raised to generic status and its diagnosis emended; specimens from New South Wales are assigned to the type species B. spicula, which is considered to be synonymous with five other Japanese species assigned to three different genera by Kobayashi and Hamada. Bumastus is tentatively recorded on the basis of a single rostral plate; the genus is otherwise known with certainty only from Laurentia and eastern Avalonia. Meraspid transitory pygidia of Bumastella and Lalax from New South Wales are up to eight times larger than those of other styginids with well documented ontogenies; transitory pygidia of large size are known also in some other Silurian effaced styginids, and it is suggested that the phenomenon may result from neoteny. The assumption that sexual maturity in trilobites coincided with the meraspid-holaspid transition is refuted. The effaced styginids from New South Wales show strong faunal affinity with those from the Upper Wenlock or Lower Ludlow of Japan.

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