New machaeridian data from the Upper Ordovician of Scotland: Palaeoecological and global palaeobiogeographical implications

New machaeridian material housed in the National Museum of Scotland alongside the type material held in the Natural History Museum London and the Sedgwick Museum of Earth Sciences, Cambridge, is documented here from the Ordovician of Girvan. The specimens are included in four taxa, three of these in open nomenclature. Syntypes and figured specimens of Plumulites scoticus by Nicholson and Etheridge (1879) and Withers (1926) are reviewed and the diagnosis emended. The description of these specimens follows standard terminology and we also introduce here a new terminology for Anterior Outer Shell Plate (AOSP). We also figure for the first time three unpublished letters from Mrs Elizabeth Gray to Thomas Henry Withers, where she criticises the illustrations of machaeridians published by Nicholson and Etheridge (1880) and Reed (1908), recognising issues identifying machaeridian plates and also clarifies a misunderstanding on some of the specimens described and illustrated in Reed (1908), being key for the distribution of Plumulites peachi. We assess the importance of machaeridians as part of the Lower Palaeozoic palaeobenthos and a key element of the palaeo-food chain. A review of the palaeogeographical distribution of the machaeridians is undertaken in the context of the Ordovician biodiversifications.