Although it has been considered a secondary, perhaps diagenetic, state of lamellar crown tissue, white matter is now widely interpreted as a primary tissue that developed in association with lamellar crown tissue. The homology, structure and ontogeny of white matter are contentious and its interpretation has been central to debate over conodont skeletal tissue homology and the phylogenetic position of conodonts. Lines parallel to model long axis delineate segmentation for radial orthotropy (see text for explanation). Black area in ( e) indicates basal constraints for both loading conditions.

( e, f) Anterior and occlusal views, showing pressure applied to dorsal edge.

( d) Occlusal view, showing pressure applied to tip. Dark grey regions delineate area of applied load and arrows indicate its approximate direction. ( d– f) Model geometry and loading for FE analysis of W. ( c) Wurmiella excavata P 1 element (ROM 61381) in anterior view, highlighting cusp in boxed area arrows indicate direction of cusp motion during draw (white arrow) and puncture (grey arrow) cuts. ( b) Scanning electron micrograph of Ozarkodina confluens S element longitudinal section showing white matter and lamellar tissue. White matter appears darker because its interstices scatter light. ( a) Wurmiella excavata P 1 element (Silurian, Tramway Netherton, UK) in transmitted light, illustrating white matter and lamellar tissue.

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