I.Rábano, R. Gozalo and D. García-Bellido (Eds.), Advances in trilobite research. Cuadernos del Museo Geominero, nº 9. Instituto Geológico y Minero de España, Madrid, 2008. ISBN 978-84-7840-759-0 © Instituto Geológico y Minero de España 229 THE CEPHALIC MEDIAN ORGAN OF TRILOBITES Rudy Lerosey-Aubril and Kenneth J. McNamara Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK. leroseyaubril@lycos.com, kmcn07@esc.cam.ac.uk Keywords: Trilobita,Arthropoda, cephalic median organ, morphofunctional anatomy, systematics. INTRODUCTION As in modern arthropods, a great variety of structures (tubercles, pits, ridges) are visible on the outer surface of the trilobite cuticle.The most obvious and/or common of them have naturally attracted particular attention (e.g. terrace ridges; see Whittington, 1997, p. 148, and references therein). Others, like the cephalic median organ (CMO), have never been subject to systematic investigations and remain almost unknown. The CMO is a small complex of structures located, as indicated by its name, on the sagittal line of the cephalon. This complex, generally composed of four pits arranged at the corners of an imaginary square, is rather common in odontopleurid and corynexochid trilobites where it has been called the ‘occipital organ’ (Whittington, 1956, 1965). However, its distribution within the Trilobita, as well as through time, and its morphological variability are virtually unknown. Here we report the preliminary results concerning these issues from a comprehensive study aimed at investigating every aspects of this enigmatic organ. MORPHOLOGY AND VARIABILITY The CMO is typically composed of four rather small pits, located on the occipital ring. An additional fifth pit, with a central position, is sometimes observed, especially in odontopleurids where it may represent a character that varies intraspecifically (e.g. Meadowtownella ascita; Whittington, 1956). Generally, the pits are equidistant, located at the corners of an imaginary square (Fig. 1a). However, other geometric shapes can be recognized, such as a rectangle (Fig. 1b, c), an isosceles trapezoid (Fig. 1d) or an inverted isosceles trapezoid (Fig. 1e). The CMO is most often situated on a flat or slightly inflated surface (Fig. 2a), occupying a roughly central position on the occipital ring (sagittally). It is rather common, however, to observe it at the apex of a more or less developed tubercle (Fig. 2b, c), such as in several odontopleurids. Even when present on a flat surface, it is always on or near the highest point of the cranidium, suggesting a particular influence of