Beating the bends: The spare ribs of Big Meg.

LISTON, JEFF; GOUIRIC CAVALLI, SOLEDAD

Oxford

Simposio; 60th Annual Symposium of Vertebrate Palaeontology and Comparative Anatomy. 21st Symposium of Palaeontological preparation and Conservation; 2012

University of Oxford

Historically, a large number of skeletal elements of the pachycormid suspension-feeder Leedsichthys have been uncertainly identified as ?rib-shaped bones?. In an attempt to resolve this, the basic principle that the shape and architecture of a bone reflects the stresses that it operates and grows within (and the mechanical environment around it) was used, in an attempt to ?reverse engineer? a likely location for these unlocated elements. The ?Big Meg? specimen of Leedsichthys (GLAHM V3363) was selected for this analysis, as it has a particularly wide range of elongate bone morphologies referred to as ?rib-shaped?. A series of thirty elements from the ?Big Meg? specimen were examined for their curvature, and the normalised results plotted, using the following equation: normalised bone curvature (ξ) = X/2L x 100 where the orthogonal distance (X, the moment arm of the axial component of force acting on a bone) was measured from the chord (2L) between the proximal and distal ends. For comparison, two partial articulated pachycormid specimens (MOZ-Pv 1160, MOZ-Pv 0281) from the Upper Jurassic Neuquén Basin of Argentina were used. In these closely related taxa the bone positions were accurately known. In MOZ-Pv 1160 the pleural ribs were measured, and in MOZ-Pv 0281 the neural and haemal spines. The plotted normalised curvatures of the three specimens were then compared. Leedsichthys elements plotted into three discrete clusters, representing grades of curvature. The Argentinian pachycormids demonstrated a growth in curvature from the tail forward, only achieving mid-grade curvature as a maximum, for mid-body pleural ribs.