Fig 1.
Sexual dimorphism in the plates of S. mjosi.
(A) The largest wide morph plate (SMA 0018). (B) The largest tall morph plate (JRDI 5ES-552).
Fig 2.
(A) The biplot of the first and second principal components for PCA of all fairly complete S. mjosi plates. The data point with the high PC2 value is the anterior dorsal plate from SMA 0092 that was later dropped from the analysis (n = 40). (B) The biplot of the first and third principal components for PCA of all fairly complete plates except the anterior dorsal plate from SMA 0092 (n = 39). Orange and black points are plates identified to be of the tall and wide morph, respectively.
Fig 3.
(A) A thin section of a tall morph plate (JRDI 5ES-357) in plane polarized light showing an external fundamental system (EFS) indicating the cessation of growth. Bone surface is to the left. (B) The same image but in crossed polarized light. (C) A CT scan of a tall morph plate (JRDI 5ES-237) in cross-section along the frontal plane. Red arrows indicate internal vascular piping that is a sign of sexual maturity.
Fig 4.
Hypothetical silhouettes of male and female S. mjosi.
The wide morph exhibits more overlap between adjacent plates than does the tall morph, leading to a more continuous display surface. Sexual dimorphism in the size and shape and plates might have also occurred with other sexual differences such as sexual dichromatism.
Fig 5.
Diagrammatic depiction of measurements.
(A) Vertical oval/triangular plates (JRDI 5ES-579). Tall morph S. mjosi plates typically fall under this category. The minor axis, rather than the anteroposterior width, was the value used for ‘width’ in PCA. (B) Horizontal oval plates (JRDI 5ES-523). Wide morph S. mjosi plates typically fall under this category. The major axis was the value used for ‘width’ in PCA. (C) Sub-right trapezoidal plates (DMNS 2818). The major axis and anteroposterior width tend to converge. The major axis was the value used for ‘width’ in PCA. (D) Shoulder plates (NHMUK R36730). These plates have a distinctive diamond shape. The distance from the center of the base to the apex and the major axis tend to converge.