Reassessment of a theropod ilium from the Kem Kem beds of Morocco and the evolution of ilia in Spinosauridae

Abstract

A theropod ilium MHNM KK04 from the Kem Kem Beds (Cenomanian) of Morocco was originally described as an abelisaurid. It is here reinterpreted as a spinosaurine spinosaurid. The phylogenetic relationships of MHNM KK04 were analyzed. A thorough and careful comparison between spinosaurids and abelisaurids was made. The Kem Kem ilium was identified as a spinosaurid based on the similarity to the Spinosaurus aegyptiacus neotype and the MSNM V6900 specimen (i.e., the lateral wall of brevis fossa is taller than the medial wall, the ventral margin of the postacetabular blade is straight, and the ventral margin of postacetabular process is posteroventrally oriented). However, MHNM KK04 is slightly different from the ilium of the Spinosaurus aegyptiacus neotype (i.e., the ilium dorsal margin is sub-horizontal to slightly anteriorly inclined, the dorsoventral height of the iliac blade at the postacetabular portion is approximately the same size as the blade height above the acetabulum, and the postacetabulum length is slightly longer than the ischial peduncle length). MHNM KK04, together with previous studies, shows that there were at least two morphotypes of spinosaurines in the Late Cretaceous of Kem Kem beds. Nevertheless, taphonomy, intraspecific or ontogenetic variation cannot be ruled out in the present work. This reclassification adds to the number of juvenile spinosaurines from the Kem Kem beds and provided evidence on the evolution of ilia in spinosaurids.