South American Sauropodomorphs: What Their Bone Histology Has Revealed to Us

Abstract

Given that bone microstructure is a very important source of paleobiological information, several paleohistological studies have been conducted on sauropodomorph dinosaurs, possibly making this clade the most studied histologically. Despite these paleohistological studies on sauropodomorph dinosaurs from South America are relatively scarce in comparison with other regions of the world, significant progress on this matter (i.e. paleohistology of South American sauropodomorphs) has been made during the last decade. Following an order from rather specific to more generalized issues, the most important advances are related to the origin of particular skeletal elements (i.e. osteoderms, sacral supraspinous ossifications and extremely elongated cervical ribs), the growth patterns of basal sauropodomorphs and the variation on sauropod growth dynamics and its relationship with gigantism. Regarding the origin of osteoderms and extremely elongated cervical ribs, these structures have been formed by metaplastic ossification of dermal and tendinous tissues, respectively. Their histological characterization has been helpful to discover that the alleged osteoderms of Agustinia ligabuei were actually dorsal and cervical ribs. The long bone histology of basal sauropodomorphs has revealed that the cyclical growth pattern assumed for this group actually showed some degree of variation. Finally, a clade of basal sauropods (i.e. lessemsaurids) was characterized by a cyclical growth pattern, not previously reported for other sauropods, combined with episodes of highly accelerated growth rates. The gigantic body sizes obtained by lessemsaurids were therefore reached through a growth strategy different from that developed by eusauropods (i.e. rapid and continuous growth).