Reconstruction of the bracing system of the trunk and tail in hyposaurine dyrosaurids (Crocodylomorpha; Mesoeucrocodylia)

Abstract

ABSTRACT The axial skeleton of hyposaurine dyrosaurids differs significantly from that of extant crocodylians, most notably in the height of the neural spines, the vertically oriented thoracic ribs and osteoderms that lack external keels. This indicates a different configuration of the epaxial musculature for hyposaurine dyrosaurids compared with extant crocodylians and possibly all other fossil species. In the trunk, m. transversospinalis tendons formed a tendinal lattice. M. tendinoarticularis and m. longissimus dorsi were divided into high, medially open myoseptal cones. M. iliocostalis of hyposaurine dyrosaurids was larger than in extant crocodylians and braced the arcade of the vertebral column during locomotion. The trunk bracing-system of hyposaurine dyrosaurids is referred to as a Hyposaurus-type bracing system, and is modeled as a segmented, hydraulically stabilized string-and-bow-construction, with the single segments consisting of inverse T-beams. The trunk bracing-system and osteoderm morphology of hyposaurine dyrosaurids indicate that the vertebral column could not be sufficiently stabilized against dorsoventral shear loads, and possibly transverse shear loads. As a consequence, the high walk and gallop could only be performed by individuals with low body mass. In the tail, the enormously elongated neural spines indicate a large cross section of the epaxial musculature, which facilitated the effective use of aquatic locomotor modes like axial and hybrid swimming.