Use of the notochordal canal as a reliable proxy for prenatal stage, a case study in Ichthyosauria

Abstract Determining embryonic stage is critical to assess heterochronic development between species and aids in our understanding of a clade’s evolution. Prenatal staging is difficult in extinct taxa due to the rarity of embryonic and fetal material. Ichthyosaurs are extinct reptiles with many prenatal specimens known. Staging in ichthyosaurs is currently based on cranial material, which is not always available and prone to interspecific heterochrony. Here, we show that the relative size of the notochordal canal compared to vertebral centrum size can be used as a proxy for developmental stage. We measured the combined height and width of the notochordal canal and vertebral centrum to obtain a measure of vertebral ossification (Notochordal Canal Index). We found that, within a fetus, the relative size of the notochordal canal is not homogeneous over the vertebral column. However, in the presacral centra, the relative size of the notochordal canal is constant. We tested the applicability of this index in several ichthyosaur species. In all species, the measured NCI is congruent with their developmental stage as inferred based on cranial ossification. The notochordal canal is a remnant found in early developmental stages of all vertebrates. The proxy is therefore potentially applicable to other vertebrate clades.