# Comparative Morphology of Skeletal Development in Homo sapiens and Raja asterias: Divergent Stiffening Patterns Due to Different Matrix Calcification Processes

## TL;DR

This paper compares how human and shark skeletons develop, focusing on how different calcification processes lead to distinct skeletal structures adapted for land and water movement.

## Contribution

The study documents divergent calcification patterns in Homo sapiens and Raja asterias, linking them to locomotion-specific skeletal adaptations.

## Key findings

- Chondrichthyans like Raja asterias retain calcified cartilage as their definitive skeletal structure, forming tesserae for aquatic locomotion.
- Mammals use calcified cartilage as a scaffold for bone deposition, which is remodeled for terrestrial locomotion.
- Dolphin flippers show endochondral ossification adaptations, including shortened bones and increased autopodial elements for aquatic demands.

## Abstract

Before calcification, the early skeletal development of both Homo sapiens and the chondrichthyan fish Raja asterias is exclusively cartilaginous. This cartilage is formed through tissue segmentation and chondrocyte mitoses. Mineral deposition in the cartilage matrix influences the stiffness and shape of the skeletal segments. In mammals, calcified cartilage serves as a scaffold for bone deposition, which is then remodeled. Conversely, chondrichthyans retain calcified cartilage as their skeletal structure, forming calcification nuclei or “tesserae”. These structures adapt to aquatic locomotion. In mammals, endochondral ossification provides limb bones with the necessary stiffness for terrestrial locomotion. X-rays of marine mammals show how endochondral ossification in dolphin flippers adapts to aquatic demands, including shortening of certain bones and an increase in elements in the autopodium’s central rays.

Before calcification begins, the early embryonic and fetal skeletal development of both mammalian Homo sapiens and the chondrichthyan fish Raja asterias consists exclusively of cartilage. This cartilage is formed and shaped through processes involving tissue segmentation and the frequency, distribution, and orientation of chondrocyte mitoses. In the subsequent developmental phase, mineral deposition in the cartilage matrix conditions the development further. The stiffness and structural layout of the mineralized cartilage have a significant impact on the shape of the anlagen (early formative structure of a tissue, a scaffold on which the new bone is formed) and the mechanical properties of the skeletal segments. The fundamental difference between the two studied species lies in how calcified cartilage serves as a scaffold for osteoblasts to deposit bone matrix, which is then remodeled. In contrast, chondrichthyans retain the calcified cartilage as the definitive skeletal structure. This study documents the distinct mineral deposition pattern in the cartilage of the chondrichthyan R. asterias, in which calcification progresses with the formation of focal calcification nuclei or “tesserae”. These are arranged on the flat surface of the endo-skeleton (crustal pattern) or aligned in columns (catenated pattern) in the radials of the appendicular skeleton. This anatomical structure is well adapted to meet the mechanical requirements of locomotion in the water column. Conversely, in terrestrial mammals, endochondral ossification (associated with the remodeling of the calcified matrix) provides limb bones with the necessary stiffness to withstand the strong bending and twisting stresses of terrestrial locomotion. In this study, radiographs of marine mammals (reproduced from previously published studies) document how the endochondral ossification in dolphin flippers adapts to the mechanical demands of aquatic locomotion. This adaptation includes the reduction in the length of the stylopodium and zeugopodium and an increase in the number of elements in the autopodium’s central rays.