Early Ontogenetic Development of Tessellated Calcified Cartilage in Chondrichthyans

Tessellated calcified cartilage (TCC) is a defining characteristic of the chondrichthyan skeleton, but its early development across the four main groups—batoids, galeomorphs, squalomorphs, and holocephalans—remains largely unexplored. Researchers analyzed 12 representative taxa to examine TCC morphology at an early ontogenetic stage and investigate whether specialized features, such as multi-layered TCC and internal mineralized trabeculae, had formed.

Batoids showcased well-developed, homogeneous, polygonal tesserae early in life, regardless of jaw structure or feeding habits. Conversely, galeomorphs exhibited high levels of morphological variability. Importantly, this study reveals the first documented case of a comprehensive trabecular network inside the TCC of a non-batoid elasmobranch, specifically Ginglymostoma cirratum, which may contribute to the mechanical resilience of suction feeding.

Additionally, the presence of voussoir tesserae in galeomorphs was discovered for the first time, with these structures showing an inverted aspect ratio (wider than tall) compared to mature forms. Surprisingly, despite being known as a durophagous feeder, Mustelus mustelus displayed limited TCC development, indicating a significant ontogenetic delay.

Squatina oculata was characterized by large, thick tesserae and extensive fused tesseral regions, potentially linked to its rapid ambush predation strategy. In contrast, Chimaera presented a poorly mineralized, mesh-like structure lacking distinct tesserae or trabeculae, consistent with earlier research findings. Multi-layered TCC was absent in all specimens, indicating that this characteristic develops later in life.

The study underscores the limitations of generalized models of TCC development based on a single group or select taxa, highlighting the need for more comprehensive investigations. Furthermore, these findings pave the way for future research and comparisons with fossil chondrichthyans, aiming to shed light on the evolutionary history of TCC.