Diverse ossification pathways in the lower jaw of Sclerocephalus (Lower Permian) as revealed by the first record of chondroid bone in Amphibia.

Dermal bones are typically thought to form directly from mesenchyme, however some can develop via a transient tissue called chondroid bone, which are well documented among fishes and amniotes. Here, we report the first occurrence of chondroid bone among non-amniote tetrapods and the stratigraphically oldest record of this tissue among vertebrates, in the temnospondyl Sclerocephalus nobilis from the Lower Permian of Germany. Histological analysis of its mandible reveals a heterogeneity of developmental pathways: the angular forms through direct intramembranous ossification, the articular ossifies endochondrally from Meckelian cartilage, and the dermal surangular uniquely contains chondroid bone, with chondrocyte-like cells embedded in the matrix composed of interwoven loop complexes. This tissue likely represents an early developmental stage, promoting rapid expansion of the bone surface while potentially providing mechanical support in regions experiencing high stress during feeding. The angular exhibits a diploë structure and is composed of lamellar and parallel-fibred bone, with remnants of avascular faint parallel-fibred bone representing the larval stage. The articular represents the only endochondral bone and consists of an early-formed periosteal bone that is parallel-fibred in nature, with prominent Sharpey's fibers and a secondary trabecular endosteal region. The discovery of chondroid bone in Sclerocephalus expands the known diversity of skeletal tissues among early tetrapods and suggests that transitional bone types may have been more widespread than previously recognized, but are likely underreported due to preservational biases and limited sampling.