Sequence of chondrocranial development in basal anurans—Let’s make a cranium

Abstract

Background The craniofacial skeleton is an evolutionary innovation of vertebrates. Due to its complexity and importance to protect the brain and aid in essential functions (e.g., feeding), its development requires a precisely tuned sequence of chondrification and/or ossification events. The comparison of sequential patterns of cartilage formation bears important insights into the evolution of development. Discoglossus scovazzi is a basal anuran species. The comparison of its chondrocranium (cartilaginous neuro- & viscerocranium) development with other basal anurans (Xenopus laevis, Bombina orientalis) will help establishing the ancestral pattern of chondrification sequences in anurans and will serve as basis for further studies to reconstruct ancestral conditions in amphibians, tetrapods, and vertebrates. Furthermore, evolutionary patterns in anurans can be studied in the light of adaptations once the ancestral sequence is established. Results We present a comprehensive overview on the chondrocranium development of D. scovazzi. With clearing and staining, histology and 3D reconstructions we tracked the chondrification of 44 elements from the first mesenchymal Anlagen to the premetamorphic cartilaginous head skeleton and illustrate the sequential changes of the skull. We identified several anuran and discoglossoid traits of cartilage development. In D. scovazzi the mandibular, hyoid, and first branchial arch Anlagen develop first followed by stepwise addition of the branchial arches II, III, and IV. Nonetheless, there is no strict anterior to posterior chondrification pattern within the viscerocranium of D. scovazzi. Single hyoid arch elements chondrify after elements of the branchial arch and mandibular arch elements chondrify after elements of the branchial arch I. Conclusions In Osteichthyes, neurocranial elements develop in anterior to posterior direction. In the anurans investigated so far, as well as in D. scovazzi, the posterior parts of the neurocranium extend anteriorly, while the anterior parts of the neurocranium, extend posteriorly until both parts meet and fuse. Anuran cartilaginous development differs in at least two crucial traits from other gnathostomes which further supports the urgent need for more developmental investigations among this clade to understand the evolution of cartilage development in vertebrates.

Embryonic pattern of cartilaginous head development in the European toad, Bufo bufo

“…Additionally, the ceratohyal chondrifies before the mandibular arch derivatives. Discoglossus scovazzi , which is closely related to Bombina orientalis partially follows the ancient anterior to posterior pattern because at least some derivatives of the mandibular arch chondrify before the ceratohyal (Lukas & Ziermann, 2022). The initial assumption, that the early development of the ceratohyal might be linked to its importance as the central element of the buccal pump and that its early development might be an apomorphic trait for the Lalagobatrachia (Lukas & Olsson, 2020) is challenged by this finding.…”

“…I evaluated a total of 75 cartilaginous structures, which were defined in my previous work (Lukas & Ziermann, 2022), of 141 specimens. A total of 45 out of 75 characters were identified in B. bufo .…”

“…If a certain state was visible within the respective structure, the whole structure was scored in this state even if parts of the structure remained in an earlier state. Additionally, I used the previously defined three different stages of development (Lukas & Ziermann, 2022). Each stage is defined by a specific combination of cartilages in different cartilaginous developmental states.…”

“…Investigations on the chondrocranial development of larval anurans are scarce, but their number is growing. The developmental sequence of three basal frog species, two discoglossoids ( Bombina orientalis , Discoglossus scovazzi ) and one pipoid ( Xenopus laevis ), was recently described and developmental patterns identified (Lukas & Olsson, 2018, 2020; Lukas & Ziermann, 2022). The development of the viscerocranial elements does not follow the anterior to posterior direction as described in other vertebrate clades.…”

“…Additionally, the development of the neurocranial elements also does not follow the presumably ancestral anterior to posterior direction. Instead, the posterior elements extend anteriorly and the anterior elements extend posteriorly until both parts meet and fuse (Lukas & Olsson, 2018, 2020; Lukas & Ziermann, 2022). In all investigated anurans so far, the neurocranium‐anchoring processes of the palatoquadrate chondrify in anterior to posterior direction; first the quadratocranial commissure, then the ascending process, and last the larval otic process.…”