Abstract
Basal tetrapods display a wide spectrum of vertebral centrum morphologies that can be used to distinguish different tetrapod groups. The vertebral types range from multipartite centra in stem-tetrapods, temnospondyls, and seymouriamorphs up to monospondylous centra in lepospondyls and have been drawn upon for reconstructing major evolutionary trends in tetrapods that are now considered textbook knowledge. Two modes of vertebral formation have been postulated: the multipartite vertebrae formed first as cartilaginous elements with subsequent ossification. The monospondylous centrum, in contrast, was formed by direct ossification without a cartilaginous precursor. This study describes centrum morphogenesis in basal tetrapods for the first time, based on bone histology. Our results show that the intercentra of the investigated stem-tetrapods consist of a small band of periosteal bone and a dense network of endochondral bone. In stereospondyl temnospondyls, high amounts of calcified cartilage are preserved in the endochondral trabeculae. Notably, the periosteal region is thickened and highly vascularized in the plagiosaurid stereospondyls. Among “microsaur” lepospondyls, the thickened periosteal region is composed of compact bone and the notochordal canal is surrounded by large cell lacunae. In nectridean lepospondyls, the periosteal region has a spongy structure with large intertrabecular spaces, whereas the endochondral region has a highly cancellous structure. Our observations indicate that regardless of whether multipartite or monospondylous, the centra of basal tetrapods display first endochondral and subsequently periosteal ossification. A high interspecific variability is observed in growth rate, organization, and initiation of periosteal ossification. Moreover, vertebral development and structure reflect different lifestyles. The bottom-dwelling Plagiosauridae increase their skeletal mass by hyperplasy of the periosteal region. In nectrideans, the skeletal mass decreases, as the microstructure is spongy and lightly built. Additionally, we observed that vertebral structure is influenced by miniaturization in some groups. The phylogenetic information that can be drawn from vertebral development, however, is limited.
Highlights
Basal tetrapods occurred from the Carboniferous up to the Early Cretaceous [1,2]
Our results show that the intercentra of the investigated stem-tetrapods consist of a small band of periosteal bone and a dense network of endochondral bone
This study addresses the ontogenetic development of vertebral centra in Paleozoic and Mesozoic basal tetrapods based on their bone histology
Summary
Basal tetrapods occurred from the Carboniferous up to the Early Cretaceous [1,2]. In the present study, we follow the taxonomic definitions and phylogenetic framework of Ruta et al [3] and Ruta & Coates [4]: “basal tetrapods” or “crown-group Tetrapoda” encompasses the Paleozoic and Mesozoic temnospondyls, anthracosaurs, seymouriamorphs and lepospondyls. Chroniosuchians form a small clade of anthracosaur-like tetrapods and are mainly known from the Permian and Triassic of Russia and Germany [15, 16] They are small to medium-sized varanid or crocodile-like animals and are characterized by the distinct morphology of their osteoderms and vertebral centra [17, 18]. Lepospondyls are a group of mostly small, but morphologically heterogeneous basal tetrapods They range from the Early Carboniferous to the Early Permian [25] with recent phylogenetic analyses indicating that they are a monophyletic group [3, 4, 26]
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