Abstract
There are two types of dermal skeletons in jawed vertebrates: placoderms and osteichthyans carry large bony plates (macromery), whereas chondrichthyans and acanthodians are covered by small scales (micromery). Fin spines are one of the last large dermal structures found on micromeric taxa and offer a potential source of histology and morphology that can be compared to those found on macromeric groups. Dermal fin spines offer a variety of morphology but aspects of their growth modes and homology are unclear. Here, we provide detailed descriptions of the microstructure and growth of a dorsal ridge spine from the acanthothoracid placoderm, Romundina stellina, using virtual three-dimensional paleohistological datasets. From these data we identify several layers of dentine ornamentation covering the lateral surfaces of the spine and reconstructed their growth pattern. We show that this spine likely grew posteriorly and proximally from a narrow portion of bone located along the leading edge of the spine. The spine is similarly constructed to the scales with a few exceptions, including the absence of polarized fibers distributed throughout the bone and the presence of a thin layer of perichondral bone. The composition of the spine (semidentine odontodes, dermal bone, perichondral bone) is identical to that of the Romundina dermal plates. These results illustrate the similarities and differences between the dermal tissues in Romundina and indicate that the spine grew differently from the dentinous fin spines from extant and fossil chondrichthyans. The morphology and histology of Romundina is most similar to the fin spine of the probable stem osteichthyan Lophosteus, with a well-developed inner cellular bony base and star-shaped odontodes on the surface. Results from these studies will undoubtedly have impact on our understanding of fossil fin spine histology and evolution, contributing to the on-going revision of early gnathostome phylogeny.
Highlights
Macromeric skeletons are characterized by large, morphologically distinctive and phylogenetically conserved dermal bones covering the head and shoulder girdle [1]
By investigating the three-dimensional architecture and growth mode of a dorsal spine of Romundina stellina, we aim to provide a basis for comparison with extant chondrichthyans in which fin spine growth has been studied directly [16], and with acanthodians and extinct chondrichthyans that can be imaged the same way, and will provide new data to address the question of fin spine homology [31,32]
We have shown that studying the 3D histology of fossils can provide new and important information about the development in extinct animals
Summary
Macromeric skeletons are characterized by large, morphologically distinctive and phylogenetically conserved dermal bones covering the head and shoulder girdle [1]. A macromeric skeleton usually consists of segmentally arranged scales of dermal bone, which sometimes carry odontodes and are pierced by sensory line canals. Micromeric skeletons do not contain large dermal bones and the fundamental unit of this type of skeleton is a small dermal scale, which extends as a relatively homogeneous squamation across the whole animal [1]. The scales almost always have a dentine crown; the base may consist of bone, osteodentine or dentine [4]. Two extinct groups of Palaeozoic jawed vertebrates, the placoderms and acanthodians, are characterized, respectively, by macromeric and micromeric skeletons
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