The humerus ofEusthenopteron: a puzzling organization presaging the establishment of tetrapod limb bone marrow

Abstract

Because of its close relationship to tetrapods, Eusthenopteron is an important taxon for understanding the establishment of the tetrapod body plan. Notably, it is one of the earliest sarcopterygians in which the humerus of the pectoral fin skeleton is preserved. The microanatomical and histological organization of this humerus provides important data for understanding the evolutionary steps that built up the distinctive architecture of tetrapod limb bones. Previous histological studies showed that Eusthenopteron's long-bone organization was established through typical tetrapod ossification modalities. Based on a three-dimensional reconstruction of the inner microstructure of Eusthenopteron's humerus, obtained from propagation phase-contrast X-ray synchrotron microtomography, we are now able to show that, despite ossification mechanisms and growth patterns similar to those of tetrapods, it also retains plesiomorphic characters such as a large medullary cavity, partly resulting from the perichondral ossification around a large cartilaginous bud as in actinopterygians. It also exhibits a distinctive tubular organization of bone-marrow processes. The connection between these processes and epiphyseal structures highlights their close functional relationship, suggesting that either bone marrow played a crucial role in the long-bone elongation processes or that trabecular bone resulting from the erosion of hypertrophied cartilage created a microenvironment for haematopoietic stem cell niches.

New Insights in the Ontogeny and Taphonomy of the Devonian Acanthodian Triazeugacanthus affinis From the Miguasha Fossil-Lagerstätte, Eastern Canada

“…Despite the excellent state of preservation of bones at the microstructural [49][50][51], histological (Figure 1h,i) and anatomical levels, our results indicate a pervasive recrystallization of bioapatite in Triazeugacanthus skeletal structures. Modification of the mineral composition of fossil bones (e.g., presence of authigenic calcite) from the Escuminac Formation has been previously reported in the placoderm Bothriolepis canadensis and the osteolepiform Eusthenopteron foordi [36].…”

“…However, such type of replacement modifies the structure of the biological material at a micrometric scale (e.g., presence of carbonate-fluorapatite infillings discordant with the biogenic Retzius striae in mammalian tooth enamel [28]). In the Miguasha Fossil-Lagerstätte, the preservation of the bone microstructures [49][50][51]64] could suggest a more subtle transformation mechanism, consistent with an early diagenetic stage, even though the formation of the "francolite"-type environment of substituted carbonates implies a transformation of apatite down to the atomic scale. Therefore, the respective contributions of early transformation and potential slow modifications [65] in the sedimentological environment of the Escuminac Formation have still to be determined.…”

The origin of novel features by changes in developmental mechanisms: ontogeny and three‐dimensional microanatomy of polyodontode scales of two early osteichthyans

“…The high-resolution PPC-SRμCT scans can produce high-quality virtual thin sections that are comparable to classical thin sections (except for the orientation and organisation of the apatite crystals within the bone matrix), with submicron structures such as osteocytes and dentine tubules being visible, while also recording the complete 3D distribution pattern of the microstructures (e.g. Sanchez et al, 2012Sanchez et al, , 2013Sanchez et al, , 2014Giles, Rücklin & Donoghue, 2013;Qu et al, 2013a). In addition, the 3D data make it possible to examine how the dermal skeletal elements of extinct vertebrates developed in space and time (e.g.…”

“…In addition, the 3D data make it possible to examine how the dermal skeletal elements of extinct vertebrates developed in space and time (e.g. Rücklin et al, 2011Rücklin et al, , 2012Qu et al, 2013a;Sanchez et al, 2014), which is a novel type of data not accessible with classical thin sectioning.…”

Morphology and histology of acanthodian fin spines from the late Silurian Ramsåsa E locality, Skåne, Swede

“…Furthermore, the use of propagation phase-contrast synchrotron X-Ray microtomography technology as a nondestructive tool for studying the 3D microstructure of fossils has made it possible to understand aspects of their overall architecture that might not be observed from traditional 2D thin section (e.g., Sanchez et al, 2012;Qu et al, 2015). This technique has been used to study the histology of a number of fossils, including scales (e.g., Qu et al, 2013Qu et al, , 2015, dermal plates (e.g., Dupret et al, 2010;Giles et al, 2013;Chen et al, 2016), fin bones and rays (Sanchez et al, 2014), and fin spines , and has provided valuable information on the morphology, vascularization, and growth of these structures.…”