Jurassic ichthyosaur bone cell traces and fibrils + juvenile phytosaur bone osteohistology

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

Recent papers:

Free pdf:

René-Paul Eustache, Alan Boyde, Xavier Jaurand & P. Martin Sander (2024)
Medusa’s Gaze: Cell traces and fibrils but no collagen in permineralized Jurassic ichthyosaur bone
iScience (advance online publication)
doi: https://doi.org/10.1016/j.isci.2024.111523
https://www.sciencedirect.com/science/article/pii/S2589004224027500


Highlights

Jurassic ichthyosaur bone preserves internal surfaces at the nanostructural level
SEM reveals forming, resting, and resorption surfaces as in modern deproteinized bone
Ichthyosaur bone and cartilage biology were the same as in extant amniotes
TEM and SEM show no evidence of collagen preserved in the fossil bone fibrils

SUMMARY

Bone is formed by specialized cells whose activity allows bone to grow, change shape and repair itself. Its composite structure of collagen fibrils and bioapatite nanocrystals gives bone exceptional mechanical strength. Using scanning electron microscopy, we show in fossil ichthyosaurs 150 and 200 million years old from the Jurassic of France and the UK abundant and direct evidence of cellular activity on the fossilized forming, resting and resorbing surfaces of bone trabeculae, as well as bone fibrils, Sharpey fibers and cartilage fibers. These features are identical to those observed in fresh deproteinized mammalian bone, including human bone. Despite the striking similarity of the fibrils to those in modern bone, we found no evidence of collagen preservation. Fossilization removed non-mineralized components and exposed trabecular surfaces at the mineralization front. Cellular activity in skeletal tissue, familiar to any medical student, preserves for >200 million years, and probably longer in vertebrate fossils.

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Free pdf:

Erika R. Goldsmith, Daniel E. Barta, Ben T. Kligman, Sterling J. Nesbitt, Adam D. Marsh, William G. Parker & Michelle R. Stocker (2024)
Osteohistological signal from the smallest known phytosaur femur reveals slow growth and new insights into the evolution of growth in Archosauria
Journal of Anatomy (advance online publication)
doi: https://doi.org/10.1111/joa.14185
https://onlinelibrary.wiley.com/doi/10.1111/joa.14185

Free pdf:
https://onlinelibrary.wiley.com/doi/epdf/10.1111/joa.14185

Fossils of embryonic and hatchling individuals can provide invaluable insight into the evolution of prenatal morphologies, heterochronies, and allometric trajectories within Archosauria but are exceptionally rare in the Triassic fossil record, obscuring a critical aspect of archosaurian biology during their evolutionary origins. Microvertebrate sampling at a single bonebed in the Upper Triassic Chinle Formation within Petrified Forest National Park has yielded diminutive archosauriform femora (PEFO 45274, PEFO 45199) with estimated and measured femoral lengths of ~31 mm and ~ 37 mm, respectively. These new specimens provide the unique opportunity to assess the preservation, body size, and growth dynamics of skeletally immature archosauriforms in North America and compare the growth dynamics of archosauromorphs within an evolutionary and ontogenetic context. We assign PEFO 45199 and PEFO 45274 to Phytosauria (Archosauriformes) based on their strongly sigmoidal shape in lateral view, the presence of proximal anterolateral and posteromedial tubera, the absence of an anteromedial tuber of the proximal end, a teardrop-shaped proximal outline, and a fourth trochanter that is not confluent with the proximal head. Osteohistological analyses of PEFO 45274 reveal a cortex comprising low vascularity, parallel-fibered bone composed of primary osteons that lacks a hatching line and any lines of arrested growth. We interpret PEFO 45274 as a slow-growing, post-hatching individual of less than 1 year of age. Surprisingly, osteohistology of some larger phytosaur femora implies faster growth rates in comparison to PEFO 45274 based on the occasional presence of woven bone and overall higher degrees of vascular density, suggesting the ontogenetic shift from rapid-to-slow growth rates might not occur simply or uniformly as expected in Phytosauria and that non-archosaurian archosauriforms may exhibit size-dependent histological characteristics. This study highlights the importance of including osteohistology from multiple body sizes to investigate non-archosaurian archosauriform ancestral growth rates given the phylogenetic position of phytosaurs near the divergence of Archosauria.