Abstract
We have applied correlative microscopy to identify the key constituents of a dorsal rib fossil from Koreanosaurus boseongensis and its hosting mudstone discovered at the rich fossil site in Boseong, South Korea, to investigate the factors that likely contributed to diagenesis and the preservation of fossil bone. Calcite and illite were the commonly occurring phases in the rib bone, hosting mudstone, and the boundary region in-between. The boundary region may have contributed to bone preservation once it fully formed by acting as a protective shell. Fluorapatite crystals in the rib bone matrix signified diagenetic alteration of the original bioapatite crystals. While calcite predominantly occupied vascular channels and cracks, platy illite crystals widely occupied miniscule pores throughout the bone matrix. Thorough transmission electron microscopy (TEM) study of illite within the bone matrix indicated the solid-state transformation of 1M to 2M without composition change, which was more evident from the lateral variation of 1M to 2M within the same layer. The high level of lattice disordering of 2M illite suggested an early stage of 1M to 2M transformation. Thus, the diagenetic alteration of both apatite and illite crystals within the bone matrix may have increased its overall density, as the preferred orientation of apatite crystals from moderate to strong degrees was evident despite the poor preservation of osteohistological features. The combined effects of rapid burial, formation of a boundary region, and diagenesis of illite and apatite within the bone matrix may have contributed to the rib bone preservation.
Highlights
The Boseong fossil site is a rich Upper Cretaceous fossil site from South Korea based on the discovery of abundant dinosaur eggs and egg clutches [1,2,3], post-cranial skeletal remains of the small basal ornithopod Koreanosaurus boseongenesis [4], and partial skeletal remains of the large anguimorph lizard Asprosaurus bibongriensis [5]
We have previously investigated structural and chemical features at micro- to nanoscale of a dorsal rib portion from Koreanosaurus [17] obtained from the holotype specimen which consist mainly the “torso” region discovered in an articulated state in a large mudstone block that may have originated directly from the main outcrop of Site 5 [4]
We identified the key constituents from a dinosaur fossil rib bone and its hosting mudstone
Summary
The Boseong fossil site is a rich Upper Cretaceous fossil site from South Korea based on the discovery of abundant dinosaur eggs and egg clutches [1,2,3], post-cranial skeletal remains of the small basal ornithopod Koreanosaurus boseongenesis [4], and partial skeletal remains of the large anguimorph lizard Asprosaurus bibongriensis [5]. While it has been presumed that rapid burial took a key role in preserving the fossils from the local region based on large-scale depositional features [2], deeper insights in specific factors that may have contributed to bone preservation has not been thoroughly explored. Analytical studies on both skeletal fossil material and the hosting geological matrix provide insights into the interaction and relationship of composing elements [6,7]. The distal region of a fully preserved seventh left dorsal rib bone was obtained from KDRC-BB2, and we have selected it based on the following reasons; i) information on exact original position, ii) simple morphology, and iii) intact hosting mudstone (S1 Fig)
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