New material ofMongolemys elegansKhosatzky and Mlynarski, 1971 (Testudines: Lindholmemydidae), from the Late Cretaceous of Mongolia with Comments on Bone Histology and Phylogeny

Abstract

Mongolemys elegans Khosatzky and Mlynarski, 1971, is a freshwater lindholmemydid turtle that is very abundant in Late Cretaceous (Maastrichtian) pond deposits from the Gobi Desert of Mongolia. Here, we present new data on the morphology, bone histology, and phylogenetic position of M. elegans based on hatchlings, juveniles, and adults collected by American Museum of Natural History and the Mongolian Academy of Sciences joint field expeditions at the Bugin Tsav locality. Phylogenetic analysis using a morphological dataset supports the placement of M. elegans as a stem testudinoid. Bone histology of M. elegans shows similar patterns of thickness and bone tissue type for the internal and external cortexes as in other freshwater turtles. Microstructural samples of fossil bone from M. elegans show exceptional preservation of osteocyte lacuno-canicular networks, and higher values of osteocyte density at the external cortex in contrast to cancellous bone and the internal cortex.

In situ SEM/EDS compositional characterization of osteocytes and blood vessels in fossil and extant turtles on untreated bone surfaces; different preservational pathways microns away

“…Osteocytes are embedded within the hard-mineralized component of bone throughout life (exceptions being when released by fracture or during remodeling) (Robling & Bonewald, 2020), providing them high preservation potential within fossil bones, which has been extensively documented in different clades of vertebrates (e.g., Bailleul, O'Connor & Schweitzer, 2019;Enlow & Brown, 1956;Pawlicki & Nowogrodzka-Zagorska, 1998;Schweitzer, 2011;Schweitzer et al, 2013;Surmik et al, 2019). Similar preservation of osteocytes-and blood vessels-like has also been documented in fossil turtles, showing that their preservation is independent of geologic time, paleoenvironment, lithology, lineages, and latitude (Cadena, 2016;Cadena, Ksepka & Norell, 2013;Cadena & Schweitzer, 2012;Cadena & Schweitzer, 2014).…”

Peer Review #2 of "In situ SEM/EDS compositional characterization of osteocytes and blood vessels in fossil and extant turtles on untreated bone surfaces; different preservational pathways microns away (v0.1)"

“…Osteocytes are embedded within the hard-mineralized component of bone throughout life (exceptions being when released by fracture or during remodeling) (Robling & Bonewald 2020), providing them high preservation potential within fossil bones, which has been extensively documented in different clades of vertebrates (e.g., Bailleul et al 2019;Enlow & Brown 1956;Pawlicki & Nowogrodzka-Zagorska 1998;Schweitzer 2011;Schweitzer et al 2013;Surmik et al 2019). Similar preservation of osteocytes-and blood vessels-like has also been documented in fossil turtles, showing that their preservation is independent of geologic time, paleoenvironment, lithology, lineages, and latitude (Cadena 2016;Cadena et al 2013;Cadena & Schweitzer 2012 Something in common to all aforementioned studies are the analytical tools used to study and characterize these fossil bone microstructures, which include principally: 1) ground sections and observation under transmitted and polarized microscopy (Cadena & Schweitzer 2012;Surmik et al 2019); 2) bone demineralization using ethylenediaminetetraacetic acid (EDTA) as a chelating agent (0.5 M, pH 8.0), facilitating release the osteocytes-, blood vessels-, and any other cells-or soft-tissue fibers-like from the bone matrix for their posterior study by transmitted and/or polarized light, scanning and/or transmission electron microscopy and any coupled elemental analyzer, Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), immunological and antibody studies (e.g., Alfonso-Rojas & Cadena 2020; Bailleul et al 2019Bailleul et al , 2020Cadena 2016;Saitta et al 2019;Schweitzer et al 2013;Surmik et al 2019;Wiemann et al 2018) The preservation of these soft-tissue microstructures (osteocytes and blood vessels) and their potential original constituents (proteins and DNA) has been questioned and considered a consequence of microbial interactions within fossil bone and its microenvironment or even as a result of cross-contamination in the laboratory (Buckley et al 2017;Kaye et al 2008;Saitta et al 2019). The 'biofilm hypothesis' as a source for soft-tissue preservation in dinosaur bones has been rigorously tested, which identified fundamental morphological, chemical and textural differences between the resultant biofilm structures and those derived from dinosaur bone, demonstrati...…”

Wutuchelys eocenican. gen. n. sp., an Eocene stem testudinoid turtle from Wutu, Shandong Province, China

“…2012), but this lacks support from the fossil record. To date, no comprehensive analysis has been run; only a few stem testudinoids have been included in the phylogenetic analyses (Claude & Tong, 2004; Cadena, Ksepka & Norell, 2013; Tong et al . 2016).…”