Soft sheets of fibrillar bone from a fossil of the supraorbital horn of the dinosaur Triceratops horridus

TEM slides of these osteocytes were made but no chromatin nor nuclear membrane were visible (Schweitzer et al., 2014). Armitage and Anderson (2013) also reported ‘nucleus-like spheres’ in Triceratops osteocytes, although no images of these structures were shown. More recently, nuclei were reported in cartilage cells of a nestling Hypacrosaurus stebingeri (another duck-billed dinosaur) from a nesting ground discovered in Montana in the 1980's (Bailleul et al., 2020).

Although, in this previous study, not all specimens sampled contained preserved soft tissue, many specimens were discovered to possess some form of “vessel” structures, collagen, or osteocytes (Schweitzer et al., 2007b). Furthermore, a Triceratops horridus revealed exceptionally well preserved tissues, including vessels and osteocytes, of which the latter contained structures resembling nuclei (Armitage and Anderson, 2013). Subsequent to the release of Schweitzer et al. (2007b), the analysis of unaltered preserved soft tissue was taken to the next level.

Regarding fossils, recovery of structures morphologically consistent with vertebrate osteocytes, blood vessels, and proteinaceous matrix has been reported from bones of a wide variety of extinct vertebrates dating as far back as the Permian (Norris et al., 2017). To date, these reports have derived from the dinosaurs Tarbosaurus (Pawlicki, 1978, 1985, 1995; Pawlicki and Nowogrodzka-Zagorska, 1998), Tyrannosaurus rex (Schweitzer et al., 2005, 2007), Brachylophosaurus (Schweitzer et al., 2007, 2009), Allosaurus (Wiemann et al., 2018), Giganotosaurus (Schweitzer, 2011), Dreadnoughtus (Schroeter, 2013), Triceratops (Schweitzer et al., 2007; Armitage and Anderson, 2013), seven other indeterminate dinosaurs (Pawlicki et al., 1966; Schweitzer et al., 2007; Peterson et al., 2010; Schweitzer, 2011; Wiemann et al., 2018), an indeterminate mosasaur (Lindgren et al., 2011), Nothosaurus (Surmik et al., 2016, 2017) and the archosauromorph Protanystropheus (Surmik et al., 2016), the synapsid Dimetrodon (Norris et al., 2017), the crocodiles Diplocynodon (Cadena, 2016) and Thoracosaurus (Boles, 2016), numerous marine and freshwater turtles (Annemis, Rhinoclemmys, Mongolemys, Allaeochelys, Neochelys, Taphrosphys, Catapleura, indeterminate pleurodires and chelonioids, an indeterminate trionychid; Cadena and Schweitzer, 2012, 2014; Cadena, 2016; Boles, 2016), the pangolin Eomanis (Cadena, 2016), and a variety of Pleistocene–Recent vertebrates (Dinornis, Mammut, Mammuthus, Bison, Balaenoptera, indeterminate manatee; Schweitzer et al., 2007; Schweitzer, 2011). These discoveries span five continents, more than 250 million years of geologic time, and a wide range of terrestrial and marine depositional environments, demonstrating that soft tissue and cellular preservation are widespread phenomena in need of further study.