A fossil sea turtle (Reptilia, Pan-Cheloniidae) with preserved soft tissues from the Eocene Fur Formation of Denmark

Abstract

A new hard-shelled sea turtle (Pan-Cheloniidae) with vestigial soft tissues from the lower Eocene (Ypresian) Fur Formation of Denmark is described and illustrated. The fossil (DK 807) comprises a partial, yet fully articulated carapace (estimated original length ∼50 cm) where the individual bones mostly are preserved in three dimensions, together with an intact sacrum, a consecutive series of articulated caudal vertebrae, a complete pelvic girdle, and both hind limbs. Primitive characters in the pelvis and limbs, along with free ribs that contact the posterior peripherals suggest affinity with the extinct pan-cheloniid Eochelone; however, because of the incomplete nature of the fossil, DK 807 is kept in open nomenclature. Associated with the skeletal elements are soft-tissue residues that include remnant epidermal scutes and a nearly complete outline of a rear paddle. The flipper-shaped halo likely represents traces of skin preserved as a dark bedding-parallel film. Its wrinkled and striated surface texture attests to an originally scaleless configuration comparable to the soft integument of living adult dermochelyid (leatherback) turtles, and unlike that of extant cheloniids. Scratches, scars and indentations on the bony carapace likely represent incompletely healed bite marks inflicted by a crocodylian or another large-sized seagoing tetrapod.

Natural external plastron mold of the Triassic turtle Proterochersis: An unusual mode of preservation

“…Due to their durable shell and frequent occurrence in faunal assemblages since the Late Triassic, turtles are relatively common in the fossil record and are usually represented by complete shells or shell fragments. In rare cases, if the taphonomic setting promotes such processes, the remains may be preserved through unusual fossilization pathways, e.g., leading to opalization [1] or fossilization of osteocytes and blood vessels [2][3][4], surrounding soft tissues [5][6][7], or epidermal scutes [7][8][9]. When the diagenetic conditions are unfavorable or the specimen is exposed to harsh environment for an extended time before it is collected, the fossilizing or fossilized bone may be subjected to dissolution or severe weathering, making it brittle and subject to destruction during recovery or preparation, or solely leaving behind an internal mold of lithified rock matrix [10].…”

Butyrumichthys henriciigen. et sp. nov.: a new stromateiform fish from the lower Eocene Fur Formation, Denmark

“…The fossil assemblage comprises: (1) more than 110 species of diatoms and other siliceous algae (Homann, 1991); (2) terrestrial plants, including seeds, fruiting structures of angiosperms and gymnosperms, tree trunks and twigs (especially Sequoia ), leaves, and a diverse microflora consisting of 42 types of spores and more than 100 types of pollen (Willumsen, 2004; Pedersen et al, 2012); (3) the earliest known Eocene insect fauna with more than 25,000 cataloged specimens belonging to more than 200 species from 15 orders, in many cases representing the earliest known lineages of several families (e.g., Larsson, 1975; Willmann, 1990, 1993; Ansorge, 1992; Rust, 1998; Archibald & Makarkin, 2006; Bechly & Rasmussen, 2019; Simonsen et al, 2022); (4) marine invertebrates, e.g., bivalves, gastropods, ophiuroids, and asteroids (Pedersen et al, 2012; Rasmussen, 1972); (5) specimens of Palaeophis Owen, 1841 also documenting a rare case of an articulated marine snake skeleton (Pedersen et al, 2012; Kristensen et al, 2012); (6) marine and freshwater turtle remains, including an almost complete specimen of Eosphargis Lydekker, 1889 and arguably the world's best preserved fossil chelonid, Tasbacka danica Karl & Madsen, 2012 (Nielsen, 1960; Karl & Lindow, 2010; Karl & Madsen, 2012; Lindgren et al, 2017; Garza et al, 2021); and (7) landbirds, including the earliest forms of stem-group parrots, ibises, trogons, turacos, and rollers (Kristoffersen, 2002; Waterhouse et al, 2008; Bertelli et al, 2010; Mayr & Bertelli, 2011; Bourdon et al, 2016). The fossil bird remains comprise imprints in the diatomite, isolated bones, feathers with preserved melanosomes, and complete articulated skeletons, counting Eocypselus vincenti Harrison, 1984, which is one of the best preserved bird fossils in the record and the earliest representative of the swifts (Dyke et al, 2004; Lindow & Dyke, 2007; Waterhouse et al, 2008; Bertelli et al, 2010; Mayr, 2010; gren et al, 2017).…”