Abstract
The holotype (MHM-K2) of the Eocene cheloniine Tasbacka danica is arguably one of the best preserved juvenile fossil sea turtles on record. Notwithstanding compactional flattening, the specimen is virtually intact, comprising a fully articulated skeleton exposed in dorsal view. MHM-K2 also preserves, with great fidelity, soft tissue traces visible as a sharply delineated carbon film around the bones and marginal scutes along the edge of the carapace. Here we show that the extraordinary preservation of the type of T. danica goes beyond gross morphology to include ultrastructural details and labile molecular components of the once-living animal. Haemoglobin-derived compounds, eumelanic pigments and proteinaceous materials retaining the immunological characteristics of sauropsid-specific β-keratin and tropomyosin were detected in tissues containing remnant melanosomes and decayed keratin plates. The preserved organics represent condensed remains of the cornified epidermis and, likely also, deeper anatomical features, and provide direct chemical evidence that adaptive melanism – a biological means used by extant sea turtle hatchlings to elevate metabolic and growth rates – had evolved 54 million years ago.
Highlights
Marine deposits of the early Eocene Fur Formation (Jutland, Denmark) have yielded a diverse biota of exceptionally preserved plant and animal fossils[1]
Five small samples representing the counterpart of MHM-K2 were collected for ultrastructural and biomolecular analysis; these were untreated with preservatives and form the basis of this study
Subsequent field emission gun scanning electron microscopy (FEG-Scanning electron microscopy (SEM)) analysis revealed that the thin layer comprises accumulations of sub-spherical to elongate microbodies with a homogenous interior and botryoidal, irregularly pitted exterior (Fig. 2a–c)
Summary
Marine deposits of the early Eocene Fur Formation (Jutland, Denmark) have yielded a diverse biota of exceptionally preserved plant and animal fossils[1]. Noteworthy finds include a speciose assemblage of early neornithes[2] and a small, but significant, collection of chelonioid turtles[3,4]. Of the latter group, by far the best preserved specimen is MHM-K2: a diminutive (about 74 mm when measured from the nuchal emargination to the posterior rim of the pygal), ontogenetically young cheloniine referred to as Tasbacka danica by Karl & Madsen[5]. Renewed preparations of the fossil in 2013 uncovered soft tissue residues from a ‘fresh’ sub-surface layer located within a sub-triangular area bordered by the dorsal process of the left scapula, proximal end of the left humerus, left costal 1, and left peripherals 1 and 2 (Fig. 1b). To determine the ultrastructural and chemical composition of the preserved soft tissues, both untreated and demineralised (using ethylenediaminetetraacetic acid, EDTA) samples were subjected to a selection of high-resolution analytical techniques, including field emission gun scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), in situ immunohistochemistry, time-of-flight secondary ion mass spectrometry (ToF-SIMS), and infrared (IR) microspectroscopy
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