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. 2020 Jul 8:8:e9479.
doi: 10.7717/peerj.9479. eCollection 2020.

Exceptionally preserved 'skin' in an Early Cretaceous fish from Colombia

Andrés Alfonso-Rojas #  1 , Edwin-Alberto Cadena #  1   2
Affiliations

Exceptionally preserved 'skin' in an Early Cretaceous fish from Colombia

Andrés Alfonso-Rojas et al. PeerJ. .

Abstract

Studies of soft tissue, cells and original biomolecular constituents preserved in fossil vertebrates have increased greatly in recent years. Here we report preservation of 'skin' with chemical and molecular characterization from a three-dimensionally preserved caudal portion of an aspidorhynchid Cretaceous fish from the equatorial Barremian of Colombia, increasing the number of localities for which exceptional preservation is known. We applied several analytical techniques including SEM-EDS, FTIR and ToF-SIMS to characterize the micromorphology and molecular and elemental composition of this fossil. Here, we show that the fossilized 'skin' exhibits similarities with those from extant fish, including the wrinkles after suffering compression stress and flexibility, as well as architectural and tissue aspects of the two main layers (epidermis and dermis). This similarity extends also to the molecular level, with the demonstrated preservation of potential residues of original proteins not consistent with a bacterial source. Our results show a potential preservation mechanism where scales may have acted as an external barrier and together with an internal phosphate layer resulting from the degradation of the dermis itself creating an encapsulated environment for the integument.

Keywords: Aspidorhynchidae; Barremian; Molecular Paleontology; Soft-tissue; South America; Zapatoca.

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Conflict of interest statement

The authors declare there are no competing interests.

Figures

Figure 1
Figure 1. Locality and other reported exceptionally preserved skin fossils from the Cretaceous.
(A) map of Colombia showing in orange the Santander department, and the fish fossil site (Zapalonga locality) very near Zapatoca. (B) outcrop view at the fish fossil site, showing the presence of mudstones and large concretions. (C) stratigraphic column along with Zapalonga locality, indicating the horizon where UR-CP-0001 was found. (D) world map with remarkable findings of exceptional preserved skin fossils through the Cretaceous: (1) Barremian, Paja Fm, Colombia (this study); (2) Barremian, Calizas de la Huérgina Fm, Spain (Martin et al., 2015); (3) Barremian-Aptian, Huajiying and Yixian formations (Xu et al., 2020) Yixian Fm, China (Lingham-Soliar & Plodowski, 2010); (4) Aptian, Clearwater Fm, Canada (Brown et al., 2017); (5) Aptian-Albian, Romulado Fm, Brazil (Martill, 1988); (6) Aptian-Albian, Haman Fm, South Korea (Paik, Kim & Huh, 2010); (7) Albian, Pietraroja, Italy (Signore et al., 2005); (8) Cenomanian, Hadjula, Lebanon (Caldwell & Sasso, 2004); (9) Cenomanian, Nobrara Fm, Kansas, United States (Lindgren, Everhart & Caldwell, 2011a); (10) Campanian, Auca Mahuevo, Argentina (Coria & Chiappe, 2007); (11) Campanian-Maastrichtian, Fruitland Fm, New Mexico, United States (Hall, Wolberg & West, 1988); (12) Maastrichtian, Hell Creek Fm, North Dakota, United States (Manning et al., 2009); (13) Maastrichtian Harrana, Jordan (Lindgren, Kaddumi & Polcyn, 2013); (14) Maastrichtian, Sânpetru Fm, Romania (Grellet-Tinner et al., 2012).
Figure 2
Figure 2. UR-CP-0001, aspidorhynchid fossil fish specimen.
(A–B) right lateral view. (C) interpreted position of UR-CP-0001 in the body of an aspidorhynchid fish. (D) left lateral view. (E) ventral view. (F) posterior view, showing the naturally preserved original 3-D volume. (G) detail of the originally preserved ‘skin’ with wrinkles and marks. (H) View of some of the ventral scales (vs) preserved. (I–J) elongated ventral flank scales (vfs). five cm scale applies for A, D, E and F; two cm for G; 1.5 cm for H and one cm for I and J.
Figure 3
Figure 3. Some ‘skin’ fragments after HCl treatment.
(A) Light micrograph of preserved ‘skin’ after treated with 15% HCl, without any infilling matrix left. (B) Enlargement of the organic patchy layer. (C–D) Fragment of the dry skin of the extant Orechromis sp. (Mojarra fish) exhibiting two layers, wrinkles and collagen fibers indicated by black arrows in d. (E) Wrinkled ‘skin’ of UR-CP-0001. (F–G) An UR-CP-0001 close-up of the two organic exterior layers and collagen fibers indicated by black arrows in g. (H) isolated tissue fragment after EDTA treatment under transmitted-light microscopy showing collagen fibers. (I–K) An UR-CP-0001 ‘skin’ fragment under transmitted-light microscope, exhibiting the two distinct inorganic (base) and organic (exterior) layers. (L–M) An UR-CP-0001 ‘skin’ fragment under transmitted-light (L) and polarized-light (M), showing low birefringence of the granular basal layer. One mm horizontal scale applies for A, C, L and M; one mm vertical scale for E and F.
Figure 4
Figure 4. SEM-EDS micrographs and elemental composition analyses of an untreated and uncoated fragment of UR-CP-0001 ‘skin’.
(A) Sample from UR-CP-0001 that contains ‘skin’ and infilling matrix. (B) detail of samples mounted over the stub. (C) SEM micrograph with point EDS analysis in the ‘skin’ region, showing the abundant content of carbon and nitrogen, with less occurrence of calcium and phosphorous. (D) SEM micrograph with point EDS analysis in the infilling matrix, showing absence of nitrogene, dominance of carbon and calcium instead. (E) SEM micrograph of the ‘skin’-infilling matrix contact before apply the EDS analysis. (F) Same micrograph as in (E) after EDS analysis, showing the extremely wrinkled organic surface of the ‘skin’, remaining intact the infilling matrix region. (G) Outline of the ‘skin’ organic and phosphatic layer, as well as the infilling matrix showed in e, which is the base of the elemental mapping. (H–L) Elemental mapping at 10 kV of the ‘skin’ infilling matrix region showing dominance of carbon (H) and oxygen (I) at the organic region, and phosphorus (K) at the boundary between the ‘skin’ and the infilling matrix; silicon (J) is very scarce in both regions, and of calcium (L) is highly abundant in the infilling-matrix.
Figure 5
Figure 5. FTIR analyses of UR-CP-0001 and the extant Orechromis sp.
(A) Composite FTIR spectra (absorbance vertical axis, wavenumber horizontal axis) of different samples: Orechromis sp. (Mojarra fish) (dark blue line) with interpretation of typical proteinaceous compounds (Amide A, I, II, III, v(C=O), C-H stretch and a phosphate) with gray bands showing potential ranges based on Boatman et al. (2019); Kong & Yu (2007); and Lee et al. (2017); an extant bacteria biofilms (black, yellow and purple lines) taken and redraw from Lee et al. (2017) and Lindgren et al. (2011b); Vinctifer comptoni (orange line) from the Cretaceous of Brazil, taken and redrawn from Sousa Filho et al. (2016); UR-CP-0001 aspidorhynchid fossil fish ‘skin’ treated with EDTA (red line); treated with HCl (green line); untreated (light blue line); and UR-CP-0001 infilling matrix (brown line). (B) Skin sample from Orechromis sp. (Mojarra fish) used for the FTIR analysis and close-up of the skin sample analyzed from this specimen (C). (D) The region from which the ‘skin’ sample of UR-CP-0001 was taken, and a close-up of the ‘skin’ fragment after EDTA treatment under a transmitted light microscope (E). (F) UR-CP-0001 sample used for the untreated analysis and a close-up of the FTIR vibrational bands (red rectangle) in (A) after deconvolution (G). (H) UR-CP-0001 infilling matrix sample and how it was grinded using a sterilized mortar and pestle (I) and placed in the FTIR machine (J).
Figure 6
Figure 6. ToF-SIMS analyses of UR-CP-0001 ‘skin’.
(A–B) Negative and Positive ion ToF-SIMS spectrum of UR-CP-0001 untreated sample (see circular photo of the sample), typical organic compounds occur in high intensities in both ions (raw data presented in Data S1). (C-H) ToF-SIMS images showing the distribution of ions CN–(C), CH4N+ (D), C4H8N+ (E), CNO–(F), C2H6N+ (G) and C3H6N+ (H).
See this image and copyright information in PMC

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