Saturday, 2 April 2016

Preserved pigment cells reveal the colour of a Colubrid Snake from the Miocene of Spain,

The fossil record reaveals a lot about the skeletal structure of extinct animals, but preservation of soft tissue is rare, and information about the colouration of past species is rarer still. Pigment is known from a few Mollusc shells, as well as some preserved Insects, and structural colouration (colour achieved by microstructures that manipulate light rather than by pigments) has also been recorded in Insects. Some recent studies have found traces of melanin pigments in the feathers of Mesozoic Birds and Dinosaurs, but otherwise the fossil record of colouration in Vertebrates is restricted to mummified Pleistocene specimens.

In a paper published in the journal Current Biology on 31 March 2016, Maria McNamara of the School of Biological, Earth and Environmental Sciences at University College Cork, Patrick Orr of the School of Earth Sciences at University College Dublin, Stuart Kearns of the School of Earth Sciences at the University of Bristol, Luis Alcala of the Fundación Conjunto Paleontológico de Teruel-Dinópolis, Pere Anadón of the Institut deCiències de la Terra ‘Jaume Almera’, and Enrique Peñalver of the Museo Geominero of the Instituto Geológico y Minero de España, describe a fossil Colubrid Snake from the Miocene Libros Lagerstätte with preserved pigment cells which reveal its colouration in life.

The Snake is preserved within the oil shales of the Libros Gypsum lacustrine sequence, making it between 11.2 and 8.7 million years old. The head is missing, preventing assignation of the specimen to a particular species, however an area of skin covering the ribs is present, preserved as a calcium phosphate replica of the original tissue. This replica material retains details of the original microstructure of the skin.

Entire specimen; inset shows anterior. Cream-colored material is fossil skin. Numerals 1–7 indicate sample locations. McNamara et al. (2016).

The preserved skin is comprised of overlapping scales, and retains a three layered structure, with an essentially featureless outer layer 6–9 μm thick, a middle layer 15-25 μm thick with mineralized fibers and oblate and spheroidal bodies and an inner layer 100-180 μm thick with a more complex fibrous structure. These layers are interpretted as an epidermis comprised entirely of keratinized cells, an upper dermal stratum spongiosum comprised of packed collagen fibers and chromatophores (pigment cells), and a lower dermal stratum compactum, comprised entirely of densely packed collagen fibers.

Scanning electron micrograph (SEM) of fractured vertical section through the skin, showing epidermis (Epi), dermis (De), basement membrane (B), stratum spongiosum (Sp) and stratum compactum (Sc). McNamara et al. (2016).

The chromatophores are extremely abundant, and can be differentiated into three types, small disc shaped bodies in the upper part of the stratum spongiosum 1-5 μm across, a middle layer of irregular spheroid to ovoid cells, with a granular interior made up of two types of granules (smaller irregular sub-spherical granules and larger rounded granules), and a lower layer comprising larger smooth surfaced ovoid cells with prominant lateral processes. These are interpretted as being different types of chromatophores based upon comparison with living Snakes, with an upper layer of iridophores (reflective, irridescent cells), a middle layer of xanthophores (cells contianing yellow carotenoid pigments) and erythrophores (cells contianing orange pteridine pigments), and a base layer comprising melanophores (cells with black or dark brown melanin pigments).

Scanning electron micrographs (SEM) of fractured vertical section through the skin, showing chromatophores (iridophores [I], melanophores [M], and xanthophores [X]), and collagen fibers (C). The voids represent structures that have separated into the counterpart of the sample during preparation. McNamara et al. (2016).

Identification of the different types of pigment cells created the possibility of determining the colour and patternation of the Libros Snake. McNamara et al. interpret the Snake as having been green in colour (the Snake has no actual green pigment cells, but many modern Snakes without green pigment cells are able to achieve a green colour by a combination of other pigments, so it is not unreasonable to assume a Miocene Snake could have done the same), with a lighter underside and a pattern of dark markings on its back.

(A) Schematic representation of the relative abundance and position of chromatophores in samples of skin from different body regions. (B) Interpative artwork by Jim Robbins. McNamara et al. (2016).

See also... ernstmayri: A new species of Worm-eating Snake from the Star Mountains of Papua New Guinea.                       Worm-eating Snakes, Toxicocalamus, are Elapid... of all modern Snakes more likely to have been a burrower than a swimmer.    Snakes are one of the most successful groups of living vertebrates, with over 3000 living species found in all but the very coldest environments on Earth. Modern snakes exhibit a wide range of habitat preferences, from... cryptamydros: A new species of Death Adder from the Kimberly Region of Western Australia and the Northern Territory. Death Adders, Acanthophis spp., are Australian Snakes closely resembling the Old World Vipers, though they are Elapid Snakes...
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1 comment:

  1. This article is really fantastic and thanks for sharing the valuable post. I like to read about snakes even those extinct like Titanoboa cerrejonensis