Phylogenetic Miniaturization and the Genesis of Keratinized Scales
Keratinized scales (Fig. 1) more or less insulate many living amniotes and all living reptiles from evaporative water loss. Anamniotes (frogs, etc.) don’t have a waterproof skin, but may have scattered osteoderms. While many mammals replace scales with fur and birds replace scales with feathers, keratinized scales are retained on the tail of the opossum and the feet of all birds. Scalation also protects against ant and termite bites.
Basal gnathostomes (basal fish) had ossified skin first and an ossified skeleton later. In teleosts and tetrapods the integumentary (skin) skeleton has undergone widespread reduction and/or modification (Vickaryous and Sire 2009). Sarcopterygians (stem tetrapods) had cosmoid scales, characterized by an intrinsic, interconnected canal system with numerous flask-shaped cavities and superficial pores. In Ichthyostega and other basal tetrapods, dentine, enameled, guanine and pore-canal systems were lost, leaving bone (osteoderms) as the remaining dermal element wherever present. Osteoderms are structurally quite variable and are found in a wide variety of tetrapods, including amphibians. Often they have been lost and independently regained. Temnospondyl amphibians, like Greererpeton, have a combination of thin and overlapping scales, granular pellets, and/or robust plates.
Amniote scale origins
While scales or their impressions are rarely preserved, Carroll (1969) reported some indication of dorsal epidermal scales in Gephyrostegus bohemicus (Fig.2). In Cephalerpeton Carroll and Baird (1972) reported that skin impressions had a slightly pebbly texture, but without evidence for discrete scales. Brough and Brough (1967) found ventral scales in tiny Brouffia (their specimen no. 1) and provided a similar description for the developing gastralia of Gephyrostegus. Not sure why the first and most substantial scales first appeared ventrally, rather than dorsally, where the sun shines, except that the ventral surface is in contact with the substrate.
The importance of scales
In basalmost amniotes forays onto land likely increased in duration. Dermal protection from desiccation is more important in smaller amniotes due to their larger surface-to-mass ratio (Hedges and Thomas, 2001). This is especially applicable to hatchlings and juveniles, some of which may have been less than 2 cm in snout/vent length because the adults were so small. Basal amniote juveniles would have rivaled certain microsaur juveniles as the smallest tetrapods of their day, and perhaps they competed in similar niches. Scales may have given the advantage to reptiles.
With regard to microsaurs
Carroll and Baird (1968) reported, “Extremely delicate dorsal scales of elongate-oval shape are present between (and occasionally overlapping) the ribs of the better-ossified United States National Museum specimen [of the microsaur Tuditanus]. If allowance is made for their insubstantial nature, the scales of Tuditanus are essentially similar to those of ther Carboniferous microsaurs (Carroll, 1966). There is no evidence of rodlike ventral scales such as occur in other lepospondyls (Baird, 1965), or wheat-shaped gastralia like those of primitive reptiles.”
Going one step further in the middle Jurassic, tiny basal mammals traded scales for insulating hair and thicker fur (assumed by phylogenetic bracketing). On the other hand, the first Jurassic dinosaurs to preserve protofeathers, like slender Sinosauropteryx (Ji and Ji, 1996), were a meter long, so not tiny. Basal volant birds became progressively smaller.
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