Dinosaur Biology - Skin, Covering and Colour

Australia: The Land Where Time Began

Dinosaur Biology - Skin, Feathers and Colour

Though the fossil bones are the only evidence known of most dinosaurs there is a growing number that are also known from fossils retaining some evidence of their integument as well allowing accumulating knowledge of their covering. The large, and some small dinosaurs show evidence of being covered with scales in a mosaic pattern, that have been preserved as impressions in the sediments before they were converted into rock, though there are also sometimes traces of keratin. Among the fossilised footprints of dinosaurs there are some that display the scales on the sole of the foot as well as foot pads. The mummies of some hadrosaurs are known that are almost complete. It appears that the overlapping scales seen is lizards are not present on dinosaurs, though the author¹ suggests there may have been some that were similar to those seen on the top of the feet of birds on the bird-like dinosaurs. The mosaic scales of dinosaurs often had a semihexagonal shape, a ring of smaller scales surrounding the larger scales that formed rosettes that were in turn surrounded by many small scales that were often flat. Small, beady scales covered tyrannosaurids. As these scales were usually small they are not apparent from about 12 ft or more. Large, projecting subconical scales were sometimes present as the central scale of a rosette that were often arranged in irregular rows. On any individual dinosaur the pattern and size of the scales varied according to their location on the animal. Triceratops has the most spectacular scales known at the present. The author¹ suggests they may have been large bristles, being as big as the palm of the hand of a large person and strongly subconical.

The author¹ describes special tissue on the backs of some dinosaurs as 'nonarmour display tissue' that came in a number of forms such as large, prominent scales, spines and frills that were either segmented or smooth-edged. There was a set of very long bristles, with a comb-like appearance, on the tails of at least psittacosaurs. The dorsal bristles on heterodontosaurs were denser and finer, extending along the back as well as on the tail. Another feature that is commonly preserved in dinosaur fossils are prominent skin folds that are believed to have had the appearance of similar-appearing structures seen on living lizards. The author¹ suggests they may have been fairly common on some dinosaurs. He also suggests that soft tissue display structures such as crests, combs, dewlaps, wattles etc. may have been more common than has been realised. According to the author¹ a fossil of an ornithomimosaur theropod has been found on which can be seen a pouch with an appearance similar to the pouch beneath the beak of a pelican. Stegosaurs and ankylosaurs have also been found that had throat pouches that were covered by a dense pavement of small ossicles, beginning at the front of the lower jaws. Hard keratin covered the armour plates, the author¹ suggesting that the horn coverings probably enlarged the plates when they were erect. Beaks, horns and claws were also lengthened by keratin, that the sauthor¹ says typically lengthens the bone core of these structures by from 1/3 to 2-fold, he says he usually adds 1/2 when estimating the total length of a bone structure that had been covered by a keratin sheath.

It is only recently, with the discovery of the Yixian heterodontosaur, that the body coverings of any small ornithischians have been found, in this case a dense coat of fibres. Though feathers have been known to be associated with bird fossils, including that of Archaeopteryx, from lake and lagoon sediments for some time, it has since been found that many small theropods had been covered by a coating of bristle protofeathers, and in the Yixian beds, pennaceous feathers that were fully developed. It has been claimed by some the the simpler bristle covering on many small dinosaurs are most likely the internal collagen fibres of feathers that have been degraded to such an extent that no trace was left of what was previously true feathers. The author¹ believes this suggestion is untenable for a number of reasons, one of the main reasons being that indications of colour has been found in some of these fibres, allowing the actual colour of the covering to be approximated.

It has also been found that on some of the small nonflying theropods there were scales on the tails and possibly on the legs, while small ornithischians, such as psittacosaurs, were largely covered by scales. The author¹ suggests that small dinosaurs were variable in which covering they had. Among modern birds ostriches have no feathers on their legs, and some mammals, such as small bats and a number of suids and humans to rhinos and elephants are essentially naked.

According to the author¹ the basal ornithischians were covered by fibres, an indication that the insulation of dinosaurs evolved once, and if so, they would all be protofeathers. As no sign of protofeathers have been found in deposits from theropods in the Triassic and Early Jurassic, it is a kind of negative evidence that the author¹ considers to be no more meaningful than the earlier belief that dinosaurs didn't have insulation because evidence of scales hadn't been discovered. He suggests insulation may still be found in basal forms, and there is the possibility that evidence may yet be found of insulation evolving more than once in the dinosaurs. He suggests there is still the question of why dinofur and feathers evolved. He suggests that the first bristles to appear on a dinosaur probably would have been too sparse to provide any insulation, so thermoregulation could not have been the reason they evolved originally. A possible use of the original bristles may have been for display, as is exemplified by psittacosaurs. If they evolved for this reason they would start to become effective as insulation as they became denser, conserving the heat generated by archosaurs that were becoming increasingly energetic. The way the fibres and feathers on some flightless dinosaurs, such as heterodontosaurs and therizinosaurs, functioned as both prominent display organs on various parts of the body and as insulation on others, supports the display-to-insulation hypothesis.

Because the pigment organelles preserve well and their shapes have been found to correlate with colours it has allowed them to be used to reconstruct the actual colours of feathered dinosaurs. Scales present a different problem, a way hasn't been found to restore the colours they had in life. The author¹ suggests that the hypothesis presented by some researchers that differences in colour correspond to differing scale patterns on a particular dinosaur is plausible, though among reptiles some are of a uniform colour regardless of the patterning of their scales. Compared with the non-scaly dull grey skin of large mammals dinosaur scales were better suited to have bold colour patterns, as seen in such animals as birds, reptiles and tigers. He suggests the colour vision of dinosaurs may have led to the evolution of colours for display and camouflage. Greens and stealth colours may have been used by dinosaurs living in forested areas, though among the large reptiles and birds tend to be earth-tinged in spite of their colour vision.

As with the bright colours of many small lizards and birds, small dinosaurs are the most likely to have bright colour patterns. Intraspecific communication or startling predators may have used specific colour displays. Natural bases for vivid, possibly iridescent display colours, especially in the breeding season, would have been provided by crests, frills, skin folds and taller neural spines. As the eyes of dinosaurs were closer to those of reptiles and birds than to mammals they probably didn't have the white surrounding the iris. The author¹ suggests dinosaur eyes my have been solid black or brightly coloured as in many reptiles and birds.

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