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Fossil of Microraptor with preserved feathers

Skeletal diagrams of Microraptor specimens

Life restorations of Microraptor, showing posture, movement, plumage and colouration evidenced by fossil remains

The life appearance of dinosaurs has been the subject of study since remains of these animals were first described in the 19th century. Features such as posture, movement, integument and colouration can be reconstructed based on several lines of evidence, mainly fossil remains, but also by deduction.

History

Scientists and artists began depicting dinosaurs in the 19th century, when these were first discovered. They combined artistic skill with knowledge of the anatomy of living animals to produce their recreations, but due to the few fossils known at the time, most of their images were composed of guesswork, and therefore quite inaccurate. The field “evolved” as more complete fossils were found, which gradually filled out gaps in the scientific knowledge. Though the artists mostly worked closely with scientists, by the 1970s, they began to diverge from widely accepted theories. The artists began drawing dinosaurs as active, bird-like animals animals, but most scientists at the time regarded this as pure fantasy, and considered dinosaurs as slow, coldblooded lizards. But reality caught up with imagination, and later studies confirmed that dinosaurs were warm-blooded creatures, like birds and mammals.

New discoveries of well-preserved fossils have therefore continuously made older reconstructions obsolete, but many old theories are still well established in popular culture. This phenomenon has been described as a kind of “cultural inertia”, where the public retains outdated ideas after they have been abandoned by scientists.

Methods

When specific skeletal elements are unknown from fossils, a method called “phylogenetic bracketing” can be used to make educated guesses. For example, internal organs are almost absent form the fossil record of dinosaurs, but by looking at their closest living relatives, birds and crocodiles, and comparing them with the fossils, scientists can get an approximate picture of how these organs would be placed inside dinosaurs. Also, if something is unknown for one group of dinosaur, one can look to related dinosaur groups where more information has been gathered, and use that to fill in the gaps.

These methods are used for reconstructing and mounting dinosaur skeletons and for creating artistic restorations (palaeoart).

General dinosaur appearance

Posture

Most dinosaurs held their necks in an s-shaped curve, and theropod dinosaurs would probably not have been possible to stretch the neck out of this pose. Dinosaurs are often shown with flexible, writhing tails. But several groups, had stiffening, bony tendons running along the tails that restricted how much they could be bent.

The fleshy nostrils are thought to have been placed at the front of the bony nares of all dinosaurs,, including those with very large or long nasal openings.

The placement and size of the eyes of dinosaurs can be inferred from the sclerotic ring, when this element is preserved. The cornea would fit within the inner diameter of the sclerotic ring.

Integument

See also: Feathered dinosaurs

Fossil feather impressions are extremely rare and they require exceptional preservation conditions to form. Therefore only a few feathered dinosaur genera have been identified. All fossil feather specimens have been found to show certain similarities. Due to these similarities and through developmental research, almost all scientists agree that feathers could only have evolved once in dinosaurs. Feathers would then have been passed down to all later, more derived species, unless some lineages lost feathers secondarily. If a dinosaur falls at a point on an evolutionary tree within the known feather-bearing lineages, then its ancestors had feathers, and it is quite possible that it did as well. This technique, called phylogenetic bracketing, can also be used to infer the type of feathers a species may have had, since the developmental history of feathers is now reasonably well-known. All feathered species had filamentaceous or plumaceous (downy) feathers, with pennaceous feathers found among the more bird-like groups.

Claws are often preserved in dinosaur fossils, and as in modern animals, claws are lengthened by keratin, the same material human hair and nails are made of. So in the case of a dinosaur, the claws would be longer than the length of the claw bones themselves, perhaps as much as twice the length.

Movement

See also: Dinosaur physiology

Sauropodomorph appearance

Posture

See also: Sauropod neck posture

In 2013, a study led by Matthew J. Cobley and published in PLOS ONE focused on the flexibility of the necks of sauropods. They compared the necks of ostriches with sauropod genera to find out how flexible the necks really were. The study noted that previous biomechanics studies found the necks to have been positioned between the extremes of a vertical, and a downward slanted neck. In conclusion, the study found that sauropod neck flexibility should not be based on osteology alone, and if it is, the results should be used with caution. Even though there is a lack of preserved muscle tissue that would determine flexibility, sauropod necks were probably less flexible than previously thought. In 2014, Mike P. Taylor analysed the flexibility in the necks of Apatosaurus and Diplodocus. He found that Cobley et al. was incorrect in the fact that vertebrae imply the neck is less flexible than in actuality. Cobley et al. found necks to be much less flexible than in reality when cartilage was added. It was found that the cartilage between the joints would have allowed for the neck to flex far past 90º. However, Taylor noted that the neck, while it could flex above the vertical, the osteological neutral pose would have been around horizontal, and the habitual pose would have held the head upwards in an alert pose.

Integument

Few remains of sauropodomorph integument have been discovered. Those known are from derived titanosaurian embryos, diplodocids, Pelorosaurus, and Tehuelchesaurus. These scarce skin fossils show that like most reptiles, these sauropods were at least partially covered in "non-overlapping scales radiating in a rosette pattern." The first known evidence of these scales in sauropods was from the forelimb of Pelorosaurus, described in 1852, from the Cretaceous of England. Later in 1935, Barnum Brown recognized currently the largest continuous scale preservation, from the Howe Quarry. Unfortunately, these were damaged during the excavation of the associated bones. In the early 1990s, similar scales were recovered from the formation, and assigned to diplodocids. These show that the group possessed keratinous dermal spines along the ridge of their backs, probably extending from the neck to the tail. In 1998, embryos of titanosaurian sauropods were uncovered from South America, showing the same radiating pattern. The scales of Tehuelchesaurus were first described in 2007, and represent some of few preserved Jurassic sauropod scales. These scales are known from the forelimb, neck and belly, and back of adults, and around the body of embryos.

Movement

Animation of the most likely way Argentinosaurus walked

In 2013, in a study published in Plos One on October 30, 2013 by Dr. Bill Sellers, Rodolfo Coria, Lee Margetts et al, Argentinosaurus was digitally reconstructed to test its locomotion for the first time. To estimate the gait and speed of Argentinosaurus, the study performed a musculoskeletal analysis. The only previous musculoskeletal analyses were conducted on hominids, terror birds, and other dinosaurs. Before they could conduct the analysis, the team had to create a digital skeleton of the animal in question, show where there would be muscle layering, locate the muscles and joints, and finally find the muscle properties before finding the gait and speed. The results of the biomechanical study revealed that Argentinosaurus was mechanically competent at a top speed of 2 m/s (5 mph) given the great weight of the animal and the strain that its joints were capable of bearing. The results further revealed that much larger terrestrial vertebrates might be possible, but would require significant body remodeling and possibly behavioral change to prevent joint collapse.