Pre-scientific history

Dinosaur fossils have been known for millennia, although their true nature was not recognized. The Chinese considered them to be dragon bones and documented them as such. For example, Huayang Guo Zhi (華陽國志), a gazetteer compiled by Chang Qu (常璩) during the Western Jin Dynasty (265–316), reported the discovery of dragon bones at Wucheng in Sichuan Province.^[39] Villagers in central China have long unearthed fossilized "dragon bones" for use in traditional medicines.^[40] In Europe, dinosaur fossils were generally believed to be the remains of giants and other biblical creatures.^[41]

Early dinosaur research

Scholarly descriptions of what would now be recognized as dinosaur bones first appeared in the late 17th century in England. Part of a bone, now known to have been the femur of a Megalosaurus,^[42] was recovered from a limestone quarry at Cornwell near Chipping Norton, Oxfordshire, in 1676. The fragment was sent to Robert Plot, Professor of Chemistry at the University of Oxford and first curator of the Ashmolean Museum, who published a description in his The Natural History of Oxford-shire (1677).^[43] He correctly identified the bone as the lower extremity of the femur of a large animal, and recognized that it was too large to belong to any known species. He, therefore, concluded it to be the femur of a huge human, perhaps a Titan or another type of giant featured in legends.^[44]^[45] Edward Lhuyd, a friend of Sir Isaac Newton, published Lithophylacii Britannici ichnographia (1699), the first scientific treatment of what would now be recognized as a dinosaur when he described and named a sauropod tooth, "Rutellum impicatum",^[46]^[47] that had been found in Caswell, near Witney, Oxfordshire.^[48]

Between 1815 and 1824, the Rev William Buckland, the first Reader of Geology at the University of Oxford, collected more fossilized bones of Megalosaurus and became the first person to describe a non-avian dinosaur in a scientific journal.^[42]^[49] The second non-avian dinosaur genus to be identified, Iguanodon, was according to legend discovered in 1822 by Mary Ann Mantell – the wife of English geologist Gideon Mantell who in fact had required remains years earlier. Gideon Mantell recognized similarities between his fossils and the bones of modern iguanas. He published his findings in 1825.^[50]^[51]

The study of these "great fossil lizards" soon became of great interest to European and American scientists, and in 1842 the English paleontologist Sir Richard Owen coined the term "dinosaur", using it to refer to the "distinct tribe or sub-order of Saurian Reptiles" that were then being recognized in England and around the world.^[7]^[8]^[9]^[52]^[53] The term is derived from Ancient Greek δεινός (deinos) 'terrible, potent or fearfully great' and σαῦρος (sauros) 'lizard or reptile'.^[52]^[54] Though the taxonomic name has often been interpreted as a reference to dinosaurs' teeth, claws, and other fearsome characteristics, Owen intended it also to evoke their size and majesty.^[55] Owen recognized that the remains that had been found so far, Iguanodon, Megalosaurus and Hylaeosaurus, shared distinctive features, and so decided to present them as a distinct taxonomic group. As clarified by British geologist and historian Hugh Torrens, Owen had given a presentation about fossil reptiles to the British Association for the Advancement of Science in 1841, but reports of the time show that Owen did not mention the word "dinosaur", nor recognize dinosaurs as a distinct group of reptiles in his address. He introduced the Dinosauria only in the revised text version of his talk published in April 1842.^[7]^[8] With the backing of Prince Albert, the husband of Queen Victoria, Owen established the Natural History Museum, London, to display the national collection of dinosaur fossils and other biological and geological exhibits.^[56]

Discoveries in North America

Edward Drinker Cope

Othniel Charles Marsh

In 1858, William Parker Foulke discovered the first known American dinosaur, in marl pits in the small town of Haddonfield, New Jersey. (Although fossils had been found before, their nature had not been correctly discerned.) The creature was named Hadrosaurus foulkii. It was an extremely important find: Hadrosaurus was one of the first nearly complete dinosaur skeletons found (the first was in 1834, in Maidstone, England), and it was clearly a bipedal creature. This was a revolutionary discovery as, until that point, most scientists had believed dinosaurs walked on four feet, like other lizards. Foulke's discoveries sparked a wave of interests in dinosaurs in the United States, known as dinosaur mania.^[57]

Dinosaur mania was exemplified by the fierce rivalry between Edward Drinker Cope and Othniel Charles Marsh, both of whom raced to be the first to find new dinosaurs in what came to be known as the Bone Wars. This fight between the two scientists lasted for over 30 years, ending in 1897 when Cope died after spending his entire fortune on the dinosaur hunt. Many valuable dinosaur specimens were damaged or destroyed due to the pair's rough methods: for example, their diggers often used dynamite to unearth bones. Modern paleontologists would find such methods crude and unacceptable, since blasting easily destroys fossil and stratigraphic evidence. Despite their unrefined methods, the contributions of Cope and Marsh to paleontology were vast: Marsh unearthed 86 new species of dinosaur and Cope discovered 56, a total of 142 new species. Cope's collection is now at the American Museum of Natural History in New York City, while Marsh's is at the Peabody Museum of Natural History at Yale University.^[58]

"Dinosaur renaissance" and beyond

World War II caused a pause in palaeontological research; after the war, research attention was also diverted increasingly to fossil mammals rather than dinosaurs, which were seen as sluggish and cold-blooded.^[59]^[60] At the end of the 1960s, however, the field of dinosaur research experienced a surge in activity that remains ongoing.^[61] Several seminal studies led to this activity. First, John Ostrom discovered the bird-like dromaeosaurid theropod Deinonychus and described it in 1969. Its anatomy indicated that it was an active predator that was likely warm-blooded, in marked contrast to the then-prevailing image of dinosaurs.^[59] Concurrently, Robert T. Bakker published a series of studies that likewise argued for active lifestyles in dinosaurs based on anatomical and ecological evidence (see § Physiology),^[62]^[63] which were subsequently summarized in his 1986 book The Dinosaur Heresies.^[64]

New revelations were supported by an increase in dinosaur discoveries. Major new dinosaur discoveries have been made by paleontologists working in previously unexplored regions, including India, South America, Madagascar, Antarctica, and most significantly China. Across theropods, sauropodomorphs, and ornithischians, the number of named genera began to increase exponentially in the 1990s.^[20] As of 2008,^[update] over 30 new species of dinosaurs were named each year.^[65] At least sauropodomorphs experienced a further increase in the number of named species in the 2010s, with an average of 9.3 new species having been named each year between 2009 and 2020. As a consequence, more sauropodomorphs were named between 1990 and 2020 than in all previous years combined.^[66] These new localities also led to improvements in overall specimen quality, with new species being increasingly named not on scrappy fossils but on more complete skeletons, sometimes from multiple individuals. Better specimens also led to new species being invalidated less frequently.^[65] Asian localities have produced the most complete theropod specimens,^[67] while North American localities have produced the most complete sauropodomorph specimens.^[66]

Prior to the dinosaur renaissance, dinosaurs were mostly classified using the traditional rank-based system of Linnaean taxonomy. The renaissance was also accompanied by the increasingly widespread application of cladistics, a more objective method of classification based on ancestry and shared traits, which has proved tremendously useful in the study of dinosaur systematics and evolution. Cladistic analysis, among other techniques, helps to compensate for an often incomplete and fragmentary fossil record.^[68]^[69] Reference books summarizing the state of dinosaur research, such as David B. Weishampel and colleagues' The Dinosauria, made knowledge more accessible^[70] and spurred further interest in dinosaur research. The release of the first and second editions of The Dinosauria in 1990 and 2004, and of a review paper by Paul Sereno in 1998, were accompanied by increases in the number of published phylogenetic trees for dinosaurs.^[71]

Soft tissue and molecular preservation

Dinosaur fossils are not limited to bones, but also include imprints or mineralized remains of skin coverings, organs, and other tissues. Of these, skin coverings based on keratin proteins are most easily preserved because of their cross-linked, hydrophobic molecular structure.^[72] Fossils of keratin-based skin coverings or bony skin coverings are known from most major groups of dinosaurs. Dinosaur fossils with scaly skin impressions have been found since the 19th century. Samuel Beckles discovered a sauropod forelimb with preserved skin in 1852 that was incorrectly attributed to a crocodile; it was correctly attributed by Marsh in 1888 and subject to further study by Reginald Hooley in 1917.^[73] Among ornithischians, in 1884 Jacob Wortman found skin impressions on the first known specimen of Edmontosaurus annectens, which were largely destroyed during the specimen's excavation.^[74] Owen and Hooley subsequently described skin impressions of Hypsilophodon and Iguanodon in 1885 and 1917.^[73] Since then, scale impressions have been most frequently found among hadrosaurids, where the impressions are known from nearly the entire body across multiple specimens.^[75]

Color restoration of Sinosauropteryx

Color restoration of Psittacosaurus

Starting from the 1990s, major discoveries of exceptionally preserved fossils in deposits known as conservation Lagerstätten contributed to research on dinosaur soft tissues.^[76]^[77] Chiefly among these were the rocks that produced the Jehol (Early Cretaceous) and Yanliao (Mid-to-Late Jurassic) biotas of northeastern China, from which hundreds of dinosaur specimens bearing impressions of feather-like structures (both closely related to birds and otherwise, see § Origin of birds) have been described by Xing Xu and colleagues.^[78]^[79] In living reptiles and mammals, pigment-storing cellular structures known as melanosomes are partially responsible for producing colouration.^[80]^[81] Both chemical traces of melanin and characteristically shaped melanosomes have been reported from feathers and scales of Jehol and Yanliao dinosaurs, including both theropods and ornithischians.^[82] This has enabled multiple full-body reconstructions of dinosaur colouration, such as for Sinosauropteryx^[83] and Psittacosaurus^[84] by Jakob Vinther and colleagues, and similar techniques have also been extended to dinosaur fossils from other localities.^[80] (However, some researchers have also suggested that fossilized melanosomes represent bacterial remains.^[85]^[86]) Stomach contents in some Jehol and Yanliao dinosaurs closely related to birds have also provided indirect indications of diet and digestive system anatomy (e.g., crops).^[87]^[88] More concrete evidence of internal anatomy has been reported in Scipionyx from the Pietraroja Plattenkalk of Italy. It preserves portions of the intestines, colon, liver, muscles, and windpipe.^[89]

Concurrently, a line of work led by Mary Higby Schweitzer, Jack Horner, and colleagues reported various occurrences of preserved soft tissues and proteins within dinosaur bone fossils. Various mineralized structures that likely represented red blood cells and collagen fibres had been found by Schweitzer and others in tyrannosaurid bones as early as 1991.^[90]^[91]^[92] However, in 2005, Schweitzer and colleagues reported that a femur of Tyrannosaurus preserved soft, flexible tissue within, including blood vessels, bone matrix, and connective tissue (bone fibers) that had retained their microscopic structure.^[93] This discovery suggested that original soft tissues could be preserved over geological time,^[72] with multiple mechanisms having been proposed.^[94] Later, in 2009, Schweitzer and colleagues reported that a Brachylophosaurus femur preserved similar microstructures, and immunohistochemical techniques (based on antibody binding) demonstrated the presence of proteins such as collagen, elastin, and laminin.^[95] Both specimens yielded collagen protein sequences that were viable for molecular phylogenetic analyses, which grouped them with birds as would be expected.^[95]^[96] The extraction of fragmentary DNA has also been reported for both of these fossils,^[97] along with a specimen of Hypacrosaurus.^[98] In 2015, Sergio Bertazzo and colleagues reported the preservation of collagen fibres and red blood cells in eight Cretaceous dinosaur specimens that did not show any signs of exceptional preservation, indicating that soft tissue may be preserved more commonly than previously thought.^[99] Suggestions that these structures represent bacterial biofilms^[100] have been rejected,^[101] but cross-contamination remains a possibility that is difficult to detect.^[102]