Uncovering the Secrets of Dinosaurs With Amazing Fossils

Scientists who study dinosaurs make new discoveries in the field, extracting previously unknown specimens from the ground. Other revelations come from surveys of historical museum collections or work in the laboratory. New technologies are continually applied to these old bones, including state-of-the-art imaging techniques, sophisticated chemical tests, and complex statistical analyses. In addition, the scientific study of dinosaurs began with only a handful of savants in nineteenth century Europe, but now there are hundreds of dinosaur specialists spread globally. As a result, we’re living in a ‘golden age’ of dinosaur research. Today, we present three fossils from the upcoming A History of Dinosaurs in Fifty Fossils that unearth the mysteries of creatures that have fascinated us for 200 years.

1. Titanosaur egg

Dinosaur eggs come in a spectacular range of shapes and sizes. Remembering that birds are part of the dinosaur radiation, the smallest dinosaur egg we know about belongs to a living animal, the bee hummingbird. Each of its tiny eggs is roughly the size of a pea and weighs only 0.0018 oz. Perhaps surprisingly, eggs at the other end of the scale did not belong to gigantic sauropods, but to a more recently extinct species, the elephant bird of Madagascar (Aepyornis), whose eggs were up to 15¾ inches long and had a volume of nearly 27½ pints (equivalent to over 22 lb, roughly seven times the size of an ostrich egg). Although the eggs of some extinct dinosaurs, such as those of giant oviraptorosaurs were longer than those of elephant birds, they were much narrower in shape, lacking the typically oval shape of bird eggs: as a result, they had much smaller volumes. One such egg type is called Macroelongatoolithus (the name meaning, literally, ‘large, elongate, fossil egg’), which reaches an impressive 24 inches in length.

Sauropod eggs were average by comparison, reaching up to 8 inches across, the size of a cantaloupe melon. This titanosaur egg from the Late Cretaceous of India is a typical example, although this particular specimen has an interesting history. It was discovered sometime in the 1840s (exact date unknown) and donated to the Natural History Museum, in London. However, dinosaurs were only established as a group in the 1840s and no one was yet thinking about dinosaur eggs. As this specimen is filled with beautiful purple and white layers of the mineral agate, which grew in the egg as it became fossilized, the original owner assumed that it was just an interesting rock. Consequently, it was never seen by the museum’s paleontologists, but was added to its Mineralogy Collection. It was only in 2021 that curators noticed some unusual features that quickly led to it being recognized as a dinosaur egg. Not only was it a dinosaur egg, but its date of collection suggests that it might have been the first example of a dinosaur egg to end up in a museum!

The features that identified this object as an egg are found in all other fossilized dinosaur eggs. Luckily, the egg is preserved in two halves so we can see its internal structure. The outer part of the specimen is formed by a narrow layer of even thickness (about 1/6 inch), which surrounds the whole structure. The mineral-filled interior is spherical in shape, matching the overall shape, and was originally hollow. The outer layer has tiny holes (called pores) and its surface is ornamented with many small, low, rounded bumps. Pores are essential for letting life-giving oxygen into the developing egg and letting the waste carbon dioxide out, and the complex surface patterns might have assisted in keeping the pores free from dirt. The spherical shape, surface patterning, and even the thickness of the outer layer are features that are consistent with features seen in fossil eggs that were found in the same area, more recently. These Late Cretaceous fossil beds in India are rich in the remains of titanosaurs, suggesting that they were the likeliest egg-layers.

This brings us to a problem. Fossil eggs are not usually found inside their mother’s bodies, but in nests (although there are two or three very rare exceptions), and almost all eggs and nests have, so far, been found in sites lacking dinosaur bones. So, how do we know which egg goes with which dinosaur? Sometimes, we are lucky and the eggs contain the fossilized remains of an unhatched baby dinosaur. In these cases, we can often identify the exact species involved or at least the dinosaur group to which the egg belonged. However, such finds are scarce (our titanosaur egg does not contain any bones, sadly). Luckily, there are other clues. It turns out that when we list features like egg size and shape, and more subtle features such as the microscopic structure of the eggshell, for those eggs that do contain hatchlings, some useful patterns emerge. These comparisons show that each major dinosaur group laid slightly different egg types, each with their own unique set of characteristics. For example, most theropod eggs are almost sausage-shaped – long and narrow, with rounded ends. By comparison, most sauropod and hadrosaur eggs are round. Microscopic structure, in particular the shapes and orientations of the individual calcite crystals of which the shell is made, is particularly useful for paleontologists, because you do not even need a whole egg, just a fragment, to identify the type of dinosaur that you are dealing with.

Detailed study of eggs can reveal other secrets of dinosaur biology. For example, living crocodiles, turtles and lizards lay plain, white eggs, whereas birds produce a variety of shades ranging from beige to vivid blues and greens, as well as eggs decorated with many different patterns. This ability assists in camouflage, reducing the risk of egg predation, and helps the parents to recognize their own eggs. Recently, a range of advanced chemical analyses were applied to dinosaur eggs, revealing their original colors. Some, like those of hadrosaurs and sauropods, were white, similar to their reptilian relatives. However, theropod eggs, like those of Deinonychus, share the same blue-green colors seen in birds, and were probably dappled. This is yet another example of a feature formerly thought to be unique to birds that originated much earlier in their dinosaur ancestors.

2. Pachycephalosaurus

The dome-heads, or pachycephalosaurs, are some of the most mysterious dinosaurs. They appear suddenly in the fossil record, near the end of the Cretaceous Period, and are known only from Asia and North America. Only around 15 species have been named and little is known of their early evolution. Most pachycephalosaur remains consist of broken partial skulls and much of our knowledge of their anatomy comes from a handful of more completely preserved specimens.

The largest, most famous pachycephalosaur is Pachycephalosaurus itself, from Canada and the USA. Reaching up to 16½ ft in length, its most impressive feature was its skull, which bore a massive, solid bony dome above the eyes that reached up to 10 inches in thickness. The edges of the dome were studded with an array of small bumps and spines, giving it a knobbly, spiky appearance. Pachycephalosaur teeth were low and triangular, showing that they were herbivores, and the few complete specimens we have confirm that they had long hind limbs and much shorter forelimbs, showing that they were bipedal.

All pachycephalosaurs had thickened skulls, with a bony shelf sticking out from the rear margin. This shelf usually bore a variety of knobs or short horn-like spikes, similar to those in Pachycephalosaurus. However, only a few pachycephalosaur species had the thick, rounded, bulbous domes for which the group is most famous. Differences in the shapes, numbers, position and sizes of these bony ornaments are used to distinguish the various species from each other. There has been continual debate about the use of the domes and spikes, with earlier generations of scientists suggesting that they might have used the domes for head-butting like mountain goats – either for competing with each other, in disputes over mates perhaps, or for warding off predators. However, detailed analyses of the dome’s bone structure using cutting-edge imaging techniques suggest that direct headon-head ramming behaviour was not likely: the domes do not seem to be strong enough to withstand major impacts, and their rounded shape would mean that the clashing heads would simply glance off each other. Despite these findings, it is still thought that these skull features were important in behaviour, with the spikes being useful for visual display and the domes and flat shelf-like areas used for slower shoving contests, or perhaps for butting into the sides of opponents.

One other ornithischian group, the ceratopsians (horned dinosaurs), also developed bony shelves at the back of their skulls. The shared possession of this feature is often used to link pachycephalosaurs and ceratopsians together into a broader group. However, as we know so little about pachycephalosaur origins, it is possible that they might be related to other ornithischians instead. Pachycephalosaur fossils should be present in older rocks, as all their potential relatives lived during the Jurassic, but so far nothing has been found. We desperately need new fossils to reveal how pachycephalosaurs gained their distinctive features and to fit them more snugly into the dinosaur tree of life.

3. Megalosaurus

The vales and hills of the Cotswolds, in the heart of southern England, are dotted with postcard-pretty villages. Straw-thatched cottages built of warm, honeycoloured limestone form tight, cosy clusters around churchyards and inns. At first glance, the rocks used to build these sought-after properties do not look remarkable. However, a closer look reveals that they did not form from the geological processes that created the granites, sandstones and other building stones used elsewhere. They had a very different origin and owe their existence to biology – the tireless work of countless corals, clams and plankton. These unsung labourers, which thrived in their billions, were part of an ancient construction industry that prospered in the warm shallow seas covering western Europe during the Middle Jurassic period, around 174–163 million years ago. By extracting the calcium dissolved in seawater, these ancient engineers built strong, intricate skeletons. These structures accumulated, cemented and compacted into rock, trapping the shells and bones of the other creatures that lived and died alongside them. Layer after layer of this organic debris arrived on the Jurassic seafloor, each with its own distinctive appearance and composition, reflecting myriad differences in the habits of their animal builders, changes in the water chemistry and in the proportions of sand and mud added by the erosion of nearby islands. Millions of years later, quarrymen and builders found that some of these limestones were ideal for building cottage walls, whereas others split more easily into thin slabs that made perfect roofing tiles.

During the eighteenth and nineteenth centuries, another biologically derived rock – coal, the product of even more ancient swamp forests – fuelled the Industrial Revolution. This, in turn, led to further exploitation of the UK’s natural resources, with economic growth stoking demand for goods, services and infrastructure. Mining for building stone, fuel and ore increased in scope and intensity. Although a farming area, the Cotswolds were also mined, with quarries opening to provide stone for the expansion of towns and cities, including the establishment of grand new colleges at the University of Oxford, the local seat of learning. Many of these building stones could be reached easily from the surface, but some – the layers providing the best roofing ‘slates’ – were in seams that were harder to reach. These required digging claustrophobically narrow tunnels deep underground, lit only by candlelight or oil lamp. This was dangerous work, as the tunnels often collapsed, but the miners excavated thousands of tonnes of slates, which still adorn the roofs of chapels and dining halls.

As the miners worked, they found strange objects similar to the bones of the horses, sheep and cattle living in the farms above, but differing in shape, and often of prodigious size. Some of these bones were acquired by local pastors and aristocrats who used their financial resources and ample leisure time to study these objects, thereby contributing to the developing sciences of biology and geology. This was a revolutionary time, when the dogma of Christian scripture was being challenged by experiments in laboratories and observations in the field. These ‘petrifactions’, the objects we now call fossils, were already known to be the remains of once living creatures, but they were often unlike anything alive today. These curiosities fuelled vicious controversies on the age of the Earth, the processes that created our landscapes, and on the life of the past.

Dozens of large, fossilized bones emerged from the slate mines around the village of Stonesfield, Oxfordshire. Many of these were acquired by the Anatomy School, in Christchurch College at the University of Oxford. Several scholars worked on this material but it was the brilliant and eccentric cleric, the Reverend William Buckland, who first brought them to public prominence. Buckland, Oxford’s first academic geologist, was initially stumped by the jaw bone, vertebrae and gigantic limb bones in these collections. However, the great French anatomist Georges Cuvier, who visited Buckland in 1818, applied his extensive knowledge of living animal anatomy to the problem, identifying the remains as those of a previously unknown gigantic reptile.

Cuvier’s insights, and those of other colleagues, enabled Buckland’s studies to progress and he presented the results of his work to a meeting of the Geological Society of London on 20 February 1824. During the proceedings he described in detail the remains of this ‘…enormous fossil animal…’, which he identified as ‘…belonging to the order of Saurians or lizards’. Buckland painted an evocative picture envisioning this creature – dubbed Megalosaurus, the ‘great lizard’ – describing it thus: ‘…the beast in question [would have] equalled in height our largest elephants, and in length fallen but little short of the largest whales’. What Buckland did not know was that he had just named the first example of what would become one of the most famous animal groups of all time: he had named a dinosaur.

Read more in A History of Dinosaurs in 50 Fossils, which is available from Smithsonian Books. Visit Smithsonian Books’ website to learn more about its publications and a full list of titles. 

Excerpt from A History of Dinosaurs in 50 Fossils by Paul M. Barrett © The Trustees of the Natural History Museum, London 2024