Have you ever been bitten by a dinosaur? That would be something worthy of Jurassic Park. In those movies it tends to be a lethal experience. But most children will get bitten by the dinosaur bug at some point in their early years. Dinosaurs fascinate; it is the excitement of a lost world, a wild place with unimaginable animals. I am the proud owner of a clutch of fossilized dinosaur eggs. Children are drawn to those eggs, with many recognizing them directly for what they are. Museums showcase dinosaur fossils – the large friendly dinosaur with its tail bones stretching along the floor is the first attraction you will see on entering the National History Museum in London. In New York, the Museum of National History similarly impresses but with a far more aggressive-looking fossil reaching far above the impressionable (and feeling very edible) public. For many, the dinosauric fascination continues into adulthood. Some become paleontologists, studying the fossils in the field. Rich people buy the fossils, showing that an abundance of money can feed the child inside. I understand them well – why else would I own those fossil eggs?

There are many sites in the world where dinosaur fossils have survived. Sometimes it is not the dinosaurs themselves but the impression they made which survives. Dinosaur footprints can be recognizable, where the massive animals depressed the surface. Soft tissue sometimes survived by impression, which is how we know about scaly skins and in some cases, feathers. Egg shells have already been mentioned as a dinosaur remnant. But mostly it is their bones, sticking out of an eroding rock.

One of the best places to see the fossils as a tourist is the Dinosaur National Monument, Utah, USA. The site, on the border between Utah and Colorado, is a gem. A viewing gallery covers a large rock wall, which at first impression is a jumble of irregular rock. A longer gaze reveals patterns in the jumble, which resolve themselves into petrified bones. Every square meter of the wall has several such bones sticking out. It is in effect a near-vertical dinosaur cemetery, the place where the old dinosaurs went to die. Excavations are still on-going, more than a century since the original discovery. If you are lucky, you will see people excavating in front of you, the paleontologists on display together with their finds. This museum is alive with the past.

The discoverer of the fossil treasure was Earl Douglas. In August 1909 he wrote: “At last in the top of the ledge where the softer overlying [sandstone] beds form a divide—a kind of saddle, I saw eight of the tail bones of a Brontosaurus in exact position. It was a beautiful sight. Part of the ledge had weathered away and several of the vertebrae had weathered out and the beautifully petrified centra lay on the ground. It is by far the best looking Dinosaur prospect I have ever found. The part exposed is worth preserving anyway”. It took some time before the true wealth of the site become clear. By November Douglas wrote: “… the ledge is full of bones… We realize that we have a stupendous task and the men are working like beavers… If we stopped to consider what a terrible lot of work there is to do we might fall over…” The area over which the bones were found measured less than 150 meters in width. The bone layer was strongly tilted, by over 60 degrees, so that bones that were originally spread out horizontally now seem to stack almost vertically. Over the following few years, complete skeletons of an apatosaurus, a diplodocus and an allosaurus were excavated. Animals had died at all ages: there were fully grown adults and young dinosaurs, including a diplodocus only 1 meter long.

Geology

Dinosaur National Monument is much more than a viewing wall, even if the description may give the impression of a Truman show with dinosaurs on a very wide screen tv. This is a national park which covers a large area, which is begging to be explored, although far harder to reach than the visitor centre with its car park. There are dinosaur remains in several places. But those are only one aspect of the park. The landscape is varied, with rocks and layers dating from a billion years ago to the much more recent past. When hiking here, or taking a river trip, you should try to take a geologist with you to explain the landscape. Rock watching may not sound as exciting as bird watching, but the Earth has spend a long time putting this place together. It is a work of art and should be given the attention it deserves, even if the view hasn’t changed much in the past 30 million years. You are not here to watch paint dry. You are admiring the end product.

But in spite of the geological riches, people are drawn mainly to the big friendly (or not so friendly) giants of their childhood. Of all those different geological layers, dinosaur remains are present in only one: the Morrison formation. This layer is a relative newcomer in the region. History here starts about a billion years ago with an ancient mountain range. It wasn’t exactly at this location, but nearby: as the rain weathered the mountains, the sediment came here. This bottom layer is a dark rock called the Uinta Mountain Group (it is named after the local mountain range of modern times). Much later, the land sank and the sea came in. The second layer, called the Lodore formation, is a sandstone that was deposited in a shallow sea around 500 million years ago. As the land sank further, 340 million years ago, the Madison Limestone and the Round Valley Limestone were deposited. At this time the sea stretched from north to south across the continent. 30 million years later, as the sea shallowed, the Morgan formation formed. This is a sandstone which forms a series of nine layers, each cycling between red sandstone, brown mudstone and light-coloured limestone. The red sandstone formed when the surface was drier, the others during wetter times. The layers can be seen in the deep canyons of the park. The layering has been attributed to an ice age which kept changing the sea levels. Now the land rose again above the sea. A vast sand desert formed: this left us the Weber Sandstone, 300 million years old, which form thick layers in which the shape of the sand dunes is sometimes visible.

Nothing now happened until 150 million years ago, as the paint dried. Any rocks that were deposited during this time have been eroded away, leaving a gap in the record. Now, a new episode of sandstone came. This is the Morrison Formation, and this is where the dinosaur fossils reside. The age is between 155 and 144 million years ago. The formation is not unique to this park. The Morrison Formation is found across several states, from New Mexico to Montana, along the east side of the Rocky Mountains. If you see a dinosaur in a US museum, it is not unlikely it came from the Morrison. The Dinosaur National Monument also has some fossils from the later Cedar Mountain formation, rougly 100 million years old, but the large majority are from the Morrison era.

Much later, the growth of the Rocky Mountains caused large scale uplift in the region. This brought with it erosion, and canyons formed. Very recently, glacial erosion has added its signature. All these events has left us a varied and deepy impressive park – which few of the visitors will ever see. They had straight for the visitor centre and the famous wall of crumbly rock-and-bone. The people don’t come for the view. They are here to celebrate an ancient Day of the Dead, a visit to the cemetery to keep the memory alive of this who have departed.

At the time of the Morrison formation, the western USA was just beginning to take shape. In Nevada, the first of the highlands were rising that were later to become the Rocky Mountains. This started 160 million years ago and is known as the Elko orogeny – Elko is a lovely town in northern Nevada, once voted the ‘best small town in America’, and recently upgraded to the ‘best city to live in the state of Nevada’, presumably after some population growth had occured. I once spend a few days in Elko and nearby Spring Creek and can confirm it was a nice place! But it wasn’t nearly as nice here during the Jurassic. A subduction zone had formed off-shore, where the Farallon oceanic plate was subducting underneath the continent, bringing with it the first of the Pacific terranes to join the west coast. An Andean-type mountain range formed above the subducted plate. Rivers were flowing east from those mountains with their load of debris, in some places sand, in other places mud to the low-lying land beyond the Elko mountains. This debris was to become the origin of the Morrison formation.

Ecology

The region east of the mountains had become a huge basin, thousand of kilometers long, and watered by those rivers. The climate was subtropical and fairly dry; the land would have been savannah, except that it didn’t look anything like a savannah because grass had not evolved yet. The area was located in the rain shadow of the newly risen highlands, and away from the rivers water would have been in limited supply, although not desert-like. There were lakes and swamps around the river, as in an ancient Pantanal. Plants were abundant along the rivers, with beds of horse tails, and on drier lands tree ferns and conifers. Across a large area of the USA, the fossils of the Morrison show an abundance of aquatic life, such as mussels and snails (both gill and lung breathing).

The sheer number of the dinosaur fossils in this Formation, including embryos, shows that this was a good environment for them. But there was much more than the occasional large dinosaurs. Much of the animal worlds would have looked somewhat familiar to us, with salamanders, frogs, turtles, and small mammals. And crocodiles.

Just like grazing animals on the African savannah, the local dinosaurs may have been migratory, following the rains and the water. In each location, they probably loitered along the rivers, where the salad bar was set for them. And of course, wherever there is a salad bar, carnivores are patiently waiting for their share. They are not after the salad bar: what they see is a walking buffet. The vegetarians would have needed to be careful eaters; any carelessness could be a recipe for disaster.

Dinosaur volcanics

Just like today, volcanoes were active in the western USA. We don’t know too much about them, but the evidence for their presence is clear in the Morrison Formation. Between 25% and 75% of the Morrison Formation consists of volcanic ash! The volcanoes themselves have not survived – this was, after all, a long time ago and much of the western USA has been remodeled as old oceans subducted, new terranes docked, basins formed and mountains grew.

The oldest Morrison beds, between 155 and 160 million years old, have fewer ash layers. They become more common in the younger layers, between 150 and 155 million years ago. After that, the deposits suddenly end. There is a time gap of perhaps 20 million years, before the Cedar Mountain Formation. By that time the Jurassic had become the Cretaceous and the dinosaurs had changed. If you liked Jurassic Park, you would have enjoyed the the Cretaceous: all the best dinosaurs from the movie were from the Cretaceous. But why the missing 20 million years? These may have been removed by erosion. However, it can be difficult to know whether something is missing because it was removed, or because it was never there.

But let’s return to the Morrisons. The eruptions that occurred over those 5 million years were massive. Ash beds in Utah are over 100 meters thick, containing at least 35 separate explosions. The size of the ash beds, far from the eruptions themselves, suggest these were caused by caldera collapses. Where did these happen? The calderas themselves have not been found and weren’t here. The ash was blown here by the wind from the southwest. This puts the calderas closer to the coast. We can envisage a volcanic arc stretching from western Utah and central Nevada to California’s Mojave. Ash can travel surprisingly far. For instance the ash of the Bishops Tuff is 1 meter thick 800 km from the source! This is not due to ash fall which would have been a few centimeters at this distance. Wind and water transported the ash over such large distances.

Some of the ash beds of Morrisons contain rhyolite and dacite, and a little andesite. The oldest beds are less evolved, the Dinosaur National Park lacks these older events. The volcanics may have changed over time.

What triggered the sudden spike in activity in a brief 5-million year period, 150 million years ago? We can only speculate. The subducting slab may have gone down steeper, causing extension in the overlying continental crust and allowing magma to rise. A slip-strike fault may have developed with an extensional component. There may have been a change in subduction direction. All of these may have occurred! In one view, there were alternating phases of contraction and extension in the region, leading to the Elko highlands, the Morrison basin, and further west the volcanic calderas. This side of the Pacific Ocean has long been a complicated place. This has since a long time been the main long-range east-facing subduction zone on Earth. As America is moving west, it is collecting all the debris that ended up in the eastern Pacific, has swallowed spreading ridges, and incorporated microcontinents. The western USA is where the Pacific ocean dumps its waste.

The volcanoes erupted and deposited thick ash over the land, over several millions of years. The ash did not always remain where it was laid down. Rivers took it down into the basin and the ash ended up in lakes and on mudflats. The interaction with water changed the minerals: sometimes clays such as smectite and illite formed, while in another place it might be albite. It depended on the chemistry of the water. Some of the SiO2 in the ash dissolved into the water. This made the local sediment and clays enriched in the other minerals which did not dissolve, while downstream the dissolved elements would become enriched. This alteration makes it more difficult to know what type of magma caused the eruption. The rare earth elements give the best indication of the original magma: they have ratios that are like rhyolite to rhyodacite, but are not as extreme as found for instance in the Bishops Tuff.

The composition confirms that the volcanoes formed in a volcanic arc along a subduction zone. There was no hot spot or spreading ridge involved. This fits with what we know from the geology.

As the ash was subjected to water in lakes and elsewhere, two important elements dissolved: uranium and vanadium. As the ash beds altered, the sediment layers that formed downstream became enriched in these elements. The sedimentary beds of the Morrison Formation are the main source of mineable deposits of these elements in the USA.

The crumbly stone of the Dinosaur Wall formed from sediments, downstream from the ash beds that fed them, which themselves were downwind from the volcanoes that caused them, which themselves were downstream of the subduction zone that caused them. A chain of cause and effect links the Wall to the distant Pacific ocean.

Dissecting the Morrisons

In the Dinosaur National Monument, the Morrison Formation contains several distinct beds, only one of which contains the famous bones. The four beds are called ‘members’, and called Windy Hill (the oldest), Tidwell, Saltwash and Brushy Basin (the youngest) – the dinosaur wall belongs to the last. Windy Hill is a marine sandstone, and was deposited when this was a sea. The others were deposited when the area had been raised above sea level: Tidwell is a mudstone, Saltwash a river sandstone and Brushy Basin is mainly clay, deposited in lakes, floodplains and in a few cases, streams. Brushy Basin contains more volcanic ash than the other members, and seems to coincide with a peak in volcanic activity. This ash interacted with the water and changed into the red or grey claystone which we now see in the landscape.

The dinosaur wall is part of the Brushy Basin member, but it is not a typical part. While the Brushy Basin is primarily clay, the Quarry bed consists of sandstone. It is present only in the immediate area around the dinosaur wall.

The sandstone layer, sandwiched between the clays, can be readily recognized in the landscape. The picture (also from Bezinsky and Kollar, 2018) shows the view east from the parking area, with the Brushy Basin indicated, and the Quarry sandstone forming a ridge above the rest. It stands out because the sandstone is harder and therefore has eroded a bit less.

The sandstone indicates a river. The clay may have formed in lakes where the sediment rained down over time, but the sand was brought by flowing water. The streams went roughly northwest to southeast. The sandstone is found along a line that runs almost west to east, a bit over 1 kilometer in length with a series of outcrops. This is not the line of the river: instead the length of the sandstone bed may correspond to the width of the river bed or it flood plain.

The sandstone is not a single deposit. The picture clearly shows layering. Twelve sandstone layers can be recognized. The layers are coarse-grained sandstone, separated by conglomerates of pebbles and granules. As you move along a layer in the (original) downstream direction, the grains become finer. One of the sandstone layers shows up as isolated, rounded features, around 4 meters in diameter, and perhaps half as thick; they are approximately lens-shaped.

The clays indicate the presence of standing water, or water-logged terrain. At other times there were rivers here in a drier environment. It was more complex than a single, fast-flowing river channel. The lenses may have been sandbanks, build up by the river.

A river runs through it

What about the dinosaurs? The abundance of bones is seen in only one of the many layers and in only one location in this layer. They are found near the bottom of the sand, in what would have been the deepest point of the channel. But the animals can hardly have died there. At least 20 individuals of 10 different species are found. That does not sound like a herd and more like panic. In fact, the location is considered a ‘death assembly’: this is not the place where dinosaurs went to die, but is where the dead dinosaurs ended up. It is the cemetery. They died elsewhere, although probably not far away as dead dinosaurs can’t easily move. If you think of a river bed, the largest pebbles move the least distance. Now think of a dinosaur as a very large pebble. It takes a big flow to move one even a small distance. A major water flow would be needed.

A carcass can actually float. This happens during part of the decomposition. Otherwise, they move by sliding. In some cases there is also evidence of rolling of the carcass by water. The completeness of the bones indicates that they fairly quickly became protected against the elements, presumably by being buried in sediment or sand. Many of the bones show insect damage, and there are indications of drying out. There are a few cases of fossilized skin. This suggests an anoxic environment, as may occur in wet soil below the carcass – but only if there is no flowing water. The alternative is a hot and dry environment, but this leads to the same conclusion: although water created the assembly, the dead dinosaurs did not remain submerged. The water went away.

It should be noted that not all bones are together. In addition to the complete animals, there are many single bones as well. We don’t know whether these belong to a few other individuals or are unrelated to each other. On the east side of the quarry, these separated bones are so dense that they would have obstructed the flow channel by capturing the sediment. And above the bone layer is a thin layer containing many bone fragments, some only millimeter size, showing signs of abrasion. They were perhaps moved by rolling across the surface. We do not know how far they would have travelled. We can’t even be sure it was done by water (although likely): abrasion by sand can also be considered.

The discussion has been going on for decades. Was this a river? It has even been given a name: the Quarry river. But did it exist, and if it did, was it here? There was certainly water, and quite a lot of it: the fossils include evidence of aquatic life, such as turtles, bivalves and gill-breathing snails. Some of the bivalves are still closed: normally they break apart after death. This suggest they were buried while still in water. Many though are disjointed, and someties even stacked. This indicates that the shells, including the closed, buried ones, died somewhere else and were brought here later. There was transport from a place with permanent water to here. It is beginning to sound very much like a river. Dinosaurs, however, would not be expected in deep, fast flowing rivers. They died outside of any river, were brought here by deep, fast water that wasn’t there when they died, and after the bodies had been moved here, at some point the water disappeared. The drying out may have taken months (or even years), but it happened before all soft tissue had decayed. Was the water seasonal? Afterwards, the water did not return to the burial site, probably for many years.

There is some evidence for scavenging of the carcasses, but not nearly as much as might be expected. Whatever happened perhaps did not leave a lot of predators around. As the vegetarians and the lost salad bar, so the carnivores and their carvery.

Death on the Quarry River

Before moving on, a discussion is needed on cause of death. This is always hard to certify in the case of fossils, and especially if the fossils are not found at the exact place where they died. Death by predation can be excluded: that never gives mass mortality. Drought has often been suggested. This is common on the African savannah, which is a similar environment. However, the animals tend to be spread out along the vanished rivers rather than concentrated, so this requires that the corpses were brought together after death. Disease can be ruled out as the main cause of death, as this would not impact such a variety of species. Migration can lead to mass mortality by drowning, if the herd feels compelled to cross a river which happens to be in spate. But this tends to happen to single-species herds, and here the great variation of species argues against it.

This leaves two main options: the deaths may have been caused by drought, and occurred over an extended period, or the dinosaurs died in a catastrophe. The former explanation is generally preferred; it explains many of the characteristics including variation in the amount of decay, but requires that the bodies were brought together by a flood. The limited evidence for scavenging may be a problem. For those who prefer catastrophe, the main choice is normally a major flood.

(The option of death by volcanic eruption (pyroclastics) may seem attractive on VC but lacks evidence and a nearby culprit. Volcanic possibilities such as CO2 poisoning (lake overturn) or a sulphur cloud from some volcano have not yet been suggested. I mention it here just to fill this gap still unclaimed by speculationists. And yes, that is a new word. You read it here first.)

Both the drought and the catastrophe require a flood as part of the explanation. That is the only thing most people would agree on – and it agrees with the presence of the remains of turtles.

The power of water

The next question becomes, what kind of flood? There are two main options: an event within a river bed, or a break-out to a flood plain.

Some of the deposits are suggestive of the latter, what happens when a stream breaks its banks and overflows the surrounding land. The illustration below is taken from the paper by Brezinsky and Kollar, 2018, already mentioned above. The first panel shows the situation at the start of the break-out. Next to the river is a splay, perhaps build up by a previous break out or by an abandoned meander of the river. The next panel shows the situation a while later. As the river broke out over the flood plain, several deep channels were formed by the fast flow, levees build up around these channels and soil developed in the standing water around the levees.

In this model, the dinosaurs could have been on the initial splay, overcome by the sudden flood and ended up at the bottom of the deep channel. The mussels, snails and turtles came from the river, and were brought by the flood. Afterwards, the break healed or the river found a new bed, the flooded area dried out, and insects (but few carnivores) feasted on the buffet. And very much later, scientists come and wondered how so many dinosaurs managed to die in such a small area.

This model explains many aspects, but is not the most popular one. Most commonly, papers argue for an event inside a river bed. A braided river system has been proposed, with many individual channels in a wide river bed. Here the dinosaurs would have gone to the river bed in times of drought, and succumbed due to lack of food or water. They remained where they died. Sometime later, the river flooded and the bodies were picked up by the flood. This idea does not require a dramatic flood, nor that all dinosaurs died in one catastrophe. It is an attractive and plausible model that has been around for a long time. The main argument raised against it is that the sand layers have the coarsest deposits at the top. That is typical for splays but not for river beds. The variations of the structure along the Quarry sandstone is also not expected for a river bed. However, this does not seem a conclusive argument, and the river bed remains attractive. The fact that the river did not return for a long time may be more problematic.

A variation of this model has suggested a meandering river. This combines features of both models and less of a problem with the subsequent drying out. This model was in fact already proposed by Douglas. It is consistent with much of the data. The main problem may be that the animals are proposed to have been deposited on the inner bend of the meander where the flow velocity are small. It might be expected that the largest, least mobile animals would have ended up in the faster channels further out. A braided river or a break-out flood may therefore seem preferred. However, this discussion is continuing and has not come to a final conclusion.

Whichever model is used, it should be able to explain the partly chaotic properties of the sandstone amidst the mud and clay, and the sheer density of the dinosaur remains.

After the flood, the remains formed a Jurassic jumble with bones and bodies thrown together in an area of perhaps 100 by 40 meters. Some were quickly buried, either partially or completely. Now the water went down again, leaving some (perhaps all) on dry land. The bones remained close to their position, showing there was little disturbance for a considerable time. They were quickly buried, and the water did not return for a very long time.

Volcanic speculation

The final question should be what caused the flood. We cannot know how extreme the flood would have been. Was it a river flow from months of rain, or a one-off event? The lack of scavenging may make the latter a bit more likely but it remains speculative.

But let me become a speculationist. The Quarry river came down from highlands covered in tephra, during an era of distant but high volcanic activity. Although there is no evidence that a volcano was directly responsible for the mass casualties, tephra is unstable and it is dangerous stuff to have near rivers. A landslide an easily happen, and this can block flow channels and cause water to pile up behind. I wonder about such an event in the Quarry river, far upstream of the park. A tephra landslide could have created a dam. Now the river was blocked and stopped flowing.

If this happened during the dry season, the animals would have been in trouble. They ran out of food and water, and many died. The land was littered with their corpses. The predators also died – there was plenty of meat near the now-empty salad bar, but they also needed their drink and the bar was closed. Vultures would have helped – but these long-distance scavengers hadn’t evolved yet, leaving a gap in the market for dead meat.

Later the rainy season returned, first in the mountains. The new lake rose, and began to overflow the dam. The river returned. But the dam was only made of tephra and it was dangerously unstable. The water eroded it, and as dams do, it gave way catastrophically. A massive flood came down the river. Much of its power was spend before it arrived here, but enough arrived to pick up the dinosaur bodies, overflow the banks, and dump them at the end of the line.

After the flood, the river went back to normal – perhaps, and the channel refilled. But the future fossils were dumped outside the usual channel and after the flood waters had dried, they remained dry. Alternatively, the flood scoured out a new path and the river never returned.

This may even have happened more than once. There are indications that the bone beds were build up by three or four events. I don’t know what the evidence is as the paper is hidden behind an insurmountable paywall. Perhaps that disqualifies the evidence – it certainly makes the proposal untestable. It would indicate that the event was not a complete one-off catastrophe, but four events over perhaps 100,000 years is rather infrequent.

Finally, although Dinosaur National Monument is most viewable dinosaur cemetary in the US, it is not unique. There are other bone beds that are equally densely populated. These were large animals, but sometimes events became too much for them. They lived in a world like ours, where events, from volcanic eruptions to floods and droughts, could happen, until the biggest accident of all ended their reign.

Dinosaurs – the end

This all happened while dinosaurs were young. They had been around since the million years of rain, 230 million years ago, when a major volcanic eruption created perfect conditions in a hostile world. Now they were having the time of their life. The Morrison Formation is the largest sedimentary formation in the USA. It formed a paradisical subtropical basin stretching from New Mexico to Canada. Volcanoes in California provided a regular dusting of fertilizer. But sometimes, even paradise has trouble and at the quarry the dinosaurs had a bad day in the office. Did the volcanoes help create their disaster? whatever it was, it created a wonderful monument to Jurassic life, one that would excite their eventual successors.

But that was far into the future. The dinosaurs would rule the world for another 85 million years, before finally they met an extremely unlikely accident. It changed the world, perhaps for the better – at least for us. But what a wonderful world it was.

Albert, April 2022