Dr Trevor Watts (UK)
In my previous articles in the series, I looked at the environments that allowed dinosaurs to flourish in the Whitby area during the Middle Jurassic and to leave their footprints. Then I considered the factors and problems in trying to match the footprints to particular species of dinosaurs. In this part, I will look at the six different forms that dinosaur footprints mostly take in the region.
The toes of theropods tend to be quite slender, they are longer than the heel and the foot is longer than it is wide. Theropods, meaning “beast-footed”, include well-known dinosaurs such as Megalosaurus, Velociraptor, Tyrannosaurus, Allosaurus, Tarbosaurus, Troodont, Deinonychus, Coelophysis and a great host of turkey-sized raptors. Most of these species were not around at this specific time and place (although Megalosaurus may well have been). However, they were principally fast-moving carnivores that hunted or scavenged. They all had sharp, serrated, meat-ripping teeth; and were mainly bipedal – that is, they ran on two strong rear legs, with much shorter and weaker forelimbs.
Their footprints are said to be “tridactyl” – a word somewhat pretentiously created in the early nineteenth century from the ancient Greek for three fingers. It loaned scientific credence and academic gravitas to this new field of study. Most of the early footprints found in the UK and along the Connecticut Valley in the USA had feet with three forward-pointing toes that were relatively long, slender and separate. In fact, they were often assumed to have been created by giant birds. Many sported a smaller, higher, hallux toe as well, which mostly did not make a mark on the ground.
The footprints are generally longer than they are wide. The smaller prints (around 7.5cm long) are frequently found in groups or short tracks across slabs of fallen rocks in the Whitby region. This is because they must have congregated together, such as in a family group and/or merely because half a dozen of them can fit on a one-metre block, but big ones can’t. Along this coast, they are found on East Cliff beach, but are more common further south in Burniston and Scalby Bays. Mostly, the footprints are raised. This indicates that they are on the underside of the bed immediately above the actual footprint layer (as in Fig. 37 of Part 1 of this article). Perhaps the surfaces with the indented impressions are more easily weathered and disappear more quickly.
The feet of sauropods tend to leave rounded footprints with a wavy fringe (or crenulation), having short toes towards the forward half of the foot.
The name Sauropoda was coined by the American palaeontologist, Othniel C Marsh, in 1878, meaning “lizard foot”. They are the largest animals ever to walk the earth. With long, whip-like tails and elongated necks, the largest may have grown 60m long (claimed for Amphicoelias) and weighed up to 122 tonnes. As a result, their legs were column-like and very strong – in some species, strong enough to allow the creature to rise on its rear legs while eating from high branches or mating. Even without doing this, their heads could be 18m above the ground. With small heads that were little broader than the necks, the necks must have had an almost eel-like appearance.
They were herbivores, browsing on whatever vegetation took their fancy along the river and marshland margins that characterised the Whitby area at the time. Travelling in extended herds, the ground must have thundered when they moved at any speed greater than a slow walk, hence the recently reinstated name, Brontosaurus or Thunder Lizard. It is believed that their defences against predators principally included their group mentality in supporting family members, in the same way many African grassland animals defend their young today. Their sheer size and weight were additional weapons, and the ability to rear up and drop their front legs onto attackers, or even, it has been suggested, to roll over and crush them. Imagine also defensive circles, using their tails as whips – with sufficient speed to break the sound barrier and opponents’ bones.
The identity of the species that left their marks in this region is not known with any certainty, because of the paucity of fossil bones that have been found here. They may have included well-known Jurassic species such as Diplodocus, Apatosaurus, Brontosaurus and Brachiosaurus. Found almost all over the world, most species are known only from single bones or disarticulated parts of fossilised skeletons.
It isn’t just Yorkshire that lacks the evidence of these gigantic beasts. However, studies in the USA indicate up to five species of sauropods were able to co-exist in the same region, with their teeth and mouth suggesting that this was possible because they browsed on different types of plants – from soft aquatic vegetation, mosses and gingko leaves to coarser conifers, cycads and ferns. The same situation most probably also applied to ornithopods and stegosaurs, and very likely to theropods of different sizes and other prey animals.
Sauropod footprints on the beaches of the Yorkshire coast are often not recognised because, at first sight, they seem to be featureless round depressions or bulges on rock surfaces. Sometimes, that is the way they still appear to be after closer examination: if a sauropod was walking sedately, its footprint may well have been an almost-circular shape, like an elephant’s. And a circular mark was all that was made.
The same thing would apply if the print is not the direct contact one, but is a “transmitted” one from a slightly deeper layer. However, the toe marks can often be seen – never long or slender, but stubby, round and short. The variations in these footprints, like all the others, probably owe as much to differences in ground conditions, or mode of movement, as they do to the particular species of sauropod that made the marks.
The footprints of ornithopods tend to form an elongated oval or rounded triangle with three toes pointing forwards, or often curved slightly inwards. The “heel” part is generally longer than the toes.
These are the “bird-footed” dinosaurs – the “ornitho-poda” – although they did not evolve into modern birds. Most had three toes, but some species had a side toe (hallux) or four true toes. Early ornithopods were quite small, perhaps a metre long, and ran rapidly on their hind legs, much like theropods, with a stiff tail to help their balance.
However, as time went by, some species grew to be very large, up to 25 tonnes and 15m long in the case of Shantungosaurus. They were herbivorous, grazing mainly on low vegetation, judging by their skeletal structure (they have a spine similar to a modern bison), evolved to walk mostly on all fours, although able to rear up to reach into trees if necessary. Later developments in ornithopods also saw the rise of dinosaurs with crested skulls, and with duck-bills – the hadrosaurs. Particularly in the case of the duck-bills, the jaw structure was highly developed for chewing, much like modern cattle.
The precise cladistic (see box: A word about cladistics – tracing the origins of shared derived characteristics) relations between the creatures in this group have not yet been fully agreed, but, included in the family of ornithopods are ankylosaurs (the ones with spikes on their armoured backs) and iguanodons (the dinosaur that was the first of the great reptiles to be recognised as such by Gideon Mantell).
|A word about cladistics – tracing the origins of shared derived characteristics|
|Zoologists and palaeontologists examine the characteristics that various creatures have – such as bony heads, long necks or feathers – and then try to look back in their ancestry to see where these features began. They then see if other creatures, which share these characteristics, can be traced back to the same common ancestor. They don’t always originate with the same common ancestor – flippers, for instance, have independently developed in fish (for example, lungfish), mammals (for example, dolphins), reptiles (for example, sea turtles) and birds (such as penguins). This is convergent evolution, in which the kind of environment where a creature lives favours a particular body design.|
The cladistics method is the presently preferred way of classifying animals (and plants). It is how experts try to decide which creatures came first and which developed later. In the case of the ornithopods, it’s extremely complex, unclear and arguable. There are several interpretations of the evolutionary history of the ornithopods and their relatives, which species came first, which might have been direct descendants, and which common ancestors some might have shared.
Cladistics can now refer to molecular and DNA characteristics, and these are further complicating the pictures of which creatures evolved first, which ones came later, and which should be grouped together. DNA is not commonly obtainable from fossils, but molecular analysis is being increasingly used. Cladistics is a much more logical way of grouping creatures than some early systems of simply placing similar characteristics together, such as thick skins, number of legs or body size. In the extreme, this would be the equivalent of ichno-species; and ultimately could result in groups that include hamsters, sparrows, trout and lizards, purely because their most obvious common characteristic is small size (like the ichno-species classifications).
I have not found any of these on Whitby beaches, but have done so north of Scarborough, where I have discovered several excellent large footprints that are very similar to the ones declared to be iguanodontids on the Isle of Wight.
Ceratopsians (with head frills and horns) and pachycephalosaurs (with very thick bony skulls) were also ornithopods. When we see an ornithopod footprint on a rock surface at any of these four locations in Whitby we can’t be sure which species the creator might have belonged to. At our imagination’s most fanciful, it might have been a 5-tonne creature the size of minibus, with a head-frill two and a half metres across. This is not really likely, as the frilled dinosaurs aren’t known from this area at this age – but with the almost complete lack of fossil bones, who actually knows? More mundanely, it would at least have been an amazing duck-bill with complexly looped nasal passages. which they used to make resonating sounds, perhaps as warnings or in mating rituals. And there it had once stood – exactly where you now stand.
In terms of sheer numbers, the variety of species and range of global locations, this was an extremely successful family. Ornithopods certainly frequented the area that is now North Yorkshire.
The feet varied greatly in size and shape, although most had three toes. In general, the toes are relatively shorter and stubbier than theropod feet, but longer than those of sauropods. The “heel” part tends to be at least as long as the toes and is usually longer. The archetypal footprints are very recognisable – big heel and shorter, forward-pointing toes. Of course, not all are the typical perfect ones: some are more rounded and seem to have four toes, so they could be sauropod footprints or the front feet of stegosaurs. Others have a relatively smaller heel and toes that are more spread out, so could be theropod toes or stegosaur rear feet. You may see “manus” used for front feet, from the Latin for hand; and “pes” for rear feet, meaning foot. Every discipline likes its jargon.
In the Whitby area, stegosaurs make up the fourth major group of dinosaur footprints that are commonly found. As with Iguanodons, they were not thought to have existed in this area at this time until fairly recently, as no fossil bones have been found, and it was not known if they had developed this early in Europe.
Their footprints display a large rounded “heel” and widespread toes. The middle is often curled aside. Stego-saurian means “lizard with a roof”, referring to the plates on its back, which were originally thought to be flat armour plates. They were mostly large creatures, the size of elongated elephants. Growing to around 10m long, with 17 plates (or scutes) on their backs and four to ten tail spikes called thagomizers. They were peaceful, family-herd creatures.
The plates were possibly a little flexible and are now considered to have been primarily for mating display purposes, although they probably also had a secondary role in temperature control and perhaps some protection. However, filled with blood vessels, they would have bled a lot when damaged. The tail spikes were mounted on fairly stiff, strong tails and must have been their principal method of defence, especially in cooperative family group situations, in which the herd was threatened by large predators.
Stegosaurs are a distinct kind of ornithischians for a number of reasons: the unique jaw structure, the rigid back, the leg posture with the splayed-out limbs, their feet and their toes. They were herbivorous, but not in the same way as other ornithischians. Their jaws had a fore-beak instead of teeth, and small peg-like teeth further back, and jaws with limited sideways movement to chew or grind their food.
With their rigidly arched back, inability to rear up and short front legs, they were most likely restricted to very low-growing vegetation such as ferns, tree-ferns, cycads, horsetails, conifers, mosses, gingkoes and fruits – probably a metre high at most. For a detailed analysis of stegosaur cladistics, see https://en.wikipedia.org/wiki/Dinosaur_classification or any of a hundred similar sites that will attempt to explain a huge range of dinosaur groups at about the same evolutionary stage as the stegosauria. No two of these sites offer the same interpretation of their origins or relationships.
Did stegosaurs really roam the tropical deltas of the Middle Jurassic hereabouts? The idea has generally been scoffed at and the literature on the subject originally discounted the notion for two reasons: no bones had ever been found outside the USA; and stegosaurs were not thought to have evolved until the Early Cretaceous, or perhaps the Late Jurassic, around 150mya – well after these Yorkshire rocks were deposited.
But then several things occurred: continuing discoveries in the Morrison Formation in Colorado confirmed both bones and footprints of stegosaurs, including a family herd with five youngsters on the famous “Dinosaur Highway” Ridge near Denver. So there was a match between bones and footprints. Then, in 2006, the first stegosaurus fossil was discovered outside North America – in Portugal. As the global supercontinent Pangea was splitting Europe and the Americas apart from about 225 to 200mya, the stegosaur population must have been widespread before that time. So stegosaurs have been in Europe since at least 200mya.
Even more telling is that the Portuguese fossil is Stegosaurus ungulates – exactly the same species as has been found in the USA. This would have been right at the end of the Triassic period and the beginning of the Jurassic. So they had at least 25myrs touring Europe before any of the Middle Jurassic beds in Yorkshire were laid down – with their footprints. Since 2006, confirmed stegosaur fossils (teeth, bones and armour plates) have been found, or reinterpreted, in France and England. These are from the Oxford Clay, making them Callovian Stage, around 165mya. As their fossils have now been found in Europe before and after the Whitby footprints, there is every chance that stegosaurs were migrating along these river banks and estuaries. It’s just that, as we have already noted, the conditions here were not conducive to bone preservation.
5. “Through” prints
Rock layers can be disturbed downwards by a foot or part of a foot making a sharp or rounded deformation of the beds, clearly seen in cross-sections. These are a surprise if you don’t know about them in advance. When a dinosaur was moving, especially over soft ground, its foot or just its claws may have dug deeply into the layers beneath the surface. This would happen more commonly and clearly if the creature was moving rapidly and putting extra thrust into its contact with the ground.
Of course, all footprints are “through” to some extent, and can be seen as shallow depressions when the rocks that carry them are broken and they are seen in profile. However, what I am referring to in this section are the ones that have dug in deeper.
It might be easier to think of them in three main forms:
- The most characteristic form is a cross-section of several layers of rock where just the toes, or claws – even only one claw – have cut into the lower layers.
- The whole foot has penetrated several layers and the complete hole has been infilled afterwards, later being exposed in cross-section.
- Frequently, footprints are found as a solid cast, where harder material has later filled in a hole made by the actual foot. As the surrounding material weathers away, so the toes are seen digging in, especially the middle toe.
6. Scratch marks
The sixth and last grouping of footprints are scratch marks. These are long, thin marks, often with a distinctly deeper and wider section at one end, and frequently in threes.
Most are aligned in a common direction across flat blocks of sandstone. Almost all the ones I have seen have been the raised part of the block, that is, the layer that was immediately above the actual surface that the dinosaurs travelled over and left their footmarks on. Judging by the claw marks, they are mostly theropod prints, although ornithopods and sauropods do occur among them. There is sometimes a continuum from long scratches, through elongated scratches with a heel, to normal footprints. Mostly, they average well under a couple of centimetres wide and less than 30cm long. Other footprints they occur with can belong to much larger or smaller creatures.
Although my wife, Chris, and I have found them at other locations along the Yorkshire coast, they occur with unusual frequency on the beach at East Cliff and under Tate Cliff among beach boulders there.
Since the mid-nineteenth century, many writers and researchers in the USA, Europe and Australia have discussed how these scratches were formed. The weight of accepted opinion at the moment is that they were made by dinosaurs that were swimming in just the right depth of water (3.2m is mentioned as being ideal in one paper), perhaps crossing an estuarine river channel or a lake.
One particular mass of such scratches in Australia was long considered to indicate a stampede of land-running dinosaurs on sloping ground (see Gilmore, Hitchcock and others, cited in Tony Thulman, Dinosaur Tracks, published in 1990, Chapman and Hall). A recent suggestion contradicts this, but gives little indication about the reasons for any new thinking. In fact, another recent group of scientists claim that they were made by dinosaur swimmers, see https://www.uq.edu.au/news/article/2013/01/australias-stampeding-dinosaurs-take-dip.
Similarly, an assemblage of scratch marks in Spain has been judged to have been made by dinosaurs struggling to swim against a current. This appears to be largely based on the evidence that they are cut into a bed of ripples – as though ripples only exist under the water. These in the Whitby area are also imagined to have been created underwater by swimming dinosaurs.
Based on my own observations here and elsewhere, I prefer the original theory, which is generally scorned nowadays, that they were formed by dinosaurs that were running. After all, theropods were hunters; and hunters run. If they were going to leave a lot of traces of their activities, then their most common activity was running, not swimming across a muddy creek.
In addition, I have trouble wondering why a dinosaur would want to be swimming upstream and having difficulties doing it. After all, didn’t theropod dinosaurs only have very small front legs that were too ineffectual to keep their heads above the water? And surely, if the current was strong enough to cause problems, would it not also be strong enough to wash the scratches away and return the ripple bed to its previous state? The bed of a river or estuarine water channel would be soft and incapable of preserving a clear, distinct print under the surface. In the summer of 2015, I happened to go fishing for minnows and sticklebacks at a local canal and reservoir.
Try as I might with sticks and the bamboo cane, I was unable to recreate anything that looked like the scratch prints that abound here. I went back with a brass toasting fork, to try it with three “claws” together. The results were just the same: in still, shallow water with a firm substratum, the best I could manage was short-lived, softened, rounded muddy marks. In moving water, there was no trace apart from a slight darkening of the lighter sandy bottom, caused by the mud disturbed from below. And that was being careful – doing it at the speed of a swimming or running creature’s foot just stirred the water and mud up formlessly.
Note also that the swimming action of animals’ legs tends to be a circular movement, with a back-thrust catching against the bottom at some unseen, unknown depth below the creature. The scratches would be long grooves with the deepest part in the middle of the circle, and they would be very irregular – because the bottom is uneven, unpredictable and muddy.
My alternative proposal seems to fit the evidence much more closely. East Cliff beach sports broad areas of rippled sand when the tide has retreated. Dogs chase after balls and sticks every day. As they accelerate and turn across the ripple beds, they leave long scratches that are almost identical to the fossilised ones supposedly made by swimming dinosaurs. Big cats chasing little deer across the Serengeti make the same claw marks. On coarse sand, they tend to be rough, but on clay they are perfect replicas of those made by dinosaurs. The claws and pads of the foot dig in at the termination of the stroke, whether made on flat beaches and muddy slopes now or in the Jurassic. There is a rule of thumb in geology to the effect that, if a process is happening now to produce a particular result, then that is also the process that operated in the geological past to produce the same outcome.
Among the long scratches, there are sometimes other footprints of various kinds. These make for interesting examinations as we try to imagine what was going on at the time – if they were there together, perhaps interacting by chasing and hunting each other?
Some examples are particularly interesting, especially those with large scratch marks among small normal footprints. The larger creature would be unlikely to need to swim if the little ones could walk (Fig. 77.). It seems more likely that the small ones were running out of its way.
Finally, in any mixed population of prey and predators, whether in the Jurassic or on the Serengeti, the predators form less than a tenth of the population. And yet the great majority of the scratches are three-toed, made by fleet-footed carnivores. Then as now, the hunters do the chasing – after a Hypsilophodon, a gazelle or a child’s ball on a beach. I can’t imagine why the theropods would do all the swimming: what theropods did was running and that is the mark they left behind.
Whatever the actual origin (or origins) of these markings, there are plenty of them on both East Cliff and Tate Cliff beaches for anyone to weigh up the evidence for themselves. Relatively few places worldwide can boast such collections of scratch footprints, so perhaps we should just be grateful that they exist here in abundance. They are easy to find and recognise, once you know what they are. And then it is easy to recognise other disturbances on the surfaces – often the prints of other, different dinosaurs that were there among the runners (or swimmers). Some blocks make for fascinating studies. Depending on what the last tide did, there are often plenty of them on East Cliff Beach.
This concludes the outline of the six different forms of footprints that we regard as being common in this area. In the fourth and final part of this article, I will discuss in detail the four locations in the close vicinity of Whitby where dinosaur footprints can readily be found.
The photographs of life-size dinosaur models were taken at various sites in the USA. Fig. 2 is from the Dinosaur Adventure Golf at Niagara, Canada. Fig. 14 is from the Oregon Museum of Science and Industry, Portland. Figs. 44 and 52 are from the Springfield Science Museum, Connecticut. And all the others were taken at Dinosaur World, Glen Rose, Texas.
Dinosaurs of the British Isles, by Dean R Lomax and Nobumichi Tamura, Siri Scientific Press, Manchester (2014), 414 pages (softback), ISBN: 978-0-9574530-5-0