The dinosaur footprints of Whitby: Part 2 – problems matching footprints to dinosaurs
Dr Trevor Watts (UK)
In the first part of this article (The dinosaur footprints of Whitby: Part 1), I considered the immediate surroundings of Whitby as a seemingly unlikely place to find many dinosaur footprints; and I looked at the environments that existed here in mid-Jurassic times; and finally discussed how the footprints came to be shaped as I find them. In this part, I look at the problems that are encountered in trying to match the footprints to particular dinosaur species, and at the idea of ‘ichno-species’. I also suggest a simple compromise in classifying the footprints.
Matching a footprint to a particular species of dinosaur isn’t easy, for several reasons.
1. Relatively few dinosaur species have been identified as living at this time or in this region
In many parts of the world, the relevant rocks have been eroded away, or are deeply buried under later beds, or no beds were laid down, or the environment was marine. On a worldwide scale, there are remarkably few places where footprints coincide with skeletal remains that might be matched with them. The Middle Jurassic is a time about which very little is known with regard to the variety, numbers and development of dinosaurs, anywhere in the world. In fact, it is the least understood part of the Jurassic.
In this particular area, it is extremely rare to find any skeletal remains and too few bones have ever been found to attempt to cross-match with the footprints. Uncountable dinosaurs walked here, and no doubt many of them died here, but their bones were very rarely preserved in the rocks. The chemical mix in the ground was too acidic and the bones dissolved away along with the soft tissue.
At the last count, hardly more than a dozen dinosaur bones are known to have been found along this coast. One single vertebral bone that was discovered in 2015 received coverage in the national press. Described as Britain’s oldest long-necked, plant-eating sauropod dinosaur, it roamed the Whitby area 176mya (see www.dailymail.co.uk/sciencetech/article-3105955/Britain-s-oldest-sauropod-dinosaur-Yorkshire-called-Alan.html#ixzz3fhzQE0IF). It also received considerable research attention (see http://dinosaursabbatical.blogspot.co.uk/).
This lack of skeletal evidence in Yorkshire makes it necessary to compare the footprints with those in other parts of the world, and with the reconstructed feet from skeletons that have been unearthed elsewhere. In this way, it becomes largely a matter of looking to see which dinosaur bones have been found in other parts of the world – China, South America, Australia, North America and Africa – and then hazarding an educated guess as to which ones might have roamed as far as England, and which ones might be the Cinderella for the footprints.
Where other evidence, such as bones, eggs or dung have been found, there might be some grounds for comparing similar footprint shapes to suggest a particular species, such as Coelophysis, Megalosaurus or Iguanodon. The preferred way now in Britain is to look at all the mass of observed prints and look for common distinct features, and to lump similar ones together. Mostly, this works well.
Although dinosaur bones are scarce along the Yorkshire coast, those of sea creatures such as ichthyosaurs and plesiosaurs are frequently found in the marine mudstones and sands. This is because the conditions were very different, and suited to bone preservation. I recently found a fine ichthyosaur vertebra lying on the beach at Port Mulgrave (16km northwest of Whitby), when I was actually looking for attractive pebbles in an idle half-hour. Acquaintances report this as being a common occurrence.
2. In this part of the world, the footprints are mostly found as single examples
Therefore, it isn’t generally possible to make a lot of inferences about the identity or habits of the creatures that made them. Some footprint trails are found on the rock platforms in some bays – Burniston, Scalby, Cornelian and Cayton, for example. But these platforms are being actively eroded by twice-daily, five-metre tides that wear them away almost as fast as they are exposed.
In the present environment at East Cliff, Whitby – of an eroding vertical cliff – slabs of rock beds are being dropped out of the cliffs one block at a time. A few display footprints, but most don’t. Even if they only fall a metre or so, they will probably be severely damaged. Even so, occasional large boulders fall from the cliffs and display several prints across their surfaces – perhaps from two or more different animals. It’s such a shame that they usually vanish to the vagaries of tidal erosion, being washed away, rolled over or buried by subsequent rock falls. Sometimes, searching among the rounded boulders, it seems that every next rock has suspicious markings on it; and it isn’t stretching the imagination to see a clear heel shape or two faint toes on so many of them.
In some countries, such as the USA, dinosaur footprints are found in helpfully-long trails – for example, at Dinosaur Ridge near Denver, or along the Paluxy River in Texas, or the Connecticut Valley in New England. In such locations, the overlying beds have been worn down relatively horizontally, and have reached the footprint levels. Therefore, long beautiful trails are exposed and it is possible to learn much from such extensive trackways.
Here in Yorkshire, the tracks must have been formed in the same way as those in the USA, and it seems disappointing that most finds are of single prints or small slabs of rock with a few prints on the surface.
This may not sound wonderful, but Yorkshire is extremely fortunate for the number and range of footprints found along this coast, and the sheer quantity – they are dropping out of the cliffs, rolling over and being exposed – all the time. As far as dinosaur footprints are concerned, Yorkshire is indeed ‘God’s own country’. And this beach under East Cliff is no exception.
3. An international system of identification and categorisation couldn’t be agreed
With little common ground or thought, researchers basically gave up and invented ‘ichno-species’ instead. In the early days of footprint study, from about 1850 onwards, specialists, such as Marsh, Owen, Hitchcock and Leidy (see http://eesc.columbia.edu/courses/v1001/dinodis3.html for a flavour of the efforts), tried to match the footprints with specific dinosaur species. However, there was a lot of argument about whether or not a particular dinosaur existed at that time, or if it roamed in that region, or if it might have been a similar one, a male or female, or a young one of another species, and the experts all contradicted each other.
If they were a little smaller they would be in a different ichno-species – Eubrontes mezzaluna and their juveniles would presumably be Grallator mezzaluna.
The answer was to invent and formalise a new kind of fossil type – ‘trace’ fossils. To sound scientific, this new branch of geology was called ‘ichnology’ (ichno = trace). Trace fossils, or ‘ichno-fossils’, are anything that an organism might have left behind that was not part of itself. These include the footprints, tail drags, burrows, starfish prints (where they settled for a time), roll marks from tumbling ammonites, and so on. In the case of footprints, they were then classified according to the attributes of each print – how big it was, how many toes it had, how widespread they were, if there was a long heel, or if it was thought to be a front foot (called a ‘manus’) or a rear foot (‘pes’).
And so a whole series of invented species of foot shapes came about – called ichno-species or ichno-genus. There was no attempt to relate them to any particular dinosaur. It is now referred to as palaeo-ichnology, to differentiate it from studies of modern trace evidence (neo-ichnology), but it has had “a somewhat controversial history” and not all specialists agree over its value (see http://www.tandfonline.com/doi/abs/10.1080/10420940.2013.843313#.VapxuvlVhBc).
Cutting a convoluted story short, the terms grallator, eubrontes and grandipus were adopted to describe footprints that were little (under 15cm), middle-sized (15 to 30cm) or big (longer than 30cm). These terms originated from individual trackways at specific sites, but became generalised, despite the problems – such as a family herd of a single species being classified under three different categories. On the other hand, a dozen different species of small or infant dinosaurs could all be classed as ‘grallator’ – little. The term ‘grallator’ actually refers to long-legged wading birds, as the first footprints found in the Connecticut Valley were thought to be bird prints.
Realising that big, medium and small was too simplistic a system, researchers again began to sub-classify the prints. And so all small prints can be called grallators regardless of how many toes they have, or how broad they are, but the descriptions have re-emerged – and I have over fifty names for different ichno-species of ‘little footprints’, such as Grallator digitgradus and Grallator magnificus and Grallator minor. Actually, some of these grallator names go back as far as 1858 (for Grallator formosus and Grallator gracilis). For instance, the latter means, or implies ‘a footprint less than six inches long, but quite graceful’. This is hardly the name of a particular species of dinosaur. See https://en.wikipedia.org/wiki/Grallator#Synonyms for a list of all the grallator footprint species names. It seems a little pointless to have one called Grallator minor, which means ‘little footprint that is little’.
Since 1998, when I joined a weekend field trip at Whitby with Phil Manning (now Professor at Manchester University), I have slowly built up a classification of 23 types of footprints that I have found, mostly along this stretch of coast. Some of these types have only a few examples attributed to them. They are just as likely to be freaks of the muddy ground or a panicking prey animal, as they are to being a rare type of dinosaur.
Other categories have more than a hundred examples in them and I have subdivided them into several sub-types within each major group – each with features that are consistently found – a particular angle of the toes, for instance, or the sideways curl of the middle toe, or the extreme breadth of some prints. I have, for instance, found at least fifty footprints – freshly exposed as well as weathered – that are shaped very much like a single-handled mezzaluna (a cook’s slicing utensil shaped like a half-moon). There is little chance that these all occur by chance of the ground’s characteristics – they must surely represent real a difference in one or more types of dinosaur.
As some experts say, the issue of ichno-species is contentious, and dubious in value (see History of Ichnology: The Origins of Trace Fossil Taxonomy and the Contributions of Joseph F. James and Walter H. Häntzschel by S. George Pemberton & James A. MacEachern. In: Ichnos: An International Journal for Plant and Animal Traces – Volume 20, Issue 4, At http://www.tandfonline.com/doi/abs/10.1080/10420940.2013.843313#.VapxulVhBc).
These are from Tate Cliff beach and East Cliff beach. They are common on many of the Yorkshire beaches, and more will be shown in the section on East Cliff in Part 4 of this article.
4. Dinosaurs’ feet differed in innumerable ways – any of which could be relevant in determining species differences
To think of dinosaur feet in the same way as human feet – pretty much all the same – is wrong. A much more apt comparison would be dinosaurs with all mammal species – the feet of horses, apes, cats, cattle, bats, dolphins, elephants and shrews.
Other factors in grouping the footprints include the following:
- The ratio between length and width (some are long and others are broad).
- How long the heel is compared with the length of the toes (Fig 9).
- The relative width and length of the ‘heel’ (which is probably the ball of the foot, not the true heel).
- If the heel is pointed, rounded or flat.
- If the heel has a small pointed tip at the back; or perhaps has a dimple.
- Whether or not there is a ‘hallux’ – a small vestigial toe on the side of the foot, like the spur of a chicken. (Cats have something similar.)
- Whether the toes are spread wide at a great angle – perhaps almost 90 degrees between toes; or if they are narrow; or even if they are parallel.
- Some are large round blobs, with a crenulated front edge indicating where the toes were.
- How big they are (some are extremely small – less than 3cm long; others are over a metre across). Size is the whole basis on the ‘Grallator, eubrontes and grandipus system’.
- The number of toes (generally between three and five, mostly three and known as ‘trydactyl’, meaning three-toed). In fact, the majority of footprints in this area show three toes – it is the variation within the trydactyl group that is most important.
- If the print is a front foot impression or rear foot (‘manus’ or ‘pes’ prints). And this can make a huge difference – imagine the disparity between a tyrannosaurus’s tiny front arms and its massive rear legs. And stegosaurs had three toes at the back and five on the front feet. (You might check your own hand and feet prints.)
5. Numerous factors that are not part of the foot’s morphology interfere with the shape of its print
Systems that try to hazard experienced guesses at the identity of the originating creature have to be approached and judged with a large pinch of common sense. As stated above, there are other major factors in determining the morphology of a footprint beyond the identity of the culprit dinosaur – the hardness or softness of the ground, for instance; the consistency of the mud or sand; how deep the soft ground was; how fast the creature was moving; if it was turning or walking in a straight line; if it settled for a time; if it was in a herd that trampled all over each other’s footmarks; going up or downhill; how it has weathered or been broken, and so on. Bearing all of this in mind, the sub-groups are bound to be partly subjective.
Some of the footprints are difficult to make out – they are simply a mass of overlapping and underlapping disturbances, or turbations. The next time you find yourself in the muddy gateway between a cow-field and the milking parlour, or strolling along the edge of a watering hole in the Serengeti, see if you can interpret the mass of jumbled and deformed footprints that you see there. It must have been the same in the Jurassic – some places were very congested. Such ground is said to be turbated, or ‘bioturbated’, to differentiate it from the mechanical kinds of turbation (disturbance to the ground) caused by landslips, waves or tectonic and volcanic disturbances. It’s the same with some of the dinosaur footprint finds – the beasts would have crowded along narrow water margins and river banks, and congregated to cross a watercourse or to get to fresh water.
Even solitary prints aren’t always clear: sometimes I study a print, then turn it round and see it differently. Often, I walk round a series of bumps and depressions on a rock several times before deciding on the most likely interpretation of the prints there – or deciding they trampled across each other’s tracks too much to make out separate shapes. I remember carrying a rounded rock the size of a football up a cliff near Whitby and along the muddy path at the top. It obviously had several raised footprints on it, but I couldn’t decide how they were linked, or separated. It slipped out of my hands and landed in firm mud. The impression that it made was perfect – and clearly revealed the actual layout of two of the prints once they were seen in depressed form – how they were created – instead of raised.
Quite often, I examine the photograph later and see an entirely different shape of print. What I thought was a four-toed dinosaur footprint might turn out to be a three-toed one that stepped partly into the print of an earlier one. Or there may be two prints overlapping in different directions. Sometimes some of the marking is ‘splurge’ – the mud that splattered widely, or squelched up around the feet, and it may seep back into the depression formed by the foot, or overflow into an adjacent one and deform it. A foot may dig down deeply into soft mud, while another foot leaves barely a mark on hard sand a few feet away. A foot may skid forward, sideways or backward, if the animal was moving quickly or on a slope. All these chance happenings alter the shape of the footprint and therefore the category it might be placed in.
There is a particular set of six footprints in Cayton Bay (40km south of Whitby), all in a line and about the same size. They were obviously made by the same creature, as they are ‘mirrored’ as left and right feet. However, by any system including my own, they would fit into four different categories because of the way each print is preserved – two have long, separated toes; one is apparently webbed; two more have very broadly-spread toes; and one has a pointed heel.
These all depend on the exact nature of the sand under one foot, or the mud, or a slight change of direction or pace. Nobody has yet come up with a perfect system for classifying dinosaur footprints, and they probably never will because of difficulties like this. For example, a print made by a young dinosaur may be classed differently from those made by its parents, or one set of footprints fits into four different categories because of chance differences in formation.
If I was starting my classification now, I would probably do it very differently – more rigorously and less intuitively. However, I started it around 1998 with only a couple of dozen finds. I can now refer to records of almost four thousand footprints or sets of footprints that I have found, measured and photographed along the Yorkshire coast.
Additionally, it has been helpful to meet with many students, researchers, fossil dealers, theme park managers, museum personnel, and landowners, especially in the USA and UK, but also France and Poland. Our judgements are also tempered by visits and references to many non-British sites, and extend to discovering previously unknown sites in Texas, Oklahoma and Alsace in France. They all help to put the Yorkshire footprints into context, but it’s too late for me to start again and re-organise the whole of my ‘twenty-three types’ system.
6. Dinosaur footprints come in a continuum of shapes, not separately distinct to each species
With records of around four thousand prints to refer to, it is not hard to spot similarities among them. Stage 1 would be to draw a sketch representing a number of very similar footprints. In Fig. 17, each shape represents such a collection of individual footprints. Stage 2 would be to see that these, in their turn, share certain characteristics, and I can put a line round them to group them together. These can be sufficiently clear and consistent to assume that they represent a genuine difference of type or species in the creatures that made the prints.
This grouping process can continue, as other similarities become more obvious and commonly seen. However, the sketches and groups don’t include all the footprints I have on record. There could be twice as many footprints on the diagram, but many would fall between one group and another. Therefore, we have a continuum – or scatter-graph – of footprint shapes, with many, but not all, being loosely groups together.
One requirement with Venn diagrams is that the criteria have to be clear. A difficulty with dinosaur footprints is that the differences between them aren’t always clear – they are mostly graduated. The other problem with footprints is that it isn’t only the characteristics of the creature’s foot that decided what its footprint would look like.
It may be possible to isolate the ones with a particular angle between the toes; or the more slender toes… or the incurving toes… or outward-spreading ones. Then, should the ones with broad heels and spreading toes be included with the broad heelers or the spreading toes? And what about the broad heels with incurved toes? Or with four toes? Or five? Or those with a dimple at the base of the heel – should they be grouped together, or among the group with the closest resemblance of toes? Or both? Those with a particularly large middle toe should be grouped with the ones with curved middle toes? Or with those that have a sharply defined angles between the toes? Each print would be in several sets; and some would be in half a dozen groups – perhaps those with a large middle toe, incurving side toes, a dimpled heel, a hallux toe and a rounded heel.
If this semi-continuum seems a little unlikely, it should be borne in mind that the ‘bird-footed’ ornithiscians didn’t give rise to birds, even though their footprints were originally mistaken for those of giant birds. It was two branches that developed much later from the theropods (which were saurischians). So there really was a great intermingling of shapes of feet among the dinosaurians. Convergent evolution was just as common as divergent evolution in the Jurassic.
This approach is valuable for research purposes, and I expect that a well-programmed computer could sort them out. However, at the moment, the Venn approach is too complex for simple straightforward identification in the field. For simplicity and tidiness, it would be good if this was a linear continuum; but it isn’t. On the other hand, what I can do is to consider the mass of shapes in the Venn diagram and chose the four basic shapes and types.
For our purposes, when I are struggling along a rugged beach in the rain, wind and seaweed, and keeping an eye on the cliff falls, loose boulders, the kids and the tide, this is enough of a division. It is also sufficient for the many people I meet on the Yorkshire beaches. Mostly, they are very interested in the idea of finding dinosaur footprints, particularly the mother or father of the accompanying primary or early secondary-age school children. So, in practice, mum or dad can point out some markings on a rock and say, “That’s a dinosaur footprint; it was travelling in that direction; it was probably a raptor; it lived around 170 million years ago when this area was a sub-tropical wetland; it wasn’t running; and it was about as tall as me.” (I loosely imagine 2.5cm foot length to equate with 30cm in height. It’s much more reliable than estimating the height of their hip from the length of pace, when you only have one footprint to look at.)
However, things are never that easy – there are two additional types of commonly-found footprint markings. They don’t fit into any of the basic four sets of: theropods and sauropods; and ornithopods and stegosaurs. These extra two are based purely on the manner in which the print was shaped, and wouldn’t be recognised as dinosaur footprints if you didn’t actually know about them. These are ‘scratch’ prints (considerably elongated by slipping) and ‘through’ prints (where the claws have dug deeply through the surface layers). These are dealt with in more detail in the third part of this article.
This is obviously not the most scientific, rigorous classification that can be found. If a parent or child is so inspired when they get home, they can look at their specimens, diagrams and/or photographs, and try the excellent site https://en.wikipedia.org/wiki/Dinosaur_classification. For even greater detail on dinosaur and footprint types, there are some very comprehensive internet sites from the USA, (for example, Kuban at http://paleo.cc/paluxy/ovrdino.htm) and within the UK (for example, Whyte and Romano and the Yorkshire Geological Society at http://pygs.lyellcollection.org/content/54/3/185.refs). Or simply Google the subject – there are hundreds of very informative sites, written by some very expert people, mostly with their own classifications, complexities and axes to grind.
The simple choice is:
- Classify footprints by their size and shape alone, and adhere strictly to a morphology system. This is the ichno-species approach that would ignore variations caused by the mode of travel or nature of the surface travelled on. So, all medium-sized footprints are grouped together and called ‘eubrontes’.
- Accept that there is a continuum of footprints that cannot be clumped together in mutually exclusive groupings – there is overlap and merging among the sets. This may seem to be unscientific, but it avoids classifying six in-line prints of a trackway in four different shape categories.
Ultimately, distinctions between the different shapes of footprints are arbitrary (see Thulborn, R.A. (1990) Dinosaur Traces. Chapman & Hall, London http://palaeo.gly.bris.ac.uk/palaeofiles/tracks/report3/ichnology.html#ichno).
So, for the reasons outlined above, I think in terms of six basic footprint types – theropods, sauropods, ornithopods and stegosaurs, plus ‘through’ prints and ‘scratch’ prints.
In Part 3 of this article I will look at each of these types in more detail. Note that I took all the photographs, almost all within about half a mile of Whitby.
Further reading
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.