A new hypsilophodont dinosaur from the Isle of Wight, England

Vectidromeus insularis by Emily Willoughby

Recently, my colleagues and I described a new species of small dinosaur from the Wessex Formation of the Isle of Wight in southern England, Vectidromeus insularis. Vectidromeus was a small plant eater, maybe two or three meters long as an adult, and weighing perhaps 40 or 50 pounds . It seems to be a member of the family Hypsilophodontidae. The hypsilophodontids are a curious family of dinosaurs- they’re a family that was once thought to be highly diverse, but which, curiously, has become less diverse over time— a sort of taxonomic extinction of the family, driven by paleontologists.

The Hypsilophodontidae

The Hypsilophodontidae has a curious history. The first member of the family, Hypsilophodon foxii, was named by evolutionary biologist Thomas Henry Huxley in 1869. Huxley was an early proponent of Darwinism, and among other things, did some important work linking birds to dinosaurs, with the small, gracile, two-legged Hypsilophodon supporting his idea that birds were related to dinosaurs. Later, in 1882, Dollo gave the dinosaur its own family, Hypsilophodontidae.

The first skeleton of Hypsilophodon, discovered in 1849. Paleontologist Richard Owen had named the Dinosauria just a few years earlier, in 1842.

           After that, however, Hypsilophodontidae became something of a wastebasket taxon- a family into which people tossed lots of species of uncertain affinities because they weren’t quite sure what else to do with them. Thescelosaurus, Parksosaurus, and Orodromeus, all from the Late Cretaceous of North America, were thrown in, as was Othnielia, a Late Jurassic species from North America, and even Heterodontosaurus, an Early Jurassic species from South Africa.

At various times the European Rhabodon and North American Tenontosaurus have been put into Hypsilophodontidae, and the south American Notohypsilophodon. Basically, if a dinosaur looked vaguely like Hypsilophodon (or even if it didn’t)- if it was a smallish, plant-eating biped- it tended to end up in Hypsilophodontidae.

The mess that is the modern neornithischian evolutionary tree. From Madzia et al. (2017). Species that were once lumped into Hypsilophodontidae fall up and down the tree, and Hypsilophodon itself emerges as a single species on the end of a long branch, the lone member of its family.

           In recent years, however, attempts to redraw the dinosaurian family tree have shown that the “Hypsilophodontidae” as traditionally conceived includes a lot of small, herbivorous, bipedal dinosaurs that (aside from being small, bipedal, and herbivorous) don’t really have much in common. Instead they seem to represent a whole range of lineages that lie up and down the stem leading to the later, bigger, more advanced plant-eaters like Iguanodon and duckbill dinosaurs.

Exactly where Hypsilophodon goes is unclear. The latest trees suggest it may actually lie below Ceratopsia, the horned dinosaurs, and outside the group Ornithopoda entirely. If so, it would have branched off from the remaining dinosaurs sometime in the Jurassic.

Curiously, the latest attempts to draw the tree have tended to produce geographic clusters. North American things- Thescelosaurus, Parksosaurus, Orodromeus, Zephyrosaurus- all go together in Thescelosauridae. South American species like Notohypsilophodon and Talenkauen tend to go together in a group called Elasmaria. The European rhabdodontids like Rhabdodon and Zalmoxes get their own group, Rhabdodontidae.

            What we ended up with is a curious situation where one of the oldest dinosaur families, Hypsilophodontidae, is also one of the least diverse- it ended up with just a single species, Hypsilophodon foxii.  

Vectidromeus insularis

The type of Vectidromeus insularis, a juvenile, shown to scale with Hypsilophodon foxii and Homo sapiens.

            Recently, pieces of a new, small herbivorous dinosaur emerged from the Isle of Wight. They weathered out of the rocks near Sudmoor Point, along the south coast of the Island. This little dinosaur was collected by an amateur collector, when another amateur, Sean Smith, noticed it and thought it might be something worth further study. It was, just not for the reasons I thought.

Our initial interest in the specimen was actually because we thought it might represent some kind of small, carnivorous dinosaur, a theropod. Preparation by Mick Green, however, revealed it to be an ornithischian, similar to Hypsilophodon- but not identical. The reason we’d initially assumed it might be a theropod in the first place was because it didn’t look right for Hypsilophodon- the hip bone, the ilium, had a different shape- it was rounded off, rather than rectangular.

While our initial guess turned out to be wrong, our hunch- that this animal wasn’t a Hypsilophodon- proved broadly correct. The ilium had a curious shape, so it seemed we had something interesting and new. Further preparation of the hips revealed that it had a distinct ilium, pubis, and ischium, suggesting it was related to, but distinct from, H. foxii.

Parts of the skeleton of Vectidromeus insularis, including the ilium, ischium, pubis, tibia, fibula, tail, and partial dorsal vertebrae.

            Eventually, working with local collectors, we were able to put together a fair amount of the animal— four different pieces were ultimately found, collected by different people at different times, including much of the hips, parts of the legs, the toes, and bit of the tail.

Partial femur and tibia(?) of Vectidromeus insularis.

They all were collected in the same place, around the same time, and represent similar sized animals, and don’t duplicate any elements. So they seem to be one skeleton; and the tibia and fibula from one block connect to another (although not a clean fit, as the bones have been damaged), we can therefore definitively  associated these two particular blocks. It’s not the most complete dinosaur in the world, but neither is it the most incomplete.

The specimen is a juvenile, which raises the possibility that some of the peculiarities of the bones are due to the immaturity of the animal, rather than it necessarily representing a new species. However, we do have small juveniles of Hypsilophodon at the Dinosaur Isle Museum in Sandown, on the Isle of Wight. They don’t look like this animal; they look like little Hypsilophodon. That suggests we have something different. Yes, it’s a juvenile, but juveniles can be diagnostic. Kittens look like cats and puppies look like dogs. You wouldn’t mix them up, and for that matter, a juvenile pug, dachshund or lab is instantly identifiably a pug, dachshund, or lab.

Vectidromeus insularis, a partial, heavily surf-worn piece of the foot and toes.

A Plethora of Species

Over the years — more than a century — a lot of hypsilophodont fossils have emerged from the Isle of Wight. Traditionally, they’ve all been classified as one species, Hypsilophodon foxii. But there’s no reason to assume that they all necessarily represent the same species.

First of all, if we look at modern small herbivores, we find there are multiple species. The Serengeti is simultaneously home to Thomson’s gazelle and Grant’s gazelle. In North America, the white-tailed deer and the closely related mule deer coexist. In Australia, the western Grey Kangaroo, eastern Grey Kangaroo, and Red Kangaroo all coexist in the southeast of the country. So there’s no particular reason to think that there couldn’t be more than one species of small plant-eating dinosaur at a time, even filling very similar niches. This is what nature and evolution do- evolution makes one species into lots of species. Species speciate. That’s just what they do.

The other reason to suspect there might be more than one species is that the Wessex Formation rocks on the Isle of Wight span a significant amount of time. Exactly how much time is represented by the beds of the Wessex is unknown. The top of the Wessex lies a little below the Aptian-Barremian boundary, which is currently dated to 121.4 million years ago. The bottom of the Wessex may span the Barremian-Hauterivian boundary and go into the Hauterivian, so it’s at least 125.77 million years old. That suggests the exposed rocks span around 3-4 million years.

This is a lot of time— that’s roughly the amount of time it took for us to evolve from Australopithecus with its chimp-sized brain into creatures with smartphones and nuclear weapons. It’s certainly enough time for one dinosaur species to evolve into a slightly different species- or even multiple species- or else for climactic and environmental changes to cause new species to move in.

We know from well-studied strata in western North America, where we have a lot of dinosaurs and good dates (based on radiometric dating of volcanic ashes) that dinosaur species didn’t last very long. In the Dinosaur Park Formation in Alberta, Canada, for example, multiple species span a period of less than two million years. At the base of the rocks we have the horned dinosaur Centrosaurus, which is replaced by Styracosaurus higher up, and finally by Pachyrhinosaurus at the top. Other species, like duckbill dinosaurs, show similar patterns of turnover.

Similarly, in the Hell Creek Formation, in the late Maastrichtian, we have Triceratops horridus at the base, and T. prorsus replacing it at the top, over a period of maybe 1-2 million years. We may also have two separate Tyrannosaurus species— T. imperator lower down, and T. rex at the very end of the Cretaceous, although this remains highly controversial. But dinosaurs turned over fairly quickly.

Partly this is driven by changes in range. Centrosaurus probably isn’t the ancestor of Styracosaurus; instead Styracosaurus is an immigrant, an invasive that displaces it (or perhaps opportunistically fills in after Centrosaurus goes extinct). Partly this is driven by evolution. Triceratops horridus seems to evolve into T. prorsus. In either case, as the environment changes, species either die out and are replaced, or evolve into new things.

Probably something similar is happening in the Wessex Formation at the Isle of Wight. It’s very unlikely that the dinosaurs at the very top of the formation— the Hypsilophodon in the Hypsilophodon beds— are the same as the ones lower down. So we may be underestimating diversity in the Wessex.

It’s probably not just the Hypsilophodon, but all the dinosaurs in the Wessex Formation that will show turnover, assuming we can get more complete, diagnostic species from the different beds. We may find there are different Iguanodon species. For that matter, unless the Iguanodon from the Isle of Wight are the exact same age- within a million years or less- as the ones from Belgium, it’s very unlikely that Iguanodon bernissartensis exists in England. Instead, it will probably turn out to be a new species, one just a little older or a little younger than the Belgian one, or even several species. What we currently call “Iguanodon bernissartensis” from England could in theory turn out to be a whole host of things that are distinct from the European species.  I suspect this will be the case, but if so, it will take decades, maybe a century, of additional sampling, preparation, study, and argument by paleontologists to sort it all out.

Implications

So what does it all mean? We had many hypsilophodonts, around the world (or so it seemed), then just the one, now… two species. What we’re slowly discovering is that there’s a lot of complexity to the ornithischian evolutionary tree. In particular, that there’s a lot of geographic signal.

Small plant eaters from the Late Cretaceous of North America tended to be related to others from the Late Cretaceous of North America. The same is true for the plant eaters of the Late Cretaceous of Europe, and South America. Closely related species tended to exist in the same places, and at the same time.

Finding a close relative of Hypsilophodon in the Early Cretaceous in England makes sense, it fits this pattern. Things that are close in time and space tend to be related to each other.

The reason for these geographic clusters is probably continental breakup and continental drift.

In the earliest Jurassic, 200 million years ago, the continents are united into a single landmass, allowing dinosaurs to readily disperse.

In the late Triassic and earliest Jurassic, the continents were united in a single, giant continent, a supercontinent called Pangaea. Over the course of the Jurassic, the supercontinent broke up into smaller landmasses, and this continued into the Cretaceous. Seas separated Europe from North America and Asia, South America, Africa, Australia, India and Madagascar drifted off to become island continents. By the end of the Cretaceous, there were (depending on exactly what one counts as a continent) maybe ten or twelve different continents.

In the early Cretaceous, 125 Mya (when Vectidromeus lived) continents are becoming isolated.

These landmasses had different evolutionary histories. Extinction hit different groups on different landmasses. Different landmasses were colonized by different lineages.

So we have dinosaurs evolving in different ways in Europe, South America, North America, with distinct lineages radiating into the small herbivore niche in each. We also see a similar pattern with the duckbill dinosaurs, with different clades radiating in different parts of the world.

Small herbivorous dinosaurs seem to disappear entirely from India and Madagascar by the end of the Cretaceous; we don’t really know what’s going on in Australia, Africa, or eastern North America at the end of the Cretaceous.

The evolutionary picture and the biogeography are complicated. Hypsilophodon and its kin- the Neornithischia- were an ancient, widespread group. Vectidromeus doesn’t radically change our picture of their evolution, rather it’s one small piece of a larger puzzle, which we’ve slowly put together over a century, one fossil, one species, one evolutionary tree at a time.

It’s also made me rethink another Isle of Wight species, the dromaeosaur Vectiraptor greeni. Originally, I thought Vectiraptor might be related to contemporary North American species. But the distinctiveness of Hypsilophodon- along with a chance to restudy Utahraptor, which is nothing at all like Vectiraptor- is making me think that maybe the dromaeosaurs of the European Early Cretaceous are doing their own thing, rather than showing close connections to North America.

Regardless, it just goes to show, we actually know surprisingly little about how the European species are related to those elsewhere in the world, despite working on them for more than a century; there’s a lot more that remains to be done.

Last one of the reasons I did this project was less because of the scientific significance— new dinosaurs are always fun, even if they aren’t Earth-shattering— than to try to build some bridges. Historically, the various people doing paleontology on the Isle of Wight- people in museums, universities, and amateurs- haven’t always worked together . Which is a shame, because amateurs often find a lot of the best fossils. There are just a lot more amateurs than professionals, and because they devote their time to fossil hunting rather than research or teaching, they also get pretty good at it. It’s actually pretty rare that a scientist finds a scientifically important fossil on the Isle of Wight.

And everyone brings a slightly different perspective to the problem, a different piece of the puzzle. As someone who’s studied anatomy for years, I probably have a better understanding of the evolutionary implications of some esoteric anatomical details— say, the shape of the fourth trochanter, or the presence of ossified tendons. But the amateurs, who have spent years collecting and preparing this stuff, know the fossils- the color of the bone, the texture of the matrix, how it weathers out, and so on- in a way that was critical to reassembling the various bits of this specimen. I do think there’s value to spending a few years doing a PhD, stuff that’s hard to learn as an amateur; but there’s also things you learn about fossils as an amateur that most PhDs never will. There are many important things we scientists will never know— we’re just far too educated.

References

Huxley, T.H., 1870. Further evidence of the affinity between the dinosaurian reptiles and birds. Quarterly Journal of the Geological Society 26, 12-31.

Huxley, T.H., 1869. On Hypsilophodon, a new genus of Dinosauria, Proceedings of the Geological Society of London.

Longrich, N.R., Martill, D.M., Jacobs, M.L., 2021. A new dromaeosaurid dinosaur from the Wessex Formation (Lower Cretaceous, Barremian) of the Isle of Wight, and implications for European palaeobiogeography. Cretaceous Research.

Longrich, N.R., Martill, D.M., Munt, M., Green, M., Penn, M., Smith, S., 2023. Vectidromeus insularis, a new hypsilophodontid dinosaur from the Lower Cretaceous Wessex Formation of the Isle of Wight, England. Cretaceous Research.

Longrich, N.R., Suberbiola, X.P., Pyron, R.A., Jalil, N.-E., 2021. The first duckbill dinosaur (Hadrosauridae: Lambeosaurinae) from Africa and the role of oceanic dispersal in dinosaur biogeography. Cretaceous Research 120, 104678.

Madzia, D., Boyd, C.A., Mazuch, M., 2018. A basal ornithopod dinosaur from the Cenomanian of the Czech Republic. Journal of Systematic Palaeontology 16, 967-979.

Mallon, J.C., Evans, D.C., Ryan, M.J., Anderson, J.S., 2012. Megaherbivorous dinosaur turnover in the Dinosaur Park Formation (upper Campanian) of Alberta, Canada. Palaeogeography, Palaeoclimatology, Palaeoecology 350, 124-138.

Martínez, R.D., 1998. Notohypsilophodon comodorensis gen. et sp. nov. un Hypsilophodontidae (Ornithischia: Ornithopoda) del Cretacico Superior de Chubut, Patagonia Central, Argentina. Acta Geologica Leopoldensia 21, 119-135.

Norman, D.B., 2015. On the history, osteology, and systematic position of the Wealden (Hastings group) dinosaur Hypselospinus fittoni (Iguanodontia: Styracosterna). Zoological Journal of the Linnean Society 173, 92-189.

Paul, G.S., Persons, W.S., Van Raalte, J., 2022. The Tyrant Lizard King, Queen and Emperor: Multiple Lines of Morphological and Stratigraphic Evidence Support Subtle Evolution and Probable Speciation Within the North American Genus Tyrannosaurus. Evolutionary Biology 49, 156-179.

Scannella, J.B., Fowler, D.W., Goodwin, M.B., Horner, J.R., 2014. Evolutionary trends in Triceratops from the Hell Creek formation, Montana. Proceedings of the National Academy of Sciences 111, 10245-10250.

Weishampel, D.B., Heinrich, R.E., 1992. Systematics of Hypsilophodontidae and basal Iguanodontia (Dinosauria: Ornithopoda). Historical Biology 6, 159-184.