Stegosaurs of the British Isles

Fig 1.- Lexovisaurus durobrivensis (= Loricatosaurus priscus)

Compared to the Stegosaurs of North America, restricted to the sole Morrison Formation and represented only by 1 or 2 genera and a handful of species, all from the Kimmeridgian-Tithonian age of the Late Jurassic, the plated dinosaurs from England appear comparatively more diversified with up to 6 genera spanning from the Middle Jurassic to the Early Cretaceous. Unfortunately, the fossils there are for the most part quite scrappy, consisting of teeth, bits of plates and other bones, making their identification difficult or even dubious.  Susannah Maidment and colleagues in their 2008 general revision of the Stegosauria, have recognized only 2 valid species to the United Kingdom, Dacentrurus armatus and the newly erected Loricatosaurus priscus, but this is certainly an underestimation. Let’s make a quick overview of stegosaurian materials found in England.

Middle Jurassic Stegosaurs

The most ancient British stegosaurs date from the Bathonian stage (165-168 MYA) of the Middle Jurassic and consist of a few isolated bones, including a massive right femur of a juvenile individual from the Cornbrah formation of Oxfordshire (Upper Batthonian), two incomplete vertebrae from the Sharp’s Hill Formation of Oxfordshire and two large dermal plates from the Chipping Norton Formation of Gloucestershire. These bones were all referred to “Lexovisaurus” vetustus (= “Omosaurus” vetustus). All that can be said is that they are the oldest recorded Stegosaurids in the world (the family to which all the most derived stegosaurs such as Stegosaurus and Kentrosaurus belong) and that a least one large species was present in Europe at that time.

Next, from the Middle Callovian (161-165 MYA) Oxford Clay Formation came Lexovisaurus durobrivensis, known from two partial skeletons. Maidment et al, 2008 have invalidated the name Lexovisaurus, on the basis that no unique character could be found in the holotype specimen. In its stead, a new genus, Loricatosaurus, was erected for the second partial skeleton that includes vertebrae, various pelvic and limb elements, and a piece of dermal armor, and folded into the species Loricatosaurus priscus. A large dermal plate named ‘Omosaurus leedsi’ from the same locality may be from the same animal. A third partial skeleton from a contemporary formation in Normandy, France is also referred to Loricatosaurus priscus. However there is no real indication that Lexovisaurus durobrivensis and Loricatosaurus priscus represent different taxa and the two might well be the same. Loricatosaurus probably measured about 5-6 meters in length. It is characterized by relatively short limbs and narrow plates and spines on the back. Shoulder spines may or may not have been present (A previously reported shoulder spine in the skeleton turns out to be a tail spine instead).

Upper Jurassic Stegosaurs

Fig 2.- Dacentrurus armatus.

From the Coralline Oolite Formation, Yorkshire, of Middle Oxfordian age (156-161 MYA), came a poorly preserved femur of a juvenile individual that has been named ‘Omosaurus phillipsi’ (= ‘Dacentrurus phillipsi’). There is no real indication that it was a stegosaur at all and the name is considered a nomen dubium (dubious name).

From the Kimmeridge Clay Formation of Wiltshire, of Lower Kimmeridgian age (151-156 MYA), came Dacentrurus armatus, a partial skeleton preserved in a large slab on exhibit at the Natural History Museum of London. Some fragmentary materials from France, Spain and Portugal were also referred to Dacentrurus. It was a large stegosaur with an estimated length of some 8 meters. Its aspect is not well known, but it probably resembled the African Kentrosaurus. From the Kimmeridge Clay also came a few dermal spines named “Omosaurus hastiger” that might belong to the same animal than Dacentrurus. 

Fig 3.- Dacentrurus armatus holotype on display at the Natural History Museum in London (Credit: Emőke Dénes, via Wikipedia)

Early Cretaceous Stegosaurs

From the Lower Cretaceous Wealden Beds, Sussex of Valanginian age (136-140 MYA), came a partial right mandible, Regnosaurus northamptoni, that was variously attributed to the Ornithopod Iguanodon (Mantell, 1841), the ankylosaur Hylaeosaurus (Owen, 1858), a scelidosaur (Lydekker, 1888), and even a sauropod (Ostrom, 1970) until it was discarded as a nomen dubium (Coombs, 1971). However, more recently, Barrett & Upchurch, 1995, resuscitated Regnosaurus as a stegosaur and found it to be a relict of the old stegosaurian lineage that included the Chinese Huayangosaurus. The dubious Craterosaurus pottonensis, known from a single incomplete and poorly preserved dorsal vertebra, has been described as a stegosaur, but that is probably more wishful thinking. Interestingly Craterosaurus from Bedfordshire is probably also of Valanginian age.

Fig 4.- Regnosaurus northamtoni was a close relative of Huayangosaurus taibaii.

This concludes our tour of the stegosaurian remains of England.

Original artworks on Paleoexhibit are copyrighted to Nobu Tamura. Do not use without permission (Email: nobu dot tamura at yahoo dot com)

References:

P. M. Barret & P. Upchurch. 1995. Regnosaurus northamptoni, a stegosaurian dinosaur from the Lower Cretaceous of Southern England. Geological Magazine 132: 213–222.

P. M. Galton and H.P. Powell, H. P. 1983. Stegosaurian dinosaurs from the Bathonian (Middle Jurassic) of England, the earliest record of the Family Stegosauridae. Geobios 16: 219-229.

P. M. Galton. 1985. British plated dinosaurs (Ornithischia, Stegosauridae). Journal of Vertebrate Paleontology 5(3):211-254.

S. C. R. Maidment, D. B. Norman, P. M. Barrett and P. Upchurch. 2008. Systematics and phylogeny of Stegosauria (Dinosauria: Ornithischia). Journal of Systematic Palaeontology 6(4):367-407.

How many Stegosaurs in the Morrison Formation?

Fig 1.- Stegosaurus ungulatus.

Stegosaurus is one of the most iconic dinosaurs from the famed Late Jurassic Morrison Formation of North America. Artists have often depicted the plated “roofed lizard” as the formidable opponent to its nemesis, the fearsome theropod Allosaurus. The first remains of a stegosaur have been unearthed in Colorado during the Bone Wars and described by Othniel Charles Marsh in 1977 as Stegosaurus armatus. Since then, numerous specimens were found and described from different areas encompassing the Morrison Formation, in Colorado, Wyoming and Utah. Of the 11 or so names that were given to stegosaur remains from the United States, Marsh has described no less than 7 species.

Fig 2.- Hesperosaurus mjosi

But subtle differences seen in the bones can either be signatures of different species or simply reflect individual variations within the same species. How to define characteristics that are unique to a given species (autapomorphies) is easier said than done when the number of specimens is limited and when remains are fragmentary (i.e. statistics is lacking).

Fig 3.- Stegosaurus stenops

In a 2008 review of all stegosaurs, Susannah Maidment and colleagues made some very drastic cuts in the number of species present in the Morrison formation, recognizing only two valid ones: Stegosaurus armatus and Hesperosaurus mjosi that they renamed Stegosaurus mjosi. The other species are either junior synonyms of S. armatus or dubious (nomen dubium). However in another case of the now classic opposition between “splitters” (scientists who think there is only a narrow range of variation inside a species, thus considering more species as valid) and “lumpers” (scientists who think that there are few species with large individual variations), Peter Galton and colleagues in a 2010 paper had a closer look on the various remains from the Morrison formation and found that up to 7 species must be considered valid. To add a little twist to the story, they infer that the type species, Stegosaurus armatus, is a nomen dubium due to the fragmentary nature of the holotype that does not allow defining any autapomorphy. But if S. armatus were unvalidated, so would be the genus name, Stegosaurus. Consequently, a petition to the ICZN (International Commission of Zoological Nomenclature) was filed to make the best-known species, Stegosaurus stenops as the new type for the genus (good choice, nobody really wants our favorite plated lizard to be renamed Hypsirophus or Diracodon !).

Fig 4.- Stegosaurus longispinus, here depicted as a Kentrosaurus grade species

In view of the diversity of Ornithopods and Sauropods in the Morrison formation, it is not that surprising to see more than one species of stegosaurs living at the same time there. If we discard the species that are either too fragmentary or not properly described, we end up with some 4 or 5 different stegosaurs in North America. Among them, Hesperosaurus mjosi is the oldest, having been found close to the base of the formation. It is easily distinguishable by the plates, which are longer than taller. Stegosaurus stenops is the best known with several almost complete skeletons.  It is characterized by three unpaired alternating dermal plates just before the spikes while Stegosaurus ungulatus has three pairs of small dermal plates there. Interestingly, remains of a stegosaur from Portugal have tentatively been assigned to this particular species. Stegosaurus sulcatus with its large based tail spikes might also be another valid species.

Stegosaurus longispinus, characterized by longer tail spines or “thagomizer” is only known from materials from the rear of the animal, which were unfortunately mostly destroyed in the early 1920s. Because of the longer tail spines, some have argued that S. longispinus might have been a Kentrosaurus-like stegosaur, which would need to be redescribed as a new genus.

Note: as I am writing this, I realized this story was already covered much more eloquently and in depth by paleontologist Darren Naish a few months earlier (see his post here)…



References:

S. C. R. Maidment, D. B. Norman, P. M. Barrett and P. Upchurch. 2008. Systematics and phylogeny of Stegosauria (Dinosauria: Ornithischia). Journal of Systematic Palaeontology 6(4):367-407.

P. M. Galton, 2010. Species of plated dinosaur Stegosaurus (Morrison Formation, Late Jurassic) of western USA: new type species designation needed. Swiss Journal of Geosciences 103, 187-198.