Tuesday, 3 May 2016

Horshamosaurus: An enigmatic Wealden ankylosaur.

I recently visited the small but excellent museum in Horsham to take a look at one of the most enigmatic dinosaurs found in the Wealden, Horshamosaurus. Of the three species of Early Cretaceous ankylosaurs currently recognised in the English Wealden, Horshamosaurus is the least known. 
The specimen was discovered in a brickworks quarry near Rudgwick, Sussex in 1985, in the Barremian-aged rocks of the Wealden Sub-basin. Somewhat typically for Wealden dinosaur remains there is not much of it and whilst Hylaeosaurus and Polacanthus foxii are known from partial skeletons, the material representing Horshamosaurus is more fragmentary and consists of only a few elements which were found associated but disartuclated.

Dorsal vertebra of Horshamosaurus.
A) dorsal B) ventral C) right lateral D) left lateral E) anterior F) posterior

The fossils were originally assigned by British ankylosaur expert Bill Blows to Polacanthus rudgwickensis, as it displays some affinities with Polacanthus foxii, the majority of which specimens come from the Wessex Formation but with one specimen having been found in the earlier Valanginian Fm near Bexhill, Surrey. Horshamosaurus shares several synampomorphies with Polacanthus: unfused caudal chevrons, astralagus fused to the tibia and similarities in the dermal armour, but the paucity of material associated with this specimen is problematic.
There are some differences between the Rudgwick ankylosaur and Polacanthus foxii too. It's suggested that Horshamosaurus is significantly bigger than the P. foxii holotype, around 30%, but this estimate must be viewed with caution and requires further testing. It also displayed differences in vertebral morphology and the length of the tibia, plus the geological occurrence of the skeleton indicated it was not P. foxii. In his recent book Polacanthid Dinosaurs of Britain (see my review here), Blows re-assigns P. rudgwickensis to Horshamosaurus and suggests it is perhaps a nodosaurid and not a polacanthid  based on a reassessment of the character differences between it and P. foxii. Of course, the existence of a monophyletic ‘polacanthid’ clade is not settled.

Scacpulocoracoid of Horshamosaurus.
A) dorsal B) ventral C) right lateral D) left lateral E) anterior F) posterior

The main parts of the specimen consist of one complete dorsal vertebra and one broken, a couple of partial caudals of which one is a left half only, some rib sections including the proximal part of a large dorsal rib, the distal end of the scapulocoracoid, two osteoderms, the distal end of left humerus and the proximal and distal sections of the right tibia; the tibia is broken into two parts and a section of unknown length is missing. The scapulocoracoid is distinctive but the pectoral girdle of Polacanthus is poorly known, with only the Bexhill specimen and a specimen in private hands having part of the scapular preserved, however this appears to be less robust than Horshamosaurus. The Horshamosaurus scapular and coracoid are fused, this condition is not known to be a characteristic of Polacanthus as the coracoids of all the other published specimens are missing..
Of the two osteoderms preserved, one is a partial roof-like, thin-walled osteoderm, the other a large keeled osteoderm with a solid base. In his 1996 paper describing the specimen Blows suggests that the thin-walled osteoderm might be indicative of an ankylsoaurid affinity for this animal; there is some support for the taxonomic utility of ankylosaur osteoderms so this could be significant but as this is not a commonly seen morphology in Wealden ankylosaurs a larger sample size of these osteoderms  is needed.

Horsham museums's dinosaur display cabinet, with Horshamosaurus on the white wall on the right.

The reassignment of Horshamosaurus by Blows in his 2015 book is tentative and hopefully more remains of this enigmatic armoured dinosaur will be unearthed in due course, which will enable us to resolve it's taxonomy and shed more light on the relationships of Wealden ankylosaurs. In the meantime, if you're passing close to Horsham I can recommend dropping into the museum to see their vertebrate palaeontology collection which is small (one cabinet), but contains the world's only known Horshamosaurus.

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