Hylaeosaurus

Credit: Mark Witton.

*Hylaeosaurus* is an extinct genus of nodosaurid dinosaur which lived in southern England during the Early Cretaceous.

## History

On July 20, 1832, Hideon Mantell wrote Benjamin Silliman about reptilian bones recovered from a gunpowder explosion in Tilgate Forest, with a local fossil dealer assembling ~50 fragments, describing them as a "great consarn of bites and boanes". Mentell purchased these and assembled them into a full skeleton, doubting their value. It was partially-articulate. Mantell had grown satisfied with this, since previous *Megalosaurus* and *Iguanodon* had consisted of single fragments, becoming the most complete non-avian dinosaur skeleton at the time, strongly wanting to name it a species of the latter. However, a visit by William Clift and John Edward Gray had changed his mind; Clift had found the spikes to be body armour. Mantell (1832) assigned it a new genus, *Hylaeosaurus*, meaning "lizard of the wood". However, he later claimed it meant "forest lizard" and "Wealden lizard". Mantell (November 30) sent it to the Geological Society of London, then travelling to London on December 5, then being informed his publication was 1/3 too long. Instead of performing a rewrite, Charles Lyell suggested he publish a book with a single chapter dedicated to *Hylaeosaurus*. In the next 3 weeks, he composed this, with Henry Da le Beche telling him he needed a specific epithet. On the 19th of December, he chose *armatus*, stating "there appears every reason to conclude that either its back was armed with a formidable row of spines, constituting a dermal fringe, or that its tail possessed the same appendage". In May 1833, The Geology of the South-East of England was published and *Hylaeosaurus* had become a valid genus and species, also publishing a lithograph of his find in the same book and another in The Wonders of Geology (1840). It is the most obscure of the first three dinosaurs used to define dinosauria. Additionally, it joins the suite of extinct animals in the Crystal Palace sculptures and was never used as a wastebasket taxon, unlike the others. Owen (1840) thought the spikes were asymmetrical, rejected that they formed a row on the back and assumed they were gastralia.

The Natural History Museum acquired the original specimen in 1838, assigned BMNH R3775 and then NHMUK 3775. It is from the Valanginian-aged Tunbridge Wells Sand Formation. It is the front of the skeleton lacking most of the head and forelimbs, measuring 135x75 centimeters. It consists of the posterior skull, possible mandibular fragments, 10 vertebrae, scapulae, coracoids and some spikes and armour; it is viewed from below. No further preparation had taken place for a while beyond Mantell's assembly and chiseling, with the museum attempting to free the remains via chemical and mechanical methods in the 21st century. However, this is difficult because the acids tend to dissolve the glue and gypsum Mantell had used to repair it, causing the blocks to disarticulate. Mark Graham (since 2003) has given limited details of this, but it was published and redescribed in 2020. Several more specimens from mainland Britain have been referred to *H. armatus*. However, Paul Barrett and Susannah Maidment (2011) concluded only the holotype is referable, since *Polacanthus* are recovered from the same layers. Specimens from France may be of *Polacanthus*, with possibly remains from Germany (DLM 537, a spike; and GPMM A3D.3, a lower humeral end) were assigned H. sp..

## Description

Gideon Mantell, based on lizards, estimated *Hylaeosaurus* was 7.6 meters (25 feet) long, with recent estimations finding up to 6 meters (20 feet). Gregory S. Paul (2010) estimates a length of 5 meters (16 feet), weighting 2 tonnes (2 long tons; 2.2 short tons). Darren Naish et al. (2001) find a length of 3-4 meters (9.8-13.1 feet). *Hylaeosaurus* has a mainly unknown build. Maidment finds two autapomorphies; the scapula does not fuse with the coracoid and 3 long spines are at the shoulder. Though, these do not distinguish it much. Mantell and Owen attributed the (lack of) fusion to ontogenetic causes, and  find the total spine count can not be seen. It is often depicted as a typical nodosaur. Kenneth Carpenter (2001) described cranial and mandibular remains, but the damaged and warped skull provides little data. The quadrate laterally bows; the quadratojugal bears a high attachment on the quadrate shaft; a triangular postorbital horn is present. In 2020, however, the presumed quadrate was fount to have been a jugal. The 2020 study found new distinguishing traits. A 120° is between the acromion and proximal plate. The acromial process is shaped like a shelf at 1/3 from the top edge that obliquely projects below and sideways. The top edge of the proximal plate curves sideways. The cervical centra have a horizontal ridge at the sides. Not included in this list are the exceptionally-concave undersides of the side processes. The shoulder spines are curved at the posterior, elongate, flattened, narrow and pointed. A shallow trough resides on the underside. The front spine is 42.5 centimeters long, where the spines shrink in height and enlarge in width. A fourth skull of about the same build, being more forwards-pointing, sits just behind the skull. Sven Sachs and Jahn Hornung (2013) suggest a configuration where 5 lateral nick spines are present, with the German spine specimen assigned the 3rd position.

## Classification

*Hylaeosaurus* is the first known ankylosaur, but its affinities are poorly known to this day. Coombs (1978) assigned it Nodosauridae, now seen as an unusual classification, as recent analyses place it a basal nodosaur and sometimes a polacanthine. In the 1990s, polacanthines were recovered as basal ankylosaurids since they believed they had small tail clubs, which is now recognized as incorrect. A 2012 study finds it to be a non-polacanthine basal nodosaurid:

## References

* https://en.wikipedia.org/wiki/Hylaeosaurus
* https://equatorialminnesota.blogspot.com/2021/10/compact-thescelosaurus-year-six.html