Polacanthus

Chapter • Meet the dinosaur

Overview

Polacanthus foxii was an armoured, four-legged plant-eater roughly four to five metres long and weighing somewhere between about 900 kilograms and two tonnes. Its body was broad and slung low to the ground, with sturdy limbs, short feet and a back carrying two rows of tall, keeled spines over the neck and shoulders. Above the hips sat a continuous sheet of bone – the sacral shield – in which countless small bony nodules had fused into a single protective plate. Rows of flat, blade-like plates ran down the tail. Unlike the Asian ankylosaurids, Polacanthus had no tail club; its defence rested entirely on armour and spikes. Like all ankylosaurs it walked with a level back and carried its tail clear of the ground, never dragging it. It was no sprinter, but its low centre of gravity and heavy armour made it hard to tip over and hard to bite through. In many encounters simply standing firm may have been enough, presenting a wall of spines to predators such as Neovenator and Eotyrannus, which shared the Isle of Wight floodplains. The skull of Polacanthus is poorly known, but relatives show that ankylosaurs had a keen sense of smell and long, looping nasal passages that may have helped cool inhaled air and possibly produced calls. Its diet consisted of low-growing vegetation: ferns, horsetails, cycads and the early flowering plants that were just beginning to spread in the Cretaceous. The mouth was held close to the ground, and the small, leaf-shaped teeth were suited to cropping soft plant material rather than grinding tough wood. A broad belly and capacious gut suggest slow fermentation of plant matter inside the body, much as in large herbivores today. The first specimen was found in 1865 at Barnes High on the southern coast of the Isle of Wight, England, by the Reverend William Fox, a clergyman who collected fossils alongside his parish duties; the species name honours him. The material was described by Richard Owen and later studied in more detail by J. W. Hulke in the 1880s. Unfortunately the holotype lacks a skull, which has made its relationships contentious: today Polacanthus is usually placed among the nodosaurids or in its own group, the polacanthines, near the base of Ankylosauria. Later finds from the Wessex Formation have filled some gaps, but the skull remains known only from fragments.

Chapter • Where it lived

Habitat

Polacanthus lived around 130–125 million years ago, during the Early Cretaceous (Barremian), in what is now southern England. Its fossils come mainly from the Wessex Formation of the Isle of Wight, with additional material from the Vectis Formation and possibly equivalent beds on the mainland. The landscape was a warm, seasonally dry floodplain crossed by meandering rivers, dotted with lagoons and covered with forests of conifers, cycads and tree ferns. Preserved charcoal layers show that wildfires were frequent, and many carcasses were buried in mud after floods.

Chapter • Wild behavior

Behavior & Ecology

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Chapter • Scientific place

Taxonomy