Thumb Spike

The story of the thumb spike begins with the earliest days of dinosaur science. In 1822, Gideon Mantell and his wife Mary Ann Mantell recovered large fossilized teeth from Sussex, England, which Mantell described in 1825 under the name Iguanodon ('iguana tooth'). As additional fragmentary material accumulated, Mantell encountered a conical bony element that he interpreted as a nasal horn, drawing analogy with the rhinoceros iguana (Cyclura cornuta). This interpretation fitted the prevailing view that dinosaurs were essentially gigantic lizards. When Richard Owen coined the name Dinosauria in 1842, using Iguanodon, Megalosaurus, and Hylaeosaurus as founding members, the nasal horn interpretation remained intact. The 1854 Crystal Palace dinosaur sculptures by Benjamin Waterhouse Hawkins, constructed under Owen's guidance, prominently featured a horn atop the snout of the Iguanodon models. These sculptures survive today in Crystal Palace Park, London, as enduring monuments to early palaeontological interpretation. It is worth noting, however, that Owen himself suggested as early as 1872 that the conical bone might actually represent an ungual phalanx of digit I of the hand, partially anticipating the correction that would come six years later.

The definitive reinterpretation came in 1878, when miners excavating a new gallery at the Fosse Sainte-Barbe coal mine near the village of Bernissart, Belgium, struck a pocket of Lower Cretaceous marl 322 metres underground. Over the following three years, under the direction of fossil preparator Louis De Pauw, approximately 30 Iguanodon skeletons were recovered, many in remarkably complete articulation. This was one of the most spectacular palaeontological discoveries of the nineteenth century.

Louis Dollo, who supervised the assembly and study of the skeletons at the Musée Royal d'Histoire Naturelle (now the Royal Belgian Institute of Natural Sciences) in Brussels, was able to definitively place the conical spike on the hand rather than on the nose. With multiple articulated forelimbs available for study, it was unambiguous that the spike belonged to digit I. Dollo published extensively on the Bernissart Iguanodon material between 1882 and 1923, establishing the bipedal 'kangaroo' posture that would dominate reconstructions for the next century, though this posture was itself later revised.

David B. Norman's landmark 1980 monograph on I. bernissartensis provides the most detailed anatomical description of the manus. The emended diagnosis states that the phalangeal count of the manus is 2-3-3-2-4. The first phalanx of digit I consists of a thin, warped plate of bone lying in a shallow recess on the proximal surface of the ungual phalanx. The ungual itself has the form of a long, curved, conical spine, which can freely articulate against the fused carpo-metacarpus. Critically, the spike and its proximal plate-like phalanx are able to rotate in a transverse plane across the wrist, and the digit can be flexed so that the ungual moves toward the palm.

The ungual phalanges of digits II and III are broad and hoof-like, twisted longitudinally, and were adapted for weight-bearing during quadrupedal locomotion. The fifth digit was elongate, flexible, and prehensile, likely serving a grasping function. The overall construction of the manus thus reflects a remarkable multifunctionality: the three central digits formed a load-bearing 'hoof', the pollex served an as-yet-uncertain specialized role, and the fifth digit provided manipulative capability.

A replica of an Iguanodon thumb spike held at National Museum Cardiff measures 14 cm in length for the bony core alone. Other sources report a range of approximately 5 to 15 cm (2 to 6 inches) depending on the individual and species, though these figures refer exclusively to the fossilized bone. The keratinous sheath that covered the bone in life would have added considerably to its overall dimensions and sharpness, but since keratin rarely fossilizes, the actual size in life remains estimated.

Despite being one of the most iconic anatomical features in palaeontology, the function of the iguanodontian thumb spike remains unresolved. Several hypotheses have been proposed.

Defense against predators is the most widely cited explanation. The idea that Iguanodon could stab an attacking theropod with its thumb spike has been a staple of popular dinosaur literature for over a century. However, as Riley Black noted in the Smithsonian Magazine, the defensive scenario requires the herbivore to place itself directly within the biting range of its attacker in order to deploy the spike, which somewhat undermines its utility. Taquet and Russell (1999), in their description of the closely related Lurdusaurus arenatus from the Lower Cretaceous of Niger, interpreted its massive thumb spike and powerfully built forearm as a 'mace-and-chain' arrangement suited for self-defense, lending some support to this hypothesis for at least some iguanodontians.

Foraging assistance has been proposed in several forms. Norman, in his section on basal Iguanodontia in the second edition of The Dinosauria, briefly suggested the spike may have been used for 'breaking into seeds and fruits.' Black proposed a comparison with the false thumbs of pandas, imagining Iguanodon running the spike along a branch to strip foliage. However, unlike the flexible wrist bones of pandas, the Iguanodon spike was relatively rigid. Furthermore, any wear patterns from such use would appear on the keratinous sheath rather than the bone, making this hypothesis exceptionally difficult to test from the fossil record.

Intraspecific combat and display represents a third hypothesis. By analogy with the wing spurs of extant birds, which Davidson (1985) documented in galliform species as serving both combative and sexual display functions, some researchers have suggested that the thumb spikes may have been used in male-male contests, territorial disputes, or as visual signals of fitness. The observation that Iguanodon is generally interpreted as not being a herd animal has been used to speculate that it may have been monogamous and territorial. However, drawing functional parallels between avian spurs (which evolved independently and relatively recently) and iguanodontian thumb spikes requires accepting convergent evolution as an explanatory framework, which is plausible but unverifiable from the fossil record alone.

Multiple or variable functions should also be considered. Mark Witton (2014) noted that the marked variation in pollex morphology across iguanodontian taxa strongly suggests that thumb spike function was not uniform across the group. A single explanation is unlikely to account for both the massive, freely articulating spike of I. bernissartensis and the tiny, barely functional spike of Mantellisaurus.

The thumb spike is not unique to Iguanodon but is a defining character of Ankylopollexia, a clade of iguanodontian ornithopods named for this very feature. However, the morphology of the pollex varies substantially among genera.

In Iguanodon bernissartensis, the spike is relatively large and conical, not fused to the carpal block, and retains the ability to rotate transversely. In Mantellisaurus atherfieldensis (formerly I. atherfieldensis), the spike is similar in basic form but drastically reduced in size, likely too small to function effectively as a weapon. In Hypselospinus fittoni, the pollex is proportionally large (approximately 40% the length of the forearm) but laterally compressed rather than truly conical, and tightly attached to the carpal block with minimal flexion possible. In Barilium dawsoni, the spike is fused to the carpal block itself and extremely short. Ouranosaurus nigeriensis possesses a stubby pollex spike. Lurdusaurus arenatus has a massive thumb spike described as larger than that of Iguanodon.

This diversity of form, size, and articulation across closely related animals strongly suggests that no single functional explanation can account for thumb spikes across all iguanodontians. In some taxa, the tight articulation of the pollex to the carpal block recalls the carpometacarpal knobs and spurs of certain modern birds, while in others the free articulation suggests different mechanical capabilities.

More basally branching iguanodontians, such as Tenontosaurus, retain a five-fingered hand without a specialized pollex spike, providing a baseline for understanding when and how this structure evolved within the iguanodontian lineage. McDonald et al. (2010), in their description of new basal iguanodonts from the Cedar Mountain Formation of Utah, explicitly addressed the evolution of 'thumb-spiked dinosaurs,' helping to clarify the phylogenetic distribution of this character.

The importance of the thumb spike extends beyond anatomy into systematics. The clade Ankylopollexia, which includes all iguanodontians more derived than Camptosaurus and encompasses both classic iguanodonts and hadrosaurids, is named directly for the stiff, cone-shaped thumb that characterizes these animals. The name derives from Greek ankylos (commonly interpreted as 'stiff' or 'fused,' though etymologically meaning 'bent' or 'curved') and Latin pollex ('thumb'). In the most derived members of this clade, the hadrosaurids, digit I is reduced or lost entirely, so the thumb spike is a feature primarily of the 'iguanodont-grade' members of Ankylopollexia.

The thumb spike of Iguanodon stands as one of the most famous case studies in the history of scientific self-correction. The progression from nasal horn (1825) to manual digit (1878) demonstrates how incomplete fossil evidence can lead to logical but ultimately incorrect interpretations, and how more complete material can radically alter understanding. The contrast between the Crystal Palace rhinoceros-like Iguanodon of the 1850s and the modern depiction of a facultatively bipedal ornithopod giving a permanent 'thumbs up' is perhaps the single most widely reproduced visual comparison in popular palaeontology, serving as a compelling illustration of how scientific knowledge evolves with new evidence.