Iguanacolossus: a Utah iguanodont built from one partial skeleton

Iguanacolossus fortis is a large early iguanodont from Utah, known from an associated but incomplete skeleton. Its bones come from one individual and include parts of the skull, spine, pelvis and limbs. The find helps document the diversity of North American iguanodonts before hadrosaurids became widespread. The name evokes a “mighty iguana”, but the proposed nine-metre body length is a comparison-based estimate, not a measurement of a complete skeleton.

Quick facts

A skeleton from Don’s Ridge

Donald DeBlieux discovered the fossils in 2005 at Don’s Ridge, near Green River in Grand County, Utah. The holotype, UMNH VP 20205, was recovered from the lower part of the Yellow Cat interval within the Cedar Mountain Formation. The site is distinct from other Yellow Cat bone beds, even though they share a broad formation name.

Andrew McDonald and colleagues described the species in 2010. The specimen is disarticulated, but its bones are similar in size and do not duplicate one another, which led the authors to interpret them as one associated individual. It includes a fragment of the predentary, part of the upper jaw, isolated teeth, a squamosal, both quadrates, vertebrae, ribs, parts of the shoulder girdle, pelvis and limbs. Much of the snout, hand and several stretches of the backbone are missing.

The genus name combines Iguanodon with the Greek-derived colossus; the species epithet fortis means strong or mighty. The name captures the size suggested by the skeleton, but it does not serve as a measurement or diagnosis. The bones and their anatomy remain the basis for identifying the animal.

Diagnostic details in individual bones

One distinctive feature is on the squamosal, a bone from the rear part of the skull: its contact surface with the supraoccipital has a wavy outline. Other traits include a semicircular plate on the neural spine of the second neck vertebra, a straight upper edge of the ilium and a front pelvic process that expands into a horizontal, shoe-like shape.

These details are more useful for diagnosis than the animal’s large size or general outline. The front of the skull is incomplete, and the hand is not preserved well enough to show a thumb spike. Artists often include one because a robust thumb spike occurs in related iguanodonts. For Iguanacolossus, it is an inference from relatives, not a directly identified bone in the holotype.

The teeth have features suited to processing plant material. No stomach contents or attributable coprolites identify the actual plants eaten. It is reasonable to describe the animal as herbivorous, but not to name a precise menu or seasonal feeding pattern from the evidence available.

How large was it?

The original description compared Iguanacolossus with the European Iguanodon and estimated a length of about nine metres. That figure scales the incomplete skeleton using a better-known relative. The preserved ilium approaches 90 centimetres, and the quadrates measure roughly 31–33 centimetres, supporting the conclusion that the individual was large.

A long body length does not reveal a single fixed gait. Strong hind limbs are compatible with bipedal movement, while comparisons with other large iguanodonts suggest that the animal may have used all four limbs while walking and risen onto its hind legs in other situations. No trackway securely attributed to Iguanacolossus records its usual posture or speed.

Body mass estimates would depend on the reconstructed proportions of the missing torso and limbs. They should not be treated as a direct reading from the skeleton. The fossil establishes a substantial animal; its exact weight and maximum adult size are less certain.

Among the early iguanodonts

The 2010 phylogenetic analysis recovered Iguanacolossus as a basal member of Styracosterna, an iguanodont branch that includes later hadrosauroids. It was not itself a hadrosaurid and is not regarded as a direct ancestor of the later duck-billed dinosaurs. The result reflects the character sample available in the original analysis; new taxa and revised scores can alter the branching pattern.

The holotype’s incompleteness matters for those analyses. Missing bones are coded as unknown, not as proof that a feature was absent. A newly discovered hand or more complete skull could add characters and refine its position. For now, the distinct skull and pelvic traits, together with the associated skeleton, support a diagnosable early iguanodont.

Age and the Yellow Cat landscape

When the species was described, the age of the lower Yellow Cat deposits was poorly constrained. Later work using detrital zircons and pollen placed the interval across the Berriasian–Valanginian and dated some levels to around 136 million years ago. That age cannot simply be assigned as an exact date to the Iguanacolossus skeleton; its horizon remains within a broader sequence.

The sediments formed on a continental plain with river channels and developed soils. The Cedar Mountain Formation contains a range of dinosaurs and smaller vertebrates, but fossils from different levels are not automatically members of one contemporaneous ecosystem. The lower and upper Yellow Cat deposits have distinct fossil assemblages and histories.

In the dinosaur catalogue, Iguanacolossus represents the large-bodied iguanodonts that lived in Early Cretaceous North America. Colour, vocalisations, social behaviour and soft tissues are not preserved, and the familiar thumb spike in artwork should remain labelled as a reconstruction.