Bolong: an early iguanodont with a remarkably young fossil

Bolong yixianensis is a small-bodied iguanodont from the Early Cretaceous of northeastern China. Its record is notable for two very different skeletons: a crushed adult holotype and an almost complete juvenile. Together they help describe an early relative of later hadrosauroids and show how some anatomical proportions changed early in growth. They do not reveal a single preferred food plant, social behaviour or the animal’s living colours.

Quick facts

Discovery in the Yixian Formation

Wenhao Wu, Pascal Godefroit and Hu Dongyu named Bolong yixianensis in 2010. The holotype, YHZ-001, came from the Batuyingzi area near Yixian County in Liaoning Province, China. It was recovered from the middle part of the Yixian Formation, usually assigned to a late Barremian–early Aptian interval of the Early Cretaceous, roughly around the 125–121-million-year range in common correlations. The exact age of any one fossil bed depends on the stratigraphic scheme being used.

The genus name combines the family names Bo Haicheng and Bo Xue, who assisted in recovering the fossil, with long, the Chinese word for “dragon”. The species name refers to the Yixian Formation. The holotype includes a strongly flattened skull with lower jaws and a partly articulated postcranial skeleton. It preserves the arrangement of bones and teeth, but compression changed the shape of some parts. A flat fossil outline is not automatically the three-dimensional form of the living head.

A juvenile from Inner Mongolia

A second specimen, ZMNH M8812, was described by Wenjie Zheng, Xingsheng Jin, Masateru Shibata and Yoichi Azuma. A local resident found the skeleton near Xitaizhi village in Ningcheng County, in what is now Inner Mongolia. It also comes from the Yixian Formation. The animal was very young: its small size, loose connections between several bones and skull proportions point to an early growth stage.

The juvenile is important because it preserves a much more complete body than the flattened adult. It also confirms that a number of features seen in the holotype belong to the same species rather than to a mixture of unrelated fossils. Similarities between the two skeletons are useful, although a young animal’s bones had not yet reached their adult proportions or final degree of fusion.

The juvenile’s teeth preserve a diagnostic feature on the inner side of the crown: a thickened margin divided by a strong vertical ridge. This is a detail observed directly on fossil enamel and dentine. It is stronger evidence for identification than attempts to infer skin colour or facial expression from an artistic reconstruction.

Skull and plant-processing teeth

The head of Bolong was relatively short and deep for an early iguanodont. A predentary bone formed the front of the lower jaw and would have supported a keratinous cropping beak in life. Behind it sat the cheek teeth, whose broad crowns and prominent primary ridges could cut and grind vegetation.

The genus is diagnosed by a combination of anatomical details rather than one striking trait. The original description noted a deep depression around the meeting of the lacrimal, maxilla and prefrontal; unusual proportions at the predentary–dentary junction; and a bend in the main ridge near the top of the maxillary teeth. A combination allows researchers to distinguish similar early iguanodonts even when their overall skull outlines look alike.

The jaws support herbivory, but they do not identify a favourite plant. The Yixian ecosystem preserves a diverse flora, yet no gut contents are known from Bolong. A detailed menu or a claim that this animal regularly fed at one particular height would go beyond the evidence.

Among early iguanodonts

A more extensive 2012 analysis recovered Bolong near the base of Hadrosauroidea. That result places it close to the branch that later included duck-billed dinosaurs, but Bolong was not a specialised hadrosaurid with a broad muzzle and complex dental batteries. It retains the anatomy of an earlier iguanodont grade.

Its precise position depends on which species and anatomical characters are included in the analysis. Later studies have rearranged several early iguanodonts within Hadrosauriformes. It is therefore more informative to call Bolong an early iguanodont close to the base of the hadrosauroid branch than to assign it an unchanging family label. The better-known Iguanodon represents another iguanodont lineage, while other hadrosaurids show how much later members of that broad branch diversified.

Body size, movement and growth

The adult has been estimated at about four metres long, but this is a reconstruction from an incomplete, compressed skeleton rather than a direct measurement of a complete animal. Mass is even more indirect because it requires scaling missing bones against other species. The juvenile must not be used as though its proportions represented an adult Bolong.

The adult skeleton has strong hind limbs, a long tail and shorter forelimbs. Those proportions are compatible with movement mainly on two legs, with possible forelimb support at a slower pace. They do not quantify how often the animal switched posture or how fast it moved. No trackway has been securely assigned to this species.

The almost complete juvenile helps document early development. Its skull was proportionally short and deep, its eye openings appear large relative to body size, and several vertebral sutures were not fused. These are growth-stage observations, not a portrait of the adult head. The two known individuals cannot establish a full growth curve or the range of variation within the species.

The Yixian environment

The Yixian Formation contains lake deposits and volcanic sediments within the Jehol Biota. The regional landscape included freshwater bodies and abundant vegetation, alongside birds, small mammals, pterosaurs and many dinosaurs. That setting describes the wider ecosystem of the formation; it does not prove that an individual Bolong spent all its time at a lake shore.

The adult holotype and the juvenile came from different localities. Together they show that the species occurred across part of northeastern China, but two skeletons cannot establish migration routes, herd structure or population size. Nor have skin impressions, pigment-bearing structures, a nest or a group of individuals been described for this genus.

What the fossils leave open

The strongest picture of Bolong is of an early iguanodont with plant-processing jaws, a distinctive tooth pattern and skeletal evidence from both adult and juvenile growth stages. The fossils do not reveal exact colour, vocalisations, nesting behaviour or a fixed body posture. The crushed adult remains require care when reconstructing three-dimensional anatomy; the young skeleton must be interpreted in light of its age.

As early iguanodont relationships are analysed with new taxa and characters, the branch occupied by Bolong may move. That does not erase the direct evidence from its jaws, teeth and two skeletons. Compare it with other species in the dinosaur catalogue, where the amount and kind of fossil evidence differ from entry to entry.