Huehuecanauhtlus: a Mexican hadrosauroid with an injured spine

Huehuecanauhtlus tiquichensis is a basal hadrosauroid from the Santonian of Michoacán, Mexico. Two partial specimens preserve parts of the jaws, spine and pelvis, but no complete skull. Its fossils became especially notable when researchers described unusual lesions on ribs and a vertebra. The bone changes show that the individual lived with a serious pathology; they do not provide a single certain medical diagnosis or a full record of its symptoms.

Quick facts

A find in Michoacán

The bones came from Barranca Los Bonetes, near Tzitzio in southwestern Mexico. The site lies in continental deposits assigned to the Santonian, approximately 86–84 million years old. The original description did not give the fossil-bearing sequence a formal formation name. Later correlations should therefore be treated carefully rather than attached to a named unit without a stratigraphic argument.

Ángel Ramírez-Velasco and colleagues named the dinosaur in 2012. The genus name draws on Nahuatl words translated as “ancient duck”, a reference to the hadrosauroid lineage. The species epithet tiquichensis honours the people of Tiquicheo, whose hospitality and support aided the field work.

Two partial specimens

The holotype, IGM 6253, preserves part of a left maxilla, a fragment of the lower jaw, four neck vertebrae and nine dorsal vertebrae, portions of the sacrum and tail, ribs, ossified tendons, and incomplete paired pelvic bones. The skeleton is not fully articulated, but the main set of bones was interpreted as one individual. It provides enough anatomy to diagnose the taxon without preserving the complete head or limbs.

A smaller specimen, IGM 6254, consists of part of a left lower jaw, two teeth and a cervical vertebra. It establishes that a second individual is represented, but it does not create a full growth series. Differences in size could reflect age, individual variation, or both. They do not by themselves demonstrate that the two specimens belong to separate species.

The overall skull is unknown. The maxilla reveals how the upper cheek teeth were arranged, but it does not show the complete snout, eye region or soft beak. Broad, duck-like faces in artwork are reconstructed from better-known relatives rather than preserved in the Mexican fossils.

Teeth, spine and an unusual pelvis

At least two teeth were active at each position in the upper and lower jaws. This arrangement foreshadows the more elaborate dental batteries of later hadrosaurids and fits repeated processing of plant material. No stomach contents or identifiable plant remains confirm a particular menu, so herbivory is supported by the dental anatomy rather than by a preserved meal.

The holotype has seven sacral vertebrae and tall neural spines on the tail vertebrae. Its ilium has a long supra-acetabular process above the hip socket, a short trapezoidal rear process and a front process that angles sharply downward. That downward bend is among the features that distinguish the pelvis from other described hadrosauroids.

These traits help researchers recognise the taxon even without a complete skull. They do not supply an exact body length: estimates scale the preserved portions against related dinosaurs. The hands, the end of the tail and many head proportions remain unknown.

Close to, but outside, the true hadrosaurids

The original cladistic analysis placed Huehuecanauhtlus close to Hadrosauridae but outside that family. In other words, it was a hadrosauroid, not a hadrosaurid like the later Hadrosaurus. It can be compared with early forms such as Eolambia, although that comparison does not imply that one genus descended directly from the other.

Phylogenetic positions can shift when new taxa and anatomical characters are added. The incomplete Mexican fossils leave many character scores unknown, which affects where the animal falls in an evolutionary analysis. Unknown anatomy should not be coded as evidence that a feature was absent.

The find expanded the known geographic range of early hadrosauroids into southern North America. The describing authors discussed alternative models for the group’s early history, including dispersal from Asia into North America by the Albian or an older split within a broadly distributed ancestral population. These are biogeographic hypotheses, not routes directly recorded in the skeleton.

Lesions on ribs and vertebra

A 2016 study examined two dorsal ribs and one dorsal vertebra from the holotype. The bones show abnormal growth and altered surface texture. The pattern indicates that the animal survived long enough for bone to remodel around the affected areas. It is direct evidence of pathology, even though the cause is less certain.

The authors compared the changes with inflammatory and degenerative conditions affecting the spine and ribs, including possible osteomyelitis. Similar-looking lesions can result from different processes, so a fossil rarely permits the certainty of a modern clinical diagnosis. The study also considered damage to the nervous system and the possibility of pain, but those effects cannot be observed directly in the bones.

The lesions do not prove that the dinosaur limped continuously, experienced severe pain every day or died because of the disease. Soft tissues and the cause of death are not preserved. The responsible conclusion is narrower: this individual had substantial bone abnormalities during life, and their exact cause remains an interpretation.

Movement and behaviour

The general anatomy of ornithopods suggests strong hind limbs, with the forelimbs available for support during slower movement in some relatives. The limbs of Huehuecanauhtlus are not sufficiently complete to calculate its gait or speed. No trackway has been securely assigned to the genus.

The tooth rows support plant eating but do not identify particular vegetation. Several individuals from one region do not establish that the animals formed a herd. Eggs, nests, skin impressions and evidence of colour are absent, leaving social behaviour and soft coverings outside the direct fossil record.

In the dinosaur catalogue, Huehuecanauhtlus is a distinctive early hadrosauroid from southwestern North America. Comparisons with European Telmatosaurus can illustrate the diversity of lineages near Hadrosauridae, but do not form a simple ladder of direct ancestors and descendants.