Polyglyphanodon

Polyglyphanodon sternbergi is a Late Cretaceous lizard from the North Horn Formation of Utah, represented by an unusually rich sample of skeletons. Its posterior teeth are widened across the jaws and carry sharp ridges, supporting an interpretation of plant processing. A 2016 reassessment of almost 30 specimens also identified a complete lower temporal bar and proposed that some skull differences could reflect sexual dimorphism. The tooth function is supported by anatomy; the interpretation of adult morphs remains a hypothesis. Polyglyphanodon belongs in the ancient lizard and snake catalogue.

Quick facts

From a local fossil find to a large sample

Charles W. Gilmore named Polyglyphanodon sternbergi in 1940 from fossils collected in the South Dragon area of Emery County, Utah. The holotype, USNM 15477, includes a skull, lower jaw and associated skeletal parts; the paratype USNM 15816 is a nearly complete articulated skeleton. George F. Sternberg found the first specimen, and the species honours him.

Gilmore’s 1942 osteological study documented additional material from the North Horn Formation. Later work expanded the anatomical picture, and Simões and colleagues examined almost 30 specimens in 2016, including nearly complete skeletons. A sample spanning size classes is unusual for Mesozoic lizards and allows researchers to compare growth-related changes with variation among individuals.

The local concentration may represent animals accumulated in a shared depositional episode, possibly a flood, but the exact cause is not certain. Even a large sample from one locality is not a census of the species across its full range or lifetime.

Teeth built for cutting

The anterior part of the tooth row differs from the posterior series. Behind the first several positions, the teeth broaden transversely and carry sharp ridges that meet across the upper and lower jaws. Randall Nydam and Richard Cifelli examined the dentition in detail and described serrated, chisel-like edges. This geometry supports cutting and processing fibrous plant material.

Herbivory is a strong functional interpretation, but the fossils do not preserve the contents of a stomach or identify a plant species. Tooth wear and shape can reveal how food was handled more readily than the exact menu. The possibility of occasional animal matter cannot be excluded, though the specialised rear teeth indicate that plant processing was important.

The teeth are attached differently in the upper and lower jaws, and later study refined details of their implantation. These small anatomical distinctions matter because the original description treated the dentition as more uniform than subsequent examination showed.

A temporal bar and a changing skull

In most living squamates, the lower temporal bar is absent, leaving the lower temporal region of the skull open. The 2016 reassessment identified a complete bony bar in Polyglyphanodon. Its relative development changes across the growth series, with younger individuals showing a less complete condition than adults. This is an unusual example of ontogenetic transformation in a fossil lizard skull.

The same study found variation in skull shape among specimens of similar size and proposed that at least some of it could represent sexual dimorphism. Because sex cannot be observed directly in fossil bones, this remains a supported interpretation rather than a confirmed distinction between males and females.

The authors’ functional analysis did not find an obvious advantage for the bar in this species. Its reappearance may reflect developmental or structural constraints rather than a direct adaptation. A bone can be anatomically striking without having a simple, demonstrable purpose.

Body, habitat and what remains open

Associated skeletons show a robust lizard with developed limbs and a long tail. Published reconstructions place adults around a metre or somewhat more in total length, but estimates depend on incomplete specimens and reconstruction. The fossil series is much better for skull anatomy than for preserving soft tissues, so skin pattern and colour remain unknown.

The North Horn Formation preserves a terrestrial community in Late Cretaceous Utah. Polyglyphanodon lived alongside dinosaurs and other reptiles, yet formation-level co-occurrence cannot establish direct encounters or competition. The fossil record supports a large-bodied lizard with teeth adapted for plant processing, not a detailed daily routine.

Polyglyphanodon is also important because it shows that the Cretaceous lizard fauna included large, specialised herbivores. The unusually rich sample makes its anatomy testable, but questions about reproductive biology, exact diet and the evolutionary advantage of its skull features remain open.