Bicuspidon

Bicuspidon numerosus is a Utah lizard known mainly from jaws and teeth recovered in the Mussentuchit Member of the Cedar Mountain Formation. Its name describes the paired cusps on the broad rear teeth, while its many specimens made it unusually well represented for an early Cretaceous lizard. The tooth row changes from simple conical crowns at the front to transversely expanded bicuspid crowns behind them. Larger jaws show more developed rear teeth, a pattern interpreted as growth-related. The fossil record does not reveal an exact diet or a complete body portrait, but the jaws make Bicuspidon a useful comparison in the ancient lizard and snake catalogue.

Quick facts

A common lizard in a microfossil fauna

Randall Nydam and Richard Cifelli named Bicuspidon numerosus in 2002 from material collected on the western flank of the San Rafael Swell in Emery County, Utah. The fossils come from the Mussentuchit Member of the Cedar Mountain Formation. Radiometric ages from localities yielding the fauna placed it near the Albian–Cenomanian boundary, approximately 98.4 million years ago in the original chronology.

The holotype, OMNH 26743, is a left maxilla. Other specimens include a nearly complete dentary with 18 tooth positions, additional jaw fragments and isolated teeth. This is a comparatively rich record for a small fossil lizard, but the material is disarticulated. It does not include a complete skull or skeleton, so the jaws carry most of the evidence for identifying the animal.

A tooth row that changes from front to back

The front teeth are conical and slightly recurved. Farther back, the crowns broaden across the jaw and carry two cusps. A ridge links the smaller inner cusp to the larger outer one. This arrangement is heterodonty: different positions in one tooth row have different shapes. It is not equivalent to the specialised incisors and molars of mammals, because the lizard jaw moved and processed food differently.

The pattern was important for distinguishing Bicuspidon from other Cretaceous lizards with transversely expanded teeth. Nydam and Cifelli argued that the second cusp was added during the evolution of this North American lineage, rather than the whole tooth crown simply rotating. Later work grouped Bicuspidon with other borioteiioid lizards, outside the living teiid family in its strict sense.

Do the teeth record growth?

Small jaws tend to have narrower, less expanded rear teeth. In larger specimens, the cusps are more widely separated and the crown spreads farther across the row. The original authors interpreted the near-continuous change among specimens as ontogenetic variation, meaning that the teeth changed as an individual matured.

That interpretation is plausible, but fossils do not preserve a labelled age series from one animal. Differences among individuals could include other sources of variation that the available sample cannot test. The secure observation is the association between jaw size and crown form; growth is the explanation best supported by the original study.

Diet: mechanics without a menu

Broad posterior teeth could have helped process firmer food, while the pointed anterior teeth could seize or hold it. These functions are inferences from shape. The species has not yielded a stomach contents fossil or a coprolite that identifies a meal, and its authors did not assign it a single specific diet. Insect prey, plant material or small animals cannot be separated confidently from the tooth row alone.

The more specialised crowns of Polyglyphanodon and the different jaw anatomy of Paramacellodus show that extinct lizards explored varied dental designs. They do not establish identical feeding habits. For Bicuspidon, the most useful evidence is how the tooth row was organised and how it changed across the sample.

What remains unknown

Most of the skull and the postcranial skeleton are absent. A reconstructed animal can be placed among small-bodied lizards with comparisons to related forms, but exact body length, limb proportions, colour and scale pattern cannot be read directly from its jaws. Likewise, the abundance of teeth in a collecting sample is not a count of complete animals.

The fossils securely document a distinctive Cretaceous lizard with a predictable transition from conical front teeth to bicuspid rear teeth. They also preserve a possible record of dental change during growth. They do not provide a complete ecology, a precise menu or a direct ancestor of living teiids.