Among non-avian dinosaurs, Heterodontosaurus tucki is unique for possessing complex dental features including both morphological and proportional heterodonty, sub-hypsodonty, tooth occlusion, and extensive low-angled wear facets—a collection of derived traits made additionally noteworthy by their appearance in one of the earliest-branching ornithischian lineages. In many taxa with similar dental characteristics, complex suites of modified dental tissues shape functional occlusal surfaces through wear. It remains unknown if H. tucki possesses similar histological complexity. Here, we investigate the histology and enamel microstructure of H. tucki maxillary cheek teeth from the Early Jurassic upper Elliot Formation of South Africa. Despite possessing a superficially complex dentition, the maxillary teeth exhibit a thin, relatively simple, three-layered enamel schmelzmuster (basal unit, columnar unit, and parallel crystallite) with enamel tubules. On the labial face, the enamel thins out drastically (<6 μm) and is discontinuous with a more simplified enamel microstructure. Surprisingly, a thick band of wear-resistant, histologically distinct dentine arises concurrent with the thinning enamel and appears to form the primary cutting crest of the functional occlusal surface, a role typically filled by enamel. This represents both the phylogenetically and chronologically earliest known acquisition of this form of modified dentine within Ornithischia.

Key words: Dinosauria, Heterodontosauridae, Heterodontosaurus tucki, enamel, microstructure, histology.

Cecilia E. Calvert [cecilia_calvert@nps.gov; ORCID: https://orcid.org/0009-0007-2745-5549] and Niall S. Whalen [nwhalen@bio.fsu.edu; ORCID: https://orcid.org/0000- 0001-9732-1182], Department of Biological Science, Florida State University, 319 Stadium Dr, Tallahassee, FL 32306, USA. Tyler C. Hunt [thunt@bio.fsu.edu; ORCID: https://orcid.org/0000-0001-6826-780X] (corresponding author) and Gregory M. Erickson [gerickson@bio.fsu.edu; ORCID: https://orcid.org/0009-0003-4881-4900], Department of Biological Science, Florida State University, Tallahassee, FL 32306, USA; Mechanical and Physical Properties Laboratory National High Magnetic Field Laboratory-Florida State University, 1800 E Paul Dirac Dr, Tallahassee, FL 32310, USA. Jonah N. Choiniere [Jonah.choiniere@wits.ac.za; ORCID: https://orcid.org/0000-0002-1008-0687], Evolutionary Studies Institute, University of Witwatersrand, 1 Jan Smuts Avenue, Johannesburg, South Africa. Mark A. Norell [mark.norell@icloud.com; ORCID: https://orcid.org/0000-0002-1084-5555], Division of Vertebrate Paleontology, American Museum of Natural History, 200 Central Park West, New York, NY, USA.