Heterodontosaurus tucki was an unordinary species of dinosaur, found initially in Southern Africa, but also on the coast of southern England, Argentina, western North America, and China (Sereno, 2012, p. 4) from the Early Jurassic Period. The name means Different-Toothed-Dinosaur. Discovered in 1962, it has since changed our knowledge of dinosaurs significantly. Heterodontosaurus tucki displayed a distinct set of teeth and jaw mechanisms leading to a better understanding of ornithischian functions generally.
Ornithischian functions throughout the latter half of the Mesozoic era, in the Jurassic and Cretaceous periods, are understood as being vastly observed over many body sizes. Ornithischian dinosaurs were those with specialized stomachs and organs for properly digesting their herbivorous plant matter. They dominated all other groups of herbivores, approximately 3:2 (Norman & Weishampel, 1985, p. 154). Het. tucki had an unusual jaw system: interlocking teeth; an elevated obtusely angular coronoid eminence, which allowed for a stronger jaw as it provided stability against stress; a synchronous shearing movement of the jaw, provided by a ventrally displaced jaw joint; and ball-and-socket mandibles, all of which allowed for a powerful bite (Norman & Weishampel, 1985, pp. 156-7). Het. tucki showcases an evolutionary stepping-stone, and a more supplementary view into how dinosaurs in the second half of the Mesozoic era lived and ate. Other such ornithischians such as hypsilophodontids, iguanodontids, and hadrosaurids had evidence of pleurokinetic hinges, or those which allow for transverse rotation in the upper jaw on either side of the skull during isognathic (having equal jaw width, evenly crushing food. Rigid and precise jaw closure) jaw closure. However, Heterodontosaurus tucki shows no evidence of this jaw closure, (Norman & Weishampel, 1985, pp. 155-6) thus distancing it from other ornithischians. This distinct jaw and teeth closing function not only gave Het. tucki its name, but additionally proved that early ornithischian dinosaurs were more complex and varied than previously thought.
Het. tucki can additionally be used as a comparison figure. If we compare this ornithischian with others, we can see the differences between the species. As its name suggests, Heterodontosaurus is marked by dental differentiation, such as a strong mandibular caniniform, three teeth in the premaxilla, 12 in the maxilla, with insufficient data on the mandible (He & Cai, 1984, p. 12). Certain specimens for Het. tucki, give us a clear vantage about both adult and juvenile skulls. Of these, there are three adult skulls, SAM-PK-K1332, SAM-PK-K337, and NM QR 1788 respectively, in various states of completion. There are two juvenile skulls, AMNH 24000 and SAM-PK-K10487 respectively, along with two partial maxilla—one left, SAM-PK-K1334, and one partial, SAM-PK-K1326. Heterodontosaurus has 11 to 12 dentary teeth which increase in size to the center of the tooth row (Sereno, 2012, pp. 6-7). This collection of specimens can give a deeper view into where other dinosaurs evolved from, and how the Het. tucki lived.
A skull discovered prior to 1913, incorrectly categorized initially as a synapsid (largest terrestrial vertebra in the Permian period. All mammals descend from this) but was later correctly known as a Het. tucki. The first formally and correctly identified specimen, found in 1962, was the holotypic skull and partial skeleton. The skull is relatively complete, though the postcranial remains appear to have been lost. From 1966-67, four additional skeletons were discovered not far from the initial specimen. These include the aforementioned anterior portion of the juvenile skull, fragmentary maxilla, a portion of the left maxilla and teeth from an adult skull, and the adult postcrania: vertebrae, partial pelvic girdle, and parts of hind and forelimbs. In 1975, a fifth specimen was discovered including a partial snout of a large (adult) Het. tucki, later identified correctly, initially identified as a sauropodomorph (related to Sauropoda clade, broad term encapsulating all of bipedal and quadpedial members) (Sereno, 2012, pp. 14-17).
A new specimen, discovered in 2021, called AM 4766 and nicknamed Tucky, provides a suite of features, unknown by other ornithischians. AM 4766 revolutionized the research and known content of Het. tucki. These unforeseen features provide more knowledge about the evolution of all dinosaurs evolved from Het. tucki, along with Heterodontosaurus itself. These features include a large, anteriorly projecting sternum, paddle-shaped sternal ribs, and a full gastral basket, which should be noted as the first recovered, in ornithischians (Radermacher et al., 2021, p. 1). This newfound information allows for more knowledge on the ventilatory, or respiratory systems in Het. tucki and other dinosaurs.
In this newfound specimen of Het. tucki, AM 4766, was manually prepared and X-rayed using a synchrotron radiation X-ray micto-computed tomography to reveal new and unexpected elements of Heterodontosaurus’s anatomy, unpreserved in other specimens. The researchers (Radermacher et al.) investigated the size and shape changes through the evolution of the ornithischian pelvic girdle (Radermacher et al., 2021, pp. 3-4). Their observations provided two hypotheses for this ornithischian breathing: the first being that Het. tucki had avian-like air sacs which did not invade the skeleton (similar to diving birds); the second hypothesis being the presence of a ventilatory method entirely different from other ornithodirans (Radermacher et al., 2021, p. 3). Modern day birds likely began evolving in the Triassic and Jurassic periods. Likely, their ancestor is the facultative (primarily quadpedial, but for short periods can be bipedal) Euparkeria from the south of Africa. The pubis of ornithischians and birds indicated a closer relationship between the two groups (Galton, 1970, pp. 448-49). The pubis of dinosaurs (ornithischians) relates to the ventilation systems of dinosaurs as it provided an anchor for breathing muscles.
Heterodontosaurus tucki has revolutionized our understanding of ornithischians. Its distinct teeth and jaw mechanisms: interlocking teeth, elevated obtusely angular coronoid eminence, shearing jaw movement, and ball-and-socket mandibles vary from the more typically found pleurokinetic hinges found on other ornithischians. These discoveries are important to the understanding of dinosaurs as they provide additional information to varied types of jaw functions, and a supplementary view into how dinosaurs in the second half of the Mesozoic era lived and ate.
References:
Butler, R. J., Porro, L. B., & Norman, D. B. (2008). A Juvenile Skull of the Primitive Ornithischian Dinosaur Heterodontosaurus tucki from the “Stormberg” of Southern Africa. Journal of Vertebrate Paleontology, 28(3), 702–711. http://www.jstor.org/stable/20490997
Galton, P. M. (1970). Ornithischian Dinosaurs and the Origin of Birds. Evolution, 24(2), 448–462. https://doi.org/10.2307/2406818
He, X., & Cai, K. (1984). The Middle Jurassic dinosaurian fauna from Dashanpu, Zigong, Sichuan: Vol. I. The ornithopod dinosaurs (W. Downs, Trans.). Sichuan Scientific and Technological Publishing House. https://www.naturalhistory.si.edu/sites/default/files/media/translated_publications/He%26amp%3BCai_84.pdf
Norman, D. B., & Weishampel, D. B. (1985). Ornithopod Feeding Mechanisms: Their Bearing on the Evolution of Herbivory. The American Naturalist, 126(2), 151–164. http://www.jstor.org/stable/2461504
Radermacher, V. J., et al. (2021). A New Heterodontosaurus Specimen Elucidates the Unique Ventilatory Macroevolution of Ornithischian Dinosaurs. eLife. https://elifesciences.org/articles/66036
