Rhabdodontidae
Rhabdodontidae is a family of herbivorous iguanodontian ornithopod dinosaurs whose earliest stem members appeared in the middle of the Lower Cretaceous. The oldest dated fossils of these stem members were found in the Barremian Castrillo de la Reina Formation of Spain, dating to approximately 129.4 to 125.0 million years ago.[1] With their deep skulls and jaws, Rhabdodontids were similar to large, robust iguanodonts. The family was first proposed by David B. Weishampel and colleagues in 2003.[2] Rhabdodontid fossils have been mainly found in Europe in formations dating to the Late Cretaceous.
The defining characteristics of the clade Rhabdodontidae include the spade-shape of the teeth, the presence of three or more premaxillary teeth, the distinct difference between the two maxillary and dentary teeth ridge patterns, and the uniquely shaped femur, humerus, and ulna.[1] This group of dinosaurs had a diet that consisted of angiosperms, herbaceous plants and fibrous plants.[3] Members of Rhabdodontidae have an adult body length of 1.6 to 6.0 meters.[4]
Rhabdodontids have a simple dentition with leaf-shaped teeth used for a powerful scissors-like shearing. These teeth are well-suited to a diet of tough and fibrous plants, examples of which include monocots, ferns and grass.[5] Each tooth has a ridge on it that is offset from the midline of the tooth. These ridges also have a specific pattern which is unique to Rhabdodontids: their dentary teeth have a central primary ridge with multiple equally spaced secondary ridges, and their maxillary teeth have no primary ridge and have similarly-sized secondary ridges.[1][4][5]
Unique characteristics are found in the femur, the humerus, and the ulna bones. The femur has a non-pendant, crested fourth trochanter.[1] The humerus lacks a proximal bicipital sulcus, and a concave border between the head and the deltopectoral crest.[1] The ulna has a large olecranon process.[1]
There are differing opinions as to the constituents of Rhabdodontidae. Originally they were defined as the last common ancestor of Zalmoxes robustus and Rhabdodon priscus.[2] Later, Paul Sereno proposed a new definition, the most inclusive clade containing Rhabdodon priscus but not Parasaurolophus walkeri.[6] More recently, a morphological diagnosis was proposed, that excluded Muttaburrasaurus, unlike Sereno's definition. The clade Rhabdodontomorpha was coined to contain the larger group.[1] The following cladogram was recovered by Dieudonné and colleagues in 2016:[1]
Rhabdodontids first appeared during the Barremian stage of the Early Cretaceous, and an extensive fossil record shows that they remained extant until the Maastrichtian stage at the end of the Late Cretaceous. During much of the Late Cretaceous, an isolated island habit in the western Tethyan archipelago contributed to the evolution of rhabdodontids in two main ways. First, the rhabdodontid dentition is relatively primitive, which is consistent with their habitat being sheltered from expansive mixing leading to a long period of dominance.[7] Second, the fossil record contains three genera of rhabdodontids – Mochlodon, Zalmoxes, and Rhabdodon – that make up two geographically separated lines in the archipelago.
