Rhabdodontidae

Comparison of rhabdodontid dentaries

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.[5] 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]

Adult body sizes of Mochlodon, Zalmoxes and Rhabdodon.

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.[6] Second, the fossil record contains three genera of rhabdodontids – Mocholdon, Zalmoxes, and Rhabdodon – that make up two geographically separated lines in the archipelago.

Traditionally, it has been thought Mochlodon and Zalmoxes were insular dwarfs. The smaller sizes of Mocholodon (1.6 to 1.8 m) and Zalmoxes (2.0 to 2.5 m) relative to Rhabodon (5.0 to 6.0 m)[6] and the island locale that all three genera shared led to the hypothesis of island nanism in the case of the former two. However, Ősi et al. (2012) proposed that Rhabdodon underwent gigantism on the mainland, as opposed to Zalmoxes and Mochlodon experiencing nanism on island habitats.[3] Ösi and colleagues used femur length to estimate body size through evolution in the rhabdodontid lineage. The conclusion was that in the eastern lineage comprising Zalmoxes and Mochlodon, the size ranges of both were too close to that of the ancestral rhabdodontid to support the hypothesis of nanism. Ösi and colleagues instead came to the conclusion that Rhabdodon in the western lineage is a case of gigantism in the group Rhabdodontidae.[3]

Rhabdodontids and Nodosaurids were overtaken by Hadrosaurids as the dominating herbivorous dinosaur group during the Maastrichtian period, 72.1–66.0 million years ago, of the Late Cretaceous. Titanosaurs coexisted with all of these groups throughout the Maastrichtian. Transylvania is the only location known thus far to have had Rhabdodontids, Nodosaurids, Titanosaurs, and Hadrosaurids all coexisting together.[6]

The Late Cretaceous is characterized by a warm temperate climate that extended to the poles, elevated sea levels, and inland seas. With a dentition adapted to shearing vegetation, members of the family Rhabododontidae were well-suited for life in the lush, vegetative environment at middle latitudes.

• Rhabdodontidae at TaxonSearch