A Tangle Hard to Untie — Re-examining Theories of Ornithischian Phylogeny

Silesaurus opolensis (圖片來源：Szymon Górnicki，採用 CC BY-SA 4.0 授權)

In traditional dinosaur classification, researchers divide dinosaurs into two major branches according to pelvic structure: Ornithischia and Saurischia (the latter further divided into Theropoda and Sauropodomorpha). This framework was first proposed by Harry Seeley in 1888. However, alternative views on dinosaur classification have also been suggested.

One hypothesis grouped sauropodomorphs and ornithischians together within an order called Phytodinosauria (“plant dinosaurs”), while theropods were interpreted as a lineage that diverged earlier. Another proposal placed theropods and ornithischians in the same clade, Ornithoscelida (“bird-legged dinosaurs”), with sauropods representing an earlier-diverging independent lineage. As recently as 2017, research by Matthew Baron supported this latter interpretation. Even today, the phylogenetic position of Ornithischia within Dinosauria remains unstable. Studies continue to support all three of these competing classification schemes, although Seeley’s traditional framework remains the most widely adopted.

In recent years, some analyses have suggested that Silesauridae, a group from the Middle to Late Triassic, may actually represent an early branch of Ornithischia. Whether ornithischians already existed during the Triassic has long remained an unresolved question. For instance, Pisanosaurus from the Late Triassic was originally regarded by Rodolfo Casamiquela as the earliest known ornithischian dinosaur. Later re-examinations of the fossil material and additional phylogenetic analyses reassigned it to Dinosauriformes instead. Similarly, Revueltosaurus, also from the Late Triassic, was once classified as a primitive ornithischian but was later recognized as belonging to Pseudosuchia, the crocodile-line archosaurs. For this reason, some researchers believe that Ornithischia had not yet appeared during the Triassic.

A 2015 study introduced another complication: Chilesaurus, from the Late Jurassic of Chile, possesses features characteristic of both saurischians and ornithischians. It was interpreted as a potential transitional form linking the two groups. However, this interpretation created a major conflict with previously estimated divergence times, and debates over its pelvic structure left its classification uncertain. In 2022, Federico Agnolin re-evaluated anatomical features of the thorax, cervical vertebrae, ilium, and wrist bones, concluding that Chilesaurus is more closely related to theropods.

The internal relationships within Ornithischia have also been subject to revision. The family Heterodontosauridae has long been regarded as a basal ornithischian lineage and has been widely incorporated into phylogenetic analyses of dinosaurs and archosaurs. In 2020, Paul Dieudonné proposed placing Heterodontosauridae within Cerapoda. Some researchers argue that analyses conducted under such restrictive assumptions could strongly influence—or even mislead—the construction of classification systems. If heterodontosaurids indeed belong within Cerapoda, then Lesothosaurus and other early members of Genasauria would become key reference taxa in phylogenetic analyses, potentially leading to substantial changes in reconstructed evolutionary trees.

In 2020, Rodrigo Müller and Maurício Garcia proposed that Silesauridae might fill the transitional stage leading to early Ornithischia. In their analysis, silesaurids were incorporated into the base of Ornithischia as a paraphyletic group. This study pushed the divergence between Saurischia and Ornithischia further back in time, citing evidence from Middle Triassic (Anisian) silesaurid species such as Lutungutali and Asilisaurus. These findings suggest that the two main dinosaur lineages may already have separated during the Early Triassic. However, the study did not include early representatives of Thyreophora, Neornithischia, or Heterodontosauridae, reducing both the quantity and diversity of samples available for analysis.

The present study therefore builds upon those earlier analyses by incorporating additional taxa, including Laquintasaura, Scutellosaurus, Scelidosaurus, Emausaurus, Hexinlusaurus, Agilisaurus, Chilesaurus, and members of Heterodontosauridae, in order to clarify the evolutionary relationships among Ornithischia, Silesauridae, and Saurischia.

Phylogenetic tree (Norman, D. B., Baron, M. G., Garcia, M. S., & Müller, R. T. (2022))

Four analytical approaches were employed under different conditions:

1. Unconstrained analysis (no predefined phylogenetic restrictions).

2. Constrained analysis assuming Silesauridae as a monophyletic group.

3. Constrained analysis assuming Ornithoscelida (Theropoda + Ornithischia) as a monophyletic group.

4. Constrained analysis forcing Chilesaurus into Theropoda.

The results of the first, unconstrained analysis closely resemble the evolutionary tree proposed by Müller and Garcia. The main difference lies in a clade formed by Sacisaurus, Lutungutali, and Kwanasaurus, which together form a lineage that stands in a paraphyletic relationship with Eucoelophysis, Pisanosaurus, Laquintasaura, and Prionodontia. In addition, Laquintasaura appears as the closest sister group to Prionodontia. Interestingly, Heterodontosauridae is positioned within a more derived branch of Prionodontia, contrasting with the traditional view that treated it as a basal ornithischian lineage. In this analysis, Chilesaurus is placed within Heterodontosauridae.

The second and third analyses place Silesauridae outside Dinosauria, though their overall results are less stable. The fourth analysis largely mirrors the first and produces relatively stable results, providing partial support for Müller and Garcia’s hypothesis that Silesauridae occupies a basal position within Ornithischia.

The origin and evolutionary relationships of Ornithischia have undergone repeated revisions. Fossils from Triassic strata have yet to reveal specimens possessing definitive ornithischian characteristics, leaving the origin of the group unclear. Known silesaurid fossils lack the distinctive ornithischian pelvic structure, so researchers must rely on other skeletal features—and even ecological roles—to infer their relationship with ornithischians. In recent years, an increasing number of studies have supported the view that Silesauridae represents a basal group related to Ornithischia.

This study also places Heterodontosauridae within Neornithischia, while reviving the term Prionodontia, originally proposed by Richard Owen in 1874, to encompass Thyreophora and Neornithischia, including heterodontosaurids. In addition, the study establishes a new clade, Parapredentata, defined as the most recent common ancestor of Silesaurus opolensis and Iguanodon bernissartensis and all of its descendants. The revised classification framework for Ornithischia proposed here may help clarify the evolutionary relationships within the group in future research.

Author: Shui Ye-You

※Edited by #白稜 & #Chenyisaurus

參考文獻：

Norman, D. B., Baron, M. G., Garcia, M. S., & Müller, R. T. (2022). Taxonomic, palaeobiological and evolutionary implications of a phylogenetic hypothesis for Ornithischia (Archosauria: Dinosauria). Zoological Journal of the Linnean Society.