A new early-branching armoured dinosaur from the Lower Jurassic of southwestern China
Abstract
The early evolutionary history of the armoured dinosaurs (Thyreophora) is obscured by its patchily distributed fossil record and by conflicting views on the relationships of its Early Jurassic representatives. Here, we describe an early-diverging thyreophoran from the Lower Jurassic Fengjiahe Formation of Yunnan Province, China, on the basis of an associated partial skeleton that includes skull, axial, limb and armour elements. It can be diagnosed as a new taxon based on numerous cranial and postcranial autapomorphies and is further distinguished from all other thyreophorans by a unique combination of character states. Although the robust postcranium is similar to that of more deeply nested ankylosaurs and stegosaurs, phylogenetic analysis recovers it as either the sister taxon of Emausaurus or of the clade Scelidosaurus +Eurypoda. This new taxon, XXXX XXXX , represents the first valid thyreophoran dinosaur to be described from the Early Jurassic of Asia and confirms the rapid geographic spread and diversification of the clade after its first appearance in the Hettangian. Its heavy build and distinctive armour also hint at previously unrealised morphological diversity early in the clade’s history.
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Evaluation Summary:
This paper reports a new species of armored dinosaur from rocks in southwestern China dated to the beginning of the Jurassic Period. This represents the first valid species of armored dinosaur from the Early Jurassic in Asia, as although the presence of armored dinosaurs in Asia has been documented for decades based on isolated jaw bones referred to Thyreophora-the group of armored dinosaurs-none that material was complete enough for diagnosis to a known or new species. This new specimen demonstrates the rapid diversification and distribution of armored dinosaurs across the northern hemisphere early in their evolutionary history.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer …
Evaluation Summary:
This paper reports a new species of armored dinosaur from rocks in southwestern China dated to the beginning of the Jurassic Period. This represents the first valid species of armored dinosaur from the Early Jurassic in Asia, as although the presence of armored dinosaurs in Asia has been documented for decades based on isolated jaw bones referred to Thyreophora-the group of armored dinosaurs-none that material was complete enough for diagnosis to a known or new species. This new specimen demonstrates the rapid diversification and distribution of armored dinosaurs across the northern hemisphere early in their evolutionary history.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1 and Reviewer #2 agreed to share their name with the authors.)
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Reviewer #1 (Public Review):
The authors describe the partial articulated skeleton of a new armoured dinosaur from an osteological and taxonomical point of view. The finds are of particular importance both for the anatomical understanding of early armoured dinosaurs and for the paleogeographical aspects of the group. The authors achieved their aims, and their results support their conclusions. A detailed comparative study confirms the existence of a new taxon. Using the data matrix of Maidment et al. (2020) and Norman (2021), they determine the phylogenetic position of the new taxon, which clearly shows its basal position among the thyreophrans.
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Reviewer #2 (Public Review):
This is a well-constructed anatomical description of an exciting specimen of a new armored thyreophoran dinosaur species from the Early Jurassic of Yunnan, China, which the authors name Yuxisaurus kopchicki. For years, the presence of Early Jurassic thyreophorans in China has been inferred from isolated fragments ultimately deemed undiagnostic (the two previously named taxa "Bienosaurus" and "Tatisaurus" are now widely considered nomina dubia). Yuxisaurus kopchicki is therefore the first valid thyreophoran taxon from the Early Jurassic not only of China but also of Asia. Yuxisaurus is the easternmost occurrence of a thyreophoran from Lower Jurassic Laurasian strata, confirming the rapid spread and diversification of armored dinosaurs throughout the northern hemisphere early in their evolution.
The authors …
Reviewer #2 (Public Review):
This is a well-constructed anatomical description of an exciting specimen of a new armored thyreophoran dinosaur species from the Early Jurassic of Yunnan, China, which the authors name Yuxisaurus kopchicki. For years, the presence of Early Jurassic thyreophorans in China has been inferred from isolated fragments ultimately deemed undiagnostic (the two previously named taxa "Bienosaurus" and "Tatisaurus" are now widely considered nomina dubia). Yuxisaurus kopchicki is therefore the first valid thyreophoran taxon from the Early Jurassic not only of China but also of Asia. Yuxisaurus is the easternmost occurrence of a thyreophoran from Lower Jurassic Laurasian strata, confirming the rapid spread and diversification of armored dinosaurs throughout the northern hemisphere early in their evolution.
The authors added Yuxisaurus to two recent datasets to evaluate its phylogenetic relationships, both of which support its referral to Thyreophora; however, both datasets heavily emphasize thyreophoran taxa and are limited in taxon sampling overall, so I think adding Yuxisaurus to a dataset emphasizing basal taxa in other ornithischian clades would have strengthened the study given the apparently basal position of Yuxisaurus within Thyreophora. That being said, I think the included analyses are sufficient for what the authors were trying to achieve.
The Early Jurassic record of thyreophorans is limited to only a handful of taxa, mostly from Europe and the USA, so this new study is especially valuable for understanding the early evolution of armored dinosaurs and will surely be referenced heavily by dinosaur researchers in the future.
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