Investigations regarding the early evolutionary history of bird-line archosaurs have undergone significant advancements in recent years. However, controversies remain, especially regarding the phylogenetic position of “silesaurids”. Whereas some hypotheses place these archosaurs as the sister-group to dinosaurs, others suggest that “silesaurids” are early ornithischian dinosaurs. Despite the phylogenetic disputes, “silesaurids” are a crucial group for comprehending the origins and early evolution of dinosaurs. In the present study, a new “silesaurid” from the Middle-Upper Triassic of Southern Brazil is described. Gondwanax paraisensis gen. et sp. nov. comes from the Dinodontosaurus Assemblage Zone of the Santa Maria Formation (Ladinian-Carnian boundary), representing one of the oldest “silesaurids” worldwide. Furthermore, Gondwanax paraisensis gen. et sp. nov is the oldest “silesaurid” with three sacral vertebrae, a feature usually observed in more derived forms. In addition, distinct from any other Triassic Pan-Aves, the new taxon bears an incipient fourth trochanter of the femur. This unique combination of features indicates a high diversity of locomotor strategies among early pan-avians. The co-occurrence of Gondwanax paraisensis gen. et sp. nov. and Gamatavus antiquus in the same Assemblage Zone represents the oldest evidence of sympatry among “silesaurids” in South America. Indeed, the unique combination of sacral and hindlimb features may suggest distinct behaviors for these species, potentially leading to niche differentiation within the same ecosystems.

中文翻译：

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来自南美洲最古老的恐龙形目床的新“硅龙”为鸟系古龙的早期进化提供了见解


近年来，关于鸟系古龙早期进化历史的研究取得了重大进展。然而，争议仍然存在，尤其是关于 “silesaurids” 的系统发育位置。虽然一些假说将这些古龙视为恐龙的姊妹群，但另一些假说则认为“硅龙类”是早期鸟臀目恐龙。尽管存在系统发育争议，但“硅龙类”是理解恐龙起源和早期进化的关键群体。在本研究中，描述了来自巴西南部中上三叠纪的一种新的“硅龙类”。Gondwanax paraisensis gen. et sp. nov.来自圣玛丽亚组（拉丁语-卡尼安边界）的恐齿龙组合区，代表了世界上最古老的“硅龙类”之一。此外，Gondwanax paraisensis gen. et sp. nov 是最古老的“骶椎”，有三个骶椎，这一特征通常以更衍生的形式观察到。此外，与任何其他三叠纪 Pan-Aves 不同，新分类单元具有股骨的早期第四转子。这种独特的特征组合表明早期泛鸟类的运动策略高度多样化。Gondwanax paraisensis gen. et sp. nov.和同一组合区的 Gamatavus antiquus 代表了南美洲“硅龙类”中最古老的同源证据。事实上，骶骨和后肢特征的独特组合可能表明这些物种的不同行为，可能导致同一生态系统内的生态位分化。