ABSTRACT

Acaenasuchus geoffreyi is a diminutive armored archosaur from the Upper Triassic Chinle Formation of northern Arizona, U.S.A., with uncertain evolutionary relationships and skeletal maturity. Known only from osteoderms, the taxon has been considered a valid taxon of aetosaur, juvenile specimens synonymous with the aetosaur Desmatosuchus spurensis, or a non-aetosaurian pseudosuchian archosaur. Here, we describe new fossils of Acaenasuchus geoffreyi that represent cranial, vertebral, and appendicular elements as well as previously unknown variations in the dorsal carapace and ventral shield. The skull bones are ornamented with the same anastomosing complex of ridges and grooves found on the paramedian and lateral osteoderms, and the appendicular skeleton resembles that of Revueltosaurus callenderi, Euscolosuchus olseni, aetosaurs, and other armored archosaurs such as erpetosuchids. Histology of osteoderms from the hypodigm of Acaenasuchus geoffreyi shows multiple growth lines, laminar tissue, and low vascularity, evidence that the individuals were close to skeletal maturity and not young juveniles. A revised phylogenetic analysis of early archosaurs recovers Acaenasuchus geoffreyi and Euscolosuchus olseni as sister taxa and members of a new clade that is the sister taxon of Aetosauria. This new phylogeny depicts a broader distribution of osteoderm character states previously thought to only occur in aetosaurs, demonstrating the danger of using only armor character states in aetosaur taxonomy and phylogeny. Acaenasuchus geoffreyi is also a good example of how new fossils can stabilize ‘wild card’ taxa in phylogenetic analyses and contributes to our understanding of the evolution of the aetosaur carapace.

摘要

Acaenasuchus geoffreyi是来自美国亚利桑那州北部上三叠统Chinle组的小型铠甲龙，具有不确定的进化关系和骨骼成熟度。仅从骨皮中得知，该分类单元被认为是有效的甲龙类，与甲龙Desmatosuchus spurensis同义的少年标本或非阿托索类伪类恐龙。在这里，我们描述的新的化石Acaenasuchus geoffreyi代表头颅，脊椎，四肢和元素以背壳，腹盾以及先前未知的变化。头骨的骨饰有相同的脊骨和沟槽的吻合复合体，位于脊骨的正中和外侧，而阑尾的骨骼类似于Revendertosaurus callenderi，Euscolosuchus olseni，甲龙和其他装甲龙，例如erpetosuchids。Acaenasuchus geoffreyi的亚种的成骨真皮组织学显示出多条生长线，层状组织和低血管性，这表明个体接近骨骼成熟而不是年轻的少年。修订的早期恐龙系统发育分析可恢复古猿（Acaenasuchus geoffreyi）和Euscolosuchus olseni作为姐妹分类单元和新进化枝的成员，这是埃托龙的姐妹分类群。这种新的系统发育描述了以前认为仅在甲龙中发生的成骨特征状态的广泛分布，这表明在甲龙分类学和系统发育中仅使用盔甲特征状态的危险。Acaenasuchus geoffreyi也是一个很好的例子，说明新化石如何在系统发育分析中稳定“野生卡”类群，并有助于我们了解甲壳动物的进化。