Field of Science

Diandongos​uchus, A New Archosaur from Triassic of China

There are lots of incredible specimens coming out of the Middle Triassic in recent years (SVP attendees will see another in a talk this year by Sterling Nesbitt and colleagues), and this amazing fossil from the Ladinian of China does not disappoint. It consists of most of a partially articulated/associated skeleton found in marine sediments not far from where the early diverging poposauroid  Qianosuchus mixtus was  recovered some time ago.

The authors have provided color figures that allow the reader to see many of the details of the skeleton; however, the semi-articulated nature of the specimen seems to prelude dismantling of the specimen to see three dimensional views of the specimen. Furthermore, because it is in a marine shale, some crushing of the specimen has occurred. This is unfortunate because it does not allow clear study of the ilia or of the ends of the femora, which would provide key characters for the taxonomic placement of the specimen. Li et al. do code the specimen into Sterling Nesbitt's recent (2011) phylogenetic matrix of Archosauriformes and it is recovered, with much support, as the earliest diverging poposauroid. This is of interest of course because it suggests that the plesiomorphic conditions for poposauroids are quadrupedal, toothed, long-snouted forms that lack an expanded dorsal sail, possess osteoderms, and are possibly semi-aquatic. Compare this to other members of the clade such as Effigia okeeffae. The amount of morphological variation in this group is incredible.

I'll admit that I'm not entirely convinced of the phylogenetic placement of this specimen, but unfortunately I cannot get into the details of why at this time. Let's just say that this is an extremely interesting taxon that should be the subject of debate for a while to come. I do really wish the ilia were better preserved or more visible, but it is really hard to complain about such a beautiful fossil.

Li, C., Wu, X.-C., Zhao, L.-J., Sato, T. and L.-T. Wang. 2012. A new archosaur (Diapsida, Archosauriformes) from the marine Triassic of China. Journal of Vertebrate Paleontology 32(5): 1064-1081. DOI:10.1080/02724634.2012.694383

Abstract - A new Middle Triassic archosaur, Diandongosuchus fuyuanensis, gen. et
sp. nov., is described on the basis of a skeleton from the Zhuganpo Member (Ladinian) of the Falang Formation, eastern Yunnan Province, China. It is primarily characterized by the nasal process of the premaxilla extending posteriorly well beyond the external naris, the super-sized coracoid foramen laterally bordered by the scapula, the ischium with a strongly expanded medial portion anteroposteriorly longer than the proximodistal height of the bone, and anteriorly notched cervical osteoderms. D. fuyuanensis is a pseudosuchian on the basis of the crocodile-normal tarsal joint and other features, such as the distal end of the ulna in posterolateral view squared off, osteoderms with a distinct anterior process, the presacral vertebrae dorsally covered by more than one osteoderm, dorsal osteoderm alignment dorsal to presacrals 10–24 staggered, the pubis-ischium contact reduced to a thin proximal contact, and the medial contact of the ischia extensive but the dorsal margins separate. It is from a marine deposit but shows few morphological adaptations of the postcranial skeleton for a semiaquatic way of life when compared with Qianosuchus from the Anisian limestone of the same area. A phylogenetic analysis derived from an existing data matrix suggests that the new archosaur occupies the basal-most position in Poposauroidea and further confirms the poposauroid status of Qianosuchus. On the basis of current information, the discovery of Diandongosuchus does not firmly underscore the affinity of the semiterrestrial vertebrate faunas between the eastern and western regions along the northern coastline of the Tethys.

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