Irisosaurus yimenensis – New Species of “Core Prosauropod” from Southern China

Researchers writing in the open access journal Scientific Reports have described a new species of Early Jurassic herbivorous dinosaur from China.  The animal, named Irisosaurus yimenensis is known from fragmentary remains and demonstrates a suite of unusual anatomical features that suggests this genus is a member of the Sauropodiformes, between early-branching “core prosauropods” and the late-branching “sauropod-like” members of the non sauropodan sauropodomorphs.  The fossilised remains come from the Fengjiahe Formation close to the village of Zhanmatian in Yunnan province.

A Life Reconstruction of Irisosaurus yimenensis

Irisosaurus yimenensis illustration.
Irisosaurus yimenensis life reconstruction.  This herbivorous dinosaur is thought to have been bipedal, measuring approximately 5 metres in length and from the Early Jurassic of China (Pliensbachian faunal stage of the Early Jurassic).

Picture credit: Ang Li

Fossil Material Found in 2018

The researchers which include Claire Peyre de Fabrègues of Yunnan University analysed the partial skeleton and concluded that Irisosaurus has a body plan close to that of the so-called “core prosauropods” in having, for instance, cervical vertebrae longer than most dorsal vertebrae, gracile forelimbs which are much shorter than the hindlimbs, a deltopectoral crest extending half of the total length of the humerus and a unique carpal-metacarpal complex.  The fossil material has been dated to approximately 185 million years ago (Pliensbachian faunal stage of the Early Jurassic), it was discovered in 2018.  A phylogenetic analysis confirmed that Irisosaurus belongs to non-sauropodan sauropodomorphs and places it as the sister taxon to Mussasaurus which lived more than thirty million years early in Argentina.

Phylogenetic Analysis places Irisosaurus as the Sister Taxon to the South American Mussaurus

Irisosaurus phylogeny.
Phylogenetic analysis places Irisosaurus as the sister taxon to the South American Mussaurus

Picture credit: Scientific Reports

On the Road to the Sauropoda

The Sauropodomorpha is defined as saurischian dinosaurs that incorporate the Sauropoda and the early diverging sauropodomorphs.  The earliest sauropodomorphs are known from the Late Triassic, with most of the described species from Gondwana.  The Laurasian record of these types of dinosaurs does consist of some Triassic forms, but the majority of the known species date from the Jurassic.  Among the fourteen Jurassic non-sauropodan sauropodomorphs from Laurasia described to date, eight are from China.  Nearly all of the non-sauropodan sauropodomorph genera currently known from China were first reported from the Lufeng Formation, which is also associated with the Yunnan province.  The Fengjiahe Formation is the southern equivalent of the Lufeng Formation, it has revealed a similar ancient dinosaur dominated biota.

Silhouette of Irisosaurus yimenensis with Some of the Fossil Material

Irisosaurus yimenensis outline and fossil material.
Silhouette of Irisosaurus yimenensis with some of the fossil material.  The most informative elements are figured; (a) Outline; (b) Tooth; (c) Left maxilla; (d) Middle cervical; (e) Posterior cervical; (f) Anterior dorsal; (g) Middle dorsal neural spine; (h) Right scapula; (i) Right humerus; (j) Right ulna; (k) Right manus.  Scale bars = 1 m (a);1 cm (b); 5 cm (c–g); 10 cm (h–k).

Picture credit: Scientific Reports

What’s in a Name (Etymology)?

The generic name refers to the famous iridescent clouds of Yunnan Province (彩云之南). The specific epithet refers to Yimen County, where the type locality is located

The scientific paper: “A new species of early-diverging Sauropodiformes from the Lower Jurassic Fengjiahe Formation of Yunnan Province, China” by Claire Peyre de Fabrègues, Shundong Bi, Hongqing Li, Gang Li, Lei Yang and Xing Xu published in Scientific Reports.

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