Pachysuchus in Yunnan

Nebulasaurus is an extinct genus of basal eusauropod dinosaur known from the early Middle Jurassic Zhanghe Formation(Aalenian or Bajocian stage) of Yunnan Province, China. It is known only from the holotype braincase LDRC-v.d.1. A phylogenetic analysis found Nebulasaurus to be a sister taxon to Spinophorosaurus from the Middle Jurassic of Africa. This discovery is significant paleontologically because it represents a clade of basal eusauropods previously unknown from Asia.

Etymology

The genus name Nebulasaurus, means “misty cloud lizard”, and is derived from the Latin word nebulae meaning “misty cloud”, a reference to “Yunnan” which means “southern misty cloudy province” and the Greek word “sauros” (σαυρος) meaning “lizard”[2] The specific name ‘’taito’’, was given in honor of the Taito Corporation of Japan, which funded the field project and is geographically near the discovery site. Nebulasaurus was described and named by Lida Xing, Tetsuto Miyashita, Philip J. Currie, Hailu You, and Zhiming Dong in 2015 and the type species is Nebulasaurus taito. Nebulasaurus was one of eighteen dinosaur taxa from 2015 to be described in open access or free-to-read journals.

Description

The only fossil material recovered was a braincase, which was in a good state of preservation.

Classification

Nebulasaurus was classified as a basal eusauropod, however its braincase bears resemblance to those of more derived neosauropods. Phylogenetic analysis showed that Nebulasaurus was most closely related to Spinophorosaurus. When compared to coeval sauropods, the discovery of Nebulasaurus demonstrates the diversity of sauropodomorph fauna in China during the Jurassic period.

Distinguishing anatomical features

A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism or group.

According to Xing et al. (2015), Nebulasaurus can be distinguished based on the following characteristics:

  • the exoccipitals are nearly excluding the supraoccipital from the foramen magnum (supraoccipital forms less than a tenth of the margin of the foramen magnum)
  • the supraoccipital is not expanded laterally between the parietal bone and the exoccipital.
  • the crista interfenestralis is incompletely partitioning the fenestra ovalis and the jugular foramen
  • the frontoparietal fenestra is located at the frontal-parietal suture and is larger than postparietal foramen[Note 2]
  • the craniopharyngeal foramen is posterior to the basal tubera

Paleoecology

Provenance and occurrence

The holotype specimen of Nebulasaurus taito LDRC-v.d.1 was recovered from the Zhanghe Formation, near Xiabanjing, in Yuanmou County of Yunnan Province, China. The specimen was collected in terrestrial sediments deposited during the Aalenian and Bajocian stagesof the Jurassic period, approximately 174 to 168 million years ago. This specimen is housed in the Lufeng Dinosaur Research Center in Yunnan Province.

Fauna and habitat

The Zhanghe Formation has produced the remains yielded one basal sauropodomorph Yunnanosaurus youngi, and two basal eusauropods Eomamenchisaurus yuanmouensis and Yuanmousaurus jiangyiensis. This diverse sauropodomorph faunal collection from the Middle Jurassic of Asia preceded the mamenchisaurid dominance that is observed in East Asia during the Late Jurassic.

Discovery and Species

Discovery

  • Pachysuchus was described by paleontologist Yang Zhongjian (also known as “C.C. Young”) in 1951.
  • It is known from a poorly preserved partial rostrum found in the Lower Lufeng Series in Yunnan.

Type Species

  • The type species is Pachysuchus imperfectus.
    • Generic Name: “Pachysuchus” translates to “thick crocodile.”
    • Specific Name: “Imperfectus” means “imperfect” in Latin.

Initial Identification

  • Yang initially identified the rostrum as belonging to a phytosaur, a long-snouted crocodile-like crurotarsan. Phytosaurs were common in the Triassic but thought to have gone extinct during the Triassic-Jurassic extinction event.

Reclassification and Doubts

Skepticism

  • The rostrum described by Yang has since been lost, and his initial identification has been questioned.
  • The poor preservation of the specimen and its presence in Jurassic beds cast doubt on it being a phytosaur.

Reclassification

  • In 2012, paleontologists Paul M. Barrett and Xu Xing relocated the holotype of Pachysuchus imperfectus, specimen IVPP V 40.
  • They identified it as belonging to a taxonomically indeterminate basal sauropodomorph rather than a phytosaur.
  • Barrett and Xu concluded that the holotype of Pachysuchus “does not bear any unique features or a unique character combination.” It differs from rostra of other Early Jurassic Chinese sauropodomorphs, but the differences could be attributed to poor preservation, making it a nomen dubium (a name of doubtful validity).

Significance

Scientific Importance

  • The case of Pachysuchus highlights the complexities and challenges of paleontological classification, especially with poorly preserved or incomplete fossils.
  • The initial misidentification and subsequent reclassification underscore the importance of careful examination and re-evaluation of fossil specimens as new techniques and knowledge become available.

Historical Context

  • Pachysuchus remains a part of the rich paleontological history of Yunnan Province, contributing to the broader understanding of dinosaur evolution during the Early Jurassic period.

Notes

  1. ^ This condition is also present in some neosauropods, but not present in Mamenchisaurus, Omeisaurus, Shunosaurus, and Spinophorosaurus, all of which are observed with a fully formed crista interfenestralis
  2. ^ This condition is absent from mamenchisaurids, which are observed to have both fenestrae absent altogether
  3. ^ A condition distinguishing it from Spinophorosaurus, where the foramen is anterior to the basal tubera