Monday, January 29, 2018

[Paleontology • 2017] Pectodens zhenyuensis • A New Diapsid from the Middle Triassic of southern China


Pectodens zhenyuensis 
Li, Fraser, Rieppel, Zhao & Wang, 2017

   DOI: 10.1017/jpa.2017.12 

Abstract
The Middle and early Late Triassic of southern China is well known for a remarkable diversity of marine vertebrates, particularly reptiles, including an abundance of intriguing new forms (e.g., Jiang et al., 2005; Hu et al., 2011; Li et al., 2016). Here we describe a new diapsid from Yunnan Province. It possesses an elongate neck that exhibits a remarkable similarity to that of many Protorosauria, yet in other respects the skull and postcranium are much less derived.

The new taxon is part of the so-called Panxian-Luoping Fauna and the deposits correspond to the Upper Member of the Guanling Formation, comprising thin to medium bedded, gray to dark-gray laminated marly limestone and limestone, with several layers of bentonite intercalated in the fossil level at Panxian (Wan, 2002; Motani et al., 2008; Jiang et al., 2009). Their age is Pelsonian (middle Anisian, Middle Triassic) as is indicated by the conodont Nicoraella kockeli Zone (Sun et al., 2006; Zhang et al., 2009). A recent U-Pb study indicates the absolute age of these middle Anisian beds to be close to 244 Ma (Wang et al., 2014).


  Life restoration of Pectodens zhenyuensis
by Yu Wang. 

Figure 1: Pectodens zhenyuensis n. gen. n. sp., Photograph of the holotype IVPP V18578. 

Systematic paleontology
Reptilia
Archosauromorpha von Huene, 1946
?Protorosauria Huxley, 1871

Family incertae sedis

Genus Pectodens new genus
Type species: Pectodens zhenyuensis n. gen. n. sp. by monotypy.

Etymology: From the Latin pecto meaning to comb and dens meaning teeth; in reference to the comb-like nature of the marginal dentition.

Occurrence: Luoping County of Yunnan Province, China; Member II of the Guanling Formation, Anisian, Middle Triassic.

Pectodens zhenyuensis new species

Holotype: IVPP V18578. Almost complete articulated skeleton.

Etymology: In honor of Zhenyu Li, who contributed greatly to the collection of the specimen from the field.

....


Conclusions: 
A new, small terrestrial tetrapod is described from the Middle Triassic of Yunnan, China. Pectodens zhenyuensis n. gen. n. sp. bears very characteristic elongate teeth forming a comb-like marginal dentition. The elongate cervicals of Pectodens zhenyuensis n. gen. n. sp. with low neural spines together with the morphology of the cervical ribs are features consistent with protorosaurs, such as Macrocnemus. However, the imperforate puboischiadic plate, simple rounded proximal tarsals, and a straight 5th metatarsal are primitive characteristics. A key protorosaurian character is the long neck with elongated cervical ribs that typically extend across intervertebral articulations. It was mostly on the basis of these characters that Dinocephalosaurus from the Middle Triassic of China was referred to the protorosaurs (Li, 2003). Another long-necked form, Fuyuansaurus, also exhibits certain affinities with protorosaurs, in particular the tanystropheids (Fraser et al., 2013). Yet both taxa also display characters that are inconsistent with at least the tanystropheids. Unlike tanystropheids, but in common with Protorosaurus (personal observation, N.C. Fraser, 2013), both lack a thyroid fenestra in the pelvis.

The Middle Triassic vertebrate faunas of southern China are largely dominated by marine reptiles and fishes, but occasional terrestrial components (e.g., Macrocnemus fuyuanensis) are recovered from localities in the Zhuganpo Member of the Falang Formation (Li et al., 2007; Jiang et al. 2011). Likewise, Pectodens zhenyuensis n. gen. n. sp. exhibits no adaptations for an aquatic lifestyle; instead the long, slender limbs with pronounced articular ends, and elongate digits together with the claw-like distal phalanges speak to an entirely terrestrial existence. No fully terrestrial vertebrates have been documented previously from the Panxian-Luoping Fauna, although the archosaur Qianosuchus mixtus exhibits a combination of terrestrial and aquatic characteristics (Li et al., 2006). Pectodens is therefore the first fully terrestrial reptile reported from the Guanling Formation. The occurrence of terrestrial reptiles such as Macrocnemus and Pectodens are indicative of the proximity of the ancient coastline at the localities where they occur.


Chun Li, Nicholas C. Fraser, Olivier Rieppel, Li-Jun Zhao and Li-Ting Wang. 2017. A New Diapsid from the Middle Triassic of southern China. Journal of Paleontology. 91(6); 1306-1312.   DOI: 10.1017/jpa.2017.12


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