Tuesday, November 19, 2013

[Paleontology • 2014] ชาละวัน ไทยแลนดิคัส • Chalawan thailandicus • A Large pholidosaurid in the Phu Kradung Formation of north-eastern Thailand; former 'Sunosuchus' thailandicus


Chalawan thailandicus (Buffetaut & Ingavat 1984)
Chalawan gen. nov. Martin, Lauprasert, Buffetaut, Liard & Suteethorn 2013
syn: Sunosuchus thailandicus Buffetaut & Ingavat 1984

ชาละวัน ไทยแลนดิคัส Chalawan thailandicus
ซูโนซูคัส ไทยแลนดิคัส Sunosuchus thailandicus


Abstract
In the early 1980s, the remains of a large crocodilian, consisting of a nearly complete lower jaw, were referred to a distinct species of Sunosuchus, S. thailandicus. The specimen was recovered from a road-cut near Nong Bua Lamphu, north-eastern Thailand, in the upper part of the continental Phu Kradung Formation, and then considered Early to Middle Jurassic in age. Since then, this age has been revised and most of the formation is now considered Early Cretaceous, although a Late Jurassic age is possible for its lowermost part. Here, we report for the first time cranial elements associated with mandibular remains assignable to ‘S’. thailandicus. An attribution to Pholidosauridae is proposed on the basis of premaxillary morphology, and the original referral of this taxon to the goniopholidid Sunosuchus is discarded. A new genus name Chalawan now designates the originally described material of S. thailandicus. Nevertheless, the newly described specimen shares a characteristic with both ‘traditional’ Goniopholididae and Pholidosauridae: the presence of a depression located on the lateral wall of the maxilla and jugal. A phylogenetic analysis confirms the inclusion of both Goniopholididae and Pholidosauridae into a common clade, Coelognathosuchia tax. nov. Although the new Thai skull is much fragmented, its original shape is reconstructed and is compared with other pholidosaurid genera, namely Elosuchus, Meridiosaurus, Oceanosuchus, Pholidosaurus, Sarcosuchus and Terminonaris. The presence of the genus Sunosuchus being highly questionable in Thailand, it cannot be used as evidence to link the Chinese and Indochinese blocks. Instead, the recognition of a freshwater pholidosaurid in a continental formation of the Indochinese block suggests that early in their evolutionary history, these crocodilians, already known from Europe, Africa and South America, were more widely distributed along the northern margin of the Tethys than previously recognized.

Keywords: crocodilia; Coelognathosuchia; Pholidosauridae; Mesozoic; Phu Kradung Formation; Thailand


SYSTEMATIC PALAEONTOLOGY

This published work and the nomenclatural act it contains have been registered in Zoobank: http://zoobank.org/References/EC1507EB-0C95-48A0-B675-56141C9393FC

Order CROCODILIA Gmelin, 1789

Suborder COELOGNATHOSUCHIA tax. nov.

Derivation of name. In reference to the depression or concavitylocated on the posterodorsal surface of the maxilla, close or par-ticipating to the jugal. The name translates from the Greek jοιkος (concave), cmάhος (jaw) andrο~tvος (crocodile).

Diagnosis. Coelognathosuchia is characterized by the fol-lowing combination of features: presence of a depressionon the posterodorsal region of the rostrum (autapomorphic), located either fully on the maxilla or on both themaxilla and jugal; subcircular supratemporal fenestrae larger than orbits; maximum frontal width as wide or widerthan the diameter of one orbit; anterior process of nasal excluded from posterior margin of external nares; in dorsalview, marked notch at the premaxillary–maxillary junction;reduced or absent antorbital fenestra; more or less developed spine at anterolateral corner of postorbital; exoccipitaland quadrate not in contact posterior to otic area; pendu-lous quadrate medial hemicondyle; posteroventrally pro-jected retroarticular process; insertion for m. pterygoideusposterior restricted to medial margin of angular.

Definition. The sister group to the most derived neosuchians (Bernissartia fagesiiand Eusuchia), incorporatingthe family Pholidosauridae and all taxa more closelyrelated to it than to Bernissartia fagesiiand Eusuchia. Under the present phylogenetic hypothesis, the family Goniopholididae is paraphyletic.


Family PHOLIDOSAURIDAE Zittel and Eastman, 1902 

Derivation of name: ชาละวัน, Chalawan, a gigantic crocodile in the epic story of the crocodile hunter Khrai Thong written by King Rama II (1768–1824).

Type species: Sunosuchus thailandicus Buffetaut and Ingavat, 1980; CAS42-20 in Sirindhorn Museum, Kalasin province (formerly TF1370 in DMR, Bangkok); a nearly complete mandible lacking only a part of the right ramus from the Phu Kradung Formation near Nong Bua Lamphu, north-eastern Thailand.

Referred specimen: PRC102-143, Skull and mandibular elements of a single individual comprising parts of the rostrum, braincase and skull table as well as various parts of the mandible from Kham Phok, Mukdahan Province.

Type locality: Nong Bua Lam Phu, Nong Bua Lam Phu Province.

Type stratum: Phu Kradung Formation.


Jeremy E. Martin, Komsorn Lauprasert, Eric Buffetaut, Romain Liard and Varavudh Suteethorn. 2013. A Large pholidosaurid in the Phu Kradung Formation of north-eastern Thailand. Palaeontology. DOI: 10.1111/pala.12086

Buffetaut, E.; and Ingavat, R. 1984. The lower jaw of Sunosuchus thailandicus, a mesosuchian crocodilian from the Jurassic of Thailand. Palaeontology. 27 (1): 199–206.

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