Ordovician Orthocerida and Pseudorthocerida (Cephalopoda: Nautiloidea) from the Lower Setul Limestone of the Langkawi Islands, Malaysia
The Lower Setul Limestone in the Langkawi Islands of Peninsular Malaysia is well known for its continuous succession for most of the Ordovician Period. It is part of an extensive carbonate platform in the West Malaya (=Sibumasu) Block that is considered to have been in low-latitude, northern Gondwana during the early Palaeozoic. Highly endemic cephalopods belonging to the two orders Orthocerida and Pseudorthocerida occur in the late Floian–early Sandbian (late Early–early Late Ordovician) sections of this limestone. Based on some 170 shells collected, 10 species out of six genera were identified: the four orthocerids, Malayorthoceras gracilentum gen. et sp. nov., Tofangoceras kedahense sp. nov., Tofangoceras staiti sp. nov. and Tofangoceras rayense sp. nov.; and the six pseudorthocerids, Andamanoceras densiseptum gen. et sp. nov., Langgunites mucronulatus gen. et sp. nov., Shanthaiceras amplum gen. et sp. nov., Sibumasuoceras langkawiense (Kobayashi, 1959), Sibumasuoceras kilimense sp. nov. and Sibumasuoceras scrivenori sp. nov. Tofangoceras, a typical Darriwilian–Sandbian element of the North China Platform, was found also to be abundant and diverse in the coeval strata of Malaysia. All of the Langkawi pseudorthocerids belong to Greenlandoceratidae, an ill-defined familial group that is here reviewed and emended comprehensively. The Greenlandoceratidae are the oldest Pseudorthocerida and occur predominantly in both South-East Asia and North China. Langgunites mucronulatus from the Floian is the earliest species of Pseudorthocerida and possesses some primitive features of the order. The poorly understood origin of Pseudorthocerida is revisited. Pseudorthocerida is traditionally believed to have split off from Orthocerida; however, we suggest an alternative view that it derived from a stock of Protactinoceratidae (order Plectoronocerida).
http://zoobank.org/urn:lsid:zoobank.org:pub:6303EBE2-8AE1-4BE2-8AA0-30650EB15AC1
