Cephalopods are a type of marine mollusks. During the Ordovician period, they differentiated strongly in the shell type and internal structure of the shell.
Lituitid cephalopods are distinguished from other Ordovician cephalopods by their specific conch shape and ornaments. Most of them have a coiled or cyrtoconic part at the early growth stage, and then develop the uncoiled or fully straight conch at the late stage.
Recently, researchers led by Dr. FANG Xiang from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NIGPAS) and their collaborators conducted phylogenic analysis on Lituitida (Cepahlopoda) based on the large amount fossil materials from South China.
The study was published in Journal of Systematic Palaeontology.
As the index fossils in Middle to Late Ordovician, the classification and phylogeny of lituitids have been long debated. The study may help to reveal the phylogeny of Lituitida.
Combining with previously published materials, the researchers selected nine genera of order Lituitida, a total of 17 species-level taxa; and two outgroup units from orthocerids.
The researchers coded 24 characteristics and conducted further analysis. They found that there were three clades of lituitids, Sinoceras, Ancistroceras and Lituites; the former group corresponds to the family Sinoceratidae, and the latter two to the family Lituitidae.
"The topology shows that the Sinoceratidae represents the basal branch, while the Lituitidae represents a monophyletic, derived clade," said Dr. FANG.
Furthermore, the researchers described new materials of four species, three of which are newly defined, in four genera of the order Lituitida from the well-exposed, Middle to Upper Ordovician of Hubei, South China. Tyrioceras longicameratum from South China is the first reported occurrence of Tyrioceras in China.
The study was supported by Chinese Academy of Sciences Strategic Priority Research Program and National Natural Science Foundation of China.
