Middle Triassic mixosaurid ichthyosaurs from SW China

Abstract

Mixosaurid ichthyosaurs (mixosaurs) are a dominant group of Middle

Triassic marine reptiles that was widely distributed in both Tethyan and

Panthalassic terranes. They are represented by many excellently preserved

specimens from all around the world and have been studied for more than one

hundred years. Even so, their morphology is not adequately known. The

systematics of mixosaurs is not well understood and their phylogeny has been

controversial. The nomination of several Chinese mixosaurs makes the situation

even worse. This thesis aims to provide a comprehensive review of Chinese

mixosaurs and discuss their phylogenetic relationships.

The detailed anatomy of only valid Chinese mixosaurid species Mixosaurus

panxianensis is well established in this thesis. Its ontogenetic changes are also

noted and a better diagnosis is given. A new morphotype of mixosaurs from a

recently discovered fossil Lagerst?tte, the Luoping biota, is described in detail.

This morphotype is similar to M. panxianensis but consistent differences are also

present. The ecological comparison to M. panxianensis demonstrates that this

morphotype represents a new taxon that is different from M. panxianensis. A

nearly complete and articulated specimen of Phalarodon atavus from the Middle

Triassic Luoping biota, Yunnan, South China is described. This is the first

specimen of P. atavus discovered outside the Germanic Basin. The discovery of

this specimen demonstrates a cosmopolitan distribution of P. atavus across the

whole Tethyan ocean. The new specimen is also the first one preserving the

postcranial anatomy of this species, providing the potential to evaluate its

swimming ability for the first time. Functional morphology shows that this

species is well adapted for a pursuit attack of prey, consistent with the

distribution pattern of the species. Tooth crown morphology suggests that P.

atavus may prefer externally soft prey such as belemnoids.

Mixosaurs are divided into three ontogenetic stages based on the humeral

morphology. The juvenile stage is characterized by a textured dorsal humeral

shaft surface. The subadult stage is characterized by a smooth humeral shaft in

dorsal view with a rugose humeral head. The adult stage is characterized by a

smooth humeral head. The consistent presence of the surface texture of ventral

humeral shaft is most probably caused by the strong muscle insertion presumably

related to the function of steering of the fore fins in mixosaurs.

Five morphotypes of Chinese mixosaurs are recognized in this study.

Twenty-five more or less new characters are introduced to evaluate the

phylogenetic relationships of mixosaurian-like specimens from China. The

monophyly of mixosaurs including all studied Chinese morphotypes is well

corroborated. Further investigation on the mixosaurs from Monte San Giorgio is

recommended to resolve the phylogenetic relationships of mixosaurs.