STUDY ON THE SWIMMING STYLES AND SWIMMING CAPABILITY OF TRIASSIC ICHTHYOPTERYGIANS

The swimming styles and capability of aquatic animals are important indicators of their behavior, physiology and locomotion mechanism. The Ichthyopterygia is a group of reptiles secondarily adapted to aquatic life. In order to understand its evolution and adaptation process, it is crucial to study its swimming styles and swimming capability. Over the past decade, abundant well-preserved ichthyopterygian materials have been found from the Triassic of South China. This provides convenience for the study of swimming among early ichthyopterygians. Species involved in this research cover all taxa from Early Triassic (Olenekian) to the Late Triassic (Carnian) on the level of family.

Applying the method of functional morphology, we analyzed the swimming styles and swimming capability of Triassic ichthyopterygians from South China based on data related to body shape and centrum shape. By comparing fineness ration and aspect ration with extant sharks, we revealed that Chaohusaurus swam in anguilliform, Xinminosaurus and Mixosaurus in carangiform, while Qianichthyosaurus, Guizhouichthyosaurus, Guanlingsaurus in thunniform. Our further study in relative centrum length (RCL) substantiated the identification of swimming styles through body shape. Moreover, it proved that at least Mixosaurus, Guanlingsaurus, Guizhouichthyosaurus and Qianichthyosaurus mainly relied on tail beat for locomotion. Taking advantage of formulas provided in the method of scaling effect, we calculated the parameters related to swimming capability of Triassic ichthyopterygians involved. Mixosaurus, Qianichthyosaurus, Guizhouichthyosaurus and Guanlingsaurus each had a swimming speed of 1.14 m/s, 1.29 m/s, 1.77 m/s and 2.09 m/s.

Triassic ichthyopterygians in swimming styles have completed the transformation from anguilliform to carangiform and to thunniform, while present a trend of improvement in their swimming capability. Changes in these two aspects of ichthyopterygians form a fine correlation with sea level fluctuation during this period. Therefore, we propose that the global regression emerging from the Middle Triassic serves as a key factor to motivate the evolution of Triassic ichthyopterygians swimming.