Platypterygius australis, an Australian Cretaceous ichthyosaur



Sufficient material has been assembled to restore the powerful pectoral girdle and large fore limbs of Platypterygius australis (M'Coy). A reasonable approximation to the line of action of dorsal and ventral muscles which principally affected the trim of the low-set fin-blades was along the middle of the fin. Increased tension brought the fin-blades nearer horizontal (to a diving position) and varying amounts of relaxation during forward movement allowed mainly water resistance to increase the tilt. The flexible blade edges were crucial in this. Unequal tension on the blades would have caused turning of the animal with whatever rapidity was desired. Allowing both fin-blades to rise together to stalling point could have checked the animal abruptly as it struck prey. Large relative and absolute increases in the portions of the coracoids adjacent to the median symphysis highlight the relative development of young individuals. Positive allometric growth in this area stopped quite suddenly as the animals approached 6 m total length, leaving only general size increase.