It's too easy to think "ichthyosaur" and automatically conjure an image of a vaguely dolphin-shaped Mesozoic reptile, represented by a famous member of the group like Ichthyosaurus or Opthalmosaurus. It becomes just another role-player in the Mesozoic seas that now exist solely in our imaginations. But these were living animals, important members of lost ecosystems. Like a host of other extinct organisms, the deeper scientists look, the more nuanced and fascinating they become.
New research published in PLoS ONE demonstrates just how much you miss if you arrive at a superficial description of these animals and stop there. The study, by the University of Bonn's P. Martin Sander and Xiaohong Chen, Long Cheng, and Xiaofeng Wang of the Wuhan Institute of Geology and Mineral Resources in Hubei, China, takes a look at new fossil material representing the genus Shastasaurus, as well as a reevaluation of S. pacificus.
The shastasaurs are an odd offshoot of the ichthyosaur family, with short, slender snouts that appear to be truly toothless. While not entirely unknown in the ichthyosaur fossil record, toothlessness this plainly obvious is a unique characteristic of Shastosaurus: the upper surface of the Shastosaurus dentary, one of the lower jawbones, is smooth with no trace of grooves or pits for teeth. In others, toothlessness may be a mere artifact of poor preservation.
In seeking an explanation for these strange features, the authors draw a comparison to a living family of the ocean dwellers, the beaked whales of the family Ziphiidae, which have reduced dentition and similarly proportioned snouts as well as enlarged hyoid bones. In the PLoS ONE paper, the team writes, "the ziphiid whales are suction feeders in which the tongue is pulled back rapidly by strong retractor muscles that are attached to hypertrophied hyoid bones. In this way, prey items are taken up by the negative pressure generated in the oral cavity, obviating the need for teeth to hold them before swallowing."
Cuvier's Beaked Whale, shared by Navy Currents Magazine on Flickr.
The Ziphiids are a poorly understood family overall, with only a few species having been the subject of study. One thing we do know is that they are some of the whale family's gold medal divers, able to hunt deep water cephalopods and fish and crustaceans from near the sea floor, sometimes as deep as 500 meters. The authors paint a picture of Triassic oceans populated by a diverse range of ichthyosaurs including swift-swimming Shastasaurus in the open ocean, diving deep to prey on bioluminescent cephalopods, among other benthic delicacies. They suggest that as oxygen levels rose at the end of the Triassic, gill-breathing fish were able to supplant many of the creatures the ichthyosaurs relied on, instigating the dramatic decrease in ichthyosaur diversity in the Jurassic, presaging the family's eventual extinction. Except, naturally, in the imagination of a bizarre primate millions of years later.
What I'm getting at here is... let's see some Shastasaurus hunting glowing cephalopods, paleoartists!