May 25, 2011
Everybody knows that chewing your food carefully is part of good table manners. No one told that to Shastasaurus. This 27-foot marine reptile was probably a suction feeder that slurped up little cephalopods in the Late Triassic seas.
Shastasaurus was not a dinosaur. Instead, this creature was an ichthyosaur, a member of a group of fish-shaped marine reptiles that became beautifully adapted to a life spent entirely at sea. Thanks to new specimens found in the 228- to 216-million-year-old strata of China, paleontologists P. Martin Sander, Xiaohong Chen, Long Cheng and Xiaofeng Wang have discovered that Shastasaurus differed from the rest of its family in a strange way. Whereas most other ichthyosaurs had long snouts filled with small, conical teeth suited to snatching fish and cephalopods, Shastasaurus had a shortened, toothless maw.
Sander and colleagues reported their findings in the journal PLoS One earlier this week. Although several species of Shastasaurus are already known from China, British Columbia and the western United States, the new study is based on fossils previously described under the name Guanlingsaurus liangae. These fossils, it turned out, were actually another species of Shastasaurus, and the specimens illustrate that the skull anatomy of this ichthyosaur was different than previously supposed.
In Richard Hilton’s 2003 book Dinosaurs and Other Mesozoic Reptiles of California, for example, two Shastasuaurus species were reconstructed with the long, toothy snouts typical of other ichthyosaurs. Since the complete snouts of these North American species were unknown, and partial fossils assigned to Shastasaurus from Mexico and Canada seemed to indicate they were long-snouted, the ichthyosaur was given the usual, toothy profile. As Sander and co-authors point out, though, it is now thought that those long-snouted fossils don’t belong to Shastasaurus at all, and the specimens from China indicate that Shastasaurus had a short snout devoid of teeth.
Naturally, this revised skull shape has implications for the way Shastasaurus fed. Modern-day beaked whales appear to be good analogs. Much like Shastasaurus, beaked whales have short skulls which, with the exception of one or two pairs of small teeth in the lower jaw, are functionally toothless. Rather than biting down on food, these whales rapidly retract their tongue, creating a small pocket of suction that draws in small prey. Since Shastasaurus has a generally similar skull anatomy, as well as equivalent sites for muscle attachments that would have allowed them to perform similar lingual maneuvers, Sander and colleagues propose that the ichthyosaur was adapted to be a suction feeder many, many millions of years before whales.
After revising the anatomy and habits of Shastasaurus, Sander and co-authors also suggest that the existence of multiple, suction-feeding ichthyosaur species over the course of millions of years during the Late Triassic indicates some underlying environmental cause. The scientists note that levels of atmospheric oxygen dropped during the time of Shastasaurus. Fish populations, strangled by the reduced oxygen in the seas, may have declined as a result, but cephalopods like squid—which are more tolerant of low-oxygen environments—may have proliferated. Since suction-feeding appears to be an adaptation to consuming small, quick prey, and soft-bodied cephalopods are known to have been an important part of the ichthyosaur diet, the scientists hint that the evolution of Shastasaurus might be attributable to a boom in squid which was itself caused by a decrease in ocean oxygen levels. This hypothesis is not delineated in detail and relies on assumptions about large-scale evolutionary patterns, though, and testing it will require detailed studies of the prehistoric atmosphere, Triassic cephalopods, prehistoric fish and ichthyosaurs.
Regardless of the impetus for the evolution of Shastasaurus, the recognition that this animal was a suction-feeder adds to the diversity of ichthyosaur types known to have existed during the Triassic. There were crushers, cutters and squid-suckers, all filling different ecological roles when the seas were very different. Some whale species occupy some of the same ecological roles today, and in the way they swim and feed, they are fuzzy echoes of a long-lost Triassic past.
Sander, P., Chen, X., Cheng, L., & Wang, X. (2011). Short-Snouted Toothless Ichthyosaur from China Suggests Late Triassic Diversification of Suction Feeding Ichthyosaurs PLoS ONE, 6 (5) DOI: 10.1371/journal.pone.0019480
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