Ichthyosaurs Didn’t Slurp Squid, After All

Ichthyosaurs didn’t suck. Not so far as paleontologists have been able to discern, anyway.

Let me back up a bit. Over 216 million years ago, in the seas of the Late Triassic, there swam immense, fish-like marine reptiles that paleontologists have named Shastasaurus. These 27 foot long ichthyosaurs have traditionally been illustrated with long jaws set with snatching, conical teeth typical of their close relatives, but in a 2011 study researchers P. Martin Sander, Xiaohong Chen, Long Cheng and Xiaofeng Wang found that Shastasaurus actually had a short, toothless snout.

Without teeth, how did Shastasaurus feed? Sander and coauthors thought suction-feeding might be the solution. Much like modern day beaked whales, the paleontologists proposed, Shastasaurus may have been able to retract its tongue with such speed to create a vacuum to slurp in unwary squid and other small prey. A handy skill to have when skewering fast-moving prey on toothy spikes isn’t an option. But, as it turns out, Shastasaurus probably wasn’t capable of such suction.

In a critical response paper, which includes Sander as an author, University of California, Davis paleontologist Ryosuke Motani and colleagues have scuttled the suction-feeding hypothesis for known ichthyosaurs. Shastasaurus and other ichthyosaurs just didn’t have the anatomical framework needed to inhale prey.

One of the tell-tale signs of a suction-feeder is an element in the throat called the hyoid. This bone can sometimes be made of cartilage, but in suction-feeders the element is a stronger anchor point that is made of the actual substance bone. Muscles attached to this bone allow beaked whales and other suction-feeders to quickly retract their tongues to create their feeding vacuum. But out of 18 species of Late Triassic and Early Jurassic ichthyosaurs examined for the study, only one was found to have a ossified hyoid bone. This one occurrence, in the ichthyosaur Hauffiopteryx, wasn’t strongly connected to the other parts of the throat apparatus thought to be important to suction feeding, nor did the ichthyosaur show other adaptations for the ability.

The shape of ichthyosaur jaws didn’t fit the suction-feeder profile, either. Compared to the jaws of beaked whales and sperm whales, ichthyosaur jaws did not become narrow enough quickly enough along their length to create a concentrated area of high suction. Ichthyosaur jaws were longer and more closely resembled those of sharks and cetaceans that open their jaws to engulf their prey in a behavior called “ram feeding.” Along with considerations of throat elements called the hyobranchial rods and the snout bluntness, Motani and collaborators found that ichthyosaurs did not possess the related features that allow suction-feeding to work.

If there were suction-feeding ichthyosaurs, paleontologists haven’t found them yet. The odds don’t look particularly good. The construction of the ichthyosaur jaw, Motani and coauthors point out, puts their jaw joint far so far back on the skull that they had to have a relatively long jaw compared to their snout length. The way their bones were arranged may have prevented them from becoming suction feeders.

Then again, suction-feeding has independently evolved several times among aquatic animals. Perhaps, at some point in their history, some ichthyosaurs did, too. While the new study has contradicted the proposal for known Triassic and Early Jurassic ichthyosaurs, the paper has also outlined a list of expected features than can be used to pick out possible suction-feeders. And there are other marine reptiles to test. Prehistoric marine crocodiles and turtles have been proposed as suction-feeders from superficial evidence, but whether or not such creatures possessed the essential traits for drawing in prey hasn’t been studied in detail.

Figuring out who sucked and who didn’t isn’t just a matter of paleontological profiles. Suction-feeding is a specialized way of hunting that relies on slow-moving prey that often aggregates in the deep sea. That’s because suction only works in close quarters – only at distances of a bit more than two inches, Montani and collaborators note. For a suction-feeder to evolve there must be a ready supply of relatively sluggish prey that the predator can almost touch before drawing them in.

While future fossil discoveries could show that there was plenty of slow and sessile food in the deep sea by the Late Triassic, no such evidence has been found yet. The lack of suction-feeding ichthyosaurs matches this part of the record. Not that ichthyosaurs were especially skilled at reaching such depths at the time. Triassic ichthyosaurs lacked the hallmarks of a deep diving lifestyle, such as large eyes and a skeleton suited to a cruising mode of swimming, seen in their later relatives. Ichthyosaurs such as the bulky Shastasaurus patrolled upper waters, overtaking prey in their toothless jaws. The deep sea’s slow food revolution had not yet evolved.

Reference:

Motani, R., Ji, C., Tomita, T., Kelley, N., Maxwell, E., Jiang, D., Sander, P. 2013. Absence of suction feeding ichthyosaurs and its implications for Triassic mesopelagic paleoecology. PLoS ONE. 8, 12: e66075. doi:10.1371/journal.pone.0066075

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: e19480. doi:10.1371/journal.pone.0019480