Dinocephalosaurus – a star gazer, benthic feeder

Earlier we looked at the odd macrocnemid, Dinocephalosaurus (Fig. 1). This story opens with the first description of Dinocephalosaurus (Li et al. 2004), then follows with a response by Peters, Demes and Krause (2005) that disputed the swimming and sucking abilities originally ascribed, proposing instead a benthic ambush predation mode.

Dinocephalosaurus. Note the very narrow cranial portion of the skull and the very wide cheeks. That, by it self, opens the orbits dorsally. Sure there's some lateral exposure, but those eyes are looking up!

Figure 1. Dinocephalosaurus. Note the very narrow cranial portion of the skull and the very wide cheeks. That, by it self, opens the orbits dorsally. Sure there’s some lateral exposure, but those eyes are looking up!

Two of the original authors, LaBarbera and Rieppel (2005) reported in their reply: “We think it unlikely that Dinocephalosaurus was a benthic ambush predator. First, we would expect that the eyes in a benthic ambush predator would be dorsally located to monitor the overlying water (as seen in living frogfish and flatfish); the eyes of Dinocephalosaurus are anteriorlaterally positioned, apparently to monitor regions to the sides and in front of the snout.”

First of all, I disagree that frogfish and flatfish have the same sort of eye orientation. But be that as it may, one look at the skull of Dinocephalosaurus makes it easy to see the narrow inter orbital region (the frontals) as compared to the much wider skull. This alone produces an orbit that looks up. Sure there’s a lateral aspect, but the dorsal aspect is plainly present to an extent not seen in related Macrocnemus and Tanystropheus specimens.

LaBarbera and Rieppel (2005) report, “Dinocephalosaurus and suggests that the relative size of the limbs indicates a “poor swimmer.” We disagree. Two pairs of 30-cm-long, flipper-shaped f ins seem more than adequate to drive a 1-m-long body. Living sea lions (Zalophus californianus) have a similar ratio of flipper to body length.”

Big limbs, really, not so much… but look at how tiny the pectoral and pelvic girdles are. That’s where the muscles anchor. That’s where the strength originates. These are not the girdles of a strong and steady swimmer.

LaBarbera and Rieppel (2005) report, “In addition, Peters’ reconstruction would have Dinocephalosaurus capture prey by sweeping its neck through the water. We find this unlikely because (i) the cervical vertebrae lack neural processes that would improve the mechanical advantage of the (necessarily small) neck muscles to drive dorsiflexion, and (ii) such motion would generate high drag forces on the neck that would tend to drag the body of the animal  along the substrate in the direction opposite to the motion of the head. (The neck would act as an oar.)”

Let’s remind ourselves that the speed of “sweeping” the neck through the water is not an issue. The neck could have arisen slowly, not moving quickly until the last moment and then just the last few inches of neck would have been involved. So, no speed, no drag, no opposite motion, which was prevented in any case by the extreme width of the extremely flattened torso (a morphology ignored originally), and wide paddles.

Dinocephalosaurus in resting, feeding and breathing modes.

Figure 2. Dinocephalosaurus in resting, feeding and breathing modes. In breathing mode the throat sac would capture air that would not be inhaled until the neck was horizontal at the bottom of the shallow sea. Orbits on top of the skull support this hypothesis.

LaBabera and Rieppel (2005) report, “Peters rejects our hypothesis that Dinocephalosaurus may have employed suction feeding (driven by expansion of the cervical ribs) as a mode of prey capture on the basis that the cervical ribs are “bound to one another.” We know of no evidence to  suggest that the cervical ribs were bound to each other; indeed, the dispersal of the cervical ribs in the only available specimen would seem to indicate that tissues that surrounded the cervical ribs were quite liable to decay and thus unlikely to have been collagenous or cartilagenous.”

Well, they were bound together by their mutual length and overlapping proximity to each other (like uncooked spaghetti noodles), surrounded by skin. There’s nothing here more elaborate than anything seen in Tanystropheus and Macrocnemus, which do not have such expansion abilities. If the cervicals were able to rotate on some sort of axis, some sort of axis should be visible, but there’s nothing there. Expansion should have occurred in the cheeks or the stomach, two regions in which some small amount of expansion is already possible. The esophagus works by peristaltic motion, squeezing food toward the stomach. There little possibility for it to expand like the cheeks of a frogfish. The structure of the neck cervicals in Dinocephalosaurus, used to strengthen the extreme length of the neck, would be compromised by any lateral expansion.

If anything, let’s look for hyoids that might expand. Perhaps that’s where the confusion lies after all.

LaBabera and Rieppel (2005) report, “We have no direct evidence that Dinocephalosaurus used the cervical ribs to expand the throat, but that hypothesis is consistent with the observed morphology and we continue to search for additional tests of the hypothesis. If cervical ribs were used to power suction feeding in this animal, that function was certainly an exaptation.”

And we all appreciate this candor.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Li C, Rieppel O and LaBarbera MC 2004. A Triassic aquatic protorosaur with an extremely long neck. Science 305:1931.
LaBarbera M and Rieppel O 2005. Response. Science 308, p. 1113.
Peters D, Demes B and Krause DW 2005. Suction feeding in Triassic Protorosaur? Science, 308: 1112-1113.

wiki/Dinocephalosaurus

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.