Diving Down Under in the Cretaceous Sea

Cyclone Yasi - the massive tropical storm that hit Queensland, Australia last week – was a Category 5 hurricane, with winds well in excess of 250 km/hr (> 150 mph) at one point. Once it reached that place where the land meets the sea, it smashed a large area from Cairns to Townsville, its eye passing over Mission Beach, then Tully, places I have visited twice with study-abroad students from Emory University. The cyclone continued inland, bringing high winds and heaps of rain to communities inland that normally stay dry: Hughenden, Winton, Richmond, Boulia, and Mt. Isa. As expected, entire beaches vanished, coastal forests were torn apart, and property damage was excessive. Fortunately, though, very few people were injured, a direct result of excellent emergency preparedness by the Australian people and government.

The path of Cyclone Yasi (top), compared to a paleogeographic map of Cretaceous Queensland (bottom). Sometimes the sea comes back to visit, however briefly and horrifically. Cyclone track map is from Wikipedia Commons, and paleogeographic map was in the Museum of Tropical Queensland, Townsville, Australia, a place that was hit hard by Yasi just last week.

So how does Cyclone Yasi relate to the Cretaceous of Queensland? These storms help to remind us that the direct influence of the modern sea on the former site of a Cretaceous seaway is only a hurricane away. Thus the shells, bones, and trace fossils of lives 100 million years past are soaked anew with marine-spawned waters, an elemental circle that has repeated many times, long after ammonites and plesiosaurs cruised through the open waters that covered what we now call Queensland.

So let’s go back to that seaway for several moments. In a previous entry, I introduced my gentle readers to some of the trace fossils of that Cretaceous sea, gastroliths, otherwise known as “stomach stones.”

For Mesozoic vertebrates, gastroliths are normally apparent as localized accumulations of rounded gravel, pebbles, or cobbles in the skeletons of dinosaurs and marine reptiles. However, in Mesozoic vertebrates that lived on land exclusively - like dinosaurs – the existence of gastroliths has been a subject of remarkable controversy. At least one paleontologist has even derisively referred to gastroliths as “gastromyths,” which I, as the founder of The Church of Ichnology, would consider sacrilege if applied to real ones.

“Those aren’t trace fossils – they’re just a bunch of rocks!” Blasphemy! I cast thee out of the Church of Ichnology! No more beer for you! Photo is of gastroliths recovered from a Cretaceous elasmosaur by “Dinosaur Dick” Suter and are on display in Boulia, Queensland, Australia.

Other paleontologists have even started to question long-supposed gastroliths in sauropod dinosaurs; nevertheless, these trace fossils are now getting recognized more often in ornithopods and theropods. Their presence in theropods, though, is unsurprising, considering how many of their living relatives (birds) also use gastroliths.

So why all of the disagreement about gastroliths in the fossil record? It has to do with distinguishing localized collections of rounded rocks that might have accumulated in a skeleton well after an animal died, versus those that were in its body before it died. Sure, skepticism is warranted when the sedimentary strata surrounding fossil skeletons also contain lots of rounded rocks that look just like the ones found inside a skeleton. But other explanations tend to whither on the vine when these stones occur only in the abdominal region of a skeleton. For example, fossil marine reptiles – like plesiosaurs – are oftentimes encased in fine-grained sedimentary rock – like shale or sandstone – with nary a pebble in sight. This means that an accumulation of pebbles inside a vertebrate animal is probably not an accident.

Recreation of the Cretaceous plesiosaur (more specifically, elasmosaur) Woolungasaurus glendowerensis, swimming through the Museum of Tropical Queensland in Townsville, Australia.

When you find a cluster of stones that look like these, they’re inside the rib cage of a Mesozoic marine reptile, and you don’t see any other such stones in the rock surrounding the skeleton, it’s probably OK to identify these as gastroliths. But why are they there? Read on. (Gastroliths at Kronosaurus Korner, Richmond, Queensland, Australia.)

Aside from debating which dinosaurs had gastroliths or not, everyone seems to agree that these rocks assisted in digestion in these animals. When enfolded in a muscular gizzard, a bunch of silica-rich stones would have helped to mechanically grind down tough-to-digest food, rather than relying just on stomach acids or enzymes. But the presence of concentrated masses of rounded rocks in non-dinosaurian vertebrates, such as Mesozoic marine reptiles, caused a little bit of pause in paleontologists before they also identified these as gastroliths. The hesitation felt by paleontologists was not so much about “what,” but more about “why,” as in “Why did marine reptiles swallow rocks?” Digestion was not considered as the sole reason, because these animals only ate seafood, most of which should not have required as much grinding as tough plants, insects, or other terrestrial foodstuff.

As a result of one idea discarded, an alternative one proposed by paleontologists was that these rocks somehow contributed to buoyancy control. For example, everyone who has done SCUBA diving knows that weight belts need to be used in combination with a buoyancy compensator (BC) vest. This human-based analogy, however imperfect, helps us to think about how these reptiles – some of which were multi-tonne animals – moved vertically within a watery environment to find food, mate, give birth, go up to the surface to breathe, and otherwise conduct marine-reptile business. To float, they had to ingest air at the surface, and to sink, they had to exhale, all of which was aided by coordinated movements of the fins and tail.

So let’s just look at some large, modern, air-breathing marine vertebrates for an example of how this is done, and the mystery will be solved, right? In fact, whales would be perfect, right? Well, no. Turns out gastroliths are absent in cetaceans (whales, dolphins, and porpoises); instead, these animals have other adaptations that allow them to easily move up and down in the water column, yet don’t require them to swallow anything other than food. Sea turtles are a bust, too: not one modern or fossil sea turtle had been found with gastroliths. Turned out the only modern marine vertebrates that have gastroliths are some pinnipeds (seals, sea lions and walruses) and crocodilians (alligators and crocodiles).  So because crocodilians are reptiles, let’s focus on those, and specifically we’ll take a look at estuarine crocodiles (Crocodylus porous) of Australia, which are affectionately known as “salties” because of their tolerance of marine salinity.

An estuarine crocodile (“salty”) in a tank at the Alice Springs Reptile Centre (Northern Territory, Australia), showing excellent buoyancy control as it hangs out on the bottom, whilst readily evoking thoughts of large marine reptiles in the Cretaceous. For perfectly rational reasons, I did not check to see whether it had any gastroliths.

The “rocks must help with buoyancy control” idea, though, took a hit with a 2003 study that showed that gastroliths make up far too little weight compared to the body weight of a crocodilian (Alligator mississippiensis): only about 1-2%. This means they have little effect on a crocodile just moving up and down in the water. So this hypothesis got tweaked a bit: maybe these were used not so much as ballast for diving, but for stabilizing while swimming. Instead of weight belts, then, think of the tiny weights placed on tires to balance them on a car, which help the tires to rotate more evenly.

OK, time for the thesis-antithesis-synthesis approach. How about, gastroliths were used for both buoyancy control (balance more than ballast) and helping to digest food. Maybe we may have a winner! (At least, for now. We scientists have this peculiar habit of changing our minds about hypotheses once we get new evidence.) Two Cretaceous elasmosaurs from Queensland, described in 2005, not only had gastroliths, they also unexpectedly contained stomach contents (fossilized, of course), which included bits of clams, snails, crustaceans, and other invertebrates that lived on the ocean bottom. This evidence was a big surprise, because paleontologists had always assumed elasmosaurs only ate fish, and never thought of them as bottom feeders. You reckon some rocks in your stomach would help to grind down those hard-shelled critters and make them easier to digest? (Hopefully you said “yes,” otherwise stop reading this right now, and go back to watching CSI.)

So now we have a new hypothesis for gastroliths in marine reptiles, which is a variation on a previous one, but also combines another one given previously for these trace fossils. In summary, the more things change, the more they stay the same, unless they change, then they’re not exactly the same, especially if they only changed a little bit.

Famed Australian singer-songwriter Kasey Chambers (and oh yeah, Shane Nicholson), perform Rattlin’ Bones, singing of “…smoke not rising, fuel not burning, the sun not shining, late-night sorrow scratching at a door”; and “dragging stones until buried beneath the ground, with dust and bones.” Lyrics that evoke Cyclone Yasi and gastroliths in Cretaceous plesiosaurs, respectively. Best wishes to all in Queensland affected by Yasi.

Further Reading on Gastroliths

Cerda, I.A. 2008. Gastroliths in an ornithopod dinosaur. Acta Palaeontologica Polonica, v. 53, p. 351-355.

Drehmer, C.J., and de Oliveira, L.R. 2003. Occurrence of gastroliths in South American sea lions (Otaria byronia) from southern Brazil. LAJAM, v. 2, 123-126.

Henderson, D.M. 2003. Effects of stomach stones on the buoyancy and equilibrium of a floating crocodilian: a computational analysis. Canadian Journal of Zoology, v. 81, p. 1346-1357.

McHenry, C.R., Cook, A.G., and Wroe, S. 2005. Bottom-feeding plesiosaurs. Science, v. 310, p. 75.

Taylor, M.A. 1993. Stomach stones for feeding or buoyancy? The occurrence and function of gastroliths in marine tetrapods. Philosophical Transactions: Biological Sciences, v. 341, p. 163-175.

Wings, O. 2007. A review of gastrolith function with implications for fossil vertebrates and a revised classification. Acta Palaeontologica Polonica, v. 52, p. 1–16.